WO2008032857A1

WO2008032857A1 - High-strength magnesium alloy and process for production thereof

Info

Publication number: WO2008032857A1
Application number: PCT/JP2007/068214
Authority: WO
Inventors: An Pang Tsai; Satoshi Ohhashi; Akira Kato
Original assignee: Toyota Jidosha Kabushiki Kaisha
Priority date: 2006-09-15
Filing date: 2007-09-12
Publication date: 2008-03-20
Also published as: JP2008069438A; CN101495660B; CN101495660A; US20090320967A1; JP4849402B2

Abstract

The invention provides a high-strength magnesium alloy which is improved in high-temperature strength without using any expensive rare earth element with cost reduction and a process for the production of the alloy. A high-strength magnesium alloy represented by the composition formula: Mg100-(a+b)ZnaXb (wherein X is at least one member selected from among Zr, Ti and Hf and a and b are contents of Zn and X respectively as represented in at% and satisfy the relationships (1), (2) and (3): (1) a/28 ≤ b ≤ a/9, (2) 2 < a < 10, and (3) 0.05 < b < 1.0), wherein Mg-Zn-X quasicrystals and approximants thereof in theform of fine particles are dispersed in a Mg mother phase; and a process for the production of the high-strength magnesium alloy which comprises melting Mg in an inert atmosphere to form molten Mg, adding Mg-Zn-X quasicrystals (wherein X is at least one of Zr, Ti and Hf) to the molten Mg to form a molten alloy, casting the molten alloy, and heat-treating the cast alloy to precipitate the quasicrystals and approximants thereof in the Mg mother phase.

Description

Description High-strength magnesium alloy and manufacturing method thereof Technical Field

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

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 in improving fuel consumption and thereby protecting resources and the environment.

Commercially available materials include: AS TM AZ 9 1 C (standard composition [wt%]: Mg—8.7A 1 — 0.7Z n— 0.13M n), ZE 4 1 A (same as: M g— 4.2Z n— 1.2RE— 0.7Z r), etc., and AZ 6 1 A (same as: Mg — 6.4A 1-1. OZ n-0.28M n) AZ 3 1 B (same as: Mg—3.0A 1-1. OZ n-0.15M n) is widely used.

Of these, AZ 9 1 C and ZE 4 1 A, which are sand-type forging alloys, are precipitation effect type alloys, and T 6 (solution + aging) or T 5 (aging only) is applied to the forging material. Adjust to the required strength. However, when exposed to room temperature or higher, especially 50 ° C or higher for a long time, aging precipitation of solid solution elements occurs, causing the alloy structure to change gradually, which causes a change in characteristics over time. As a result, the thermal stability of the structure and properties was low, and there was a drawback that it was not possible to stably obtain high high-temperature strength.

In addition, AZ 6 1 A and AZ 3 1 B, which are wrought alloys, use the grain refinement by processing and recrystallization during rolling, extrusion, etc. as a strengthening mechanism. ing. However, when the temperature is higher than 100 ° C, remarkable grain boundary slippage unique to Mg occurs, so the refinement of crystal grains conversely causes a decrease in strength due to an increase in the generation site of grain boundary slippage. . In addition, when exposed to high temperatures, crystal grains grow and the effect of refining is lost, causing a decrease in room temperature strength. As a result, not only high temperature strength could not be secured, but also room temperature strength was thermally unstable.

In order to improve the disadvantages of the above-mentioned conventional commercially available materials and ensure high high-temperature strength, Japanese Patent Application Laid-Open No. 2002-309332 discloses Mg-1 to 10 at% Zn— in which a solid solution matrix is dispersed and strengthened with quasicrystalline particles. 0. l-3at% Y alloy is disclosed. In the forged structure, a quasicrystalline eutectic structure is formed at the α-Mg grain boundary, and the quasicrystal is finely and uniformly dispersed by hot working. Because quasicrystals are much harder than crystalline compounds of approximate composition, magnesium with excellent strength and stretchability can be obtained. However, although the thermal stability has increased, the strength itself is comparable to that of a commercially available alloy with a similar composition such as Z E 41, and there is a limit that a higher high-temperature strength cannot be obtained.

