WO2006033458A1 - Magnesium alloy - Google Patents

Abstract

Disclosed is a magnesium alloy comprising, in atomic percent, 2.0-10% of zinc, 0.05-0.2% of zirconium, 0.2-1.50% of a rare earth element and the balance of magnesium and unavoidable impurities. Such an Mg-Zn-RE alloy is improved in strength, especially in high-temperature strength.

Description

Magnesium Alloy Technology

 The present invention relates to a magnesium alloy excellent in high temperature strength. Specifically, it relates to a fine particle-dispersed magnesium alloy with excellent high temperature strength.

 Light

Background art

 Rice field

 Magnesium has a specific gravity of 1.74, which is the lightest industrial metal material, and its mechanical properties are not inferior to those of aluminum alloys. It has been attracting attention as a material that responds to the transformation.

 For example, magnesium alloy is already used as a head cover material for automobile wheels and engines. Recently, there is a strong demand for weight reduction of all components, and the scope of application of magnesium alloys is expanding further. For example, it is considered that a magnesium alloy is applied to structural parts such as engine blocks that become high temperature and functional parts such as pistons. For example, when the Biston is made from aluminum alloy to magnesium alloy, not only the weight of the part itself but also the weight of other parts can be further reduced by reducing the inertia weight.

 Magnesium alloy products usually consist of forged products, including die-cast products

Of the conventional magnesium alloys, Mg_A1 series alloys (ASTM standard 1 AM 60 B, AM 50 A, AM 20 A, etc.) contain 2 to: 12% A 1 and a small amount M n is added, the Mg side is α _ Mg solid solution and β — Mg! ₇ A 1! _It is a eutectic system of _two compounds. ₇ A 1 age hardening by the _second interphase precipitation occurs. Also, the solution improves strength and toughness. Also, A l 5 ~: 10%, Z n :! In Mg-A 1 -Zn system (AS TM standard 1 AZ 9 1 D etc.) containing ~ 3%, there is a wide _α -solid solution region on the Mg side, and Mg-A 1- Crystallize. Although it is as-made, it is tough and excellent in corrosion resistance, but mechanical properties are improved by aging heat treatment, and the compound phase precipitates at the grain boundary in a pearlite state by quenching and tempering.

In Mg-Ζn-based alloys, when 2% Zn is added to Mg, the highest strength and elongation can be obtained with forging, but the forging property is improved and a healthy deposit is obtained. To obtain it, a larger amount of Zn is added. Mg _ 6% Z n alloy has a tensile strength of ² units of 17 kg Zmm when fabricated, and is improved by T 6 treatment, but is considerably inferior to Mg-A 1 system. . An example of the M g ^ Z n system is ZCM 6 3 0 A (M g-6% Z n-3% C u-0.2 M n).

 On the other hand, a magnesium alloy that has excellent heat resistance and is suitable for use at high temperatures was searched for. Alloys added with rare earth elements (R.E.) are slightly inferior to aluminum alloys in mechanical properties at room temperature. It has been found that properties comparable to aluminum alloys can be obtained at high temperatures up to 0 0 ° C. For example, as a practical alloy containing R.E., an EK30A alloy that does not contain Zn (2.5 to 4% R.E.—0.2% Zr), that contains Zn ZE 4 1 A alloy (1% R. E, — 2.0% Z n — 0.6% Z r) has been put to practical use.

 The strength of these Mg alloys is increased as follows.

(1) In Japanese Patent Laid-Open No. 2 0 0 2-3 0 9 3 3 2, a quasi-crystal with _α — Mg is formed by hot forming after forging an Mg — Z n — Y alloy. The crystal phase is finely and uniformly dispersed in the structure. Quasicrystals are quasicrystalline phase-reinforced magnesium alloys that are much harder than crystalline compounds of approximate composition and have excellent strength and stretchability. The composition is limited to Mg—1 to 10 at% Z n −0.1 to 3 at ° / ₀ Y. The forged structure of the Mg- gη-— alloy has a quasicrystalline eutectic structure at the α-Mg crystal grain boundary. By hot forming this, the quasicrystal is finely and uniformly dispersed to increase the strength. (2) Mg alloys for sand-type forging such as AZ 9 1 C and ZE 4 1 have a predetermined strength by heat treatment such as T 6 and Τ 5 after alloy fabrication. These alloys are precipitation hardened alloys. Therefore, heat treatments such as Τ6 and Τ5 are required to adjust the strength to a predetermined level and to stabilize the characteristics over a long period of time. In addition, when exposed to room temperature or higher (generally 50 ° C or higher) for a long time, aging precipitation of dissolved elements occurs, and the alloy structure gradually changes, so the characteristics may change.

