Patent Application
20130039805
The present invention relates to a magnesium alloy a main component of which is magnesium.
Magnesium alloys having high strength have recently been developed, and the developed materials can substitute for aluminum alloys. The application of the magnesium alloys to materials constituting automobiles, aircrafts, and the like has drawn attention.
However, when the conventional magnesium and its alloys are used as such industrial materials, the magnesium alloys have a problem that is poor secondary processing. Many studies have been performed to solve this issue, but the problem has not yet been solved.
For example, for improving the ductility of magnesium alloy, the wrought process, such as extrusion, is one of the effective ways. In the case of extruded magnesium and its alloy, however, it is difficult to improve the compressive strength, and further the deformation anisotropy ratio, which is a ratio of the compressive yield stress to the tensile yield stress, is increased. Therefore, it is also difficult to use the wrought processed magnesium and its alloys as a light weight structural material.
On the other hand, patent document 1 shows that the control of the crystalline structure is one of the effective ways to develop the magnesium alloys, which improves the secondary processing with a high strength property. However, further improvement of the magnesium alloy in high strength and high ductility is desired from a practical point of view.
In view of the above, an object of the present invention is to provide a magnesium alloy that can achieve both high strength and high ductility which have conventionally been considered impossible.
As to solve the problem above-mentioned, the present invention provides a magnesium alloy which contains magnesium as a main component and other elements added has a microstructure in which grains surrounded by high angle grain boundaries are formed by subgrains and fine particles are dispersed into the subgrains.
In the magnesium alloy, it is preferred that an average grain size of the grains is 5 μm or less and a size of the subgrains is 1.5 μm or less.
In the magnesium alloy, it is preferred that a fraction of the grains having a particle diameter of 5 μm or less constitute 70% or more.
In the magnesium alloy, it is preferred that the fine particles have an average particle diameter of 10 nm to 1 μm.
In the magnesium alloy, it is preferred that the amount of the fine particles in the subgrains is 15% or less (with the proviso that the amount is not 0%).
The magnesium alloy can achieve both high strength and high ductility which have conventionally been considered impossible.
Similar to the conventional magnesium alloys, the magnesium alloy has a chemical composition which contains magnesium as a main component and one element or two or more elements, such as zinc or aluminum further added thereto. For example, the magnesium alloy is a Mg—Zn binary alloy, a ternary alloy, or multicomponent system alloys comprising magnesium, zinc, and another or other metal elements. When the chemical composition is represented by Mg-A mass % Zn—B mass % Z (Mg: magnesium, Zn: zinc, Z: another metal element), it is preferred that 0<A<10 and 0<B<10. When Zn and other element are added in an amount of 10 mass % or more, the fraction of the below-mentioned fine particles, which is dispersed into the subgrains, these magnesium alloys are difficult to be produced and show lower ductility. In the above chemical composition, with respect to Z, examples include metal elements, such as Al, Zr, Ca, Sn, Li, and Ag, and rare earth elements, such as Y, Ho, Gd, Tb, Dy, and Er.
On the other hand, the magnesium alloy has characteristic features, particularly with respect to the crystal structure thereof. The crystal structure of the magnesium alloy is based on the following features:
1) the magnesium alloy has a high angle grain boundary,
2) the inside of crystal grains surrounded by the high angle grain boundary, namely, the inside of high angle grain boundary consists of the subgrains, and
3) the fine particles, such as quasicrystal particles or second phase deposited particles, disperse into the subgrains.
The “high angle grain boundary” is defined as a grain boundary having a misorientation angle of 15 degrees or more. The high angle grain boundary is confirmed by crystal orientation mapping using a SEM/EBSD (Scanning Electron Microscopy/Electron Back-Scattered Diffraction) or a misorientation measurement using a transmission electron microscope, etc.
