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Mechanical Behavior of Shape Memory Alloys: 2022
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Mechanical Behavior of Shape Memory Alloys: 2022

A special issue of Materials (ISSN 1996-1944). This special issue belongs to the section "Metals and Alloys".

Deadline for manuscript submissions: closed (20 November 2022) | Viewed by 38723

Printed Edition Available!
A printed edition of this Special Issue is available here.

Special Issue Editor

Dr. Salvatore Saputo

E-Mail Website
Guest Editor
Department of Mechanical and Aerospace Engineering, Politecnico di Torino, Corso Duca degli Abruzzi 24, 10129 Torino, Italy
Interests: CDM; delamination; dynamic analysis

Special Issue Information

Dear Colleagues,

Over recent years, interest in shape material alloys has continuously increased in several fields, such as aerospace, automotive, naval, civil, and biology. The features that make shape memory alloys attractive are the ability to recover a deformation after heating and the pseudoelastic stress–strain behavior for large deformations, as well as the biocompatibility that makes these alloys extremely interesting for the bioengineering application. To effectively use shape memory alloys, an accurate description of certain characteristics such as the critical transformation temperature and stress values is mandatory. All researchers working on shape memory materials are invited to contribute their work to this Special Issue on the “Mechanical Behavior of Shape Memory Alloys: 2021”, which will cover different topics concerning the behavior of shape memory materials from an analytical, experimental, and numerical perspective. Contributions on manufacturing processes with shape memory materials and smart structures are welcome.

The following topics will be covered in this Special Issue, among others:

  • Smart materials;
  • Smart structure and devices;
  • Piezoelectric materials;
  • Shape memory alloys (SMAs);
  • Shape memory effect (SME);
  • Analytical and numerical smart materials models;
  • Smart materials properties and characterizations;
  • Self-recovering materials;
  • SMA manufacturing, testing, and design;
  • SMA thermomechanical behavior;
  • SMA thermoelectric behavior.

Dr. Salvatore Saputo
Guest Editor

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Keywords

  • shape memory alloy (SMAs)
  • shape memory effect (SME)
  • superelasticity
  • smart materials
  • constitutive models
  • simulation and modeling
  • smart devices

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Published Papers (16 papers)

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Research

16 pages, 11407 KiB  
Article
Structure and Mechanical Properties of the NiTi Wire Joined by Laser Welding
by Tomasz Goryczka, Karol Gryń, Adrian Barylski and Barbara Szaraniec
Materials 2023, 16(7), 2543; https://doi.org/10.3390/ma16072543 - 23 Mar 2023
Cited by 1 | Viewed by 1745
Abstract
Joining wires made of NiTi alloys with shape memory effect and pseudoelasticity causes many technical and structural problems. They result from unwanted phase interactions that occur in high temperatures and negatively affect the characteristics of these materials. Such obstacles are challenging in terms [...] Read more.
Joining wires made of NiTi alloys with shape memory effect and pseudoelasticity causes many technical and structural problems. They result from unwanted phase interactions that occur in high temperatures and negatively affect the characteristics of these materials. Such obstacles are challenging in terms of welding. Hence, an attempt was made to join NiTi wires via an economical and reliable basic laser welding technique which does not require complicated equipment and gas protection. The parameters such as spot diameter and pulse time were constant and only the laser power, calculated as a percentage of the total power, was optimized. The wires were parallelly connected with overlapping seam welds 10 mm long. The welds were examined regarding their microstructure, chemical and phase composition, reversible martensitic transformation, microhardness, and pseudoelasticity. The obtained results showed that the joint was completed at the 12–14% power. The weld revealed good quality with no voids or pores. As the laser power increased, the microhardness rose from 282 (for 4%) to 321 (for 14%). The joint withstood the stress-inducing reversible martensitic transformation. As the transformation was repeated cyclically, the stress value decreased from 587 MPa (initial wire) to 507 MPa (for the 14% power welded wire). Full article
(This article belongs to the Special Issue Mechanical Behavior of Shape Memory Alloys: 2022)
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Figure 1

Figure 1
<p>As-delivered NiTi wire (<b>a</b>) and an example of the overlapping seam weld (<b>b</b>).</p> Full article ">Figure 2
<p>Scheme of mounting the welded wires in the tensile test holders.</p> Full article ">Figure 3
<p>Images of the wire’s cross-sections: (<b>a</b>) transverse (SEM-SE); (<b>b</b>) longitudinal (SEM-BS).</p> Full article ">Figure 4
<p>SEM image of the transverse wire’s cross-sections with Ti<sub>2</sub>Ni particles (<b>a</b>) as well as the measured EDS spectra (<b>b</b>).</p> Full article ">Figure 5
<p>X-ray diffraction patterns measured for the as-received (black-line) and the welded wires with the Curr power of 14% (blue-line).</p> Full article ">Figure 6
<p>Scheme of the formation of the weld cross-section depending on the applied laser power.</p> Full article ">Figure 7
<p>Microscopic image of the weld bead with an example of the weld profile modelling (marked as a blue line) (<b>a</b>), the 3D model of the welded area (<b>b</b>) and the modelled weld profile (<b>c</b>).</p> Full article ">Figure 8
<p>DSC cooling (<b>a</b>) and heating (<b>b</b>) curves measured for the as-received wire and the welded one with various laser powers.</p> Full article ">Figure 9
<p>SEM-BS images observed on the transverse cross-section of the wires welded with the power of 4% (<b>a</b>) and 14% (<b>b</b>).</p> Full article ">Figure 10
<p>Results of the indentation test done on the wire’s cross-section (<b>a</b>) and the weld obtained with the laser power of 10% (<b>b</b>).</p> Full article ">Figure 11
<p>SEM-BS image of the exemplary indentations in the wire.</p> Full article ">Figure 12
<p>SEM-BS images of the indentations done in the fusion zone for Point 1 (<b>a</b>) and Point 3 in the wires welded with the laser power of 10% (<b>b</b>).</p> Full article ">Figure 13
<p>Stress-strain curves measured for the as-received as well as for the welded ones at the various percentages of the maximum (100%) laser beam power.</p> Full article ">Figure 14
<p>The critical stress triggering the martensitic transformation for the forward (<b>a</b>) and reverse course (<b>b</b>).</p> Full article ">
11 pages, 7046 KiB  
Article
An SMA Transducer for Sensing Tactile Sensation Focusing on Stroking Motion
by Ryusei Oya and Hideyuki Sawada
Materials 2023, 16(3), 1016; https://doi.org/10.3390/ma16031016 - 22 Jan 2023
Cited by 3 | Viewed by 2142
Abstract
The authors have developed a micro-vibration actuator using filiform SMA wire electrically driven by periodic electric current. While applying the SMA actuators to tactile displays, we discovered a phenomenon that the deformation caused by a given stress to an SMA wire generated a [...] Read more.
The authors have developed a micro-vibration actuator using filiform SMA wire electrically driven by periodic electric current. While applying the SMA actuators to tactile displays, we discovered a phenomenon that the deformation caused by a given stress to an SMA wire generated a change in the electrical resistance. With this characteristic, the SMA wire works as a micro-force sensor with high sensitivity, while generating micro-vibration. In this paper, the micro-force sensing ability of an SMA transducer is described and discussed. Experiments are conducted by sliding the SMA sensor on the surface of different objects with different speeds, and the sensing ability is evaluated to be related with human tactile sensation. Full article
(This article belongs to the Special Issue Mechanical Behavior of Shape Memory Alloys: 2022)
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Figure 1

Figure 1
<p>Physical properties of the deformation of an SMA.</p> Full article ">Figure 2
<p>SMA vibration actuator.</p> Full article ">Figure 3
<p>Temperature characteristics of SMA wire.</p> Full article ">Figure 4
<p>SMA sensor for sensing micro-force.</p> Full article ">Figure 5
<p>Sliding the SMA sensor on material surface.</p> Full article ">Figure 6
<p>Tactile sensing system using a SMA sensor.</p> Full article ">Figure 7
<p>SMA sensor mounted on a linear plotter.</p> Full article ">Figure 8
<p>SMA driving circuit.</p> Full article ">Figure 9
<p>Selected five materials with different textures.</p> Full article ">Figure 10
<p>Examples of sensing data with different stroking speeds: (<b>a</b>) sliding speed 1 cm/s, (<b>b</b>) sliding speed 3 cm/s, (<b>c</b>) sliding speed 5 cm/s, (<b>d</b>) sliding speed 7.5 cm/s, (<b>e</b>) sliding speed 10 cm/s, (<b>f</b>) sliding speed 12.5 cm/s, (<b>g</b>) sliding speed 15 cm/s, (<b>h</b>) sliding speed 20 cm/s.</p> Full article ">Figure 10 Cont.
<p>Examples of sensing data with different stroking speeds: (<b>a</b>) sliding speed 1 cm/s, (<b>b</b>) sliding speed 3 cm/s, (<b>c</b>) sliding speed 5 cm/s, (<b>d</b>) sliding speed 7.5 cm/s, (<b>e</b>) sliding speed 10 cm/s, (<b>f</b>) sliding speed 12.5 cm/s, (<b>g</b>) sliding speed 15 cm/s, (<b>h</b>) sliding speed 20 cm/s.</p> Full article ">Figure 11
<p>Ten materials for tactile classification experiment.</p> Full article ">Figure 12
<p>Confusion matrix of classification for ten different materials at sliding speed 10 cm/s.</p> Full article ">Figure 13
<p>Contact situation between a tactile pin tip and material surface.</p> Full article ">
12 pages, 5016 KiB  
Article
Production, Mechanical and Functional Properties of Long-Length TiNiHf Rods with High-Temperature Shape Memory Effect
by Roman Karelin, Victor Komarov, Vladimir Cherkasov, Vladimir Yusupov, Sergey Prokoshkin and Vladimir Andreev
Materials 2023, 16(2), 615; https://doi.org/10.3390/ma16020615 - 9 Jan 2023
Cited by 1 | Viewed by 1452
Abstract
In the present work, the possibility of manufacturing long-length TiNiHf rods with a lowered Hf content and a high-temperature shape memory effect in the range of 120–160 °C was studied. Initial ingots with 1.5, 3.0 and 5.0 at.% Hf were obtained by electron [...] Read more.
In the present work, the possibility of manufacturing long-length TiNiHf rods with a lowered Hf content and a high-temperature shape memory effect in the range of 120–160 °C was studied. Initial ingots with 1.5, 3.0 and 5.0 at.% Hf were obtained by electron beam melting in a copper water-cooled stream-type mold. The obtained ingots were rotary forged at the temperature of 950 °C, with the relative strain from 5 to 10% per one pass. The obtained results revealed that the ingots with 3.0 and 5.0 at.% Hf demonstrated insufficient technological plasticity, presumably because of the excess precipitation of (Ti,Hf)2Ni-type particles. The premature destruction of ingots during the deformation process does not allow obtaining high-quality long-length rods. A long-length rod with a diameter of 3.5 mm and a length of 870 mm was produced by rotary forging from the ingot with 1.5 at.% Hf. The obtained TiNiHf rod had relatively high values of mechanical properties (a dislocation yield stress σy of 800 MPa, ultimate tensile strength σB of 1000 MPa, and elongation to fracture δ of 24%), functional properties (a completely recoverable strain of 5%), and a required finishing temperature of shape recovery of 125 °C in the as-forged state and of 155 °C after post-deformation annealing at 550 °C for 2 h. Full article
(This article belongs to the Special Issue Mechanical Behavior of Shape Memory Alloys: 2022)
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Figure 1

