U.S. patent application number 13/635491 was filed with the patent office on 2013-02-14 for magnesium alloy.
The applicant listed for this patent is Toshiji Mukai, Hidetoshi Somekawa. Invention is credited to Toshiji Mukai, Hidetoshi Somekawa.
Application Number | 20130039805 13/635491 |
Document ID | / |
Family ID | 44649029 |
Filed Date | 2013-02-14 |
United States Patent
Application |
20130039805 |
Kind Code |
A1 |
Somekawa; Hidetoshi ; et
al. |
February 14, 2013 |
MAGNESIUM ALLOY
Abstract
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 consist of subgrains and
fine particles are dispersed into the subgrains.
Inventors: |
Somekawa; Hidetoshi;
(Ibaraki, JP) ; Mukai; Toshiji; (Ibaraki,
JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Somekawa; Hidetoshi
Mukai; Toshiji |
Ibaraki
Ibaraki |
|
JP
JP |
|
|
Family ID: |
44649029 |
Appl. No.: |
13/635491 |
Filed: |
March 7, 2011 |
PCT Filed: |
March 7, 2011 |
PCT NO: |
PCT/JP2011/055271 |
371 Date: |
November 2, 2012 |
Current U.S.
Class: |
420/408 ;
420/411 |
Current CPC
Class: |
C22C 23/02 20130101;
C22C 23/06 20130101; C22C 23/04 20130101; C22F 1/06 20130101 |
Class at
Publication: |
420/408 ;
420/411 |
International
Class: |
C22C 23/02 20060101
C22C023/02 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 17, 2010 |
JP |
2010-060144 |
Claims
1-5. (canceled)
6. A magnesium alloy having a chemical composition represented by
Mg-A mass % Zn--B mass % Z (Mg: magnesium, Zn: zinc, Z: one or more
elements other than Mg and Zn) and having an unavoidable impurity,
wherein when 6.ltoreq.A<10, Z is at least one element of Al, Zr,
Ca, Sn, Li, Ag, and a rare earth element and 0<B<10, and
wherein when 0<A<6, Z is Al and 6.ltoreq.B<10, the
magnesium alloy rolled by groove rolling with a strain at a
cross-section reduction ratio of 90% or more and having high angle
grain boundaries, wherein grains surrounded by the high angle grain
boundaries consist of subgrains into which fine particles are
dispersed.
7. The magnesium alloy according to claim 6, wherein the grains
have an average particle diameter of 5 .mu.m or less and the
subgrains have an average particle diameter of 1.5 .mu.m or
less.
8. The magnesium alloy according to claim 7, wherein the grains
having a particle diameter of 5 .mu.m or less possess a fraction of
70% or more.
9. The magnesium alloy according to claim 8, wherein the fine
particles have an average particle diameter of 10 nm to 1
.mu.m.
10. The magnesium alloy according to claim 9, wherein the amount of
the fine particles in the subgrains is 15% or less with the proviso
that the amount is not 0%.
Description
TECHNICAL FIELD
[0001] The present invention relates to a magnesium alloy a main
component of which is magnesium.
BACKGROUND ART
[0002] 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.
[0003] 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.
[0004] 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.
[0005] 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. [0006] Patent
document 1: WO2009/044829 A1
SUMMARY OF THE INVENTION
Problems that the Invention is to Solve
[0007] 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.
Means for Solving the Problems
[0008] 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.
[0009] In the magnesium alloy, it is preferred that an average
grain size of the grains is 5 .mu.m or less and a size of the
subgrains is 1.5 .mu.m or less.
[0010] In the magnesium alloy, it is preferred that a fraction of
the grains having a particle diameter of 5 .mu.m or less constitute
70% or more.
[0011] In the magnesium alloy, it is preferred that the fine
particles have an average particle diameter of 10 nm to 1
.mu.m.
[0012] 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%).
Advantage of the Invention
[0013] The magnesium alloy can achieve both high strength and high
ductility which have conventionally been considered impossible.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] FIG. 1 is a microstructure of the magnesium alloy in Example
1 observed by a transmission electron microscope;
[0015] FIG. 2 is a nominal stress-nominal strain curve obtained
from a tensile test at room temperature of the magnesium alloy;
[0016] FIG. 3 is a microstructure of the magnesium alloy in Example
2 observed by a transmission electron microscope;
[0017] FIG. 4 is a microstructure of the magnesium alloy in Example
3 observed by a transmission electron microscope;
[0018] FIG. 5 is a microstructure of the magnesium alloy in Example
6 observed by a SEM/EBSD;
[0019] FIG. 6 is a microstructure of the magnesium alloy in
Comparative Example 4 observed by a transmission electron
microscope;
[0020] FIG. 7 is a photograph of the appearance of a magnesium
alloy during a process;
[0021] FIG. 8 is a microstructure of the magnesium alloy in Example
8 observed by a transmission electron microscope; and
[0022] FIG. 9 is a microstructure of the magnesium alloy in Example
8 observed by a SEM/EBSD.
EMBODIMENTS
[0023] 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.
[0024] 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:
[0025] 1) the magnesium alloy has a high angle grain boundary,
[0026] 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
[0027] 3) the fine particles, such as quasicrystal particles or
second phase deposited particles, disperse into the subgrains.
[0028] 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.