 (3) In processing Mg alloys such as A Z 6 1 A and AZ 3 1 B, the crystal grains are refined and reinforced by recrystallization accompanying strong processing such as rolling and extrusion. The main strengthening mechanism of these alloys is grain refinement. At a high temperature of 100 ° C or higher, however, strong grain boundary slip, which is peculiar to Mg, occurs, and grain refinement becomes a cause of strength reduction. In addition, since grain growth occurs at high temperatures, once these alloys are exposed to high temperatures, they may not return to their original strength even when the temperature drops. Disclosure of the invention

 Mg-Zn-Y alloy disclosed in Japanese Patent Laid-Open No. 2 0 0 2-3 0 9 3 3 2 The forged material is a common eutectic alloy, and the strength is a commercially available alloy with a similar composition such as ZE 4 1 It was equivalent. In addition, aging alloys such as A Z 9 1 C and Z E 4 1 are always aging above room temperature because of the low thermal stability of precipitates. In addition, Mg alloys for processing such as A Z 6 1 A and A Z 3 1 B do not have a mechanism to pin grain boundaries or suppress grain growth at high temperatures <=

 The high-strength magnesium alloy of the present invention has been made in view of such circumstances. In other words, the objective is to improve the strength of Mg-Zn-RE alloys, particularly the high temperature strength.

By replacing a part of RE of the Mg_Zn-RE alloy with a specific element, the present inventors can obtain nanoparticles having a complicated structure derived from a quasicrystal in a crystalline magnesium matrix. A high-strength magnesium alloy with a dispersed structure is obtained. The present invention has been reached.

 That is, the present invention is an invention of a high-strength magnesium alloy. In atomic%, zinc is 2.0 to 10%, zirconium is 0.05 to 0.2%, and rare earth elements are 0.2 to 1.5. %, With the balance being magnesium and inevitable impurities.

 As the rare earth element (R E), yttrium (Y) is preferably exemplified.

 The magnesium alloy of the present invention has the following general formula:

M g! 0 0-( _a + _b + c) Z n _a Z r _b RE _c

It is represented by Here, R E is a rare earth element, and a, b, and c are atomic% of zinc (Z n), zirconium (Z r), and rare earth element (R E), respectively, and α a

—≤b + c≤―

 one two Three

1. 5 <a <1 0

 It is preferable to satisfy the relationship of 0.05 <b <0.2 <25c.

 The magnesium alloy of the present invention having the above composition has the following characteristics. (1) α-Mg crystal grains occupy 50% or more of the volume, and α-Mg crystal grain boundaries have nanoparticles with a complex structure such as quasicrystals or approximate crystals. Here, a quasicrystal is a new ordered structure that does not have translational properties and is not allowed in crystallography, and has 5 or 10 times symmetry and a quasiperiodic arrangement. A 1 — P d 1 Mn, A 1 Cu-Fe, Cd-Yb, Mg-Zn-Y, etc. are known as alloys that produce quasicrystals. Due to its unique structure, it has unique properties such as high hardness, high melting point, and low μ compared to ordinary crystals.

(2) Fine precipitates (1 μm or less) are uniformly dispersed in α-Μg crystal grains. This fine precipitate improves the strength of the magnesium alloy of the present invention. (3) The main fine precipitates are approximate crystals and Mg-Y intermetallic compounds. Here, the approximate crystal is a quasicrystal (an intermetallic compound having a structure and composition similar to that of MggZneYj, and is a relative of the quasicrystal.