Figure 1
<p>Images of TiNiHf initial ingots obtained by electron beam melting in a copper water-cooled stream-type mold: 1.5 at.% of Hf (<b>a</b>), 3.0 at.% of Hf (<b>b</b>), and 5.0 at.% of Hf (<b>c</b>).</p> Full article ">Figure 2
<p>Image of TiNiHf sample with 3.0 at.% Hf after several passes of rotary forging.</p> Full article ">Figure 3
<p>X-ray diffractograms of TiNiHf SMA rods with a diameter of 3.5 mm after RF and RF + PDA 550 °C for 2 h.</p> Full article ">Figure 4
<p>Microstructure of the TiNiHf SMA rods with a diameter of 3.5 mm after RF (<b>a</b>) and RF + PDA 550 °C for 2 h (<b>b</b>). Light optical microscopy.</p> Full article ">Figure 5
<p>SEM images of TiNiHf rods after RF (<b>a</b>,<b>c</b>) and RF + PDA (<b>b</b>).</p> Full article ">Figure 6
<p>Elemental EDS mapping of Ti, Ni and Hf in TiNiHf rods after RF (<b>a</b>) and with higher resolution (<b>b</b>).</p> Full article ">Figure 7
<p>Calorimetric curves of the TiNiHf initial ingots in as-cast state and after annealing at 1000 °C for 1 h: 1.5 at.% Hf—(<b>a</b>,<b>c</b>), 3.0 at.% Hf—(<b>b</b>,<b>d</b>), and 5.0 at.% Hf—(<b>e</b>,<b>f</b>).</p> Full article ">Figure 8
<p>Calorimetric curves of the TiNiHf rods after RF (<b>a</b>) and RF + PDA at 550 °C for 2 h (<b>b</b>).</p> Full article ">Figure 9
<p>Representative stress–strain diagrams of TiNiHf rods after RF and RF + PDA at 550 °C for 2 h.</p> Full article ">
9 pages, 2420 KiB  
Article
Evolution of Structure and Properties of Nickel-Enriched NiTi Shape Memory Alloy Subjected to Bi-Axial Deformation
by Victor Komarov, Roman Karelin, Irina Khmelevskaya, Vladimir Cherkasov, Vladimir Yusupov, Grzegorz Korpala, Rudolf Kawalla, Ulrich Prahl and Sergey Prokoshkin
Materials 2023, 16(2), 511; https://doi.org/10.3390/ma16020511 - 5 Jan 2023
Cited by 2 | Viewed by 1722
Abstract
The effect of a promising method of performing a thermomechanical treatment which provides the nanocrystalline structure formation in bulk NiTi shape memory alloy samples and a corresponding improvement to their properties was studied in the present work. The bi-axial severe plastic deformation of [...] Read more.
The effect of a promising method of performing a thermomechanical treatment which provides the nanocrystalline structure formation in bulk NiTi shape memory alloy samples and a corresponding improvement to their properties was studied in the present work. The bi-axial severe plastic deformation of Ti-50.7at.%Ni alloy was carried out on the MaxStrain module of the Gleeble system at 350 and 330 °C with accumulated true strains of e = 6.6–9.5. The obtained structure and its mechanical and functional properties and martensitic transformations were studied using DSC, X-ray diffractometry, and TEM. A nanocrystalline structure with a grain/subgrain size of below 80 nm was formed in bulk nickel-enriched NiTi alloy after the MaxStrain deformation at 330 °C with e = 9.5. The application of MaxStrain leads to the formation of a nanocrystalline structure that is characterized by the appearance of a nano-sized grains and subgrains with equiaxed and elongated shapes and a high free dislocation density. After the MaxStrain deformation at 330 °C with e = 9.5 was performed, the completely nanocrystalline structure with the grain/subgrain size of below 80 nm was formed in bulk nickel-enriched NiTi alloy for the first time. The resulting structure provides a total recoverable strain of 12%, which exceeds the highest values that have been reported for bulk nickel-enriched NiTi samples. Full article
(This article belongs to the Special Issue Mechanical Behavior of Shape Memory Alloys: 2022)
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Figure 1

Figure 1
<p>Deformation scheme on the MaxStrain module.</p> Full article ">Figure 2
<p>Scanning electron microscopy image of the NiTi SMA sample in the place of destruction after deformation at 330 °C with <span class="html-italic">e</span> = 9.5.</p> Full article ">Figure 3
<p>X-ray line{110}<sub>B2</sub> profiles (<b>a</b>) and FWHM (<b>b</b>) of NiTi at room temperature after various treatments.</p> Full article ">Figure 4
<p>Structure of NiTi alloy after reference treatment, light optical microscopy (<b>a</b>), after MS deformation at 350 °C, <span class="html-italic">e</span> = 6.6 (<b>b</b>), and 330 °C, <span class="html-italic">e</span> = 9.5 (<b>c</b>); both of them involved transmission electron microscopy. Left, bright-field images; center, dark-field images; right, selected area electron diffraction patterns.</p> Full article ">Figure 5
<p>DSC curves of NiTi at room temperature after various treatments.</p> Full article ">
17 pages, 18699 KiB  
Article
Effect of Severe Plastic Deformation and Post-Deformation Heat Treatment on the Microstructure and Superelastic Properties of Ti-50.8 at.% Ni Alloy
by Tae-Jin Lee and Woo-Jin Kim
Materials 2022, 15(21), 7822; https://doi.org/10.3390/ma15217822 - 5 Nov 2022
Cited by 6 | Viewed by 1970
Abstract
Severe plastic deformation via high-ratio differential speed rolling (HRDSR) was applied to the Ni-rich Ti-50.8Ni alloy. Application of HRDSR and a short annealing time of 5 min at 873 K leads to the production of a partially recrystallized microstructure with a small grain [...] Read more.
Severe plastic deformation via high-ratio differential speed rolling (HRDSR) was applied to the Ni-rich Ti-50.8Ni alloy. Application of HRDSR and a short annealing time of 5 min at 873 K leads to the production of a partially recrystallized microstructure with a small grain size of 5.1 μm. During the aging process for the annealed HRDSR sample at 523 K for 16 h, a high density of Ni3Ti4 particles was uniformly precipitated over the matrix, resulting in the formation of an R phase as the major phase at room temperature. The aged HRDSR sample exhibits excellent superelasticity and superelastic cyclability. This achievement can be attributed to an increase in strength through effective grain refinement and particle strengthening by Ni3Ti4 and a decrease in the critical stress for stress-induced martensite (B19′) due to the presence of the R-phase instead of B2 as a major phase at room temperature. The currently proposed method for using HRDSR and post-deformation heat treatment allows for the production of Ni-rich NiTi alloys with excellent superelasticity in sheet form. Full article
(This article belongs to the Special Issue Mechanical Behavior of Shape Memory Alloys: 2022)
►▼ Show Figures

