阅读说明：本技术 基于剧烈塑性变形的可提高铝合金耐腐蚀性和力学性能的方法及高性能耐腐蚀铝合金 (Method for improving corrosion resistance and mechanical property of aluminum alloy based on severe plastic deformation and high-performance corrosion-resistant aluminum alloy ) 是由 陈莹 张厚安 石祥文 杨益航 卢博沁 陈佳帆 于 2021-09-08 设计创作，主要内容包括：本发明提供一种基于剧烈塑性变形的可提高铝合金耐腐蚀性和力学性能的方法及高性能耐腐蚀铝合金,该方法通过将预变形处理铝合金放入高压扭转系统中进行周期性扭转变形,可得到高性能耐腐蚀铝合金。其中,周期性扭转变形的一个扭转变形周期为先顺时针扭转180°,然后在逆时针扭转180°,且周期性扭转变形的扭转周次N≥1。本发明通过高压扭转系统的周期性高压扭转变形可改变铝合金的析出相分布、晶粒尺寸、位错密度等,使得铝合金的晶粒尺寸细化,进而提高铝合金的机械性能。另一方面,通过周期性高压扭转变形还可改变铝合金的耐腐蚀性能,进而获得高性能耐腐蚀铝合金。(The invention provides a method for improving corrosion resistance and mechanical property of aluminum alloy based on severe plastic deformation and high-performance corrosion-resistant aluminum alloy. Wherein, one torsional deformation period of the periodic torsional deformation is that the periodic torsional deformation is firstly twisted by 180 degrees clockwise and then twisted by 180 degrees anticlockwise, and the twisting frequency N of the periodic torsional deformation is more than or equal to 1. The invention can change the precipitated phase distribution, the grain size, the dislocation density and the like of the aluminum alloy through the periodic high-pressure torsional deformation of the high-pressure torsion system, so that the grain size of the aluminum alloy is refined, and the mechanical property of the aluminum alloy is further improved. On the other hand, the corrosion resistance of the aluminum alloy can be changed through periodic high-pressure torsional deformation, and then the high-performance corrosion-resistant aluminum alloy is obtained.)

1. A method for improving corrosion resistance and mechanical property of aluminum alloy based on severe plastic deformation is characterized by comprising the following steps:

s1, obtaining a pre-deformation treatment aluminum alloy;

s2, putting the pre-deformed aluminum alloy into a high-pressure torsion system, and performing periodic high-pressure torsion deformation to obtain a high-performance corrosion-resistant aluminum alloy; wherein, one torsional deformation period of the periodic high-pressure torsional deformation is that the periodic high-pressure torsional deformation is firstly twisted by 180 degrees clockwise and then twisted by 180 degrees anticlockwise, and the twisting cycle N of the periodic high-pressure torsional deformation is more than or equal to 1.

2. The method as claimed in claim 1, wherein the pre-deformation treated aluminum alloy is a T351-2024 aluminum alloy having a grain size of 3.0 to 4.5 μm.

3. The method according to claim 1, wherein the pre-deformation treated aluminum alloy has a pre-elongation deformation amount of 2 to 5%.

4. The method of claim 2, wherein the T351-2024 aluminum alloy comprises, by mass, 4.6-4.7% Cu, 0.14-0.16% Fe, 1.45-1.55% Mg, 0.6-0.7% Mn, 0.07-0.09% Si, 0.04-0.06% Zn, and less than 0.05% of other elements, and the balance Al.

5. The method of claim 1, wherein the high pressure torsion system comprises an upper anvil and a lower anvil, each of the upper anvil and the lower anvil having a groove formed therein, wherein the high pressure torsion system is configured such that when the upper anvil and the lower anvil are closed, the two grooves form a cavity for receiving the pre-deformed aluminum alloy and the cavity is substantially coupled to the pre-deformed aluminum alloy.

6. The method of claim 5, wherein the diameter of the groove is 9 to 11mm and the height of the groove is 0.5 to 0.7 mm.

7. The method of claim 6, wherein the pre-deformed aluminum alloy is an aluminum alloy wafer having a diameter of 8.8 to 10.8mm and a height of 0.82 to 0.85 mm.

8. The method according to claim 1, wherein the high pressure torsion system has a torsion deformation pressure of 1-6 GPa and a torsion deformation speed of 1-3 r/min.

