Search Results (36)

Focused Ion Beam (FIB) has been used to create single α-β colony micro-pillars from a polycrystalline commercial Ti-6Al-4V (Ti-64) sample. Each pillar was selected to have either a single alpha phase, a single beta phase, or two α lamella separated by a thin β phase filet. Then, utilizing a diamond flat tip as a compression platen, uniaxial micro-compression tests were performed on the single crystal α and β pillars as well as a tri-crystal α/β/α pillar using a nano-indenter. Then, utilizing a diamond flat tip as a compression platen, uniaxial micro-compression tests were performed on the single crystal alpha and beta pillars as well as a tri-crystal α/β/α pillar using a nano-indenter. Through the use of Electron Back Scattering Diffraction (EBSD) to choose the crystal orientation along the micro-pillar, three distinct unique slip systems have been selectively triggered by maximizing the Schmid factor for each system. The potential to localize a single crystal volume that can be characterized after deformation is one benefit of the micro-compression approach over traditional mechanical testing. The sample strengths compare well with published data. The mechanical properties of the α-β colonies and the single α and β phases have been compared in order to elucidate the role of the α/β interfaces in determining the critical resolved shear stress. Full article

Metallic biomaterials in a solid form cause stress-shielding in orthopedic applications. Such implants also suffer from limited tissue attachment to become a part of the living system. In view of that, hydroxyapatite (HA) coating reinforced with titanium oxide (TiO₂) was deposited in a beta (β)-Titanium (Ti-35Nb-7Ta-5Zr) substrate by plasma spray. This allows us to exploit the best of the two materials, namely the relatively low modulus of β-Ti, together with the porous and bone-like structure/composition of the HA to facilitate cell growth. This is foreseen to be used as an implant, particularly for musculoskeletal-related disability. Detailed scanning electron microscopy (SEM) investigation shows the lamellar structure of the coating that is composed of different phases and some porosities. Transmission electron microscopy (TEM) confirms the co-existence of both the amorphous and crystalline phases that build up the coating structure. In situ micro-mechanical tests revealed that the HA-TiO₂ coating was low in strength and modules compared to that of the substrate material, together with lower ductility. The yield stress and modulus of elasticity of the coating were about 877 ± 174 MPa and 447 ± 24 MPa, respectively. In contrast, the beta (β)-Ti substrate possesses about 990 ± 85 MPa of yield stress and 259 ± 19 MPa modulus of elasticity. The deformation mechanism was also quite different, where the coating crumbled under compressive loading, featuring limited ductility with cleavage (brittle)-type fracture, and the substrate showed plastic flow of materials in the form of slip/shear planes with extended ductility. Full article

Two different microstructures of an Al-12Si (wt. %) alloy were produced, respectively, via a powder laser bed fusion (P-LBF) additive manufacturing and casting. Compared to casting, additive manufacturing of Al-based alloy requires extra care to minimize oxidation tendency. The role of the microstructure on the mechanical properties of Al-12Si (wt. %) alloy was investigated by in situ compression of the micro-pillars. The microstructure of additively manufactured specimens exhibited a sub-cellular (~700 nm) nature in the presence of melt-pool arrangements and grain boundaries. On the other hand, the microstructure of the cast alloy contains typical needle-like eutectic structures. This striking difference in microstructure had obvious effects on the plastic flow of the materials under compression. The yield and ultimate compressive strength of the additively manufactured alloy were 23.69–27.94 MPa and 75.43–81.21 MPa, respectively. The cast alloy exhibited similar yield strength (31.46 MPa); however, its ultimate compressive strength (34.95 MPa) was only half that of the additively manufactured alloy. The deformation mechanism, as unrevealed by SEM investigation on the surface as well as on the cross-section of the distorted micro-pillars, confirms the presence of ductile and quasi-ductile facture of the matrix and the Si needle, respectively, in the case of the cast alloy. In contrast, the additively manufactured alloy exhibits predominantly ductile fractures. Full article