The JP 2 0 0 5 1 1 3 2 3 5 JP contrast, as an alloy with improved high temperature _{strength, M g, 0 0., a} + b) Z n a Y b, a / 1 2≤ b ≤ a Z 3 and 1.5 ≤ a≤ 1 0, and its approximate crystal with Mg ₃ Z n ₆ quasicrystal in Mg parent phase (complex structure phase derived from quasicrystal) A magnesium alloy in which and are dispersed in the form of fine particles is disclosed.

Further, JP-2 0 0 6 - 8 9 7 7 2 _{_{discloses, M g 1 M _ (a}} + b + c) Z n a Z r b Y. , A / 1 2 ≤ (b + c) ≤ a / 3 and 1.5 ≤ a ≤ 1 0, 0. 0 5 <b <0. 2 5 c, similar to Mg matrix A magnesium alloy in which fine particles of crystals are dispersed is disclosed. Japanese Laid-Open Patent Publication No. 2 0 0 5-1 1 3 2 3 4 describes that M g o- + b + c) Z n _a A 1 _b Y _c , (a + b) / 1 2 ≤ c ≤ ( a + b) Z 3 and 1.5 ≤ a ≤ 1 0, 0. 0 5 5 a <b <0. 2 5 a, and in the Mg matrix, Mg ₃ Z n ₆ A magnesium alloy in which the approximate crystal (complex structure phase derived from a quasicrystal) is dispersed in the form of fine particles 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) As an essential component, there is a problem that cost increases cannot be avoided. Disclosure of the invention

An object of the present invention is to provide a high-strength magnesium alloy that is inexpensive and improves high-temperature strength without using expensive rare earth elements, 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 M g _{1 () ()} -( _{a + b} ) Z n _a X _b , where X is Z r, T 1 or more selected from i and H f, a and b are the contents of Zn and X, respectively, expressed in at%, and the relationship of the following formulas (1), (2) and (3): a / 2 8 ≤ b≤ a / 9---(1)

2 <a <1 0 (2)

0. 0 5 <b <1.0 (3)

And

It is characterized in that the Mg-Zn-X quasicrystal and its approximate crystal are dispersed in the form of fine particles in the Mg matrix.

The method of the second invention for producing the high-strength magnesium alloy of the first invention is as follows: A process of melting Mg in an inert atmosphere to form a molten Mg, in which the Mg-Zn-X series quasicrystal (X is one of Zr, Ti, Hf) A process of forming a molten alloy,

Forging the molten alloy,

The obtained forged product is heat treated to precipitate the quasicrystal and its approximate crystal in an Mg matrix.

The high-strength magnesium alloy of the present invention is a conventional rare earth element obtained by dispersing Mg-Zn-X quasicrystals in the form of fine particles and their approximate crystals as reinforcing particles in an Mg matrix. Can achieve high strength, especially high temperature strength, equivalent to alloys in which quasicrystals and approximate crystals are dispersed using

Brief Description of Drawings

1, M g manufactured according to this invention, is a photograph of electron beam diffraction pattern of Z n ₈₃ Z r ₆ quasicrystal.

FIG. 2 is a transmission electron micrograph showing the metal structure of the Mg alloy produced according to the present invention. BEST MODE FOR CARRYING OUT THE INVENTION

The high-strength magnesium alloy of the present invention has a composition range as defined above, so that the Mg parent phase contains —Z n—: (one or more of X = Zr, Ti, Hf). A structure in which crystals and their approximate crystals are dispersed can be obtained.

A quasicrystal is a compound that has a regular structure (typically 5-fold symmetry) in the short range, but does not have the translational symmetry characteristic of ordinary crystals. The compositions that produce quasicrystals are known as A 1 — P d — M n, A 1 — C u — F e, C d — Y b, M g — Z n — Y, etc. It is. Because of its unique structure, it has various unique properties such as high hardness, high melting point, and low friction coefficient compared to crystalline intermetallic compounds of approximate composition.

An approximate crystal is a crystalline compound that has a complex structure derived from a quasicrystal, partially having the same structure as the quasicrystal, and has the same properties as the quasicrystal from which it was derived.

The composition of the Mg-Zn-X quasicrystal is preferably the at% ratio between Zn and X, a: b = 8 3: 6, and M gu Z n Z r M gjj Z n ₈₃ T i ₆ , Mg! , Z n ₈₃ H f ₆ or more. However, it is not necessary to limit to these, and it may be a quasicrystal having a composition allowed within the alloy composition range specified above, and its approximate crystal is also allowed. The fine particles have a particle size of about several tens of nm to several hundreds of nm.

