JP4849402B2 - High strength magnesium alloy and method for producing the same - Google Patents

Description

  The present invention relates to a high-strength magnesium alloy, in particular, a magnesium alloy with an increased high-temperature strength and a method for producing the same.

  Magnesium alloys are being applied to various structural members by taking advantage of their light weight. In particular, when applied to automobiles, it is effective for improving fuel efficiency and protecting resources and the environment.

  Commercially available materials include ASTM AZ91C (standard composition [wt%]: Mg-8.7Al-0.7Zn-0.13Mn), ZE41A (same: Mg-4.2Zn-1.2RE-0.7Zr), etc. as magnesium alloys for sand casting. However, AZ61A (Mg-6.4Al-1.0Zn-0.28Mn), AZ31B (Mg-3.0Al-1.0Zn-0.15Mn), etc. are widely used as the magnesium alloy for extension.

  Among these, AZ91C and ZE41A, which are sand casting alloys, are precipitation effect type alloys, and are adjusted to the required strength by applying T6 (solution treatment + aging) or T5 (aging only) to the cast material. However, when exposed to room temperature or higher, particularly 50 ° C. or longer, aging precipitation of solid solution elements occurs, the alloy structure gradually changes, and this causes changes in characteristics over time. As a result, there is a drawback that the thermal stability of the structure and properties is low, and high high-temperature strength cannot be obtained stably.

  Further, AZ61A and AZ31B, which are wrought alloys, utilize the grain refinement by processing and recrystallization during rolling, extrusion, and the like as a strengthening mechanism. However, when the temperature is higher than 100 ° C., remarkable grain boundary slippage peculiar to Mg is generated. Therefore, the refinement of crystal grains causes a decrease in strength due to an increase in grain boundary slip generation sites. Further, when exposed to high temperature, crystal grains grow and the effect of refining is lost, which causes a decrease in room temperature strength. As a result, there is a disadvantage that not only high high-temperature strength cannot be secured, but also room temperature strength is thermally unstable.

  In order to improve the drawbacks of the above-mentioned conventional commercially available materials and ensure high high-temperature strength, Patent Document 1 (Japanese Patent Laid-Open No. 2002-309332) discloses Mg-1 to 10 atm in which a solid solution matrix is dispersion strengthened by quasicrystalline particles. % Zn-0.1-3 at% Y alloy is disclosed. The cast structure has a quasicrystalline eutectic structure formed at the α-Mg grain boundaries, and the quasicrystal is finely and uniformly dispersed by hot working. Since the quasicrystal has a hardness much higher than that of a crystalline compound having an approximate composition, magnesium having excellent strength and stretchability can be obtained. However, although the thermal stability is increased, the strength itself is comparable to that of a commercially available alloy having a similar composition such as ZE41, and there is a limit that a higher high-temperature strength cannot be obtained.

On the other hand, Patent Document 2 describes Mg _{100- (a + b)} Zn _a Y _b , a / 12 ≦ b ≦ a / 3 and 1.5 ≦ a ≦ 10 as an alloy with improved high-temperature strength. There is disclosed a magnesium alloy having a composition that satisfies, in which Mg ₃ Zn ₆ Y ₁ quasicrystal and its approximate crystal (complex structure phase derived from quasicrystal) are dispersed in the form of fine particles in the Mg matrix.

Further, Patent Document 3 describes Mg _{100- (a + b + c)} Zn _a Zr _b Y _c , a / 12 ≦ (b + c) ≦ a / 3 and 1.5 ≦ a ≦ 10, 0.05 <b. A magnesium alloy in which fine particles of approximate crystals are dispersed in an Mg matrix with a composition satisfying <0.25c is disclosed.

Patent Document 4 _discloses Mg _{100- (a + b + c)} Zn _a Al _b Y _c , (a + b) / 12 ≦ c ≦ (a + b) / 3 and 1.5 ≦ a ≦ 10, 0.05a The composition satisfying <b <0.25a, and the Mg ₃ Zn ₆ Y ₁ quasicrystal and its approximate crystal (complex structure phase derived from the quasicrystal) are dispersed in the form of fine particles in the Mg matrix. A magnesium alloy is disclosed.

  All of the above prior art magnesium alloys achieve high high-temperature strength by dispersing quasicrystals and their approximate crystals as fine reinforcing particles in the Mg matrix, but rare earth elements (Y) are essential components. Therefore, there was a problem that an increase in cost was inevitable.