This review explores the mechanical behavior of high-strength concrete-filled steel tubes (CFSTs), focusing on their structural integrity and failure mechanisms. This study highlights the crucial role of the steel tube in providing passive confinement, which limits crack progression and enhances the ductility of the concrete. The concept of concrete as a structural system composed of micro- and mini-pillars, derived from rock mechanics, can be a useful approach to understanding CFST behavior. The review identifies that the strength index (SI) can, in some cases, decrease with an increase in the confinement factor (ξ), particularly in high-strength and ultrahigh-strength concrete (HSC and UHSC), which seems to be different to the common understanding of confinement. The experimental results show that different crack patterns and concrete compositions significantly impact the CFST performance. For example, silica fume in concrete mixtures can reduce the strength enhancement despite increasing the unconfined compressive strength. This work advocates for a mechanistic approach to better comprehend the interaction between concrete and steel tubes, emphasizing the need for optimized concrete mixtures and improved mechanical interaction. Future research should focus on the potential of HSC and UHSC in CFST, addressing factors such as crack progression, confinement effects, and concrete–steel interaction. Full article

NiCoCrAlY high entropy alloy (HEA) coating (47.1 wt.% Ni, 23 wt.% Co, 17 wt.% Cr, 12.5 wt.% Al, and 0.4 wt.% Y) was deposited on a stainless steel subtract by atmospheric plasma spraying (APS). The as-deposited coating was about 300 μm thickness with <1% porosity. The microstructure of the coating consisted of dispersed secondary phases/intermetallics in the solid solution. The stress–strain behaviour of this coating was investigated in micro-scale with the help of in situ micro-pillar compression. The experimental results show that yield and compressive stress in the cross-section of the coating was higher (1.27 ± 0.10 MPa and 2.19 ± 0.10 GPa, respectively) than that of the planar direction (0.85 ± 0.09 MPa and 1.20 ± 0.08 GPa, respectively). The various secondary/intermetallic phases (γ′–Ni₃Al, β–NiAl) that were present in the coating microstructure hinder the lattice movement during compression, according to Orowan mechanism. In addition to that, the direction of the loading to that of the orientation of the phase/splat boundaries dictate the crack propagation architecture, which results in difference in the micro-mechanical properties. Full article

Graphite IG-110 is a synthetic polycrystalline material used as a neutron moderator in reactors. Graphite is inherently brittle and is known to exhibit a further increase in brittleness due to radiation damage at room temperature. To understand the irradiation effects on pre-existing defects and their overall influence on external load, micropillar compression tests were performed using in situ nanoindentation in the Transmission Electron Microscopy (TEM) for both pristine and ion-irradiated samples. While pristine specimens showed brittle and subsequent catastrophic failure, the 2.8 MeV Au²⁺ ion (fluence of 4.378 × 10¹⁴ cm⁻²) irradiated specimens sustained extensive plasticity at room temperature without failure. In situ TEM characterization showed nucleation of nanoscale kink band structures at numerous sites, where the localized plasticity appeared to close the defects and cracks while allowing large average strain. We propose that compressive mechanical stress due to dimensional change during ion irradiation transforms buckled basal layers in graphite into kink bands. The externally applied load during the micropillar tests proliferates the nucleation and motion of kink bands to accommodate the large plastic strain. The inherent non-uniformity of graphite microstructure promotes such strain localization, making kink bands the predominant mechanism behind unprecedented toughness in an otherwise brittle material. Full article

The deformation aspects associated with the micro-mechanical properties of the powder laser bed fusion (P-LBF) additively manufactured stainless steel 316L were investigated in the present work. Toward that, micro-pillars were fabricated on different planes of the stainless steel 316L specimen with respect to build direction, and an in situ compression was carried out inside the chamber of the scanning electron microscope (SEM). The results were compared against the compositionally similar stainless steel 316L, which was fabricated by a conventional method, that is, casting. The post-deformed micro-pillars on the both materials were examined by electron microscopy. The P-LBF processed steel exhibits equiaxed as well as elongated grains of different orientation with the characteristics of the melt-pool type arrangements. In contrast, the cast alloy shows typical circular-type grains in the presence of micro-twins. The yield stress and ultimate compressive stress of P-LBF fabricated steel were about 431.02 ± 15.51 − 474.44 ± 23.49 MPa and 547.78 ± 29.58 − 682.59 ± 21.59 MPa, respectively. Whereas for the cast alloy, it was about 322.38 ± 19.78 MPa and 477.11 ± 25.31 MPa, respectively. Thus, the outcome of this study signifies that the AM-processed samples possess higher mechanical properties than conventionally processed alloy of similar composition. Irrespective of the processing method, both specimens exhibit ductile-type deformation, which is typical for metallic alloys. Full article