9. A high performance corrosion resistant aluminum alloy produced by the method of any one of claims 1 to 8.

10. The high performance corrosion-resistant aluminum alloy of claim 9, wherein the high performance corrosion-resistant aluminum alloy has a grain size on the order of 100nm and a corrosion potential of-0.693 to-0.609V.

Technical Field

The invention relates to the technical field of nanocrystalline metal materials, and particularly relates to a method for improving corrosion resistance and mechanical property of an aluminum alloy based on severe plastic deformation and a high-performance corrosion-resistant aluminum alloy.

Background

The nanocrystalline material is a material with a grain size of 1-100 nm. Since the crystals are extremely fine, the grain boundaries may account for 50% or more of the entire material. Their atomic arrangement differs from ordered crystalline states and from disordered amorphous states. Due to the structural particularity, the nanocrystalline material has many excellent performances and wide application prospects.

2024 aluminum alloy is currently a commonly used lightweight metal structural material. Heating the molten 2024 aluminum alloy (comprising 4.5% of Cu, 1.5% of Mg, 0.5% of Mn and the balance of Al in percentage by mass) subjected to homogenizing annealing and pre-plastic deformation extrusion to 493 ℃, preserving heat for 5 hours, carrying out solution treatment, and then carrying out water quenching to room temperature. The average grain size of the solution treated 2024 aluminum alloy was measured from the optical microstructure to be about 390 μm. The transmission electron microscope analysis of the solution treated 2024 aluminum alloy shows that almost no residual second phase particles are observed, and almost all alloy elements are dissolved in an Al matrix to form a single-phase solid solution coarse-grained 2024Al alloy. Since the concentrations of the above alloying elements Cu, Mg and Mn are all higher than their solubilities at room temperature, the alloying elements are supersaturated and dissolved in the Al matrix.

The high pressure twisting method is an important method for preparing nano-structured materials. The sample is placed between an upper die and a lower die, and positive pressure and torsional force are applied to the sample under high pressure, so that the strain of the sample is gradually increased, and the sample finally becomes a material with fine grains and uniform tissues. At room temperature, the single-phase solid solution coarse-grained 2024 aluminum alloy with the supersaturation dissolved alloy elements is placed into a high-pressure torsion system with a groove on a lower anvil and a boss on an upper anvil (the boss of the upper anvil is inserted into the groove of the lower anvil with the residual space after the material to be deformed is placed), the high-pressure torsion is carried out for 20 circles under the pressure of 6GPa, and transmission electron microscope observation indicates that the single-phase solid solution nanocrystalline 2024 aluminum alloy consisting of nanocrystalline grains is formed and the average grain size is 67 nm. Finally, the single-phase solid solution nanocrystalline 2024Al alloy consisting of the nanocrystalline grains is placed into a pyrophyllite (pressure transmission medium) cavity in a high-pressure space formed by six top hammers of a cubic press for aging. The existing high-pressure torsion method is mainly to obtain the nanocrystalline metal material by carrying out torsion deformation along the same direction under high pressure, but the influence of a high-pressure torsion deformation mode on the material performance is not involved. With the rapid development of modern aerospace, automobile and other industries, the demand for high-specific-strength aluminum metal is increasing day by day. Therefore, the research on how to improve the mechanical properties, corrosion resistance and other properties of the aluminum alloy material has extremely important significance.

Disclosure of Invention

The invention aims to provide a method for improving the corrosion resistance and mechanical property of an aluminum alloy based on severe plastic deformation.

Another object of the present invention is to provide a high performance corrosion resistant aluminum alloy, which is a nanocrystalline material with high mechanical properties and corrosion resistance.

The technical problem to be solved by the invention is realized by adopting the following technical scheme.

The invention provides a method for improving corrosion resistance and mechanical property of aluminum alloy based on severe plastic deformation, which comprises the following steps:

s1, obtaining a pre-deformation treatment aluminum alloy;

s2, putting the pre-deformed aluminum alloy into a high-pressure torsion system, and performing periodic high-pressure torsion deformation to obtain a high-performance corrosion-resistant aluminum alloy; wherein, one torsional deformation period of the periodic high-pressure torsional deformation is that the periodic high-pressure torsional deformation is firstly twisted by 180 degrees clockwise and then twisted by 180 degrees anticlockwise, and the twisting cycle N of the periodic high-pressure torsional deformation is more than or equal to 1.