JP 2002-309332 A JP 2005-113235 A Japanese Patent Laid-Open No. 2006-89772 JP 2005-113234 A

  An object of the present invention is to provide a high-strength magnesium alloy that is reduced in price without using an expensive rare earth element and improved in high-temperature strength, and a method for producing the same.

In order to achieve the above object, the high-strength magnesium alloy of the first invention is represented by the composition formula Mg _{100- (a + b)} Zn _a X _b , wherein X is one selected from Zr, Ti, and Hf Where a and b are the contents of Zn and X expressed in at%, respectively, and the relationship of the following formulas (1), (2) and (3):
a / 28 ≦ b ≦ a / 9 (1)
2 <a <10 (2)
0.05 <b <1.0 (3)
And satisfy
And approximate crystal Mg-Zn-X system quasicrystals and its the Mg mother phase is characterized by being dispersed in the form of fine particles.

The method of the second invention for producing the high strength magnesium alloy of the first invention is:
A step of melting Mg in an inert atmosphere to form a molten Mg,
Adding a Mg—Zn—X-based quasicrystal (X is one or more of Zr, Ti, and Hf) to the molten Mg to form a molten alloy;
Casting the molten alloy,
It includes a step of heat-treating the obtained casting to precipitate the quasicrystal and its approximate crystal in the Mg matrix.

  The high-strength magnesium alloy of the present invention is a quasicrystal using a rare earth element in the past by dispersing Mg-Zn-X quasicrystals in the form of fine particles and their approximate crystals as reinforcing particles in the Mg matrix. In addition, high strength, particularly high temperature strength, equivalent to an alloy in which approximate crystals are dispersed can be achieved.

  The high-strength magnesium alloy of the present invention has a quasicrystal composed of Mg—Zn—X (X = Zr, Ti, or Hf) in the Mg matrix and its approximate crystal by setting the composition range as specified above. And a dispersed structure can be obtained.

A quasicrystal is a compound having a regular structure (typically 5-fold symmetry) in the short range, but having no translational symmetry, which is a characteristic of ordinary crystals. Al-Pd-Mn, Al-Cu-Fe, Cd-Yb, Mg-Zn-Y, and the like have been known as compositions that produce quasicrystals. Because of its unique structure, it has various unique properties such as high hardness, high melting point, low friction coefficient, etc., compared to crystalline intermetallic compounds of approximate composition.
An approximate crystal is a crystalline compound having a complex structure derived from a quasicrystal, partially having the same structure as the quasicrystal, and having the same properties as the quasicrystal from which it was derived.

The composition of the Mg—Zn—X quasicrystal is desirably an at% ratio of Zn and X: a: b = 83: 6, and Mg ₁₁ Zn ₈₃ Zr ₆ , Mg ₁₁ Zn ₈₃ Ti ₆ , Mg ₁₁ Zn ₈₃ One or more selected from Hf ₆ . However, it is not necessary to limit to these, and it may be a quasicrystal having a composition allowed within the above-defined alloy composition range, and its approximate crystal is also allowed. The fine particles have a particle size of about several tens nm to several hundreds nm.

  Quasicrystals and approximate crystals are very hard and stable without being decomposed to about 230 ° C., so if they are dispersed as fine particles in the Mg matrix, they interact strongly with dislocations and are extremely high. Demonstrates dispersion strengthening and improves strength at normal and high temperatures. In particular, the fine particles present at the α-Mg crystal grain boundaries suppress grain boundary sliding at high temperatures and contribute to high high temperature strength.

  The alloy of the present invention uses Zr, Ti, Hf instead of the rare earth of the conventional alloy as a constituent of the quasicrystal and the approximate crystal. However, these elements are difficult to melt with the molten Mg which is the main component of the alloy. It cannot be produced by directly melting a molten alloy having a final composition such as an alloy containing rare earth elements. That is, even if the raw materials of each alloy component are weighed according to the final alloy composition, charged together into a melting furnace and heated to form a molten metal, high melting point Zr, Ti, and Hf are contained in this molten metal. It does not melt and remains as a solid. The melting point of these high melting point elements is very high and higher than the boiling point of Mg.