 (4) During solution, quasicrystals and approximate crystal phases at the a-Mg grain boundary pin the movement of the grain boundary. Therefore, since the growth of crystal grains is suppressed, even if the temperature is maintained at a high temperature of 300 ° C. or higher, the strength is not reduced due to the coarsening of the crystal grains.

 (5) Due to aging after solution treatment, approximate crystals with a grain size of 100 nm or less precipitate at a high number density. As a result, precipitates with a particle size of several tens to several hundreds of nm are dispersed in α-Mg grains at a high concentration together with the crystallized product during fabrication. These precipitates interact strongly with dislocations and do not decompose until around 2300 ° C. In addition, quasicrystals and approximate crystals existing at the crystal grain boundaries suppress the grain boundary slip at high temperatures. Due to these synergistic effects, the high-temperature strength is very excellent.

 By forging the magnesium alloy of the present invention having the above composition, the c_Mg phase occupies 50% or more of the volume, and has quasicrystals and similar crystal grains at the a-Mg grain boundary. Since these grains pin the movement of grain boundaries, the grain growth is suppressed. For this reason, strength reduction due to crystal coarsening does not occur even at high temperatures. In addition, fine crystals are precipitated in the grains. The main fine precipitates are approximate crystals and Mg_Y intermetallic compounds. Brief Description of Drawings

 FIG. 1A shows an S-organ structure photograph of Example 1, and FIG. 1B shows an SEM structure photograph of a comparative example. FIG. 2 shows an enlarged photograph of the grains of Example 1 of Mg 6 Ζη — 0.1 Zr — O. 9 Y (atomic%) forged material. Fig. 3 is an enlarged photograph of the grain boundary (strictly speaking, the eutectic-like part) of Example 1 M g— 6 η η— 0. l Z r — 0.9 9 Y (atomic%) Indicates. BEST MODE FOR CARRYING OUT THE INVENTION

In order to produce the magnesium alloy of the present invention, all the predetermined additive elements are added to molten Mg and mixed uniformly, and then formed into a mold. Where The method is not limited, and methods such as gravity fabrication, die casting, and rheocasting are adopted.

 In order to improve the strength, the magnesium alloy of the present invention is preferably not only forged, but also heat-treated after forging and accompanied by hot working and heat treatment steps after forging.

 The rare earth elements constituting the magnesium alloy of the present invention include scandium (Sc), yttrium (Y), lanthanum (La), cerium (Ce), praseodymium (Pr ), Neodymium (N d), promethium (P m), samarium (S m), plutonium (E u), gadolinium (G d), terbium (T b), dysprosium (D y), Examples include holmium (H o), erbium (E r), thulium (T m), ytterbium (Y b), and lutetium (L u). Of these, yttrium (Y) is preferred.

 Examples of the present invention and comparative examples are shown below.

 [Example 1]

 M g— 6 Z n — 0.1 Z r-0.9 Y (atomic%) A forged alloy was produced by the following process.

 (1) Raw material

Pure Mg (9 9. 9%): 1 6 4 9 g

Pure Zn (9 9. 9 9%): 2 8 6 g

Pure Zr (9 9. 9%): 6.7 g

Pure Y (9 9. 9%): 5 8 g

 (2) Dissolution

Dissolving pure M g at iron crucible, holding the molten metal 7 0 0 ^D C. Add other constituent materials to the molten metal and stir until all of the molten metal is melted and uniform while maintaining the molten metal temperature at about 700 ° C. The order in which the constituent raw materials are added to the molten Mg does not affect the characteristics and is not specified.

 (3) Forging

The molten alloy kept at about 700 ° C was made into a JIS No. 4 boat type preheated to about 100 ° C. [Comparative example]

 Except for the following raw materials, the same conventional material Mg-3Zn-0. 5Y was fabricated as in Example 1.

Pure Mg (99. 9%): 1 8 1 4 g

Pure Zn (9 9. 9 9%): 1 5 1. 4 g

Pure Y (9 9. 9%): 3 4.6 g

 [Comparison of Example 1 and Comparative Example]

Fig. 1A shows the SEM structure photograph of Example 1, and Fig. 1B shows the SEM structure photograph of the comparative example. Example 1 has a woven KyoAkiragumi in overview similar to Comparative Example, alpha-M g grain boundaries approximate crystal (Example 1) or M g ₃ Z n ₆ quasicrystal (Comparative Example). However, the shape of the eutectic structure differs between Example 1 and the comparative example, and in Example 1, the eutectic structure 'is finely and uniformly dispersed throughout.