The invention provides a high-performance corrosion-resistant aluminum alloy which is prepared according to the method.

The method for improving the corrosion resistance and the mechanical property of the aluminum alloy based on the severe plastic deformation and the high-performance corrosion-resistant aluminum alloy have the beneficial effects that:

according to the invention, the pre-deformation aluminum alloy is placed into a high-pressure torsion system for periodic torsion deformation, so that the high-performance corrosion-resistant aluminum alloy can be obtained. Wherein, one torsional deformation period of the periodic torsional deformation is that the periodic torsional deformation is firstly twisted by 180 degrees clockwise and then twisted by 180 degrees anticlockwise, and the twisting frequency N of the periodic torsional deformation is more than or equal to 1. The invention can change the precipitated phase distribution, the grain size, the dislocation density and the like of the aluminum alloy through the periodic high-pressure torsional deformation of the high-pressure torsion system, so that the grain size of the aluminum alloy is refined, and the mechanical property of the aluminum alloy is further improved. On the other hand, the corrosion resistance of the aluminum alloy can be changed through periodic high-pressure torsional deformation, and then the high-performance corrosion-resistant aluminum alloy is obtained.

Drawings

In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings needed to be used in the embodiments will be briefly described below, it should be understood that the following drawings only illustrate some embodiments of the present invention and therefore should not be considered as limiting the scope, and for those skilled in the art, other related drawings can be obtained according to the drawings without inventive efforts.

FIG. 1 is a schematic view of a periodic high pressure torsional deformation of the present invention;

FIG. 2 is a TEM image of the high performance corrosion-resistant aluminum alloy of example 1 of the present invention and the T351-2024 aluminum alloy of comparative example 1;

FIG. 3 is a graph of the grain size distribution of the high performance corrosion-resistant aluminum alloy of example 1;

FIG. 4 is a Tafel plot of electrochemical testing of the high performance corrosion resistant aluminum alloys of examples 1 and 5 and the T351-2024 aluminum alloy of comparative example 1.

Detailed Description

In order to make the objects, technical solutions and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention will be clearly and completely described below. The examples, in which specific conditions are not specified, were conducted under conventional conditions or conditions recommended by the manufacturer. The reagents or instruments used are not indicated by the manufacturer, and are all conventional products available commercially.

The method for improving the corrosion resistance and mechanical properties of the aluminum alloy based on severe plastic deformation and the high-performance corrosion-resistant aluminum alloy of the embodiment of the invention are specifically described below.

The method for improving the corrosion resistance and the mechanical property of the aluminum alloy based on the severe plastic deformation comprises the following steps:

and S1, obtaining the pre-deformation aluminum alloy.

Further, in the preferred embodiment of the present invention, the pre-deformation treated aluminum alloy is T351-2024 aluminum alloy, and the grain size is 3.0-4.5 μm. The T351-2024 aluminum alloy used in the present invention is commercially available. For example, T351-2024 aluminum alloy is commercially available from Dongguan City Stone, Inc. of auspicious metallic materials, Inc.

Further, in a preferred embodiment of the present invention, the pre-deformation amount of the pre-deformation treated aluminum alloy is 2 to 5%.

Further, in a preferred embodiment of the invention, the T351-2024 aluminum alloy comprises, by mass, 4.6-4.7% of Cu, 0.14-0.16% of Fe, 1.45-1.55% of Mg, 0.6-0.7% of Mn, 0.07-0.09% of Si, 0.04-0.06% of Zn, less than 0.05% of other elements, and the balance of Al.

S2, putting the pre-deformed aluminum alloy into a high-pressure torsion system, and performing periodic high-pressure torsion deformation to obtain a high-performance corrosion-resistant aluminum alloy; wherein, one torsional deformation period of the periodic high-pressure torsional deformation is that the periodic high-pressure torsional deformation is firstly twisted by 180 degrees clockwise and then twisted by 180 degrees anticlockwise, and the twisting cycle N of the periodic high-pressure torsional deformation is more than or equal to 1 (shown in figure 1).