Figure 1

Figure 1
<p>The inverse pole figure, KAM and GB maps for the (<b>a</b>–<b>c</b>) as-received (AR) and (<b>d</b>–<b>f</b>) HRDSR samples. In the GB map, low-angle boundaries (2–5°) are in red, intermediate angle boundaries (5–15°) are in green and high-angle boundaries (&gt;15°) are in blue.</p> Full article ">Figure 2
<p>(<b>a</b>) The DSC curve for the AR and HRDSR samples. (<b>b</b>) The XRD curves for the AR and HRDSR samples. Identification of phases was made based on the data from JCPDF cards (01−076 −3614, 01−076−7519 and 01−076−4263).</p> Full article ">Figure 3
<p>The IPF maps for the AR samples (<b>a</b>) annealed for 5 min at 873 K, (<b>b</b>) annealed for 5 min at 873 K and then aged at 523 K for 16 h, (<b>c</b>) annealed for 120 min at 873 K, (<b>d</b>) annealed for 120 min at 873 K and then aged at 523 K for 16 h. The IPF maps for the HRDSR samples (<b>e</b>) annealed for 5 min at 873 K, (<b>f</b>) annealed for 5 min at 873 K and then aged at 523 K for 16 h, (<b>g</b>) annealed for 120 min at 873 K, (<b>h</b>) annealed for 120 min at 873 K and then aged at 523 K for 16 h.</p> Full article ">Figure 4
<p>(<b>a</b>) The grain size of the AR and HRDSR samples before and after heat treatment. (<b>b</b>) The fraction of recrystallized grains in the AR and HRDSR samples before and after heat treatment determined using GOS.</p> Full article ">Figure 5
<p>The DSC curves for (<b>a</b>) the annealed and (<b>b</b>) the aged AR and HRDSR samples. (<b>c</b>) The DSC curves for the aged AR and HRDSR samples magnified in the temperature range between 220 and 300 K upon cooling.</p> Full article ">Figure 6
<p>The XRD curves for the (<b>a</b>) AR and (<b>b</b>) HRDSR samples after annealing or annealing plus aging. Identification of phases was made based on the data from JCPDF cards (01−076−3614, 01−076−7519 and 01−076−4263).</p> Full article ">Figure 7
<p>The KAM maps for the AR samples (<b>a</b>) annealed for 5 min at 873 K, (<b>b</b>) annealed for 5 min at 873 K and then aged at 523 K for 16 h, (<b>c</b>) annealed for 120 min at 873 K, (<b>d</b>) annealed for 120 min at 873 K and then aged at 523 K for 16 h. The IPF maps for the HRDSR samples (<b>e</b>) annealed for 5 min at 873 K, (<b>f</b>) annealed for 5 min at 873 K and then aged at 523 K for 16 h, (<b>g</b>) annealed for 120 min at 873 K, (<b>h</b>) annealed for 120 min at 873 K and then aged at 523 K for 16 h.</p> Full article ">Figure 8
<p>The average KAM values of the AR and HRDSR samples after annealing or annealing plus aging.</p> Full article ">Figure 9
<p>The TEM micrographs for the aged HRDSR sample (after annealing for 5 min): (<b>a</b>) near grain boundaries and (<b>b</b>) grain interior. The (<b>c</b>) TEM and (<b>d</b>) SEM micrographs for the aged HRDSR sample (after annealing for 120 min). The regions marked by yellow and red circles represent the regions where B2+<span class="html-italic">R</span> phases and B2+Ni<sub>4</sub>Ti<sub>3</sub> phases are identified to exist, respectively.</p> Full article ">Figure 10
<p>The [233]<sub>B2</sub>//RD, [111]<sub>B2</sub>//RD and [011]<sub>B2</sub>//RD texture components mapped on the EBSD-generated microstructures of the AR samples (<b>a</b>) annealed for 5 min at 873 K and (<b>b</b>) annealed for 5 min at 873 K and then aged at 523 K for 16 h, and the HRDSR samples (<b>c</b>) annealed for 5 min at 873 K and (<b>d</b>) annealed for 5 min at 873 K and then aged at 523 K for 16 h.</p> Full article ">Figure 11
<p>The total fraction of grains with the texture components of [233]<sub>B2</sub>//RD, [111]<sub>B2</sub>//RD and [011]<sub>B2</sub>//RD texture components for the AR and HRDSR samples after annealing or annealing plus aging.</p> Full article ">Figure 12
<p>The Vickers hardness of the (<b>a</b>) AR and (<b>b</b>) HRDSR samples after annealing or annealing plus aging.</p> Full article ">Figure 13
<p>Superelastic cyclic tests up to the strain of 6% for (<b>a</b>) the AR sample, (<b>b</b>) the AR sample annealed at 873 K for 5 min, (<b>c</b>) the AR sample annealed at 873 K for 120 min, (<b>d</b>) the HRDSR sample, (<b>e</b>) the HRDSR sample annealed at 873 K for 5 min and (<b>f</b>) the HRDSR sample annealed at 873 K for 120 min.</p> Full article ">Figure 14
<p>Residual strain as a function of cyclic number for the AR and HRDSR samples after annealing or annealing plus aging.</p> Full article ">Figure 15
<p>Superelastic cyclic tests up to the strain of 6% for (<b>a</b>) the aged AR sample (after annealing at 873 K for 5 min), (<b>b</b>) the aged AR sample (after annealing at 873 K for 120 min), (<b>c</b>) the aged HRDSR sample (after annealing at 873 K for 5 min) and (<b>d</b>) the aged HRDSR sample (after annealing at 873 K for 120 min).</p> Full article ">Figure 16
<p>(<b>a</b>) The difference between the yield strength and the critical stress for phase martensitic transformation (Δσ) and (<b>b</b>) the relationship between Δσ and residual strain.</p> Full article ">
18 pages, 6060 KiB  
Article
Thermal–Optical Evaluation of an Optimized Trough Solar Concentrator for an Advanced Solar-Tracking Application Using Shape Memory Alloy
by Nasir Ghazi Hariri, Kamal Mohamed Nayel, Emad Khalid Alyoubi, Ibrahim Khalil Almadani, Ibrahim Sufian Osman and Badr Ahmed Al-Qahtani
Materials 2022, 15(20), 7110; https://doi.org/10.3390/ma15207110 - 13 Oct 2022
Cited by 4 | Viewed by 3309
Abstract
One of the modern methods for enhancing the efficiency of photovoltaic (PV) systems is implementing a solar tracking mechanism in order to redirect PV modules toward the sun throughout the day. However, the use of solar trackers increases the system’s electrical consumption, hindering [...] Read more.
One of the modern methods for enhancing the efficiency of photovoltaic (PV) systems is implementing a solar tracking mechanism in order to redirect PV modules toward the sun throughout the day. However, the use of solar trackers increases the system’s electrical consumption, hindering its net generated energy. In this study, a novel self-tracking solar-driven PV system is proposed. The smart solar-driven thermomechanical actuator takes advantage of a solar heat collector (SHC) device, in the form of a parabolic trough solar concentrator (PTC), and smart shape memory alloy (SMA) to produce effective mechanical energy for solar tracking applications from sun rays. Furthermore, a thermal–optical analysis is presented to evaluate the performance of the solar concentrator for the simulated weather condition of Dammam City, Saudi Arabia. The numerical results of the thermal and optical analyses show the promising feasibility of the proposed system in which SMA springs with an activation temperature between 31.09 °C and 45.15 °C can be utilized for the self-tracking operations. The work presented adds to the body of knowledge an advanced SMA-based SHC device for solar-based self-actuation systems, which enables further expansions within modern and advanced solar thermal applications. Full article
(This article belongs to the Special Issue Mechanical Behavior of Shape Memory Alloys: 2022)
►▼ Show Figures

