This application is a U.S. national stage of International Application No. PCT/JP2009/060188 filed Jun. 3, 2009.
The present invention relates to an Mg-based alloy of which the yield anisotropy has been reduced.
Magnesium is a lightweight and provides rich resources, and thus, magnesium is specifically noted as a material for weight reduction for electronic devices, structural members, etc.
On the other hand, in order to apply to the structural parts, i.e., rail ways and auto mobiles, the alloy needs to show the high strength, ductility and toughness, from the viewpoints of safety and reliability for the human been.
However, when magnesium alloys are produced by wrought process through rolling, extrusion, there is a problem that the alloy has a strong texture due to the process. Therefore, a conventional wrought magnesium alloy could have a high tensile strength at room temperature; however this alloy shows a low compression strength. Accordingly, when a conventional wrought magnesium alloy is applied to mobile structural parts, there is a large defect; the part, which is applied the compressive strain, occurs brittle fracture and the lacks of isotropic deformation.
Recently, it has been found that the formation of a specific phase, i.e., quasi-crystal phase, which possesses five-fold symmetry and is very different from crystalline phases, has discovered in an Mg—Zn-RE alloy (where RE=Y, Gd, Dy, Ho, Er, Tb).
The quasi-crystal phase has a good matching to a magnesium matrix interface, i.e., the interface between magnesium and quasi-crystal phase is coherency. Therefore, the dispersion of a quasi-crystal phase in a magnesium matrix causes to the reduction of the basal texture and can enhance the compression strength with high tensile strength. In addition, this alloy can reduce the yield anisotropy, which is an unfavorable characteristic to apply the structural parts.
However, in order to form a quasi-crystal phase in a magnesium alloy, there is a serious problem that the addition of a rare earth element is indispensable. The rare earth element is an element that is rare and valuable. Therefore, if the alloy with the addition of rare earth elements could exhibit good properties, its material cost is expensive; not advantage from the industrial point of views.
Concretely, Patent References 1 to 3 merely specify that, the addition of a rare earth element (especially yttrium) is necessary to form the quasi-crystal phase in magnesium.
Patent Reference 4 merely shows that, the addition of yttrium and other rare earth element is indispensable to form the quasi-crystal phase in magnesium. The problem that the wrought magnesium alloy shows the yield anisotropy, could be solved due to the dispersion of quasi-crystal phase and the grain refinement.
Patent Reference 5 merely specifies that the addition of yttrium and other rare earth element is indispensable to form the quasi-crystal phase in magnesium. This reference shows the working conditions (working temperature, speed, etc.) at the secondary forming using the magnesium alloys with dispersion of quasi-crystal phase.
Non-Patent References 1 and 2 describe the formation of a quasi-crystal phase of Mg—Zn—Al alloy. However, since the phase is a quasi-crystal single phase, an Mg matrix does not exist in this alloy.
In Non-Patent Reference 3, the size of the Mg matrix is at least 50 μm since the alloys are produced by a casting method. Therefore, this reference does not show that the alloy exhibit high strength/high toughness properties on the same level as or higher than that of the above-mentioned, rare earth element-added (Mg—Zn-RE) alloys. In addition, it would involve technical difficulties (see
Patent Reference 1: JP-A 2002-309332
Patent Reference 2: JP-A 2005-113234
Patent Reference 3: JP-A 2005-113235
Patent Reference 4: Japanese Patent Application No. 2006-211523
Patent Reference 5: Japanese Patent Application No. 2007-238620
Non-Patent Reference 1: G. Bergman, J. Waugh, L. Pauling: Acta Cryst. (1957) 10 254
Non-Patent Reference 2: T. Rajasekharan, D. Akhtar, R. Gopalan, K. Muraleedharan: Nature (1986) 322 528
Non-Patent Reference 3: L. Bourgeois, C. L. Mendis, B. C. Muddle, J. F. Nie: Philo. Mag. Lett. (2001) 81 709
The present invention has been made in consideration of the above-mentioned situation, and its object is to make it possible to reduce the yield anisotropy, which is a serious problem of the wrought magnesium alloys, by using additive elements which are easily obtained in place of a rare earth element while maintaining a high tensile strength.
For solving the above-mentioned problems, the present invention is characterized by the following:
The Mg-base alloy of the invention is an Mg-base alloy containing Zn and Al added to magnesium, comprising a composition represented by (100-a-b) wt % Mg-a wt % Al-b wt % Zn and satisfying 0.5 b/a.
In the Mg-base alloy, 5≦b≦55 and 2≦a≦18 are preferable.
In the Mg-base alloy, a quasi-crystal phase or its approximate crystal phase is preferably dispersed in the magnesium matrix.
In the Mg-base alloy, the size of the Mg matrix is preferably at most 40 μm.
According to the invention, uses of Zn and Al elements in place of a rare earth element expresses that the alloy with using of Zn and Al elements can reduce the yield anisotropy to the same level as or to a higher level than that in the alloy with a rare earth element.
The invention will be described in detail.
When the composition of the present invention represented by (100-a-b) wt % Mg-a wt % Al-b wt % Zn satisfies 0.5≦b/a, the results, which describe in below, show that the yield anisotropy could reduce. In the present invention, preferably, 1≦b/a, more preferably 1.5≦b/a.