Further, in a preferred embodiment of the present invention, the high-pressure torsion system comprises an upper anvil and a lower anvil, each of the upper anvil and the lower anvil has a groove formed thereon, and the high-pressure torsion system is configured such that after the upper anvil and the lower anvil are covered, two of the grooves form a cavity for placing the pre-deformation-treated aluminum alloy, and the cavity is substantially coupled to the pre-deformation-treated aluminum alloy. In the invention, the grooves of the upper and lower anvils are basically coupled with the pre-deformation treatment aluminum alloy after the pre-deformation treatment aluminum alloy is placed, and a small amount of allowance space exists.

Further, in the preferred embodiment of the present invention, the diameter of the groove is 9-11 mm, and the height of the groove is 0.5-0.7 mm. Preferably, the diameter of the groove is 10 mm.

Further, in a preferred embodiment of the present invention, the pre-deformed aluminum alloy is an aluminum alloy wafer having a diameter of 8.8 to 10.8mm and a height of 0.82 to 0.85 mm. Preferably, the diameter of the pre-deformation treatment aluminum alloy is 9.8mm, and the height of the pre-deformation treatment aluminum alloy is 0.82-0.85 mm. The method comprises the steps of putting a pre-deformation aluminum alloy with the diameter of 9.8mm and the height of 0.82-0.85 mm into a groove with the diameter of 10mm, and carrying out periodic high-pressure torsional deformation on the pre-deformation aluminum alloy to reduce the thickness of an aluminum alloy sheet to 0.4-0.5 mm. After severe plastic deformation, part of the aluminum alloy overflows from the edge of the groove.

Further, in the preferred embodiment of the present invention, the torsional deformation pressure of the high pressure torsion system is 1 to 6GPa, and the torsional deformation speed is 1 to 3 r/min. Preferably, the high pressure torsion system has a torsion deformation pressure of 6GPa and a torsion deformation speed of 1 r/min.

The invention firstly proposes that aluminum metal is subjected to torsional deformation in a periodic high-pressure torsional deformation mode. Wherein, the aluminum alloy is firstly twisted by 180 degrees clockwise and then twisted by 180 degrees anticlockwise to define a torsional deformation period. The precipitated phase distribution, the grain size, the dislocation density and the like of the aluminum alloy can be changed through the transformation of the high-pressure torsional deformation mode, so that the grain size of the aluminum alloy is refined, and the mechanical property of the aluminum alloy is improved. On the other hand, the corrosion resistance of the aluminum alloy can be changed through periodic high-pressure torsional deformation, and the high-performance corrosion-resistant aluminum alloy is finally obtained.

The invention also provides a high-performance corrosion-resistant aluminum alloy which is prepared according to the method.

Further, in the preferred embodiment of the invention, the grain size of the high-performance corrosion-resistant aluminum alloy can reach 100nm grade, and the corrosion potential is-0.693V to-0.609V. The grain size can be refined by periodically carrying out high-pressure torsional deformation on the aluminum alloy, so that the nanocrystalline metal material is obtained. On the other hand, the corrosion potential of the aluminum alloy can be changed by adopting periodic high-pressure torsional deformation, so that the corrosion resistance of the aluminum alloy is improved.

The features and properties of the present invention are described in further detail below with reference to examples.

Example 1

The embodiment provides a high-performance corrosion-resistant aluminum alloy which is obtained according to the following method:

(1) obtaining a T351-2024 aluminum alloy: the T351-2024 aluminum alloy comprises, by mass, 4.63% of Cu, 0.15% of Fe, 1.51% of Mg, 0.66% of Mn, 0.08% of Si, 0.05% of Zn, less than 0.05% of other elements, and the balance of Al. The T351-2024 aluminum alloy is a circular disk with a diameter of 9.8mm, a height of 0.82mm and a pre-stretching deformation of 2%.

(2) The T351-2024 aluminum alloy is placed in a high-pressure torsion system with grooves in both an upper pressing anvil and a lower pressing anvil, the diameter of each groove is 10mm, the T351-2024 aluminum alloy is basically coupled after the T351-2024 aluminum alloy is placed between the upper pressing anvil and the lower pressing anvil, and a small amount of margin space exists. And (3) carrying out high-pressure torsional deformation under the conditions that the high-pressure torsional deformation pressure is 6Gpa and the torsional deformation speed is 1r/min, wherein the torsional cycle is 1 time, so as to obtain the high-performance corrosion-resistant aluminum alloy. Wherein, one torsional deformation period is that the torsion is firstly 180 degrees clockwise and then 180 degrees anticlockwise.