Figure 1

Figure 1
<p>Conceptual design of the solar-driven SMA thermomechanical actuator.</p> Full article ">Figure 2
<p>SMA-based actuator phases; (<b>a</b>) activated and (<b>b</b>) deactivated arrangements.</p> Full article ">Figure 3
<p>Thermomechanical actuator (<b>a</b>) before sunset, (<b>b</b>) at noon, and (<b>c</b>) after sunrise.</p> Full article ">Figure 4
<p>CAD model of the designed thermomechanical SMA actuator.</p> Full article ">Figure 5
<p>Thermal resistance for the thermomechanical SMA actuator.</p> Full article ">Figure 6
<p>Imported CAD model of thermomechanical SMA actuator within the CFD software.</p> Full article ">Figure 7
<p>The external natural convection process for (<b>a</b>) upper horizontal plate and (<b>b</b>) inclined plate.</p> Full article ">Figure 8
<p>Physics-controlled mesh sequence in the multiphysics interface solution tool for the thermomechanical SMA actuator.</p> Full article ">Figure 9
<p>Surface plot of the inward heat flux through the projected area of the lower surface of the triangular SHC without a reflector at (<b>a</b>) 9:00; (<b>b</b>) 12:00; and (<b>c</b>) 15:00 on 1 July 2022.</p> Full article ">Figure 10
<p>Surface plot of the inward heat flux through the projected area of the lower surface of the triangular SHC with a reflector at (<b>a</b>) 9:00; (<b>b</b>) 12:00; and (<b>c</b>) 15:00 on 1 July 2022.</p> Full article ">Figure 11
<p>Variation in the amount of inward heat flux through the lower surface of the triangular SHC during a whole day with the effect of using the reflector.</p> Full article ">Figure 12
<p>Three-dimensional plot of the temperature gradient of the circular SHC with a reflector at (<b>a</b>) 9:00, (<b>b</b>) 12:00, and (<b>c</b>) 15:00 and the temperature gradient of the triangular SHC with a reflector at (<b>d</b>) 9:00, (<b>e</b>) 12:00, and (<b>f</b>) 15:00.</p> Full article ">Figure 13
<p>Midpoint temperature for different shapes of SHC.</p> Full article ">Figure 14
<p>Midpoint temperature for triangular SHC for different values of aperture width.</p> Full article ">Figure 15
<p>Midpoint temperature for triangular SHC for different values of focal length.</p> Full article ">Figure 16
<p>Temperature profile of the triangular SHC with a reflector at (<b>a</b>) 9:00; (<b>b</b>) 12:00; (<b>c</b>) and 15:00 on 1 July 2022.</p> Full article ">Figure 17
<p>Maximum and minimum daily midpoint temperature and ambient temperature variation throughout 2022G.</p> Full article ">
21 pages, 10653 KiB  
Article
A State-of-the-Art Self-Cleaning System Using Thermomechanical Effect in Shape Memory Alloy for Smart Photovoltaic Applications
by Nasir Ghazi Hariri, Ibrahim Khalil Almadani and Ibrahim Sufian Osman
Materials 2022, 15(16), 5704; https://doi.org/10.3390/ma15165704 - 18 Aug 2022
Cited by 8 | Viewed by 2662
Abstract
This research aims to present a state-of-the-art cleaning technology solution that effectively overcomes the dust accumulation issue for conventional photovoltaic systems. Although continuous innovations and advanced developments within renewable energy technologies have shown steady improvements over the past years, the dust accumulation issue [...] Read more.
This research aims to present a state-of-the-art cleaning technology solution that effectively overcomes the dust accumulation issue for conventional photovoltaic systems. Although continuous innovations and advanced developments within renewable energy technologies have shown steady improvements over the past years, the dust accumulation issue remains one of the main factors hindering their efficiency and degradation rate. By harvesting abundant solar thermal energy, the presented self-cleaning system uses a unique thermomechanical property of Shape Memory Alloys to operate a solar-based thermomechanical actuator. Therefore, this study carries out different numerical and experimental validation tests to highlight the promising practicability of the developed self-cleaning system from thermal and mechanical perspectives. The results showed that the system has a life expectancy of over 20 years, which is closely equivalent to the life expectancy of conventional photovoltaic modules while operating under actual weather conditions in Dammam city. Additionally, the thermal to mechanical energy conversion efficiency reached 19.15% while providing average cleaning effectiveness of about 95%. The presented outcomes of this study add to the body of knowledge an innovative methodology for a unique solar-based self-cleaning system aimed toward smart and modern photovoltaic applications. Full article
(This article belongs to the Special Issue Mechanical Behavior of Shape Memory Alloys: 2022)
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<p>Conceptual design model of the cleaning system.</p> Full article ">Figure 2
<p>Basic conceptual procedures of the thermomechanical solar-based linear actuator.</p> Full article ">Figure 3
<p>Overview of developed CAD models for (<b>a</b>) actuator A, (<b>b</b>) actuator B, and (<b>c</b>) actuator C.</p> Full article ">Figure 4
<p>Detailed CAD model of the PV cleaning system based on (<b>a</b>) Actuator A, (<b>b</b>) Actuator B, and (<b>c</b>) Actuator C.</p> Full article ">Figure 5
<p>(<b>a</b>) Arbitrary CAD example of dusty PV modules string, and (<b>b</b>) coverage area of multiple actuator arrangements used for cleaning.</p> Full article ">Figure 6
<p>Implemented rapid prototyping techniques with (<b>a</b>) 3D printer, (<b>b</b>) laser cutting machine, and (<b>c</b>) fully assembled actuator.</p> Full article ">Figure 7
<p>A prepared uncleaned PV module for the cleaning effectiveness test.</p> Full article ">Figure 8
<p>Mechanical models of the assessment platforms for the (<b>a</b>) SMA spring, and (<b>b</b>) actuator’s assembly tests.</p> Full article ">Figure 9
<p>(<b>a</b>) Block diagram of the process gain scheduling PID controlled system, and (<b>b</b>) flow chart diagram of the controller working principle.</p> Full article ">Figure 10
<p>Maximum stresses developed within rack and pinion gear arrangement.</p> Full article ">Figure 11
<p>Example of 1-day temperature distributions for the designed SHC through (<b>a</b>) morning, (<b>b</b>) zenith, (<b>c</b>) afternoon, and (<b>d</b>) night periods.</p> Full article ">Figure 12
<p>Temperature profiles inside the SHC over an entire year.</p> Full article ">Figure 13
<p>Histogram plot for the daily temperature variation of the three actuator designs.</p> Full article ">Figure 14
<p>Activation temperature lines for the proposed three actuators.</p> Full article ">Figure 15
<p>Gain scheduling PID controller’s Response to (<b>a</b>) step, (<b>b</b>) square-wave, (<b>c</b>) staircase, and (<b>d</b>) sinusoidal-wave command signals.</p> Full article ">Figure 16
<p>Long-duration displacement response of a square wave command signal.</p> Full article ">Figure 17
<p>Displacement and force vs. temperature (hysteresis behavior).</p> Full article ">Figure 18
<p>Actual setup for the smart PV cleaning system.</p> Full article ">Figure 19
<p>The cleaning effectiveness of the presented SMA-driven cleaning method under different dust densities, where (<b>a</b>) the actual performance of the self-cleaning operations, and (<b>b</b>) the quantitative outcome of the cleaning effectiveness percentages versus dust densities.</p> Full article ">
13 pages, 3954 KiB  
Article
Microwave versus Conventional Sintering of NiTi Alloys Processed by Mechanical Alloying
by Rodolfo da Silva Teixeira, Rebeca Vieira de Oliveira, Patrícia Freitas Rodrigues, João Mascarenhas, Filipe Carlos Figueiredo Pereira Neves and Andersan dos Santos Paula
Materials 2022, 15(16), 5506; https://doi.org/10.3390/ma15165506 - 11 Aug 2022
Cited by 6 | Viewed by 2059
Abstract
The present study shows a comparison between two sintering processes, microwave and conventional sintering, for the manufacture of NiTi porous specimens starting from powder mixtures of nickel and titanium hydrogenation–dehydrogenation (HDH) milled by mechanical alloying for a short time (25 min). The samples [...] Read more.
The present study shows a comparison between two sintering processes, microwave and conventional sintering, for the manufacture of NiTi porous specimens starting from powder mixtures of nickel and titanium hydrogenation–dehydrogenation (HDH) milled by mechanical alloying for a short time (25 min). The samples were sintered at 850 °C for 15 min and 120 min, respectively. Both samples exhibited porosity, and the pore size results are within the range of the human bone. The NiTi intermetallic compound (B2, R-phase, and B19′) was detected in both sintered samples through X-ray diffraction (XRD) and electron backscattering diffraction (EBSD) on scanning electron microscopic (SEM). Two-step phase transformation occurred in both sintering processes with cooling and heating, the latter occurring with an overlap of the peaks, according to the differential scanning calorimetry (DSC) results. From scanning electron microscopy/electron backscatter diffraction, the R-phase and B2/B19′ were detected in microwave and conventional sintering, respectively. The instrumented ultramicrohardness results show the highest elastic work values for the conventionally sintered sample. It was observed throughout this investigation that using mechanical alloying (MA) powders enabled, in both sintering processes, good results, such as intermetallic formation and densification in the range for biomedical applications. Full article
(This article belongs to the Special Issue Mechanical Behavior of Shape Memory Alloys: 2022)
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<p>Schematic of the adopted mechanical alloying and sintering processes.</p> Full article ">Figure 2
<p>SEM morphology of the elementary powders: (<b>a</b>) nickel and (<b>b</b>) titanium. (<b>c</b>) Micrography of the MA-processed powders. (<b>d</b>) XRD patterns of the starting powders and the MA-processed powders. (<b>e</b>) DTA heating curve of MA-processed powders.</p> Full article ">Figure 3
<p>DSC curves showing the phase transformations for (<b>a</b>) MW and (<b>b</b>) CS samples. Dot line: room temperature (20 °C).</p> Full article ">Figure 4
<p>XRD patterns (<b>a</b>) MW and (<b>b</b>) CS sample.</p> Full article ">Figure 5
<p>SEM micrographs of the sample porous NiTi: (<b>a</b>) MW sample and (<b>b</b>) CS sample before etching. (<b>c</b>–<b>f</b>) After etching to reveal the possible phases. MW sample indicated by arrows 1 and 2: (<b>c</b>) general aspect and (<b>d</b>) magnification from arrow 1; CS sample: (<b>e</b>) general aspect and (<b>f</b>) magnification from the square, showing the martensitic phase, indicated by the arrows.</p> Full article ">Figure 6