Bulk metallic glasses (BMGs) display excellent strength, high hardness, exceptional wear resistance and corrosion resistance owing to its amorphous structure. However, the manufacturing of large-sized and complex shaped BMG parts faces significant difficulties, which seriously hinders their applications. Laser powder bed fusion (LPBF) is a typical additive manufacturing (AM) technique with a cooling rate of up to 10⁸ K/s, which not only allows for the formation of amorphous structures but also solves the forming problem of complex-shaped BMG parts. In recent years, a large amount of work has been carried out on the LPBF processing of BMGs. This review mainly summarizes the latest progress in the field of LPBF additively manufactured BMGs focusing on their mechanical properties. We first briefly review the BMG alloy systems that have been additively manufactured using LPBF, then the mechanical properties of LPBF-fabricated BMGs including the micro- and nano-hardness, micropillar compressive performance, and macro-compressive and tensile performance are clarified. Next, the relationship between the mechanical properties and microstructure of BMGs produced via LPBF are analyzed. Finally, the measures for improving the mechanical properties of LPBF-fabricated BMGs are discussed. This review can provide readers with an essential comprehension of the structural and mechanical properties of LPBF-manufactured BMGs. Full article

The present work investigates the formation and microstructural and micro-mechanical characterization of the recast layer that formed on Inconel 718 alloy in the course of the wire electro-discharge machining (WEDM). The as-machined surface contains globules, shallow cracks, and re-deposition of molten materials, together with the elements from the decomposition of wire electrode and electrolyte, which does not exceed beyond the surface of the recast layer. Under presently investigated machining parameters, the recast layer was about 6.2 ± 2.1 µm thick. There was no presence of a heat-affected zone (HAZ), as otherwise indicated for other hard-to-cut materials. The transmission electron microscopy (TEM) and electron back-scattered diffraction (EBSD) investigations show that the microstructure of the recast layer is similar to that of bulk alloy. Micro-mechanical characterizations of the recast layer were investigated via in-situ micro-pillar compression on the micro-pillars fabricated on the recast layer. The strength of the superficial layer (1151.6 ± 51.1 MPa) was about 2.2 times higher than that of the base material (523.2 ± 22.1 MPa), as revealed by the in-situ micro-pillar compression. Full article

The equiatomic CoCrFeNiMn high-entropy alloy (HEA) possesses excellent properties including exceptional strength–ductility synergy, high corrosion resistance, and good thermal stability. Selective laser melting (SLM) additive manufacturing facilitates the convenient fabrication of the CoCrFeNiMn HEA parts with complex geometries. Here, the SLM process optimization was conducted to achieve a high relative density of as-built CoCrFeNiMn HEA bulks. The mechanisms of process-induced defects and process control were elucidated. The microscale mechanical behaviors were analyzed through in situ scanning electron microscopy observation during the compression tests on micro-pillars of the as-built HEA. The stress–strain characteristics by repeated slip and mechanism of “dislocation avalanche” during the compression of micro-pillars were discussed. The high-cycle fatigue tests of the as-built HEA were performed. It was found that a large number of nano-twins were induced by the fatigue, causing a non-negligible cycle softening phenomenon. The effects of promoted ductility due to the fatigue-induced nano-twins were illustrated. This work has some significance for the engineering application of the SLM additively manufactured CoCrFeNiMn HEA parts. Full article