The grains surrounded by the high angle grain boundary preferably have an average size of 5 μm or less, more preferably 3 μm or less. Further, it is preferred that the grains having a particle diameter of 5 μm or less constitute 70% or more of the whole crystal grains. When the grains surrounded by the high angle grain boundary have the average size of 5 μm or less constitute 70% or more of the whole crystal grains, twinning is difficult to be formed during the plastic deformation and, therefore, the yield strength in compression is improved. For this reason, the magnesium alloy can more surely achieve isotropic deformation (yield strength in tensile is the same as yield strength in compression). These grains consist of the magnesium phase, and the composition of the magnesium phase is specifically magnesium atoms and solute atoms which can dissolve into magnesium.
The “subgrains” are defined as grains having a grain boundary having a misorientation angle of 5 degrees or less. The size of subgrains is 1.5 μm or less, preferably 1 μm or less, more preferably 0.5 μm or less. The crystal subgrains are also formed from a magnesium matrix phase, but a difference between the subgrains and the above-mentioned grains surrounded by high angle grain boundary is that the misorientation angle between the adjacent grains is 5 degrees or less.
The fine particles dispersed in the crystal subgrains preferably have an average particle diameter of 10 nm to 1 μm, more preferably 25 nm to 500 nm. When the average particle diameter of the fine particles is less than 10 nm, it is likely that the particles cannot fully contribute to the increase of strength. Furthermore, when the average particle diameter of the fine particles exceeds 1 μm, the particles easily become an origin of the microvoid during the plastic deformation and hence these alloys show lower ductility. The amount of the fine particles in the crystal subgrains is preferably 15% or less, more preferably 10% or less (with the proviso that the density is not 0%). When the mount of the fine particles exceeds 15%, the particles easily become an origin of the microvoid during the plastic deformation and hence these alloys show lower ductility. When the average particle diameter and mount of the fine particles exceed the respective preferred upper limits, for example, as shown in
The magnesium alloy having the above-described crystal structure shows tensile strength of 330 MPa or more. Further, the magnesium alloy shows yield stress (A) of 300 MPa or more and a compressive yield stress (B) of 220 MPa or more and achieves a yield stress anisotropy ratio (B/A) of 0.7 or more.
Thus, the magnesium alloy achieves both high strength and ductility which have conventionally been considered impossible. The reason for such excellent properties of the magnesium alloy is presumably that the presence of the subgrains enables the deformation into grain interior and prevents the grain boundary sliding. Furthermore, the presence of the fine particles plays a role of prevention of dislocations during the plastic deformation. In the magnesium alloy, by controlling the above-mentioned crystal structure, both the increase in strength and the reduction in deformation anisotropy i.e., isotropic deformability are achieved.
The applying to processing strain, such as severe plastic deformation, is an effective to produce the magnesium alloys.
The “processing strain” is defined as permanent deformation made by applying a load at the setting temperature. The introduction of such a processing strain is realized by, for example, groove rolling, extrusion at a high extrusion ratio, rolling under a high reduction, or high strain shearing processing, such as ECAE (Equal-channel-angular-extrusion).
The groove rolling is rolling using a roll having grooves with a cross-section of a triangular shape, for example. In the roll having a triangular cross-section, when an upper roll and a lower roll are in contact with a material, holes with a diamond like shape are formed. The groove rolling is a preferred method to obtain the magnesium alloys. Examples of shape of groove roll include those which can form holes of the above-mentioned diamond like shape, and those which can form holes of a hexagonal shape, an elliptic shape, or the like. The rolling speed is preferably in the range of from 1 to 50 m/minute. It is preferred that, before the groove rolling, the material is subjected to heat treatment at a temperature in the range of from 100 to 500° C. for 5 to 120 minutes.
In the case of applying to a processing strain using various methods including the above-mentioned groove rolling, it is preferred that the billet is heated and maintained at a temperature before the process. After that, a strain is repeatedly applied to the billet. The cross-section reduction ratio in the material can be appropriately selected in association with the conditions for the introduction of a processing strain into the material. In other words, the cross-section reduction ratio can be selected according to conditions such that the above-mentioned structure is formed. For example, the cross-section reduction ratio can be set at 92%, 95%, or the like. As a processing strain is introduced at a cross-section reduction ratio of 90% or more, the strength can be remarkably increased without suppression of the excellent ductility. When a strain is repeatedly introduced, it is preferred that the introduction of strains is continuously performed. In this case, a strain is introduced per single pass so that the total cross-section reduction ratio becomes 90% or more, for example, it is a satisfactory for a strain at a cross-section reduction ratio of about 10 to 20%.