<p>The image quality (IQ) map: (<b>a</b>) MW and (<b>b</b>) CS sample. EBSD phase maps + IQ map: (<b>c</b>) MW sample with the R-phase in green, B2 in red, B19′ in blue, and TiNi<sub>3</sub> in cyan; (<b>d</b>) CS sample with B2 in red, B19′ in blue, Ti<sub>3</sub>Ni<sub>4</sub> in cyan, and Ti<sub>2</sub>Ni in yellow.</p> Full article ">Figure 7
<p>Force vs. depth (loading and unloading curves). (<b>a</b>,<b>b</b>) MW sample and (<b>c</b>,<b>d</b>) CS sample; maximum forces of 1.0 gf/9.81 mN and 20.0 gf/196.1 mN, respectively.</p> Full article ">Figure 8
<p>Total, elastic, and plastic works: maximum forces of (<b>a</b>) 1.0 gf/9.81 mN and (<b>b</b>) 20.0 gf/196.1 mN.</p> Full article ">
10 pages, 3385 KiB  
Article
The Study of New NiTi Actuators to Reinforce the Wing Movement of Aircraft Systems
by Rafael Braga, Patrícia Freitas Rodrigues, Hélder Cordeiro, Pedro Carreira and Maria Teresa Vieira
Materials 2022, 15(14), 4787; https://doi.org/10.3390/ma15144787 - 8 Jul 2022
Cited by 5 | Viewed by 1925
Abstract
Actuators using Shape Memory Alloy (SMA) springs could operate in different mechanical systems requiring geometric flexibility and high performance. The aim of the present study is to highlight the potential of these actuators, using their dimensional variations resulting from the phase transformations of [...] Read more.
Actuators using Shape Memory Alloy (SMA) springs could operate in different mechanical systems requiring geometric flexibility and high performance. The aim of the present study is to highlight the potential of these actuators, using their dimensional variations resulting from the phase transformations of NiTi springs (SMA) to make the movements of the system’s mobile components reversible. This reversibility is due to thermal-induced martensitic transformation of NiTi springs. The transformation promotes the extended and retracted of the springs as the phase changing (martensite–austenite) creates movement in part of the system. Therefore, the phase transition temperatures of NiTi, evaluated by differential scanning calorimetry (DSC), are required to control the dimensional variation of the spring. The influence of the number of springs in the system, as well as how impacts on the reaction time were evaluated. The different numbers of springs (two, four, and six) and the interspaces between them made it possible to control the time and the final angle attained in the mobile part of the system. Mechanical resistance, maximum angle, and the system’s reaction time using different NiTi springs highlight the role of the actuators. Fused Deposition Modelling (FDM)/Material Extrusion (MEX) or Selective Laser Sintering (SLS) was selected for shaping the composite matrix system. A new prototype was designed and developed to conduct tests that established the relationship between the recoverable deformation of the matrix suitable for the application as well as the number and distribution of the actuators. Full article
(This article belongs to the Special Issue Mechanical Behavior of Shape Memory Alloys: 2022)
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<p>The geometrical design of a NiTi coil spring, thickness (d), outer diameter (D), length (L), number of active coils (n), and initial pitch angle (α).</p> Full article ">Figure 2
<p>Geometry and assembly of prototype: (<b>a</b>) nylon support; (<b>b</b>) scheme; (<b>c</b>) support with SMA wire and springs; (<b>d</b>) aircraft component system before and after SMA actuator effect.</p> Full article ">Figure 3
<p>DSC curve of NiTi spring.</p> Full article ">Figure 4
<p>Experimental setup for tensile tests of the NiTi SMA spring.</p> Full article ">Figure 5
<p>Geometry and dimensions of the specimens for tensile tests.</p> Full article ">Figure 6
<p>Prototype (one spring): (<b>a</b>) schema; (<b>b</b>) support with SMA springs.</p> Full article ">Figure 7
<p>Force vs. time of the thermal cycle of the NiTi spring (tensile test) (10 s (S10), 20 s (S20), and 30 s (S30)).</p> Full article ">Figure 8
<p>Visual aspect after thermal resistance tests on the supports (one spring): (<b>a</b>) PLA; (<b>b</b>) ABS; (<b>c</b>) PA12.</p> Full article ">Figure 9
<p>Prototype with six springs and heating system (across different points).</p> Full article ">Figure 10
<p>Evaluation of angle of inclination of the prototype.</p> Full article ">Figure 11
<p>Maximum angle of inclination of the prototype (blue) and extended time as a function of the number of NiTi springs (black) in PA12 support.</p> Full article ">
9 pages, 28886 KiB  
Article
In Situ Observation of Thermoelastic Martensitic Transformation of Cu-Al-Mn Cryogenic Shape Memory Alloy with Compressive Stress
by Zhenyu Bian, Jian Song, Pingping Liu, Farong Wan, Yu Lei, Qicong Wang, Shanwu Yang, Qian Zhan, Liubiao Chen and Junjie Wang
Materials 2022, 15(11), 3794; https://doi.org/10.3390/ma15113794 - 26 May 2022
Cited by 4 | Viewed by 2071
Abstract
The thermoelastic martensitic transformation and its reverse transformation of the Cu-Al-Mn cryogenic shape memory alloy, both with and without compressive stress, has been dynamically in situ observed. During the process of thermoelastic martensitic transformation, martensite nucleates and gradually grow up as they cool, [...] Read more.
The thermoelastic martensitic transformation and its reverse transformation of the Cu-Al-Mn cryogenic shape memory alloy, both with and without compressive stress, has been dynamically in situ observed. During the process of thermoelastic martensitic transformation, martensite nucleates and gradually grow up as they cool, and shrink to disappearance as they heat. The order of martensite disappearance is just opposite to that of their formation. Observations of the self-accommodation of martensite variants, which were carried out by using a low temperature metallographic in situ observation apparatus, showed that the variants could interact with each other. The results of in situ synchrotron radiation X-ray and metallographic observation also suggested there were some residual austenites, even if the temperature was below Mf, which means the martensitic transformation could not be 100% accomplished. The external compressive stress would promote the preferential formation of martensite with some orientation, and also hinder the formation of martensite with other nonequivalent directions. The possible mechanism of the martensitic reverse transformation is discussed. Full article
(This article belongs to the Special Issue Mechanical Behavior of Shape Memory Alloys: 2022)
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<p>The Ms temperature of Cu-Al-Mn shape memory alloys.</p> Full article ">Figure 2
<p>In situ observation apparatus of cryogenic metallographic with deformation excitation unit. (<b>a</b>) the pipe connecting the sample table and the liquid nitrogen tank, through which liquid nitrogen is introduced into the sample stage (<b>b</b>) sample stage with deformation excitation unit and (<b>c</b>) the metallographic microscope is equipped with micro image processing system (MIPs).</p> Full article ">Figure 3
<p>A schematic diagram of the area of observation with external stress. (<b>a</b>) Schematic diagram of observation window of copper sample table; (<b>b</b>) the picture of area of observation and (<b>c</b>) schematic diagram of compressive stress.</p> Full article ">Figure 4
<p>In situ observation of the thermoelastic martensitic transformation (1–5) and reverse transformation (6–10) without compressive stress. ‘A’ is austenite and ‘M’ is martensite.</p> Full article ">Figure 5
<p>In situ observation of the thermoelastic martensitic transformation (1–5) and reverse transformation (6–10) with compressive stress. ‘A’ is austenite and ‘M’ is martensite.</p> Full article ">Figure 6
<p>Synchrotron radiation X-ray diffraction spectrum of Cu-Al-Mn alloy at 293 K (<b>a</b>) and 77 K (<b>b</b>).</p> Full article ">
9 pages, 5503 KiB  
Article
The Effect of Heat Treatment on Damping Capacity and Mechanical Properties of CuAlNi Shape Memory Alloy
by Ivana Ivanić, Stjepan Kožuh, Tamara Holjevac Grgurić, Ladislav Vrsalović and Mirko Gojić
Materials 2022, 15(5), 1825; https://doi.org/10.3390/ma15051825 - 28 Feb 2022
Cited by 11 | Viewed by 2293
Abstract
This paper discusses the effect of different heat treatment procedures on the microstructural characteristics, damping capacities, and mechanical properties of CuAlNi shape memory alloys (SMA). The investigation was performed on samples in the as-cast state and heat treated states (solution annealing at 885 [...] Read more.
This paper discusses the effect of different heat treatment procedures on the microstructural characteristics, damping capacities, and mechanical properties of CuAlNi shape memory alloys (SMA). The investigation was performed on samples in the as-cast state and heat treated states (solution annealing at 885 °C/60′/H2O and after tempering at 300 °C/60′/H2O). The microstructure of the samples was examined by light microscopy (LM) and scanning electron microscopy (SEM) equipped with a device for energy dispersive spectrometry (EDS) analysis. Light and scanning electron microscopy showed martensitic microstructure in all investigated samples. However, the changes in microstructure due to heat treatment by the presence of two types of martensite phases (β1′ and γ1′) influenced alloy damping and mechanical properties by enhancing alloy damping characteristics. Heat treatment procedure reduced the alloys’ mechanical properties and increased hardness of the alloy. Fractographic analysis of the alloy showed a transgranular type of fracture in samples after casting. After solution annealing, two types of fracture mechanisms can be noticed, transgranular and intergranular, while in tempered samples, mostly an intergranular type of fracture exists. Full article
(This article belongs to the Special Issue Mechanical Behavior of Shape Memory Alloys: 2022)
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<p>Schematic illustration of the tensile test sample.</p> Full article ">Figure 2
<p>Light micrographs of the CuAlNi shape memory alloys in the as-cast state (<b>a</b>), after solution annealing at 885 °C/60′/H<sub>2</sub>O (<b>b</b>), and after solution annealing at 885 °C/60′/H<sub>2</sub>O and tempering at 300 °C/60′/H<sub>2</sub>O (<b>c</b>), magnification 200×.</p> Full article ">Figure 3
<p>SEM micrographs of the CuAlNi shape memory alloys in the as-cast state (<b>a</b>), after solution annealing at 885 °C/60′/H<sub>2</sub>O (<b>b</b>), and after solution annealing at 885 °C/60′/H<sub>2</sub>O and tempering at 300 °C/60′/H<sub>2</sub>O (<b>c</b>).</p> Full article ">Figure 4
<p>DMA spectrum of the CuAlNi shape memory alloy in the as-cast state.</p> Full article ">Figure 5
<p>DMA spectrum of the CuAlNi shape memory alloy after solution annealing at 885 °C/60′/H<sub>2</sub>O.</p> Full article ">Figure 6
