WO2008088082A1

WO2008088082A1 - Mg alloy

Info

Publication number: WO2008088082A1
Application number: PCT/JP2008/051015
Authority: WO
Inventors: Tetsuya Shoji; Akira Kato; Toshiji Mukai; Hidetoshi Somekawa
Original assignee: Toyota Jidosha Kabushiki Kaisha; National Institute For Materials Science
Priority date: 2007-01-17
Filing date: 2008-01-17
Publication date: 2008-07-24

Abstract

An Mg alloy provided with high strength and high ductility by matching the strength and ductility in tensile deformation and compressive deformation at equal levels, that is, an Mg alloy characterized by consisting of Y: 2.0 to 3.0 at% and a balance of Mg and unavoidable impurities and by having high Y regions of Y concentrations higher than an average concentration distributed at a nanometer order of size and intervals.

Description

DESCRIPTION

Mg ALLOY

TECHNICAL FIELD

The present invention relates to an Mg alloy, more particularly relates to an Mg alloy improved in isotropy of deformation.

BACKGROUND ART An Mg alloy is light weight, gives strength at room temperature and high temperature, and is improved in corrosion resistance as well, so is being increasingly used for various applications. However, to improve the toughness as a structure and the plastic workability, the ductility has to be improved.

For example, Japanese Patent Publication (A) No. 2002-256370 proposes Mgi_Oo-_a-bLn_aM_b, where Ln is at least one of Y, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Tb, Lu, and a misch metal, M is at least one of Al and Zn, 0.5<a<5, 0.2<b<4, and 1.5<a+b<7, where the crystal grain size is less than 2000 nm (=2 μm) so as to obtain high strength and high ductility. However, with a Zn content larger ,than 1 at%, the solid solubility limit in the Mg is exceeded, so Mg-Zn-based intermetallic compounds are produced and a high ductility is liable not to be realizable.

Further, Japanese Patent Publication (A) No. 5- 306424 proposes Mg_balX_aLn_b, where X is at least one of Zn, Ni, and Cu, Ln is at least one of Y, La, Ce, and a misch metal, l≤a≤lO, and l≤b<20, where the average size of the crystal grains is 5 μm or less and the average grain size of the intermetallic compounds is 5 μm or less to provide strength, toughness, and secondary workability.

Japanese Patent Publication (A) No. 7-3375 proposes Mg_aZn_bX_C/ where X is at least one element of Y, Ce, La,

Nd, Pr, Sm, and a misch metal, 87at%<a<98at%, b and c are in the ranges shown in FIG. 1, 0<Y<4.5at%, O≤Ce, La, Nd, Pr, Sm, misch metal <3at%, where the microstructure is composed of a matrix phase of fine crystals in which Mg- Zn-based and Mg-X-based intermetallic compounds are dispersed so as to obtain high strength and high toughness .

International Patent Publication WO2004/085689 proposes including Zn in an amount of a at%, including at least one rare earth element selected from the group of La, Ce, and misch metals in a total of b at%, having a balance of Mg, with a and b satisfying the following expressions (1) to (3): (1) 0.2<a<3.0, (2) 0.3<b<1.8, and

(3) -0.2a+0.55<b<-0.2a+1.95 so as to obtain a high strength and high toughness. Japanese Patent Publication (A) No. 2005-113235 proposes Mgioo-_a-_bZn_aY_b, where a/12≤b<a/3 and 1.5<a<10, where the microstructure is an aged precipitated phase of Mg3Zn6Yl quasi-crystals and their similar crystals dispersed in the state of microparticles so as to improve the high temperature strength.

Japanese Patent Publication (A) No. 2006-2184 proposes an Mg-based alloy containing 1- to 8 wt% of rare earth elements and 1 to 6 wt% of Ca and having a microstructure in which the maximum crystal grain size of Mg is 30 μm or less, the maximum grain size of intermetallic compounds is 20 μm or less, and the Mg is dispersed in the crystal grains and at the crystal grain boundaries so as to improve the strength and ductility at room temperature and the high temperature strength and fatigue strength near 200⁰C.

However, in each of the above, the difference in strength and ductility between tensile deformation and compressive deformation was not considered at all. DISCLOSURE OF THE INVENTION The present invention has as its object the provision of an Mg alloy provided with both high strength and high ductility by making the strength and ductility in tensile deformation and compressive deformation equal levels . To achieve the above object, the Mg alloy of the present invention is characterized by consisting of Y: 2.0 to 3.0 at% and a balance of Mg and unavoidable impurities, where high Y regions where the Y concentration is higher than the average concentration are dispersed at nanometer order sizes and intervals_^.