Regarding the grain structure surrounded by high angle grain boundary, when the cross-section reduction such as applying strain is increased, the fraction of grains which is the size of 5 μm is increased. When the cross-section reduction ratio is more than 90%, the fraction of grains which is the size of 5 μm is more than 90%. Further, when the cross-section reduction ratio is 90% or more, the average size of subgrains is 1.5 μm or less. In addition, the fine particles having an average particle diameter of 10 nm to 1 μm are dispersed into the subgrains with fraction of 15% or less (with the proviso that the density is not 0%).
The processing strain can be applied to materials which have a large cross-sectional area and long length with a complicated shape. In addition, since the materials can produce a large billet, this technique is useful and practical in view of the industrial points.
Hereafter, the magnesium alloy will be described in more detail with reference to the following Examples. Needless to say, the following Examples should not be construed as limiting the scope of the invention.
Mg—7.5 mass % Zn—1.7 mass % Y alloy was casted. The casted alloy was subjected to solution heat treatment, followed by machining, to prepare a billet for rolling having a diameter of 40 mm. The billet for rolling was maintained in a furnace at a temperature of 350° C. for a while, and then subjected to groove rolling. The rolling surface was room temperature, and the roll speed was 30 m/minute. The cross-section reduction ratio was 18% per one pass, and the groove rolling was repeated 19 times. The total cross-section reduction ratio was 95%.
The microstructure off the rolled alloy was observed using a transmission electron microscope (TEM). The observed site was the cross-section of the material taken along the direction parallel to the rolling direction.
A tensile test specimen and a compression test specimen were taken from the rolled alloys. The gauge section in the tensile specimen was a diameter of 3 mm and length of 15 mm and the compression test specimen was a diameter of 4 mm and a height of 8 mm. The direction of taking each test specimen out was parallel to the rolling direction. The initial tensile and compressive strain speed was 1×10−3 s−1.
Mg—8 mass % Zn—4 mass % Al alloy was casted. The casted alloy was subjected to solution heat treatment, followed by machining, to prepare a billet for rolling having a diameter of 40 mm. Then, a rolled material was prepared in substantially the same manner as in Example 1 except that the processing temperature was changed to 200° C.
Mg—6 mass % Zn—3 mass % Al alloy was casted. The casted alloy was subjected to solution heat treatment, followed by machining, to prepare a billet for rolling having a diameter of 40 mm. Then, a rolled alloy was prepared in substantially the same manner as in Example 1 except that the processing temperature was changed to 200° C.
Using a commercial extruded ZK60 magnesium alloy (ZK60: Mg—6 mass % Zn—0.5 mass % Zr alloy), a billet for rolling having a diameter of 40 mm was prepared by machining. Then, a rolled alloy was prepared in substantially the same manner as in Example 1 except that the processing temperature was changed to 200° C. With respect to the rolled alloy, tensile and compression tests were performed under the same conditions as those in Example 1. The results are shown in Table 1. With respect to the magnesium alloy in Example 4, the yield anisotropy ratio is a value close to 1, which shows that the rolled alloy has isotropic deformation. Further, the magnesium alloy in Example 4 exhibits a 5% higher tensile strength than that in Comparative Example 4, which indicates that the dispersion of the fine particles contributes to a further increase of the strength. The fine particles are second phase particles, i.e., Mg2Zn, which have incoherent interface between the particle and the matrix (magnesium).