When 5≦b≦55 and 2≦a≦18, a quasi-crystal phase and/or the close to the structure of the quasi-crystal phase is formed in magnesium.
More preferably, 2≦b/a≦10, and when 6≦b≦20 and 2≦a≦10, a quasi-crystal phase and/or the close to the structure of the quasi-crystal phase is formed in magnesium.
In order to reduce the yield anisotropy, i.e., showing the ratio of compression tensile yield stress of 0.8, the size of the magnesium matrix is preferably at most 40 μm, more preferably at most 20 μm, even more preferably at most 10 μm. The volume fraction of the quasi-crystal phase or the close to the structure of quasi-crystal phase is preferably from 1% to 40%, more preferably from 2% to 30%. The size of the quasi-crystal phase particles and the close to the structure of quasi-crystal phase particles is preferably at most 5 μm, more preferably at most 1 μm, and its limit is preferably at least 50 nm.
In order to obtain the above-mentioned microstructures and mechanical properties, the applied strain is at least 1, and the temperature is from 200° C. to 400° C. (at intervals of 50° C.—the same shall use hereafter).
In general, in order to reduce the fraction of dendrite structures, the alloys with the addition of rare earth elements have homogenized at a temperature of at most 460° C. for at least 4 hours before the extrusion or severe plastic deformation. However, in the present invention, uniform dispersion of the quasi-crystal phase could be attained without the heat treatment before the extrusion or severe plastic deformation.
The formation of the Quasi-crystal phase and the close to the structure of quasi-crystal phase is greatly influenced by the cooling speed during solidification. In the case of the present alloy, the quasi-crystal phase and the phase close to the structure of the quasi-crystal phase are possible to form even at the cooling rate. Therefore, the casted alloy is possible to be produced by not only the conventional casting process with a low cooling rate, but also die casting or rapid solidification with a high cooling rate.
The invention will be described in more detail with reference to the following Examples. However, the invention is not limited at all by the Examples.
Pure magnesium (purity, 99.95%), 8 wt. % zinc and 4 wt. % aluminium (hereinafter this is referred to as Mg—8 wt. % Zn—4 wt. % Al) were melted to produce a casted alloy. The casted alloy was machined to prepare an extrusion billet having a diameter of 40 mm. The extrusion billet was put into an extrusion container heated up to 300° C., kept therein for ½ hours, and then hot-extruded at an extrusion ratio of 25/1 to produce an extruded alloy having a diameter of 8 mm.
The microstructural observation and X-ray analysis were carried out in the extruded alloy. The observed position was the parallel to the extrusion direction. Also, the microstructural observation by a transmission electronic microscope (TEM) and X-ray analysis were carried out in the casted alloy.
The results of the microstructural observation in the casted and extruded alloys were shown in
A tensile test specimen has a diameter of 3 mm and a length of 15 mm and a compression test specimen has a diameter of 4 mm and a height of 8 mm. These specimens were machined from each material such as to make the tensile and compression axis parallel to the extrusion direction; and the initial tensile/compression strain rate was 1×10−3 see.
As a comparative example, the nominal stress-nominal strain curves of a typical wrought magnesium alloy, extruded Mg—3 wt. % Al—1 wt. % Zn (initial crystal particle size: about 15 μm) is also shown in
Pure magnesium (purity, 99.95%), 8 wt. % zinc and 4 wt. % aluminum were melted to prepare a casted alloy. The casted alloy was machined to prepare an extrusion billet having a diameter of 40 mm. The extrusion billet was put into an extrusion container heated up to 200° C., kept therein for ½ hours, and then hot-extruded at an extrusion ratio of 25/1 to produce an extruded alloy having a diameter of 8 mm. The microstructural observation and the tensile/compression tests at room temperature were performed Under the same condition as in Example 1 described above.
From
To add to the above-mentioned Examples 1 and 2 and Comparative Example 1, other samples were produced in the same procedures as above but changing the amount of Zn and Al elements. The mechanical properties were evaluated, and the results were listed in Table 1. The data in Table 1 obtained by the above-mentioned methods.
In
The alloys having a quasi-crystal phase or the close to the structure of quasi-phase show the reduction of yield anisotropy. On the other hand, it is known that the alloys having a quasi-crystal phase, i.e., Example 9 and 10, have a higher yield strength.
In Table 1, ZA means a composition of Zn and Al (b wt. %, a wt. %); and in Examples 1 to 14, (b wt %, a wt %)=(8, 4), (8, 4), (4, 2), (6, 1.5), (6, 2), (6, 3), (8, 2), (10, 2.5), (10, 5), (12, 2), (12, 4), (12, 6), (16, 4), (20, 2).
Number | Date | Country | Kind |
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2008-145520 | Jun 2008 | JP | national |
2009-069660 | Mar 2009 | JP | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/JP2009/060188 | 6/3/2009 | WO | 00 | 12/1/2010 |
Publishing Document | Publishing Date | Country | Kind |
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WO2009/148093 | 12/10/2009 | WO | A |
Number | Date | Country |
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0 548 875 | Jun 1993 | EP |
5-171330 | Jul 1993 | JP |
5-311310 | Nov 1993 | JP |
2007-113037 | May 2007 | JP |
Number | Date | Country | |
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20110076178 A1 | Mar 2011 | US |