In Figure 2, the M g Example 1 - 6 Z n - 0. l Z r - 0. 9 Y ( atomic _^0/0) shows an enlarged photograph of the grains of铸造material. α—Mg phase and Mg Y-based intermetallic compounds that appear to be Mg ₂₄ Y ₅ or Mg 2Y and other phases that cannot be identified.

In Figure 3, the M g- 6 Z n- 0. l Z r Example 1 - 0. 9 Y (atomic _^0/0) 鎳造material grain boundaries (strictly speaking eutectic-like portion) An enlarged photograph is shown. Phases that cannot be identified are W phase (orthogonal Zn ₃ Mg ₃ Y ₂ ) and Zn ₆ Y ₄ binary compounds, hexagonal compounds, and others.

 [Intensity comparison between Example and Comparative Example]

From JIS No. 4 boat type ingot of Example 1 (Mg_6Zn—0.1Zr-0.9Y) and comparative example (Mg—3ηη—0.5Υ) described above, Round bar tensile test specimens with a parallel part φ 5 X 25 mm were collected and subjected to a tensile test at room temperature and 150 ° C. Similarly, tensile tests were performed on Examples 2 to 4 with different composition ratios and AZ 9 1 C—T 6 and ZE 4 1 A—T 5 which are conventional materials. The test conditions were AG-2550 kND made by Shimadzu Corporation as the tensile tester, and the tensile speed was 0.8 mm / mi η. The results are shown in Table 1 below. Table 1

第 1表の結果よ り、実施例 1 〜 4の錶造材は、比較例等の従来の錶造材 と比較して 1 5 0 °Cにおける引張り強さが優れている。 又、室温→ 1 5 0 °Cの温度上昇に伴う強度低下が非常に小さい。 原因と して、 -M g結 晶粒内の微小析出物の増加等が考えられる。 近似結晶及び M g Y系金属 間化合物をはじめとする微小析出物は熱安定性が高いため、これらが 1 5 0 °Cにおいても転位の障壁と して有効に機能していると考えられる。 産業上の利用可能性

From the results in Table 1, the forged materials of Examples 1 to 4 are superior in tensile strength at 150 ° C. as compared with conventional forged materials such as comparative examples. In addition, the decrease in strength accompanying the temperature increase from room temperature to 1550 ° C is very small. The cause may be an increase in fine precipitates in the -Mg crystal grains. Fine precipitates such as approximate crystals and Mg Y-based intermetallic compounds have high thermal stability, so they are considered to function effectively as dislocation barriers even at 150 ° C. Industrial applicability

The magnesium alloy of the present invention has nanoparticles derived from quasicrystals at the Mg crystal grain boundaries, and fine crystals are precipitated in the grains, so that strength reduction due to crystal coarsening does not occur even at high temperatures. As a result, high strength can be maintained even at high temperatures. Normally, increasing the rare earth element content increases the cost but increases the high-temperature strength. For example, WE 54 has high strength, though it is extremely expensive, due to the rare earth content of nearly 10% and T 6 heat treatment. The present invention can produce high-temperature strength as high as that of the conventional heat-treated material without any heat treatment.

Claims

The scope of the claims beta 3 atoms _^0/0 Zinc 2. 0~: L 0%, 0. 0 5~ 0. 2% to Jirukoniumu includes a rare-earth element 0.2 to 1.5 0% magnesium the balance and inevitable not Magnesium alloy characterized by consisting of pure material.  2.  2. The magnesium alloy according to claim 1, wherein the rare earth element is yttrium.  3.  General formula M g _{1 0 0} _ _{(a + b + c)} Z n _a Z r _b RE _c ., RE is a rare earth element, and a, b and c are atoms of Z n, Z r and RE, respectively. % ≤b + c 5 <a <1 0 3. The magnesium alloy according to claim 1 or ^{2, wherein} the relationship 0 5 <b <0. is satisfied.