<p>DMA spectrum of the CuAlNi shape memory alloy after solution annealing at 885 °C/60′/H<sub>2</sub>O and tempering at 300 °C/60′/H<sub>2</sub>O.</p> Full article ">Figure 7
<p>Stress vs. strain in the CuAlNi SMA.</p> Full article ">Figure 8
<p>Fracture surface morphology after tensile testing in the as-cast state (<b>a</b>), after solution annealing at 885 °C/60′/H<sub>2</sub>O (<b>b</b>), and after solution annealing at 885 °C/60′/H<sub>2</sub>O and tempering at 300 °C/60′/H<sub>2</sub>O (<b>c</b>).</p> Full article ">
20 pages, 3484 KiB  
Article
Optimized Neural Network Prediction Model of Shape Memory Alloy and Its Application for Structural Vibration Control
by Meng Zhan, Junsheng Liu, Deli Wang, Xiuyun Chen, Lizhen Zhang and Sheliang Wang
Materials 2021, 14(21), 6593; https://doi.org/10.3390/ma14216593 - 2 Nov 2021
Cited by 9 | Viewed by 1955
Abstract
The traditional mathematical model of shape memory alloy (SMA) is complicated and difficult to program in numerical analysis. The artificial neural network is a nonlinear modeling method which does not depend on the mathematical model and avoids the inevitable error in the traditional [...] Read more.
The traditional mathematical model of shape memory alloy (SMA) is complicated and difficult to program in numerical analysis. The artificial neural network is a nonlinear modeling method which does not depend on the mathematical model and avoids the inevitable error in the traditional modeling method. In this paper, an optimized neural network prediction model of shape memory alloy and its application for structural vibration control are discussed. The superelastic properties of austenitic SMA wires were tested by experiments. The material property test data were taken as the training samples of the BP neural network, and a prediction model optimized by the genetic algorithm was established. By using the improved genetic algorithm, the position and quantity of the SMA wires were optimized in a three-storey spatial structure, and the dynamic response analysis of the optimal arrangement was carried out. The results show that, compared with the unoptimized neural network prediction model of SMA, the optimized prediction model is in better agreement with the test curve and has higher stability, it can well reflect the effect of loading rate on the superelastic properties of SMA, and is a high precision rate-dependent dynamic prediction model. Moreover, the BP network constitutive model is simple to use and convenient for dynamic simulation analysis of an SMA passive control structure. The controlled structure with optimized SMA wires can inhibit the structural seismic responses more effectively. However, it is not the case that the more SMA wires, the better the shock absorption effect. When SMA wires exceed a certain number, the vibration reduction effect gradually decreases. Therefore, the seismic effect can be reduced economically and effectively only when the number and location of SMA wires are properly configured. When four SMA wires are arranged, the acceptable shock absorption effect is obtained, and the sum of the structural storey drift can be reduced by 44.51%. Full article
(This article belongs to the Special Issue Mechanical Behavior of Shape Memory Alloys: 2022)
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<p>Characteristic points of austenite SMA constitutive curve.</p> Full article ">Figure 2
<p>Stress-strain curve of austenite SMA wire with different cycles.</p> Full article ">Figure 3
<p>Stress-strain curve of austenite SMA wire with different strain amplitudes.</p> Full article ">Figure 4
<p>Stress-strain curve of austenite SMA wire with different loading rates.</p> Full article ">Figure 5
<p>Stress-strain curve of austenite SMA wire with different diameters.</p> Full article ">Figure 6
<p>Flow chart of BP network optimized by genetic algorithm.</p> Full article ">Figure 7
<p>BP neural network topology.</p> Full article ">Figure 8
<p>Validation Performance Chart.</p> Full article ">Figure 9
<p>Comparison of BP network prediction curve and test curve (<b>a</b>) unoptimized BP network and (<b>b</b>) optimized BP network.</p> Full article ">Figure 10
<p>Comparison and corresponding error between test curve and prediction curves of un-optimized and optimized BP network by GA, (<b>a</b>) constitutive curves at 30 mm/min, (<b>b</b>) error at 30 mm/min, (<b>c</b>) constitutive curves at 90 mm/min, (<b>d</b>) error at 90 mm/min.</p> Full article ">Figure 11
<p>Possible locations of austenitic SMA wires and structural node numbers.</p> Full article ">Figure 12
<p>Objective function values with the change of evolutionary generations under different amounts of SMA wire, (<b>a</b>) 4 SMA wires, (<b>b</b>) 8 SMA wires, (<b>c</b>) 12 SMA wires, (<b>d</b>) 20 SMA wires.</p> Full article ">Figure 13
<p>Spatial structure model with 4 austenite SMA wires (<b>a</b>) optimal position and (<b>b</b>) shaking table test.</p> Full article ">Figure 14
<p>Time history curves of seismic response of spatial model structure with and without control, (<b>a</b>) storey drift of first floor, (<b>b</b>) interlayer acceleration of first floor, (<b>c</b>) storey drift of third floor, (<b>d</b>) interlayer acceleration of third floor.</p> Full article ">
15 pages, 5858 KiB  
Article
Modelling of SMA Vibration Systems in an AVA Example
by Waldemar Rączka, Jarosław Konieczny and Marek Sibielak
Materials 2021, 14(19), 5905; https://doi.org/10.3390/ma14195905 - 8 Oct 2021
Cited by 1 | Viewed by 1862
Abstract
Vibration suppression, as well as its generation, is a common subject of scientific investigations. More and more often, but still rarely, shape memory alloys (SMAs) are used in vibrating systems, despite the fact that SMA springs have many advantages. This is due to [...] Read more.
Vibration suppression, as well as its generation, is a common subject of scientific investigations. More and more often, but still rarely, shape memory alloys (SMAs) are used in vibrating systems, despite the fact that SMA springs have many advantages. This is due to the difficulty of the mathematical description and the considerable effortfulness of analysing and synthesising vibrating systems. The article shows the analysis of vibrating systems in which spring elements made of SMAs are used. The modelling and analysis method of vibrating systems is shown in the example of a vibrating system with a dynamic vibration absorber (DVA), which uses springs made of a shape memory alloy. The formulated mathematical model of a 2-DOF system with a controlled spring, mounted in DVA suspension, uses the viscoelastic model of the SMA spring. For the object, a control system was synthesised. Finally, model tests with and without a controller were carried out. The characteristics of the vibrations’ transmissibility functions for both systems were determined. It was shown that the developed DVA can tune to frequency excitation changes of up to ±10%. Full article
(This article belongs to the Special Issue Mechanical Behavior of Shape Memory Alloys: 2022)
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<p>The damping of the spring as a function of frequency for selected temperatures.</p> Full article ">Figure 2
<p>The spring rate <math display="inline"><semantics> <mi>k</mi> </semantics></math> as a function frequency for selected temperatures.</p> Full article ">Figure 3
<p>Calculation diagram of the vibration reduction system.</p> Full article ">Figure 4
<p>Vibration transmissibility function of the absorber as a function of frequency and temperature, the transfer function <math display="inline"><semantics> <mrow> <msub> <mi>G</mi> <mrow> <mi>z</mi> <mn>2</mn> <mi>z</mi> <mn>1</mn> </mrow> </msub> </mrow> </semantics></math>.</p> Full article ">Figure 5
<p>Absorber phase shift as a function of frequency and temperature, the transfer function <math display="inline"><semantics> <mrow> <msub> <mi>G</mi> <mrow> <mi>z</mi> <mn>2</mn> <mi>z</mi> <mn>1</mn> </mrow> </msub> </mrow> </semantics></math>.</p> Full article ">Figure 6
<p>Vibration transmissibility functions of the absorber as a function of frequency for selected temperatures 25 °C, 60 °C, 80 °C, the transfer function <math display="inline"><semantics> <mrow> <msub> <mi>G</mi> <mrow> <mi>z</mi> <mn>2</mn> <mi>z</mi> <mn>1</mn> </mrow> </msub> </mrow> </semantics></math>.</p> Full article ">Figure 7
<p>The absorber phase shifts as a function of frequency for selected temperatures 25 °C, 60 °C, 80 °C, the transfer function <math display="inline"><semantics> <mrow> <msub> <mi>G</mi> <mrow> <mi>z</mi> <mn>2</mn> <mi>z</mi> <mn>1</mn> </mrow> </msub> </mrow> </semantics></math>.</p> Full article ">Figure 8
<p>Vibration transmissibility function of disturbance <math display="inline"><semantics> <mrow> <msub> <mi>z</mi> <mi>w</mi> </msub> </mrow> </semantics></math> to the protected mass <math display="inline"><semantics> <mrow> <msub> <mi>m</mi> <mn>1</mn> </msub> </mrow> </semantics></math> as a function of frequency and temperature, the transfer function <math display="inline"><semantics> <mrow> <msub> <mi>G</mi> <mrow> <mi>z</mi> <mn>1</mn> <mi>z</mi> <mi>w</mi> </mrow> </msub> </mrow> </semantics></math>.</p> Full article ">Figure 9
<p>The phase shift of protected mass as a function of frequency and temperature, the transfer function <math display="inline"><semantics> <mrow> <msub> <mi>G</mi> <mrow> <mi>z</mi> <mn>1</mn> <mi>z</mi> <mi>w</mi> </mrow> </msub> </mrow> </semantics></math>.</p> Full article ">Figure 10
<p>Vibration transmissibility functions of disturbance <math display="inline"><semantics> <mrow> <msub> <mi>z</mi> <mi>w</mi> </msub> </mrow> </semantics></math> to the protected mass <math display="inline"><semantics> <mrow> <msub> <mi>m</mi> <mn>1</mn> </msub> </mrow> </semantics></math> as a function of frequency for selected temperatures 25 °C, 60 °C, 80 °C, the transfer function <math display="inline"><semantics> <mrow> <msub> <mi>G</mi> <mrow> <mi>z</mi> <mn>1</mn> <mi>z</mi> <mi>w</mi> </mrow> </msub> </mrow> </semantics></math>.</p> Full article ">Figure 11
<p>Absorber phase shift as a function of frequency for selected temperatures 25 °C, 60 °C, 80 °C, the transfer function <math display="inline"><semantics> <mrow> <msub> <mi>G</mi> <mrow> <mi>z</mi> <mn>2</mn> <mi>z</mi> <mi>w</mi> </mrow> </msub> </mrow> </semantics></math>.</p> Full article ">Figure 12
<p>Vibration transmissibility function of disturbance <math display="inline"><semantics> <mrow> <msub> <mi>z</mi> <mi>w</mi> </msub> </mrow> </semantics></math> to the absorber mass <math display="inline"><semantics> <mrow> <msub> <mi>m</mi> <mn>2</mn> </msub> </mrow> </semantics></math> as a function of frequency and temperature, the transfer function <math display="inline"><semantics> <mrow> <msub> <mi>G</mi> <mrow> <mi>z</mi> <mn>2</mn> <mi>z</mi> <mi>w</mi> </mrow> </msub> </mrow> </semantics></math>.</p> Full article ">Figure 13