The Mg alloy of the present invention can be deformed in directions other than along the bottom face of the Mg hexagonal crystal due to the above defined composition and structure. The yield strength and work hardening rate at the tensile deformation and compressive deformation are matched, so a high ductility can be realized.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is (1) a view of the crystal structure using a scanning electron microscope (SEM) and electron back scatter diffraction (EBSD) showing the crystal grain structure of an Mg-2.2at%Y alloy extruded material according to the present invention and (2) a reverse pole figure of the same field. FIG. 2 is an atom probe view of an Mg-2.2at%Y alloy extruded material according to the present invention. FIG. 3 is a true stress-true strain diagram in a static tensile test and static compression test of an Mg- 0.6at%Y alloy extruded material as a comparison. FIG. 4 is a true stress-true strain diagram in a static tensile test and static compression test of an Mg- 1.5at%Y alloy extruded material as a comparison.

FIG. 5 is a true stress-true strain diagram in a static tensile test and static compression test of an Mg- 2.0at%Y alloy extruded material according to the present invention.

FIG. 6 is a true stress-true strain diagram in a - -

static tensile test and static compression test of an Mg- 2.2at%Y alloy extruded material according to the present invention.

FIG. 7 is a true stress-true strain diagram in a static tensile test and static compression test of an Mg- 3.0at%Y alloy extruded material according to the present invention.

BEST MODE FOR CARRYING OUT THE INVENTION

The inventors newly discovered that by adding 2.0 at% or more of Y to Mg, it is possible to make the deformation behavior in tensile deformation and compressive deformation match extremely well and possible to achieve high deformation isotropy and thereby completed the present invention. In a conventional Mg alloy, the plastic deformation near normal temperature is performed by slip deformation due to the motion of dislocations in the close packed crystal plane, that is, the so-called basal plane of an Mg hexagonal crystal. If slip deformation other than the direction along the basal plane is hard to occur in this way, in particular in compressive deformation, deformation by twinning easily occurs. That is, in compressive deformation, deformation due to twinning occurs with priority over slip deformation due to dislocations. Specifically, in a stress-strain diagram, the phenomenon occurs where the yield strength and the work hardening rate after yielding fall in compressive deformation compared with tensile deformation.

If the deformation behavior differs between tensile deformation and compressive deformation in this way, that is, so-called deformation anisotropy occurs, when an external force acts on a 3D structure made of the Mg alloy, twinning deformation will occur at the locations acted on by the compressive stress, so deformation will start by a lower stress than the locations acted on by tensile stress and, further, the work hardening rate will be small, so fracture will occur at a low stress and small strain.

Therefore, in the past, the strength characteristics of an Mg alloy in the final analysis ended up having a deformation degree limited to low levels due to the strength characteristics in compression.

In the Mg alloy of the present invention, to match the deformation behaviors in tensile deformation and compressive deformation and achieve isotropy of deformation, 2.0 at% to 3.0 at% of Y is added. To achieve isotropy of deformation, the Y content must be 2.0 at% or more. Further, by adding the amount of the lower limit value 2.0 at% of Y, the deformation behavior in tension and compression match extremely well and an already sufficiently good isotropy is achieved. In the conventional wrought Mg alloys produced by extrusion, rolling, or other working processes, a rapid work hardening occurs during deformation by compression and appears as a change in the slope of stress-strain curve. For example, a change in slope can be seen in Fig. 3, in which the curve in the portion after yielding has two slopes denoted as the first compression work hardening rate (1st WHR) and the second compression work hardening rate (2nd WHR) with a ratio of the second rate to the first rate being more than 5 to form a downwardly convex shaped curve. Thus, the curve has a shape quite different from that occurring during tensile deformation and causes deformation anisotropy to occur.

However, as can be seen from Table 1, with the addition of Y in an amount of 2.0at% or more, the second- to-first WHR ratio (D/C) can be reduced to 2 or less to provide a stress-strain curve under compression having a shape close to that occurring during tensile deformation, thereby achieving deformation isotropy.

Therefore, other than the case where improvement of the strength characteristics, corrosion resistance, etc. at room temperature and high temperature by Y is particularly aimed at, there is no need to add a further - -

larger amount of Y. In particular, even if Y is added by an amount over 3 at%, at least from the viewpoint of achieving isotropy of deformation targeted by the present invention, the effect of addition is not further increased but ends up becoming saturated, just expensive Y is wastefully consumed and the cost is raised.

Therefore, the Y content of the Mg alloy of the present invention is 2.0 at% to 3.0 at% in range.

In a preferred embodiment, the matrix phase of the present inventive Mg alloy has an average crystal grain size of 8 μm or more, further preferably 8 μm or more and up to 12 μm.