Example 2

The embodiment provides a high-performance corrosion-resistant aluminum alloy which is obtained according to the following method:

(1) obtaining a T351-2024 aluminum alloy: the T351-2024 aluminum alloy comprises, by mass, 4.63% of Cu, 0.15% of Fe, 1.51% of Mg, 0.66% of Mn, 0.08% of Si, 0.05% of Zn, less than 0.05% of other elements, and the balance of Al. The T351-2024 aluminum alloy is a circular disk with a diameter of 9.8mm, a height of 0.82mm and a pre-stretching deformation of 2%.

(2) The T351-2024 aluminum alloy is placed in a high-pressure torsion system with grooves in both an upper pressing anvil and a lower pressing anvil, the diameter of each groove is 10mm, the T351-2024 aluminum alloy is basically coupled after the T351-2024 aluminum alloy is placed between the upper pressing anvil and the lower pressing anvil, and a small amount of margin space exists. And (3) carrying out high-pressure torsional deformation under the conditions that the high-pressure torsional deformation pressure is 6Gpa and the torsional deformation speed is 1r/min, wherein the torsional cycle is 2 times, so as to obtain the high-performance corrosion-resistant aluminum alloy. Wherein, one torsional deformation period is that the torsion is firstly 180 degrees clockwise and then 180 degrees anticlockwise.

Example 3

(1) Obtaining a T351-2024 aluminum alloy: the T351-2024 aluminum alloy comprises, by mass, 4.63% of Cu, 0.15% of Fe, 1.51% of Mg, 0.66% of Mn, 0.08% of Si, 0.05% of Zn, less than 0.05% of other elements, and the balance of Al. The T351-2024 aluminum alloy is a circular disk with a diameter of 9.8mm, a height of 0.82mm and a pre-stretching deformation of 2%.

(2) The T351-2024 aluminum alloy is placed in a high-pressure torsion system with grooves in both an upper pressing anvil and a lower pressing anvil, the diameter of each groove is 10mm, the T351-2024 aluminum alloy is basically coupled after the T351-2024 aluminum alloy is placed between the upper pressing anvil and the lower pressing anvil, and a small amount of margin space exists. And (3) carrying out high-pressure torsional deformation under the conditions that the high-pressure torsional deformation pressure is 6Gpa and the torsional deformation speed is 1r/min, and twisting for 3 times to obtain the high-performance corrosion-resistant aluminum alloy. Wherein, one torsional deformation period is that the torsion is firstly 180 degrees clockwise and then 180 degrees anticlockwise.

Example 4

(1) Obtaining a T351-2024 aluminum alloy: the T351-2024 aluminum alloy comprises, by mass, 4.63% of Cu, 0.15% of Fe, 1.51% of Mg, 0.66% of Mn, 0.08% of Si, 0.05% of Zn, less than 0.05% of other elements, and the balance of Al. The T351-2024 aluminum alloy is a circular disk with a diameter of 9.8mm, a height of 0.82mm and a pre-stretching deformation of 2%.

(2) The T351-2024 aluminum alloy is placed in a high-pressure torsion system with grooves in both an upper pressing anvil and a lower pressing anvil, the diameter of each groove is 10mm, the T351-2024 aluminum alloy is basically coupled after the T351-2024 aluminum alloy is placed between the upper pressing anvil and the lower pressing anvil, and a small amount of margin space exists. And (3) carrying out high-pressure torsional deformation under the conditions that the high-pressure torsional deformation pressure is 6Gpa and the torsional deformation speed is 1r/min, wherein the torsional cycle is 4 times, so as to obtain the high-performance corrosion-resistant aluminum alloy. Wherein, one torsional deformation period is that the torsion is firstly 180 degrees clockwise and then 180 degrees anticlockwise.

Example 5

(1) Obtaining a T351-2024 aluminum alloy: the T351-2024 aluminum alloy comprises, by mass, 4.63% of Cu, 0.15% of Fe, 1.51% of Mg, 0.66% of Mn, 0.08% of Si, 0.05% of Zn, less than 0.05% of other elements, and the balance of Al. The T351-2024 aluminum alloy is a circular disk with a diameter of 9.8mm, a height of 0.82mm and a pre-stretching deformation of 2%.