  The rare earth elements such as Y used in the conventional alloys themselves have a much higher melting point than Mg, but they are alloyed by contact with the molten Mg previously produced at low temperature during melting, making it easier to melt. In order to melt into the molten Mg, the conventional alloy was able to directly melt the molten metal having the final composition.

  Since the alloy of the present invention cannot directly melt the molten alloy of the final composition as described above, in the method of the present invention, only Mg is dissolved to form a molten Mg, and a quasicrystal is added thereto. It was possible to form molten alloy. The final alloy composition is adjusted by the amount of molten Mg, the composition of quasicrystals, and the amount added. The molten alloy is sufficiently stirred to obtain a uniform molten metal.

  The obtained molten alloy is cast by a usual method. The obtained casting is heat-treated to precipitate quasicrystals and approximate crystals as fine particles in the Mg matrix.

  As a result, a structure is finally obtained in which fine particles of quasicrystals and approximate crystals are dispersed in the Mg matrix.

The formation mechanism of fine particles of quasicrystals and approximate crystals has not been fully elucidated at present, but is thought to be generated as follows.
The molten alloy that has been sufficiently stirred with the addition of quasicrystals in the molten Mg is visually uniform, but microscopically there is a fluctuation of the composition in the molten metal, and the fine region where the specific elements are unevenly distributed is the entire molten metal. In the cooling process at the time of casting, the quasicrystal and the approximate crystal or the precursor composition thereof are finely crystallized with the finely distributed region as a nucleus. The solidified casting is a state in which Zn and X (one or more of Zr, Ti, and Hf), which are constituent elements of quasicrystals or approximate crystals, are supersaturated in Mg matrix, and are finely processed by heat treatment. Quasicrystals or approximate crystals are precipitated as fine particles with the crystallized substance as a nucleus.
In other words, the final microstructure of the alloy has quasicrystals and approximate crystal fine particles crystallized during the cooling process during casting, and quasicrystals and approximate crystal fine particles precipitated by subsequent heat treatment. Both contribute to strength improvement by strengthening dispersion.

[Example 1]
According to the present invention, an Mg—Zn—Zr magnesium alloy having a final composition of the whole alloy shown in Table 1 was prepared, and a metal structure observation and a tensile test were performed.

(1) Production of quasicrystal Pure Mg (99.9%), pure Zn (99.99%), pure Zr (99.5%) metals (the values in parentheses are the purity), Mg ₁₁ Zn ₈₃ Zr ₆ The quasicrystalline composition was weighed to a total amount of 5 g, set in an alumina Tamman tube (φ12 mm × 10 mm), and sealed in a quartz tube. The inside of the quartz tube was replaced with high-purity argon.

The temperature was raised from room temperature to 700 ° C. over 5 hours, held for 12 hours, then lowered to 500 ° C. and held for 48 hours. Thereby, a Mg ₁₁ Zn ₈₃ Zr ₆ quasicrystal was obtained. As shown in the electron diffraction pattern of FIG. 1, a typical five-fold symmetry was confirmed.
The obtained massive quasicrystal was pulverized to a particle size of several tens of μm and used below.

(2) Melting of alloy Pure Mg (purity 99.99%) was charged into a graphite crucible and melted by heating to 700 ° C. in a high-frequency melting furnace held in an argon atmosphere to form a molten Mg.
In the molten Mg, the quasicrystal produced in the above (1) is added in an amount adjusted so that the final alloy composition becomes the Mg—Zn—Zr composition shown in Table 1, and the molten metal temperature is set to 700 ° C. While being held, the mixture was stirred until it was completely dissolved and uniform, and then held for 2 hours.

(3) Casting The obtained molten alloy was kept at 700 ° C. and cast into a cast iron JIS No. 4 ship mold (70 mm × 70 mm × 300 mm) preheated to about 100 ° C.

(4) Heat treatment The casting obtained above was heat-treated at 500 ° C for 48 hours in an argon atmosphere.

<Observation of metal structure>
About the sample after heat processing, the metal structure was observed with the transmission electron microscope (TEM).
As a result, as shown in FIG. 2, fine precipitates of several tens of nanometers were observed as white spots in α-Mg crystal grains. This precipitate was confirmed to be a Mg ₁₁ Zn ₈₃ Zr ₆ quasicrystal and its approximate crystal.