Using the same material as in Example 2, a rolled alloy was prepared in substantially the same manner as in Example 1 except that the processing temperature was changed to 300° C., and the groove rolling was repeated 15 times, which corresponds to the cross-section reduction ratio of 92%. With respect to the rolled alloy, tensile and compression tests were performed under the same conditions as those in Example 1. The results are shown in Table 1. The magnesium alloy in Example 5, in which fine quasicrystal particles are dispersed into the subgrains, shows the improvement of the yield anisotropy compared to that in Comparative Example 5, in which the total cross-section reduction ratio is the same as in Example 5, but no fine particle is present. In the commercial extruded magnesium alloy extruded material in Comparative Example 5, since the zinc and aluminum atoms are dissolved into the magnesium, it is difficult to form and disperse the fine particles.
Using the same material as in Example 3, a rolled alloy was prepared in substantially the same manner as in Example 1 except that the processing temperature was changed to 300° C., and the groove rolling was repeated 15 times, which corresponds to the total cross-section reduction ratio of 92%.
Using the same material as in Example 4, a rolled alloy was prepared in substantially the same manner as in Example 1 except that the processing temperature was changed to 200° C., and the groove rolling was repeated 15 times, which corresponds to the total cross-section reduction of 92%. With respect to the rolled alloy, tensile and compression tests were performed under the same conditions as those in Example 1. The results are shown in Table 1. The magnesium alloy in Example 7, in which fine particles are dispersed in the crystal subgrains, exhibits a 6% higher tensile strength than that in Comparative Example 5 in which the total cross-section reduction ratio is the same as in Example 7 without the existence of the fine particle. This alloy is recognized to improve the yield anisotropy, as compared with that in Comparative Example 5. The fine particles are second phase deposited particles (Mg2Zn) which is an incoherent interface between the particle and magnesium matrix.
As a commercial extruded magnesium alloy (AZ61: Mg—6 mass % Al—1 mass % Zn) is used, a billet with a diameter of 40 mm was prepared by machining. Then, a rolled alloy was prepared under substantially the same conditions as those in Example 1 except that the processing temperature was changed to 200° C. and the groove rolling was repeated 15 times, which corresponds to the total cross-section reduction of 92%. With respect to the rolled alloy, tensile and compression tests were performed under the same conditions as those in Example 1. The results are shown in Table 1. The yield anisotropy ratio is a value close to 1. This result indicates that the alloy shows isotropic deformation.
Mg—7.5 mass % Zn—1.7 mass % Y alloy was produced by casting. The casted alloy was subjected to solution heat treatment, followed by machining, to prepare a billet for extrusion having a diameter of 40 mm. The billet for extrusion was placed in an extrusion container at a temperature elevated to about 230° C. for 30 minutes and then subjected to warm extrusion at an extrusion ratio of 25:1, which corresponds to the cross-section ratio 94%. The diameter of extruded bar was 8 mm. With respect to the extruded alloy, a tensile test was performed under the same conditions as those in Example 1. The results are shown in
Mg—8 mass % Zn—4 mass % Al alloy was produced by casting. The subsequent processing was the same as in Comparative Example 1. With respect to the extruded alloy, a tensile test was performed under the same conditions as those in Example 1. The results are shown in
Mg—6 mass % Zn—3 mass % Al alloy was produced by casting. The subsequent processing was the same as in Comparative Example 1. With respect to the extruded alloy, a tensile test was performed under the same conditions as those in Example 1. The results are shown in
As a commercial extruded magnesium alloy (AZ31: Mg—3 mass % Al—1 mass % Zn alloy) is used, a billet for rolling having a diameter of 40 mm was prepared by machining. Then, a rolled alloy was prepared in substantially the same manner as in Example 1 except that the processing temperature was changed to 200° C.
Using the same alloy as in Comparative Example 4, a rolled alloy was prepared under substantially the same conditions as those in Example 1 except that the processing temperature was changed to 200° C. and that the groove rolling was repeated 15 times, which corresponds to the total cross-section reduction ratio of 92%. With respect to the rolled alloy, tensile and compression tests were performed under the same conditions as those in Example 1. The results are shown in Table 1.
The magnesium alloy is improved in both strength and ductility and has a more practical material. In addition, the magnesium alloy has a possibility for using as an industrial material.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2010-060144 | Mar 2010 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2011/055271 | 3/7/2011 | WO | 00 | 11/2/2012 |