(2) The T351-2024 aluminum alloy is placed in a high-pressure torsion system with grooves in both an upper pressing anvil and a lower pressing anvil, the diameter of each groove is 10mm, the T351-2024 aluminum alloy is basically coupled after the T351-2024 aluminum alloy is placed between the upper pressing anvil and the lower pressing anvil, and a small amount of margin space exists. And (3) carrying out high-pressure torsional deformation under the conditions that the high-pressure torsional deformation pressure is 6Gpa and the torsional deformation speed is 1r/min, wherein the torsional cycle is 5 times, so as to obtain the high-performance corrosion-resistant aluminum alloy. Wherein, one torsional deformation period is that the torsion is firstly 180 degrees clockwise and then 180 degrees anticlockwise.

Comparative example 1

This comparative example provides a T351-2024 aluminum alloy, including, by mass, 4.63% Cu, 0.15% Fe, 1.51% Mg, 0.66% Mn, 0.08% Si, 0.05% Zn, and less than 0.05% of other elements, with the remainder being Al. The T351-2024 aluminum alloy is a circular disk with a diameter of 9.8mm, a height of 0.82mm and a pre-stretching deformation of 2%.

Test example 1

The grain sizes of the high performance corrosion-resistant aluminum alloy of example 1 and the T351-2024 aluminum alloy of comparative example 1 were analyzed using transmission electron microscopy. In FIG. 2, (a) is a TEM image of the T351-2024 aluminum alloy of comparative example 1, and (b) is a TEM image of the high performance corrosion-resistant aluminum alloy of example 1. As can be seen from FIGS. 2 and 3, after a period of high-pressure torsional deformation is carried out, the average grain size of the high-performance corrosion-resistant aluminum alloy can reach 115nm, 75% of the grain size falls between 50nm and 150nm, and the period torsional deformation can enable the grain size to reach the 100nm grade.

Test example 2

Vickers hardness tests were performed on the high-performance corrosion-resistant aluminum alloys of examples 1 and 5, respectively, and on the T351-2024 aluminum alloy of comparative example 1. The hardness of the T351-2024 aluminum alloy is 130 HV. The hardness of the high-performance corrosion-resistant aluminum alloy obtained by 1-week torsional deformation at the center is about 190HV, and the hardness of the edge area is about 235 HV. The hardness of the high-performance corrosion-resistant aluminum alloy obtained by 5-cycle torsional deformation at the center is about 170HV, and the hardness of the edge area is about 220 HV. It follows that the periodic torsional deformation increases the hardness of the aluminum alloy, and the hardness of the edge region is higher than that of the central region. In addition, the hardness of the high-performance corrosion-resistant aluminum alloy obtained by 1-week torsional deformation is slightly higher than that of the high-performance corrosion-resistant aluminum alloy obtained by 5-week torsional deformation.

Test example 3

Electrochemical corrosion tests were performed on the high performance corrosion resistant aluminum alloys of examples 1 and 5, respectively, and the T351-2024 aluminum alloy of comparative example 1. FIG. 4 is a Tafel plot for electrochemical testing of the high performance corrosion-resistant aluminum alloy and the T351-2024 aluminum alloy, where 1 is the Tafel plot for electrochemical testing of the high performance corrosion-resistant aluminum alloy of example 1, 2 is the Tafel plot for electrochemical testing of the high performance corrosion-resistant aluminum alloy of example 5, and 3 is the Tafel plot for electrochemical testing of the T351-2024 aluminum alloy of comparative example 1. As can be seen from FIG. 4, the corrosion resistance potential of the aluminum alloy before and after the periodic high-pressure torsional deformation is E_corr,N1>E_corr,N5>E_corr,T351Corrosion current I_corr,T351<I_corr,N1<I_corr,N5. Therefore, the corrosion resistance of the high-performance corrosion-resistant aluminum alloy with 1-week torsional deformation is better than that of the aluminum alloy with 5-week torsional deformation and better than that of the undeformed T351-2024 aluminum alloy. Therefore, the corrosion potential of the aluminum alloy can be changed by adopting periodic high-pressure torsional deformation, and the corrosion resistance of the aluminum alloy is further improved.

The embodiments described above are some, but not all embodiments of the invention. The detailed description of the embodiments of the present invention is not intended to limit the scope of the invention as claimed, but is merely representative of selected embodiments of the invention. All other embodiments, which can be derived by a person skilled in the art from the embodiments given herein without making any creative effort, shall fall within the protection scope of the present invention.