Quasicrystals and approximate crystals are very hard and stable without decomposition up to about 230 ° C, so if they are dispersed as fine particles in the Mg matrix, they strongly interact with dislocations. Acts and exhibits extremely high dispersion strengthening action, improving the strength at normal and high temperatures. In particular, the fine particles present in the Hi-Mg grain boundary suppress grain boundary sliding at high temperatures and contribute to high high-temperature strength.

The alloy of the present invention uses Zr, Ti, and Hf as constituents of the quasicrystal and the approximate crystal in place of the conventional rare earth alloy, but these elements are combined with the molten Mg, which is the main component of the alloy. Since it is difficult to melt, it cannot be produced by directly melting a molten alloy of the final composition like a conventional 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 in a melting furnace and heated to form a molten metal, the high melting points Zr, Ti, and Hf are It does not dissolve in the molten metal and remains as a solid. These refractory elements have very high melting points, higher than the boiling point of Mg. The rare earth elements such as Y used in conventional alloys themselves have a much higher melting point than Mg, but they are alloyed by contact with the molten Mg produced earlier at low temperatures during melting. The conventional alloy was able to directly melt the molten metal with the final composition because it was easily converted into Mg melt.

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 melted to form a Mg melt, and a quasicrystal It was possible to form molten alloy by adding. The amount of Mg melt, the composition of the quasicrystal and the amount added, to adjust the final alloy composition. Thoroughly stir the molten alloy into a uniform melt.

The obtained molten alloy is formed by a usual method. The resulting fabricated product is heat treated to precipitate quasicrystals and approximate crystals as fine particles in the Mg matrix.

This finally yields a structure 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 with quasicrystals added and sufficiently stirred in the molten Mg is visually uniform, but microscopically there is a composition fluctuation in the molten metal, and there is a fine region in which the specific elements are unevenly distributed. The quasicrystal and approximate crystal or its precursor composition crystallize finely in the cooling process during fabrication, with this finely distributed region as the nucleus. The solidified product is a supersaturated solid solution of Zn and X (one or more of Z r, T i, and H f), which are constituent elements of quasicrystals or approximate crystals, in the Mg matrix. By heat treatment, quasi-crystals or approximate crystals are precipitated as fine particles with fine crystals as nuclei.

In other words, in the final microstructure of the alloy, There are quasi-crystal and approximate crystal fine particles crystallized during the cooling process, and quasi-crystal and approximate crystal fine particles precipitated by the subsequent heat treatment, both of which contribute to strength improvement by dispersion strengthening. Example

Example 1

According to the present invention, an Mg-Zn—Zr magnesium alloy having the final composition of the entire alloy shown in Table 1 was prepared, and the metal structure was observed and a tensile test was performed.

(1) Preparation of quasicrystal

Net M g (99.9%), pure Z n (99.99%), the respective metal (values in parentheses purity) of the pure Z r (99.5%), M g in at% ratio,, Z n ₈₃ Z r ₆ Weighed so that the quasicrystalline composition was as follows. The total amount was set to 5 g, set in an alumina Tamman tube (Φ 12 mm XI 0 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 cooled to 500 ° C. and held for 48 hours. Thus, M g, the i Z n ₈₃ Z r ₆ quasicrystal obtained. As shown in Fig. 1, the electron diffraction pattern shows typical five-fold symmetry.

The obtained massive quasicrystal was pulverized to a particle size of several tens; m and used for the following.

(2) Melting of alloys

Pure Mg (purity 99.99%) was charged into a graphite crucible and melted by raising the temperature to 70 0 in a high-frequency melting furnace held in an argon atmosphere to form an Mg melt.

In the Mg melt, add the above-mentioned quasicrystals prepared in U in an amount adjusted so that the final alloy composition becomes the Mg-Zn-Zr composition shown in Table 1. In addition, the molten metal temperature was kept at 700 ° C., and the mixture was stirred until it was completely dissolved and uniform, and then kept for 2 hours.

(3) Forging

The obtained molten alloy was kept at 700 ° C. and poured into a pig iron J IS 4 No. 4 ship type (70 mm × 70 mm × 300 mm) preheated to about 100 ° C.