Below, specific examples will be used to explain in further detail the present invention including the mechanism of achieving deformation isotropy.

EXAMPLES

Yttrium and pure magnesium (purity 99.95 wt%) were completely melted in an argon atmosphere and cast into iron molds to prepare five Mg-Y alloys with Y contents of 0.6 at%, 1.5 at%, 2.0 at%, 2.2 at%, and 3.0 at% . The alloys with 0.6at%Y and 1.5at%Y are comparative examples outside the range of the present invention, while the alloys with 2.0at%Y, 2.2at%Y, and 3.0at%Y are invention examples inside the range of the present invention.

The obtained cast alloys were held at a temperature of 500⁰C for 24 hours in the atmosphere, then water cooled to solution treat them.

After this, the alloys were machined to prepare cylindrical materials having a diameter of 40 mm and a length of 70 mm.

These cylindrical materials were held in containers (in the atmosphere) at a temperature of 425°C for 30 minutes, then extruded by an extrusion ratio of 25:1. The average equivalent plastic strain determined from the rate of reduction of cross-section was 3.7. <Observation of Structure>

FIG. 1(1) shows the crystal structure of the cross- section parallel to the direction of extrusion of the obtained extruded material for an Mg-2.2at%Y alloy as a representative example of the present invention as observed by a scanning electron microscope (SEM) and electron back scatter diffraction (EBSD) . As illustrated, the crystal grain structure was an equiaxed grain structure free from flow structures caused by working. Further, as a result of analysis by electron back scatter diffraction (EBSD) , no texture was observed and, as shown by the reverse pole figure of FIG. 1(2), the individual crystal grains had random orientations. From these results, it is learned that the structure has a high isotropy with the crystal grain size of several μm to tens of μm.

If the conventional typical wrought Mg alloy AZ31 is rolled, forged, extruded, or otherwise hot worked, it strongly tends to form a texture with the close packed crystal plane of the crystal lattice (basal plane of hexagonal crystal) oriented parallel to the working direction and aggravates the anisotropy of deformation. As opposed to this, in the alloy of the present invention, even in the state as hot extruded as above, the crystal grain structure becomes an equiaxed grain structure, no texture due to working is observed, and a structure advantageous for achieving isotropy of deformation is obtained. Note that in this example, the hot working was performed by extrusion, but rolling, forging, or other hot working methods may also be used. Furthermore, the results of atom probe observation of an Mg-2.2at%Y alloy extruded material are shown in FIG. 2. In the figure, the bright gray colored (substantially white colored) spots are high Y regions having Y concentrations of 3 at% or more - which is higher than the average concentration of 2.2 at%. It is confirmed that high Y regions of a size of the order of several nm are distributed at intervals of about 10 nm.

Note that FIG. 2 shows the case of 3 at% or more high Y regions as a typical example of observation, but in general high Y regions 3.0% or more higher than the average concentration and conversely low Y regions 3.0% or so lower than the average concentration are alternately distributed in sizes and intervals of the order of several nm.

Further, by further detailed observation, it was learned that such nanometer order high Y regions are uniformly distributed in the crystal grains, but the density of distribution is also high at the crystal grain boundaries .

<Static Tensile Test and Static Compression Test> For the prepared five types of Y content Mg alloys, test pieces taken from the above extruded materials were subjected to a static tensile test and compressive test at room temperature at a strain rate of 10^~3/sec. The true stress-true strain diagrams obtained for the alloys are shown in FIGS. 3 to 7. Note that the results of the tensile test show the data up to the maximum load (or maximum nominal stress) of the load-displacement (or nominal stress-nominal strain) curve.

In FIG. 3 and FIG. 4, the comparative examples Mg- 0.6at%Y alloy (FIG. 3) and Mg-1.5at%Y alloy (FIG. 4) where the Y contents are lower than the lower limit 2.0 at% of the range of the present invention have a lower yield stress and sections where the rise in stress after yielding slows in the case of compressive deformation compared with tensile deformation. This shows that in compressive deformation, deformation due to the occurrence of twinning occurs with priority over slip deformation due to dislocations. That is, the anisotropy of deformation is remarkable. In FIG. 5, the Mg-2.0at%Y alloy where the Y content is at the lower limit of the range of the present invention have substantially overlapping tension and compression graphs and have deformation behaviors which match extremely well, i.e. very high deformation isotropy is realized. Specifically, the yield stress expressing the elastic deformation behavior shown by the straight line part from the origin of the figure and the work hardening rate representing the plastic deformation behavior after yielding match extremely well in tensile deformation and in compressive deformation. Some delay of the rise in stress after yielding is observed in the curve of the compressive deformation, but the overall plastic deformation characteristic is not substantially affected.