A micropillar compression study with two different techniques was performed on proton-irradiated additively manufactured (AM) 316L stainless steels. The sample was irradiated at 360 °C using 2 MeV protons to 1.8 average displacement per atom (dpa) in the near-surface region. A comparison study with mechanical test and microstructure characterization was made between planar and cross-sectional pillars prepared from the irradiated surface. While a 2 MeV proton irradiation creates a relatively flat damage zone up to 12 µm, the dpa gradient by a factor of 2 leads to significant dpa uncertainty along the pillar height direction for the conventional planar technique. Cross-sectional pillars can significantly reduce such dpa uncertainty. From one single sample, three cross-sectional pillars were able to show dpa-dependent hardening. Furthermore, post-compression transmission electron microscopy allows the determination of the deformation mechanism of individual micropillars. Cross-sectional micropillar compression can be used to study radiation-induced mechanical property changes with better resolution and less data fluctuation. Full article

Emerging high-entropy alloy (HEA) films achieve high strength but generally show ineludible brittle fractures, strongly restricting their micro/nano-mechanical and functional applications. Nanolayered (NL) CoCrFeNi/graphene composites are elaborately fabricated via magnetron sputtering and the transfer process. It is uncovered that NL CoCrFeNi/graphene composite pillars exhibit a simultaneous ultra-high strength of 4.73 GPa and considerable compressive plasticity of over 20%. Detailed electron microscope observations and simulations reveal that the monolayer graphene interface can effectively block the crack propagation and stimulate dislocations to accommodate further deformation. Our findings open avenues for the fabrication of high-performance, HEA-based composites, thereby addressing the challenges and unmet needs in flexible electronics and mechanical metamaterials. Full article

Micromechanics techniques, such as nano-indentation and micro-pillar compression, can be applied to study hydrogen-charged zirconium alloys at elevated temperatures, which is highly relevant for the nuclear industry. Such experiments are often conducted inside a scanning electron microscope (SEM) under high-vacuum conditions (10⁻⁵ mbar). The combination of a high-temperature and high-vacuum environment causes some hydrogen to escape from the sample into the chamber. Although this effect is evident at temperatures above 600 °C, the extent of hydrogen desorption at lower temperatures is still unclear. In the presented study, the desorption of hydrogen was assessed in zirconium cladding tube material under temperature and hydrogen content conditions comparable to those faced by used nuclear fuel during dry storage. The measured hydrogen loss due to the high vacuum was compared to the simulations obtained using an extended version of a hydrogen behavior tool developed at PSI. Full article

Nickel–cobalt alloys were prepared by alloy electrodeposition with a sulfamate bath, and the mechanical properties on the micro-scale were evaluated for the application as micro-components in miniaturized electronic devices. Nickel bromide and a commercially available surface brightener were used as the additives. The cobalt content increased from 21.5 to 60.1 at.% after addition of nickel bromide into the bath, and the grain size refined from 21.1 to 13.2 nm when the surface brightener was used. The mechanical properties on the micro-scale were evaluated by micro-compression test using micro-pillar type specimens fabricated by a focused ion beam system to take the sample size effect into consideration. The yield strength of the nickel–cobalt alloy having an average grain size at 13.9 nm and cobalt content of 66.6 at.% reached 2.37 GPa, revealing influences from the sample size, grain boundary strengthening, and solid solution strengthening effects. Full article

A concurrent multiscale model coupling discrete dislocation dynamics to the finite element method is developed to investigate the plastic mechanism of materials at micron/submicron length scales. In this model, the plastic strain is computed in discrete dislocation dynamics (DDD) and transferred to the finite element method (FEM) to participate in the constitutive law calculation, while the FEM solves the complex boundary problem for DDD simulation. The implementation of the whole coupling scheme takes advantage of user subroutines in the software ABAQUS. The data structures used for information transferring are introduced in detail. Moreover, a FE mesh-based regularization method is proposed to localize the discrete plastic strain to continuum material points. Uniaxial compression tests of single crystal micropillars are performed to validate the developed model. The results indicate the apparent dependence of yield stress on sample size, and its underlying mechanisms are also analyzed. Full article