(4) Heat treatment

The fabricated product obtained above was heat-treated at 50 ° C. for 48 hours under 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 nm were observed as white spots in α-Mg crystal grains. This precipitate is Mg! ] Was confirmed to be a Z n ₈₃ Z r ₆ quasi-crystals and their approximate crystal.

Tensile test>

From the heat-treated sample, a round bar tensile test piece having a parallel portion of ψ 5 X 25 mm was collected and subjected to a tensile test at room temperature, 1550 ° C and 2100 ° C. Using an A G-2500 k ND tensile tester manufactured by Shimadzu Corporation, the tensile speed was 0.8 mm / min.

Further, a tensile test was similarly performed on the sample extruded after the heat treatment. The extrusion conditions were as follows: extrusion temperature 2550 ° (: extrusion ratio 10: 1).

In addition, a tensile test was conducted in the same manner with respect to a comparative example whose composition was out of the scope of the present invention.

Example 2

According to the present invention, the final composition of the entire alloy is the composition shown in Table 1 Mg One Z n—Ti magnesium alloy was fabricated and fx of metal structure observation and tensile tests were performed.

As a quasicrystal, Mg! except for adding to prepare a i Z n _{8 3} T i ₆ quasicrystal pure M g melt was manufactured in a similar procedure as in Example 1.

After the heat treatment, fine precipitates of several tens of nm were observed as white spots in 1 Mg crystal grains using a transmission electron microscope. This precipitate was confirmed to be a M gu Z n _{8 3} Ti ₆ quasicrystal and its approximate crystal.

Tensile test results were performed under the same conditions as in Example 1 for the samples after the heat treatment and the samples extruded under the same conditions as in Example 1.

Similarly, a tensile test was performed on a comparative material having a composition outside the range of the present invention.

Further, a tensile test was performed on the conventional material in the same manner.

The above test results are summarized in Table 1.

table 1

The material of the present invention is particularly excellent in tensile strength at 150 ° C compared to the conventional material. In addition, the decrease in strength accompanying the temperature increase from room temperature to 1550 ° C is very small. This is because — Fine particles composed of quasicrystals and approximate crystals precipitated and dispersed in Mg grains are very thermally stable, so they interact strongly with dislocations even at high temperatures of 150 ° C. This is because it functions effectively as a barrier to dislocation movement. Comparative materials with a composition outside the scope of the present invention do not produce quasicrystals or approximate crystals, or even if they are produced, the amount of dispersion strengthening by these produced phases is hardly obtained. High strength cannot be obtained. Industrial applicability

According to the present invention, there are provided a high-strength magnesium alloy which is reduced in price without using an expensive rare earth element and improved in high-temperature strength and a method for producing the same.

Claims

1. Composition formula ( _{a + b} ) Z n _a X _b , where X is one or more selected from Z r, T i, and H f, and a and b are each expressed as at% Z n , X content, and the relationship of the following formula (1) (2) (3): a / 2 8≤ b≤ a / 9--· (1)

Contract

2 <a <1 0 (2)

0. 0 5 <b <1. 0 (3)

And

Enclosed high-strength magnesium alloy characterized in that Mg-Zn-X quasicrystals and their approximate crystals are dispersed in the form of fine particles in the Mg matrix.

2. In claim 1, said quasicrystals _{_{M g HZ n 83 Z r 6}} , g! , Z n ₈₃ T, Mg HZ n H fs, one or more selected from high strength magnesium alloys.

3. A method for producing the high-strength magnesium alloy according to claim 1 or 2, comprising the following steps:

A step of melting Mg in an inert atmosphere to form a molten Mg, a step of adding a Mg-Zn-X-based quasicrystal to the molten metal to form a molten alloy,

Forging the molten alloy,

A method for producing a high-strength magnesium alloy, comprising a step of heat-treating the obtained forged product to precipitate the quasicrystal and its approximate crystal in an Mg matrix.

4. The method of claim 3, wherein the preparation of the Mg-Z'n-X-based quasicrystal is performed by weighing each of the g, Zn, and X raw materials 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 quasicrystals exist,

Step of cooling from the temperature of the single phase region to room temperature

A process for producing a high-strength magnesium alloy characterized by being carried out by a method comprising