Patent Application
20100202916
This invention relates to an Mg base alloy containing Mg as a main material, realization of which is desired as a lightweight material which is a replacement for Al.
With respect to this Mg base alloy, there have hitherto been developed various alloys disclosed in the following Patent Documents 1 to 8.
In Patent Documents 2, 3, 4, 6 and 8, in order to contrive to improve strength, a rare earth element, scandium or lithium is added. However, since such a rare earth element is a rare element which is hardly obtainable on the earth, alloys thereof are high in the price and low in the multiplicity of use.
Patent Document 1 is concerned with a quinary alloy containing from 0.3 to 3% by mass of Ca and simultaneously containing Al, Sr and Mn. In such an alloy, a precipitate (crystal) is formed on the grain boundary of Mg.
Patent Document 2 is concerned with an Mg alloy which contains 0.3% or more and not more than 1.0% of Zr and which in the case of containing Ca, contains 0.2% or more and not more than 2.0% of Ca (% means % by mass).
Patent Document 8 discloses an Mg alloy developed as a casting material, which contains from 3 to 8% by weight of Zn and from 0.8 to 5% by weight of Ca.
During the process of experiments of this invention, it has become clear that a grain boundary precipitate is formed because of an excess of the content of Ca, whereby ductility at room temperature is lowered. From this fact, in all of the foregoing Patent Documents 1, 2 and 8, the ductility at room temperature is poor, too.
An alloy of Patent Document 7 is an alloy developed as a casting material. Specifically, it is disclosed that Ca is zero or 0.5% by weight, and Zn is from 1% by weight to 7% by weight; and in combinations of zero, in the case where Ca is zero or 0.5% by weight, when Zn is zero, then the alloy has a 0.2% proof stress of less than 75 MPa, whereas when Zn is from 1% by weight to 7% by weight, then the alloy has a 0.2% proof stress of 75 MPa or more and less than 100 MPa. Thus, it is demonstrated that when such an alloy is used as a structural material, its strength is insufficient. Also, according to the foregoing knowledge obtained in the experiments of this invention made by the present inventors, there is nothing other than estimation that an alloy containing Ca in a high concentration is low in the ductility.
Patent Document 5 discloses an Mg base alloy containing Mn and Zn as main components of additive materials, and in order to obtain a high strength, it is described to perform a solution heat treatment. However, there is involved such a problem that the process is complicated because, for example, an additional heat treatment of two-stage aging is required.
In Patent Document 8, an alloy in which not more than 10% by weight of Cu is added is developed. However, the addition of Cu encounters a defect that the corrosion resistance of the Mg alloy is remarkably lowered.
In summary, most of members in which an Mg alloy is used are currently manufactured by a casting or die casting method. In the future, though applications of an Mg alloy to transportation equipment such as automobiles, aircrafts, etc. are expected, the casting method involves such defects that the structure of the material is coarse; the ductility is low; and the size is limited so that the Mg alloy is not applicable to plate materials, rod materials, pipe materials, etc. On the other hand, a wrought Mg alloy which is practically useful includes Mg—Al—Zn (AZ based alloy) and Mg—Zn—Zr (ZK based alloy). However, the strength of such a wrought Mg alloy is insufficient, and a proof stress to be used for strength design is largely different between the case where a tensile load is applied and the case where a compression load is applied due to influences of the texture to be formed at the time of hot working (in commercially available AZ31 alloy rolled materials, the compression proof stress is about 50% of the tensile proof stress). Therefore, it is difficult to use such a wrought Mg alloy as it is. Up to date, in order to realize a high strength of an Mg alloy, there have been adopted methods for adding a rare earth element or adding large amounts of alloying elements.
However, since the rare earth element is expensive, its multiplicity of use is low; and furthermore, the addition of large amounts of alloying elements is accompanied with the formation of a coarse compound phase and involves such a defect that though a high strength is obtained, the ductility is impaired. Then, the development of a new wrought Mg alloy which is free from a rare earth element and which is excellent in strength and ductility by the addition of inexpensive alloying elements is being demanded.
In view of such circumstances, an object of this invention is to provide an Mg base alloy having not only a practically sufficient strength but good ductility at room temperature to an extent that it has hitherto been unable to be desired and having small anisotropy in strength characteristics.
An Mg base alloy of Invention 1 is characterized in that Ag is contained in an amount of not more than 1.98 at % as an additive material other than Zn.
Invention 2 is characterized in that in the Mg base alloy as set forth in Invention 1, Ca is further contained in an amount of not more than 0.61 at % as an additive material other than Zn and Ag.