<p>Protect mass phase shift between displacements <math display="inline"><semantics> <mrow> <msub> <mi>z</mi> <mi>w</mi> </msub> </mrow> </semantics></math> and <math display="inline"><semantics> <mrow> <msub> <mi>z</mi> <mn>2</mn> </msub> </mrow> </semantics></math> as a function of frequency and temperature, the transfer function <math display="inline"><semantics> <mrow> <msub> <mi>G</mi> <mrow> <mi>z</mi> <mn>2</mn> <mi>z</mi> <mi>w</mi> </mrow> </msub> </mrow> </semantics></math>.</p> Full article ">Figure 14
<p>Vibration transmissibility functions of disturbance <math display="inline"><semantics> <mrow> <msub> <mi>z</mi> <mi>w</mi> </msub> </mrow> </semantics></math> to the absorber mass <math display="inline"><semantics> <mrow> <msub> <mi>m</mi> <mn>2</mn> </msub> </mrow> </semantics></math> as a function of frequency for selected temperatures 25 °C, 60 °C, 80 °C, the transfer function <math display="inline"><semantics> <mrow> <msub> <mi>G</mi> <mrow> <mi>z</mi> <mn>2</mn> <mi>z</mi> <mi>w</mi> </mrow> </msub> </mrow> </semantics></math>.</p> Full article ">Figure 15
<p>Absorber phase shift functions between displacements <math display="inline"><semantics> <mrow> <msub> <mi>z</mi> <mi>w</mi> </msub> </mrow> </semantics></math> and <math display="inline"><semantics> <mrow> <msub> <mi>z</mi> <mn>2</mn> </msub> </mrow> </semantics></math> as a function of frequency for selected temperatures 25 °C, 60 °C, 80 °C, the transfer function <math display="inline"><semantics> <mrow> <msub> <mi>G</mi> <mrow> <mi>z</mi> <mn>2</mn> <mi>z</mi> <mi>w</mi> </mrow> </msub> </mrow> </semantics></math>.</p> Full article ">Figure 16
<p>Chart of the natural frequency <math display="inline"><semantics> <mrow> <msub> <mi>f</mi> <mi>n</mi> </msub> </mrow> </semantics></math> (solid line) and the resonance frequency <math display="inline"><semantics> <mrow> <msub> <mi>f</mi> <mi>r</mi> </msub> </mrow> </semantics></math> (dashed line).</p> Full article ">Figure 17
<p>Control system block scheme.</p> Full article ">Figure 18
<p>Vibration transmissibility functions of the passive absorber for selected temperatures of 25 °C, 60 °C, 80 °C and the controlled absorber (black). The transfer function <math display="inline"><semantics> <mrow> <msub> <mi>G</mi> <mrow> <mi>z</mi> <mn>2</mn> <mi>z</mi> <mn>1</mn> </mrow> </msub> </mrow> </semantics></math> describes the object.</p> Full article ">Figure 19
<p>Vibration transmissibility functions of disturbance <math display="inline"><semantics> <mrow> <msub> <mi>z</mi> <mi>w</mi> </msub> </mrow> </semantics></math> to the protected mass <math display="inline"><semantics> <mrow> <msub> <mi>m</mi> <mn>1</mn> </msub> </mrow> </semantics></math> of the passive absorber for selected temperatures of 25 °C, 60 °C, 80 °C and the controlled absorber (black). The transfer function <math display="inline"><semantics> <mrow> <msub> <mi>G</mi> <mrow> <mi>z</mi> <mn>1</mn> <mi>z</mi> <mi>w</mi> </mrow> </msub> </mrow> </semantics></math> describes the object.</p> Full article ">Figure 20
<p>Vibration transmissibility functions of disturbance <math display="inline"><semantics> <mrow> <msub> <mi>z</mi> <mi>w</mi> </msub> </mrow> </semantics></math> to the mass <math display="inline"><semantics> <mrow> <msub> <mi>m</mi> <mn>2</mn> </msub> </mrow> </semantics></math> of the passive absorber for selected temperatures of 25 °C, 60 °C, 80 °C and the controlled absorber (black). The transfer function <math display="inline"><semantics> <mrow> <msub> <mi>G</mi> <mrow> <mi>z</mi> <mn>2</mn> <mi>z</mi> <mi>w</mi> </mrow> </msub> </mrow> </semantics></math> describes the object.</p> Full article ">
13 pages, 3302 KiB  
Article
Investigations of Effects of Intermetallic Compound on the Mechanical Properties and Shape Memory Effect of Ti–Au–Ta Biomaterials
by Wan-Ting Chiu, Kota Fuchiwaki, Akira Umise, Masaki Tahara, Tomonari Inamura and Hideki Hosoda
Materials 2021, 14(19), 5810; https://doi.org/10.3390/ma14195810 - 4 Oct 2021
Cited by 10 | Viewed by 2062
Abstract
Owing to the world population aging, biomedical materials, such as shape memory alloys (SMAs) have attracted much attention. The biocompatible Ti–Au–Ta SMAs, which also possess high X–ray contrast for the applications like guidewire utilized in surgery, were studied in this work. The alloys [...] Read more.
Owing to the world population aging, biomedical materials, such as shape memory alloys (SMAs) have attracted much attention. The biocompatible Ti–Au–Ta SMAs, which also possess high X–ray contrast for the applications like guidewire utilized in surgery, were studied in this work. The alloys were successfully prepared by physical metallurgy techniques and the phase constituents, microstructures, chemical compositions, shape memory effect (SME), and superelasticity (SE) of the Ti–Au–Ta SMAs were also examined. The functionalities, such as SME, were revealed by the introduction of the third element Ta; in addition, obvious improvements of the alloy performances of the ternary Ti–Au–Ta alloys were confirmed while compared with that of the binary Ti–Au alloy. The Ti3Au intermetallic compound was both found crystallographically and metallographically in the Ti–4 at.% Au–30 at.% Ta alloy. The strength of the alloy was promoted by the precipitates of the Ti3Au intermetallic compound. The effects of the Ti3Au precipitates on the mechanical properties, SME, and SE were also investigated in this work. Slight shape recovery was found in the Ti–4 at.% Au–20 at.% Ta alloy during unloading of an externally applied stress. Full article
(This article belongs to the Special Issue Mechanical Behavior of Shape Memory Alloys: 2022)
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<p>The X–ray diffraction patterns of the (<b>a</b>) Ti–4Au–20Ta alloy and the (<b>b</b>) Ti–4Au–30Ta alloy at RT under ambient.</p> Full article ">Figure 2
<p>SEM images of the (<b>a</b>) Ti–4Au–20Ta alloy and the (<b>b</b>) Ti–4Au–30Ta alloy. Elemental mapping results of (<b>c</b>) Ti, (<b>d</b>) Au, and (<b>e</b>) Ta elementals of the (<b>b</b>) Ti–4Au–30Ta alloy. The elemental mapping analyzed regime of the (<b>b</b>) Ti–4Au–30Ta alloy is surrounded by a dashed square.</p> Full article ">Figure 3
<p>Bending tests of the (<b>a</b>) Ti–4Au–20Ta alloy and the (<b>b</b>) Ti–4Au–30Ta alloy for the examinations of the shape memory effect and shape recovery rates. Stage (<b>i</b>) corresponds to after bending deformation while stage (<b>ii</b>) indicates shape recovery upon heating process.</p> Full article ">Figure 4
<p>Stress–strain (SS) curves of the (<b>a</b>) Ti–4Au–20Ta alloy and the (<b>b</b>) Ti–4Au–30Ta alloy via the continuous tensile tests. The cross symbols at the end of the curves suggest fractures of the specimens. A stress plateau in the (<b>a</b>) Ti–4Au–20Ta alloy was indicated by the vertical dashed lines and the horizontal solid arrows.</p> Full article ">Figure 5
<p>Functional mapping of (<b>a</b>) yielding stress (black squares), (<b>b</b>) ultimate tensile strength (UTS) (red circles), and (<b>c</b>) fracture strain (blue triangles) as a function of the Ta addition concentration in the alloys. (Left <span class="html-italic">y</span>–axis: stress (MPa); Right <span class="html-italic">y</span>–axis: fracture strain (%)) (Note that the results of the binary Ti–4Au alloy was cited from our preliminary research [<a href="#B35-materials-14-05810" class="html-bibr">35</a>]).</p> Full article ">Figure 6
<p>Cyclic loading–unloading tensile tests of the (<b>a</b>) Ti–4Au–20Ta alloy and the (<b>b</b>) Ti–4Au–30Ta alloy at RT under ambient.</p> Full article ">Figure 7
<p>(<b>a</b>) Stress for the first yielding stress (i.e., stress for SIMT) of the Ti–4Au–20Ta alloy, (<b>b</b>) the ninth cycle in the cyclic loading–unloading tensile test of the Ti–4Au–20Ta alloy, (<b>c</b>) the ninth cycle in the cyclic loading–unloading tensile test of the Ti–4Au–30Ta alloy, and (<b>d</b>) the illustration for the explanations of the terms in the SS curve. Where <span class="html-italic">ε</span><sub>A</sub> indicates the total applied strain, <span class="html-italic">ε</span><sub>E</sub> suggests the elastic shape recovery strain, <span class="html-italic">ε</span><sub>PE</sub> corresponds to shape recovery strain brought by pseudoelasticity, and <span class="html-italic">ε</span><sub>R</sub> represents the remaining strain after unloading.</p> Full article ">
15 pages, 7004 KiB  
Article
Strain Rate Effect upon Mechanical Behaviour of Hydrogen-Charged Cycled NiTi Shape Memory Alloy
by Fehmi Gamaoun
Materials 2021, 14(16), 4772; https://doi.org/10.3390/ma14164772 - 23 Aug 2021
Cited by 3 | Viewed by 2341
Abstract
The rate dependence of thermo-mechanical responses of superelastic NiTi with different imposed strain rates after cycling from 1 to 50 cycles under applied 10−5s−1, 10−4s−1 and 10−3s−1 strain rates, immersion for 3 h [...] Read more.
The rate dependence of thermo-mechanical responses of superelastic NiTi with different imposed strain rates after cycling from 1 to 50 cycles under applied 10−5s−1, 10−4s−1 and 10−3s−1 strain rates, immersion for 3 h and ageing has been investigated. The loaded and unloaded as-received NiTi alloy under an imposed strain of 7.1% have shown an increase in the residual deformation at zero stress with an increase in strain rates. It has been found that after 13 cycles and hydrogen charging, the amount of absorbed hydrogen (291 mass ppm) was sufficient to cause the embrittlement of the tensile loaded NiTi alloy with 10−5s−1. However, no premature fracture has been detected for the imposed strain rates of 10−4s−1 and 10−3s−1. Nevertheless, after 18 cycles and immersion for 3 h, the fracture has occurred in the plateau of the austenite martensite transformation during loading with 10−4s−1. Despite the higher quantity of absorbed hydrogen, the loaded specimen with a higher imposed strain rate of 10−3s−1 has kept its superelasticity behaviour, even after 20 cycles. We attribute such a behaviour to the interaction between the travelling distance during the growth of the martensitic domains while introducing the martensite phase and the amount of diffused hydrogen. Full article
(This article belongs to the Special Issue Mechanical Behavior of Shape Memory Alloys: 2022)
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<p>Loading protocol of the monotonic and cyclic loading: (<b>a</b>) Schematic representation of the Instron tensile machine, (<b>b</b>) used part of the as-received alloy, and (<b>c</b>) clamping method of the NiTi archwire.</p> Full article ">Figure 2
<p>(<b>a</b>) Scheme of hydrogen charging (<b>b</b>) setup by electrolysis.</p> Full article ">Figure 3