In FIG. 6 and FIG. 7, the Mg-2.2at%Y alloy (FIG. 6) and Mg-3.0at% alloy (FIG. 7) where the Y contents are near the lower limit and at the upper limit of the range of the present invention exhibit the above features of the present invention more clearly. The graphs of tensile deformation and compressive deformation substantially overlap. From the above results, in the present invention, the minimum value where a good deformation isotropy can be obtained, that is, 2.0 at%, is the lower limit value of the Y content.

In this way, even if the Y content is near the lower limit of 2.0 at%, so long as it is in the range of the present invention, a sufficiently good deformation isotropy is achieved, while even with an Mg-3.0at%Y alloy where the Y content is at the upper limit value of the range of the present invention (FIG. 7), the isotropy of deformation (match of graphs of tension and compression) is equivalent. Note that the solid solubility limit concentration in the equilibrium state was 3.4 at%, so as the limit concentration for severe working, 3.0 at% is the upper limit. Further, from the purely technical viewpoint, there is no need to make the upper limit of the Y content 3.0 at%, but even if increasing Y further over 3.0 at%, no remarkable improvement in the isotropy of deformation can be expected and just expensive Y is wastefully consumed, so a rise in costs is merely invited. Therefore, in the present invention, the upper limit of the Y content is 3.0 at%.

Table 1 below summarizes the observed values of the grain size, tensile yield stress (A) , compressive yield stress (B) , yield stress ratio (B/A) , first compressive work hardening rate (C) , second compressive work hardening rate (D) , and work hardening rate ratio (D/C) of Mg-Y alloys with different Y contents.

Table 1

In this way, in the Mg alloy of the present invention, it is believed that a high deformation isotropy was achieved due to the following mechanism.

Due to the presence of the nanometer order high Y regions where large atom size Y concentrates, the crystal lattice remarkably becomes distorted, so it becomes difficult for the dislocations to pass through the high Y regions when moving through the basal plane of the hexagonal crystal. As a result, slip no longer occurs preferentially at the basal plane and the slip system at the crystal planes other than the basal plane becomes active.

The presence of remarkable strain of the crystal lattice due to the high Y regions as explained above not only increases the number of the slip deformation directions and increases the freedom of slip deformation, but at the same time twinning deformation predicated on the presence of a specific prescribed relationship in the arrangement of atoms of the crystal lattice becomes - -

difficult. As a result, in a conventional alloy, no twinning deformation which had occurred in compressive deformation occurs and plastic deformation due to slip deformation easily occurs. The reason why anisotropy of deformation occurred was the occurrence of twinning due to compressive deformation. Therefore, in the alloy of the present invention where the occurrence of twinning is prevented, the difference in deformation behavior in tension and compression is greatly reduced or completely eliminated and the isotropy of deformation remarkably rises.

Furthermore, the lattice strain due to the distribution of nanometer order high Y regions preventing the occurrence of twinning in the above way simultaneously functions as resistance to motion of the dislocations responsible for slip^deformation, so act extremely effectively as an alloy strengthening mechanism. The strengthening mechanism in action here is not just strengthening in the grains due to lattice strain in the crystal grains. It also effectively acts for strengthening of the crystal grain boundaries at which the high Y regions are distributed at a higher density than in the grains and contributes to improvement of the ductility of the alloy due to the prevention of intergranular fracture. Of course, grain boundary strengthening is also effective for improving the creep strength at high temperatures. <Impact Compression Test> As a representative example of the present invention, for Mg-2.2at%Y alloy, a test piece was taken from the above extruded material and subjected to an impact compression test at room temperature at a strain rate of 10³/sec. A compressive load was applied up to a nominal strain of 27%, but no cracks etc. occurred in the side faces of the test piece and the piece uniformly deformed. If twinning occurs, as shown in the above FIG. 3 and FIG. 4, a part where the rise in stress slows - 1 -

appears in the stress-strain diagram, cracks and wrinkles occur in the side faces of the test piece, and uniform deformation no longer occurs. Therefore, from the above results, it is learned that even in impact compressive deformation, deformation behavior equal to that in tensile deformation occurred without twinning. INDUSTRIAL APPLICABILITY

According to the present invention, by making the strength and ductility at tensile deformation and compressive deformation match at equal levels, an Mg alloy provided with both high strength and high ductility is provided.

Claims

1. An Mg alloy characterized by consisting of Y: 2.0 to 3.0 at% and a balance of Mg and unavoidable impurities and by having high Y regions of Y concentrations higher than an average concentration distributed at a nanometer order of size and intervals.

2. An Mg alloy as set forth in claim 1, characterized by being an equiaxed grain structure and not having texture.