Invention 3 is characterized in that in the Mg base alloy as set forth in Invention 2, Zr is further contained in an amount of not more than 0.17 at % as an additive material other than Zn, Ag and Ca.
Invention 4 is characterized in that in the Mg base alloy as set forth in any one of Inventions 1 to 3, a crystal grain size thereof is from 0.1 μm to 25 μm.
According to Inventions 1 to 4, it has become possible to provide an Mg base alloy having excellent strength and ductility to an extent that both of them have hitherto been unable to be desired and having small anisotropy in strength, by the addition of inexpensive alloying elements.
Furthermore, since alloying elements which impair corrosion resistance, such as Cu, etc., are not used, excellent durability can also be expected.
The alloy of this invention has an average Schmid factor in the bottom slip direction against the load application direction of 0.2 or more and has uniform distribution of the Schmid factor as compared with extruded materials of the existing AZ91 alloy (Mg-9 mass % Al-1 mass % Zn alloy) which is a practical Mg alloy. Namely, the alloy of this invention is characterized in that a degree of integration of the bottom parallel to the extrusion direction is weak.
The alloy of this invention has such excellent mechanical properties that the compression proof stress is 75% or more of the tensile proof stress and that anisotropy in strength is small.
It is noted from the following Examples that according to the invention of this application, age hardening properties are enhanced by adding trace amounts of Ag, Ca and Zr, each of which is free from a rare earth element and is relatively easily available. Also, it is noted that even by merely hot extruding the alloy, a fine grain structure having a fine precipitate dispersed therein is formed and that the subject alloy is an Mg base alloy which is excellent in not only strength but ductility and which has small anisotropy in strength as compared with the conventional alloys. Also, in view of the Examples and the technical common knowledge, it can be expected that the foregoing effects are displayed within the following ranges.
With respect to Zn, the maximum solubility of Zn in Mg is 2.4 at %.
So far as the composition range of Zn is 0.75 at % or more, age hardening is achieved. However, in order to contrive to realize a high strength by dispersing a rod-like β′-precipitate which works as a strengthening phase of an Mg—Zn based alloy, it is necessary that the Zn content is as high as possible, and the Zn content is preferably 1.52 at % or more.
In order to disperse this rod-like β′-precipitate in a larger amount and finely, the Zn content is preferably 1.92 at % or more.
With respect to Ag, the solubility of Ag in Mg is large, and its maximum solid solution amount is 3.82 at %.
In the case where a solution heat treatment after casting is performed at 400° C., when the Ag content exceeds 1.98 at %, there is a concern that a coarse precipitate is formed, thereby deteriorating mechanical properties.
When the Ag content exceeds 0.2 at %, even by increasing the addition amount, age hardening properties do not change too much. Therefore, in order to inhibit the formation of a compound phase with Zn or Ca or Zr as a constituent element, in a sense of suppressing the Ag content, an upper limit thereof is preferably 0.2 at %.
Also, when the Ag content Is 0.08 at % or more, a work for promoting the formation of a nucleus of the precipitate is revealed, and therefore, a lower limit value thereof is preferably 0.08 at % or more.
With respect to Ca, the maximum solubility of Ca in Mg is 0.82 at %.
In the case where a solution heat treatment after casting is performed at 400° C., when the Ca content exceeds 0.61 at %, a grain boundary precipitate is formed, thereby impairing mechanical properties.
Therefore, its upper limit was specified to be not more than 0.61 at %.
Also, as shown in
Also, when the Ca content is 0.08 at % or more, a work for promoting the formation of a nucleus of the precipitate is revealed, and therefore, a lower limit value thereof is preferably 0.08 at % or more.
With respect to Zr, the maximum solubility of Zr in Mg is 1.04 at %.
However, when the Zr content exceeds 0.17 at %, a peritectic reaction exists in the vicinity of 650° C., and a coarse precipitate is formed. Therefore, the Zr content was specified to be not more than 0.17 at %.
When the Zr content is 0.08 at % or more, an effect for inhibiting grain coarsening in the solution heat treatment and hot extrusion is expected due to a fine precipitate or the Zr atom itself. Therefore, a lower limit of the Zr content is preferably 0.08 at % or more.
The foregoing specific addition amount of each of the elements is distributed on the basis of results of the following Examples such that an average grain size of the fine grain structure is as small as possible and that orientation properties of the crystal grain are weakened.
Respective elements were blended so as to have an alloy composition shown in Table 1 and smelted in a high-frequency melting furnace using an iron-made crucible in an argon atmosphere.