<p>Typical engineering stress–strain curves of superelastic Ni-Ti alloy immersed for 3 h and aged for 24 h at different strain rates of 10<sup>−5</sup>s<sup>−1</sup>, 10<sup>−4</sup>s<sup>−1</sup> and 10<sup>−5</sup>s<sup>−1</sup> and as-received at 10<sup>−5</sup>s<sup>−1</sup>.</p> Full article ">Figure 4
<p>Simplified NiTi phase diagram showing the increase in the martensite starting stress from σ<sub>1</sub> to σ<sub>2</sub> when the temperature goes up from T<sub>1</sub> to T<sub>2</sub><sub>,</sub> respectively.</p> Full article ">Figure 5
<p>Typical strain cycling curve after 50 cycles for deformed specimen until 7.1%, at imposed strain rate of (<b>a</b>) 10<sup>−</sup><sup>5</sup>s<sup>−1</sup>, (<b>b</b>) 10<sup>−</sup><sup>4</sup>s<sup>−1</sup> and (<b>c</b>) 10<sup>−</sup><sup>3</sup>s<sup>−1</sup>, showing reduction in phase-transformation yield stress and increase in residual strain.</p> Full article ">Figure 6
<p>Evolution of residual strain at zero stress as a function of number of cycles for cyclic deformed specimen with 7.1% strain after different imposed strain rates of 10<sup>−</sup><sup>5</sup>s<sup>−1</sup>, 10<sup>−</sup><sup>4</sup>s<sup>−1</sup> and 10<sup>−</sup><sup>3</sup>s<sup>−1</sup> (represents the fitting curve).</p> Full article ">Figure 7
<p>Hydrogen thermal desorption curves for specimens immersed for 3 h and aged for 24 h at room temperature after 13 cycles with imposed deformation of (<b>a</b>) 10<sup>−5</sup>s<sup>−1</sup>, (<b>b</b>) 10<sup>−4</sup>s<sup>−1</sup> and (<b>c</b>) 10<sup>−3</sup>s<sup>−1</sup>, showing an increase in the amount of absorbed hydrogen with stain rates.</p> Full article ">Figure 8
<p>Amount of absorbed hydrogen vs. number of cycles of loaded and unloaded specimens at imposed 10<sup>−5</sup>s<sup>−1</sup>, 10<sup>−4</sup>s<sup>−1</sup>, and 10<sup>−3</sup>s<sup>−1</sup> strain rates and hydrogen charging for 3 h (represents the fitting curve).</p> Full article ">Figure 9
<p>(<b>a</b>) Tensile curve at 10<sup>−5</sup>s<sup>−1</sup> showing fracture after 13 cycles with imposed strain rate of 10<sup>−5</sup>s<sup>−1</sup>, and (<b>b</b>) amount of absorbed hydrogen after the same number of cycles at 10<sup>−5</sup>s<sup>−1</sup>, 10<sup>−4</sup>s<sup>−1</sup>, and 10<sup>−3</sup>s<sup>−1</sup>.</p> Full article ">Figure 10
<p>(<b>a</b>) Tensile curves obtained after 18 cycles and immersion for 3 h showing the embrittlement of loaded specimen with imposed strain rates of 10<sup>−5</sup>s<sup>−1</sup> and 10<sup>−4</sup>s<sup>−1</sup> and superelastic behaviour of loaded specimens with 10<sup>−3</sup>s<sup>−1</sup>, and (<b>b</b>) comparison between critical amount of absorbed hydrogen causing fracture at 10<sup>−5</sup>s<sup>−1</sup> and quantity of absorbed hydrogen after 18 cycles with 10<sup>−3</sup>s<sup>−1</sup> and immersion.</p> Full article ">
13 pages, 3256 KiB  
Communication
On the Decrease in Transformation Stress in a Bicrystal Cu-Al-Mn Shape-Memory Alloy during Cyclic Compressive Deformation
by Tung-Huan Su, Nian-Hu Lu, Chih-Hsuan Chen and Chuin-Shan Chen
Materials 2021, 14(16), 4439; https://doi.org/10.3390/ma14164439 - 8 Aug 2021
Cited by 7 | Viewed by 2923
Abstract
The evolution of the inhomogeneous distribution of the transformation stress (σs) and strain fields with an increasing number of cycles in two differently orientated grains is investigated for the first time using a combined technique of digital image correlation and [...] Read more.
The evolution of the inhomogeneous distribution of the transformation stress (σs) and strain fields with an increasing number of cycles in two differently orientated grains is investigated for the first time using a combined technique of digital image correlation and data-driven identification. The theoretical transformation strains (εT) of these two grains with crystal orientations [5 3 26]β and [6 5 11]β along the loading direction are 10.1% and 7.1%, respectively. The grain with lower εT has a higher σs initially and a faster decrease in σs compared with the grain with higher εT. The results show that the grains with higher σs might trigger more dislocations during the martensite transformation, and thus result in greater residual strain and a larger decrease in σs during subsequent cycles. Grain boundary kinking in bicrystal induces an additional decrease in transformation stress. We conclude that a grain with crystal orientation that has high transformation strain and low transformation stress (with respect to loading direction) will exhibit stable transformation stress, and thus lead to higher functional performance in Cu-based shape memory alloys. Full article
(This article belongs to the Special Issue Mechanical Behavior of Shape Memory Alloys: 2022)
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<p>The digital image correlation (DIC) technique and data-driven identification (DDI) method were employed to measure the strain and stress distributions, respectively, at the surface of the specimen to characterize the cyclic behavior of the superelasticity of the bicrystal Cu-Al-Mn SMAs. The cyclic compression–unloading test was performed under the strain-controlled mode. The strain fields in the area of interest (AOI) can be obtained using the DIC technique. Based on the experimentally determined strain fields, the stress fields in the AOI can be computed using the DDI method. Finally, three parameters (i.e., transformation stress (<math display="inline"><semantics> <mrow> <msub> <mi>σ</mi> <mi mathvariant="normal">s</mi> </msub> </mrow> </semantics></math>), residual strain (<math display="inline"><semantics> <mrow> <msub> <mi>ε</mi> <mi mathvariant="normal">r</mi> </msub> </mrow> </semantics></math>), and transformation strain (<math display="inline"><semantics> <mrow> <msub> <mi>ε</mi> <mrow> <mi>tr</mi> </mrow> </msub> </mrow> </semantics></math>)) can be computed from the stress–strain responses.</p> Full article ">Figure 2
<p>(<b>a</b>) Geometry of the bicrystal Cu-Al-Mn SMA. The loading directions of the top and bottom grains are shown in the inverse pole figure. (<b>b</b>) Average stress–strain curves of the top grain (<math display="inline"><semantics> <mrow> <msub> <mi>ε</mi> <mi mathvariant="normal">t</mi> </msub> </mrow> </semantics></math>), bottom grain (<math display="inline"><semantics> <mrow> <msub> <mi>ε</mi> <mi mathvariant="normal">b</mi> </msub> </mrow> </semantics></math>), and the entire specimen (<math display="inline"><semantics> <mrow> <msub> <mi>ε</mi> <mi mathvariant="normal">g</mi> </msub> </mrow> </semantics></math>). The bicrystal sample was loaded to a gauge strain (<math display="inline"><semantics> <mrow> <msub> <mi>ε</mi> <mi mathvariant="normal">g</mi> </msub> </mrow> </semantics></math>) of 5% during cyclic deformation. Local virtual strain gauges <math display="inline"><semantics> <mrow> <msub> <mi>ε</mi> <mi mathvariant="normal">t</mi> </msub> </mrow> </semantics></math> and <math display="inline"><semantics> <mrow> <msub> <mi>ε</mi> <mi mathvariant="normal">b</mi> </msub> </mrow> </semantics></math> were used to measure the average strains in the top and bottom grains, respectively (inset of (<b>b</b>)).</p> Full article ">Figure 3
<p>Distribution of (<b>a</b>) axial strain fields <math display="inline"><semantics> <mrow> <msub> <mi>ε</mi> <mrow> <mi>yy</mi> </mrow> </msub> </mrow> </semantics></math> during loading toward and unloading away from the gauge strain <math display="inline"><semantics> <mrow> <msub> <mi>ε</mi> <mi mathvariant="normal">g</mi> </msub> </mrow> </semantics></math> of 5% and (<b>b</b>) transformation stress fields <math display="inline"><semantics> <mrow> <msub> <mi>σ</mi> <mi mathvariant="normal">s</mi> </msub> </mrow> </semantics></math> in the bicrystal Cu-Al-Mn SMA sample for selected compression–unloading cycles: C1, C10, and C20. Points A, B, and C are probing points for recording the local axial stress–strain responses <math display="inline"><semantics> <mrow> <mrow> <mo>(</mo> <mrow> <msub> <mi>σ</mi> <mrow> <mi>yy</mi> </mrow> </msub> <mo>,</mo> <msub> <mi>ε</mi> <mrow> <mi>yy</mi> </mrow> </msub> </mrow> <mo>)</mo> </mrow> </mrow> </semantics></math> as shown in <a href="#materials-14-04439-f004" class="html-fig">Figure 4</a>a. (<b>c</b>) Transformation stress difference <math display="inline"><semantics> <mrow> <mo>∆</mo> <msub> <mi>σ</mi> <mi mathvariant="normal">s</mi> </msub> </mrow> </semantics></math>, which is the difference in transformation stress between cycles 1 and 20, shown in the plot in <a href="#materials-14-04439-f003" class="html-fig">Figure 3</a>b.</p> Full article ">Figure 4
<p>(<b>a</b>) Local axial stress–strain responses (<math display="inline"><semantics> <mrow> <msub> <mi>σ</mi> <mrow> <mi>yy</mi> </mrow> </msub> <mo>,</mo> <mo> </mo> <msub> <mi>ε</mi> <mrow> <mi>yy</mi> </mrow> </msub> <mo stretchy="false">)</mo> </mrow> </semantics></math> recorded by the probing points (according to <a href="#materials-14-04439-f003" class="html-fig">Figure 3</a>b) along the axial centerline for several selected compression–unloading cycles (C1, C5, C10, and C20). The evolution of the (<b>b</b>) transformation stress <math display="inline"><semantics> <mrow> <msub> <mi>σ</mi> <mi mathvariant="normal">s</mi> </msub> </mrow> </semantics></math> and (<b>c</b>) residual strain <math display="inline"><semantics> <mrow> <msub> <mi>ε</mi> <mi mathvariant="normal">r</mi> </msub> </mrow> </semantics></math> with respect to the number of cycles. These values are computed from the local axial stress–strain responses shown in (<b>a</b>).</p> Full article ">Figure 5
<p>(<b>a</b>) Thermal analysis of the bottom grain after 20 compression cycles. (<b>b</b>,<b>c</b>) TEM bright field images of the bottom grain, which show the formation of dislocations and residual martensite after cyclic compression, respectively.</p> Full article ">Figure 6
<p>(<b>a</b>) Distribution of horizontal strain fields <math display="inline"><semantics> <mrow> <msub> <mi>ε</mi> <mrow> <mi>xx</mi> </mrow> </msub> </mrow> </semantics></math> during loading toward and unloading away from the gauge strain <math display="inline"><semantics> <mrow> <msub> <mi>ε</mi> <mi mathvariant="normal">g</mi> </msub> </mrow> </semantics></math> of 5% in the bicrystal Cu-Al-Mn SMA sample for selected compression–unloading cycles: C1, C10, and C20. (<b>b</b>) The evolution of average incompatibility strain <math display="inline"><semantics> <mrow> <mo>∆</mo> <msubsup> <mi>ε</mi> <mrow> <mi>xx</mi> </mrow> <mrow> <mi>avg</mi> </mrow> </msubsup> </mrow> </semantics></math> in the regions (R2-R1 and R4-R3) with respect to the number of cycles. These values are computed from the strain fields multiplied by transformation matrix based on the angle between loading direction and normal direction of the grain boundary (inset of (<b>b</b>)).</p> Full article ">
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