<Tensile test>
A round bar tensile test piece having a parallel part φ5 × 25 mm was collected from the heat-treated sample and subjected to a tensile test at room temperature, 150 ° C. and 200 ° C. Using an AG-250kND tensile tester manufactured by Shimadzu Corporation, the tensile rate was 0.8 mm / min.

  Furthermore, the tensile test was similarly performed about the sample which extruded after the said heat processing. The extrusion conditions were an extrusion temperature of 250 ° C. and an extrusion ratio of 10: 1.

  Moreover, the tensile test was similarly done about the comparative example whose composition is outside the range of the present invention.

[Example 2]
According to the present invention, an Mg—Zn—Ti magnesium alloy having a final composition of the entire alloy shown in Table 1 was prepared, and a metal structure observation and a tensile test were performed.
As a quasicrystal, an Mg ₁₁ Zn ₈₃ Ti ₆ quasicrystal was prepared and the same procedure as in Example 1 was performed except that it was added to a pure Mg molten metal.

After the heat treatment, fine precipitates of several tens of nm were observed as white spots in the α-Mg crystal grains by a transmission electron microscope. This precipitate was confirmed to be an Mg ₁₁ Zn ₈₃ Ti ₆ quasicrystal and its approximate crystal.

The tensile test result was performed on the sample after the heat treatment and the sample extruded under the same conditions as in Example 1 under the same conditions as in Example 1.
Moreover, the tensile test was similarly done about the comparative material whose composition is outside the range of the present invention.
Furthermore, the tensile test was similarly performed about the conventional material.
The above test results are summarized in Table 1.

The material of the present invention is particularly excellent in tensile strength at 150 ° C. as compared with the conventional material. Moreover, the strength reduction accompanying the temperature rise from room temperature to 150 ° C. is very small. This is because fine particles composed of quasicrystals and approximate crystals precipitated and dispersed in α-Mg crystal grains have a very high thermal stability, and thus strongly interact with dislocations even at a high temperature of 150 ° C. Because it functions effectively as a barrier to exercise. The comparative material having a composition outside the scope of the present invention does not produce quasicrystals or approximate crystals, or even if they are produced, the amount of dispersion strengthening by these produced phases is hardly obtained, and high strength is obtained. I can't.

  According to the present invention, a high-strength magnesium alloy and a method for producing the same are provided which are inexpensive and improve high-temperature strength without using expensive rare earth elements.

Is a photograph of electron beam diffraction patterns of Mg ₁₁ Zn ₈₃ Zr ₆ quasicrystal manufactured according to this invention. It is a transmission electron micrograph which shows the metal structure of Mg alloy produced by this invention.

Claims (4)

Compositional formula Mg _{100- (a + b)} Zn _a X _b , X is one or more selected from Zr, Ti, Hf, a and b are the contents of Zn and X expressed in at%, respectively. The relationship of the following formulas (1), (2) and (3):
a / 28 ≦ b ≦ a / 9 (1)
2 <a <10 (2)
0.05 <b <1.0 (3)
And satisfy
High strength magnesium alloy in the Mg mother phase and the approximate crystal Mg-Zn-X system quasicrystals and its characterized in that it dispersed in the form of fine particles. According to claim 1, said quasicrystals _{Mg 11 Zn 83 Zr 6, Mg} 11 Zn 83 Ti 6, Mg 11 Zn 83 high strength magnesium alloy, characterized in that at least one member selected from Hf _6. A method for producing the high-strength magnesium alloy according to claim 1, wherein the following steps are performed:
A step of melting Mg in an inert atmosphere to form a molten Mg,
Adding a Mg-Zn-X quasicrystal to the molten Mg to form a molten alloy;
Casting the molten alloy,
A method for producing a high-strength magnesium alloy, comprising a step of heat-treating the obtained casting to precipitate the quasicrystal and its approximate crystal in an Mg matrix. In Claim 3, preparation of the said Mg-Zn-X type | system | group quasicrystal is carried out.
A step of weighing each raw material of Mg, Zn, and X so as to have a quasicrystalline composition;
A step of putting each weighed raw material into a crucible and melting it in an inert atmosphere to form a molten metal,
Lowering the temperature of the molten metal, and maintaining the temperature in a single-phase region where only the quasicrystal is present,
A method for producing a high-strength magnesium alloy, which is performed by a method including a step of cooling from the temperature of the single-phase region to room temperature.