After sealing in a PYREX (registered trademark) tube together with an argon gas, the melted material was subjected to a homogenization heat treatment at 340° C. for 48 hours. The resulting sample was cut out and sealed in a PYREX (registered trademark) tube together with an argon gas; and thereafter, the sample was subjected to a solution heat treatment at 400° C. for one hour and then quenched in ice water.
The quenched material was aged at a temperature of 160° C. and 200° C., respectively using an oil bath. Its hardness by aging was measured by a Vickers hardness meter under a condition at a load of 1 kg for a holding time of 15 seconds.
Microstructure observation was executed using a transmission electron microscope (TEM). Details of experimental procedures are shown in
The age hardening properties become better by adding Ag, (Ag+Ca) or (Ag+Ca+Zr) to the Mg-2.3Zn alloy.
The maximum hardness of an alloy obtained by adding (Ag+Ca+Zr) to the Mg-2.3Zn alloy is the highest and reaches 100 Hv.
In an alloy obtained by adding (Ag+Ca) to the Mg-2.3Zn alloy, the aging hardness of each of alloys obtained by increasing the addition amount of each of the elements by 0.2 at % is examined.
However, even when the addition amount is increased, a distinct difference in the aging characteristics is not observed.
With respect to each of the alloys shown in
Each of
In all of the aging structures, a rod-like precipitate extending to the c-axis direction of Mg is observed.
By adding Ag, (Ag+Ca) or (Ag+Ca+Zr) to the Mg-2.3Zn alloy, the precipitate becomes fine.
This refinement of the precipitate is considered to cause an increase of the peak aging hardness.
In conclusion, good age hardening properties are obtained in alloys by the combined addition of (Ag+Ca) and (Ag+Ca+Zr), respectively.
Details of experimental procedures are shown in
Also, the sample after extrusion was subjected to microstructure observation by an optical microscope and TEM.
Comparison was made between a material having been subjected to a solution heat treatment after casting and a material having been subjected to a solution heat treatment after hot extrusion. As a result, the both are substantially identical to each other with respect to the maximum hardness and age hardening characteristics.
Comparison was made between a material having been subjected to a solution heat treatment after casting and a material having been subjected to a solution heat treatment after hot extrusion. As a result, the both are not distinctly different from each other with respect to the maximum hardness and age hardening characteristics.
In the optical micrograph of
The grain size of the hot extruded Mg-2.3% Zn-0.1% Ag-0.1% Ca-0.17% Zr alloy was measured from
With respect to the Mg-2.3% Zn-0.1% Ag-0.1% Ca alloy and the Mg-2.3% Zn-0.1% Ag-0.1% Ca-0.17% Zr alloy each having excellent age hardening properties, a tensile test at room temperature and a compression test at room temperature were executed in parallel to the extrusion direction. A tensile specimen was a JIS 14B specimen and had a gauge length of 20 mm. A compression specimen had a diameter of 9.5 mm and a height of 14.3 mm. The tensile test and the compression test were performed under a condition at an initial strain rate of 10−3 s−1.
(Initial strain rate: 10−3 s−1; shape of tensile specimen: JIS 14B (gauge length: 20 mm); and shape of compression specimen: 9.5 mm in diameter and 14.3 mm in height)
Table 12 summarizes results obtained in a tensile test and a compression test regarding an Mg-2.3 Zn-0.1% Ag-0.1% Ca alloy and an Mg-2.3% Zn-0.1% Ag-0.1% Ca-0.17% Zr alloy as extruded at 300° C. and 350° C., respectively.
It is noted from these results that the hot extruded Mg-2.3% Zn-0.1% Ag-0.1% Ca alloy and Mg-2.3% Zn-0.1% Ag-0.1% Ca-0.17% Zr alloy are a material having both high strength and high ductility and having small anisotropy in proof stress.
It is considered that revealment of such excellent mechanical properties with small anisotropy in strength as well as high strength and high ductility has a relationship with a fine grain, a lowering in a degree of the basal texture and a fine precipitate within the grain.
The material of this invention has high strength and high ductility and can be used for transportation equipment which is expected to realize weight reduction as a replacement for Al alloys, such as automobiles, motorcycles, aircrafts, etc. Furthermore, since the mechanical properties of the material of this invention can be obtained without necessity of an additional heat treatment after the hot working, the material of this invention is also expected as a replacement for currently used wrought Mg alloys. Also, in view of the fact that the samples after hot extrusion at 350° C. display an ultrafine grain structure of about 500 nm in terms of an average grain size, there is a possibility that the material of this invention is applicable as a superplastic material.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2007 124879 | May 2007 | JP | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2008/058677 | 5/9/2008 | WO | 00 | 4/19/2010 |