[0029] The grains surrounded by the high angle grain boundary
preferably have an average size of 5 .mu.m or less, more preferably
3 .mu.m or less. Further, it is preferred that the grains having a
particle diameter of 5 .mu.m 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 .mu.m 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.
[0030] 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 .mu.m or less, preferably 1 .mu.m or less,
more preferably 0.5 .mu.m 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.
[0031] The fine particles dispersed in the crystal subgrains
preferably have an average particle diameter of 10 nm to 1 .mu.m,
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 .mu.m, 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 FIG. 7, a fracture
and a crack occur during the materials. In such a case, the
production of a sound billet having features 1) and 2)
above-mentioned is quite difficult. The interface between the fine
particle and the magnesium matrix phase may be either a coherency
or an in coherency. Whether the interface between them is a
coherency or an incoherency is determined by the chemical
composition constituting the fine particles or a method for forming
the material. The fine particles, e.g., intermetallic compounds,
are formed from solute atoms which are not capable of being
dissolved into magnesium or are not completely dissolved into
magnesium. Further, the fine particles include quasicrystal
particles. In the quasicrystal particles, not only the interface
between the quasicrystal and the magnesium matrix phase is the
coherency, but also the arrangement of crystals is not present. The
chemical composition of the quasicrystal particles is represented
by Mg--Zn--RE, Mg--Zn--Al, or the like.
[0032] 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.
[0033] 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.
[0034] The applying to processing strain, such as severe plastic
deformation, is an effective to produce the magnesium alloys.
[0035] 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).
[0036] 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.degree. C. for 5 to 120
minutes.
[0037] 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%.
[0038] 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 .mu.m
is increased. When the cross-section reduction ratio is more than
90%, the fraction of grains which is the size of 5 .mu.m is more
than 90%. Further, when the cross-section reduction ratio is 90% or
more, the average size of subgrains is 1.5 .mu.m or less. In
addition, the fine particles having an average particle diameter of
10 nm to 1 .mu.m are dispersed into the subgrains with fraction of
15% or less (with the proviso that the density is not 0%).
[0039] 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.
[0040] 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.
Example 1
[0041] 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.degree. 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%.
[0042] 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. FIG. 1 is an example of the microstructure
of the grains surrounded by high-angle grain boundary. It is found
that the grain boundary is not clear and that the microstructure is
formed from subgrains having a small misorientation angles. In FIG.
1, the symbol of S indicates subgrains. Regions having a similar
pattern and contrast also indicate crystal subgrains. Further, it
is found that the crystal subgrains have an average particle
diameter of about 1 .mu.m. In FIG. 1, the symbol of 1 indicates a
quasicrystal particle, which confirms that fine particles having a
particle diameter of 100 nm are dispersed in the subgrains.
[0043] 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.times.10.sup.-3 s.sup.-1. FIG. 2 shows a nominal
stress-nominal strain curve obtained from a tensile test at room
temperature. Further, the mechanical properties are shown in Table
1. As a yield stress, an offset yield stress at a 0.2% strain was
used. The yield anisotropy ratio is close to 1, which shows that
the rolled magnesium alloy shows isotropic deformation. The
magnesium alloy in Example 1 has been subjected to strain
processing at almost the same cross-section reduction ratio as that
in the below-mentioned Comparative Example 1, but it is found that
the magnesium alloy in Example 1 exhibits a 34% higher tensile
strength than that of the magnesium alloy in Comparative Example 1.
In addition, as compared to the below-mentioned Comparative Example
4 in which the magnesium alloy has a structure which has subgrains
without any dispersion of fine particle, the magnesium alloy in
Example 1 show improvement of the yield anisotropy. In the
commercial extruded magnesium alloy, in Comparative Example 4, zinc
and aluminum atoms are dissolved in magnesium and, therefore, it is
presumed that fine particles are unlikely to be deposited in
Comparative Example 4.
Example 2
[0044] 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.degree. C. FIG. 3 is an example of the
microstructural observation by TEM. A microstructure is similar to
that shown in FIG. 1, i.e., a microstructure formed from subgrains
S. Further, the average size of the subgrains is about 0.5 .mu.m
and the quasicrystal particles I as fine particles having an
average particle diameter of about 50 nm are found to be disposed
in the subgrains. 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 FIG. 2 and Table 1. It is
found that the magnesium alloy in Example 2 exhibits a 37% higher
tensile strength than that in the below-mentioned Comparative
Example 2. This is resulted that Comparative Example 2 was produced
by warm extrusion. Therefore, it is presumed that subgrains in
Comparative Example 2 were unlikely to be formed, and the amount of
the subgrains in the alloy was small. Further, the magnesium alloy
in Example 2 exhibits a 6% higher tensile strength than that in
Comparative Example 4 in which the magnesium alloy has a
subgrainstructure without dispersion of fine-particle. This result
indicates that the presence of the fine particles contributes to a
further increase of the strength.
Example 3
[0045] 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.degree. C. FIG. 4 is an example of the
microstructural observation by TEM. A microstructure similar to
that shown in FIG. 1, i.e., a microstructure formed from subgrains.
Further, the average size of the subgrains is about 0.5 .mu.m and
the quasicrystal particles as fine particles having an average
particle diameter of about 50 nm are found to be dispersed into the
subgrains. 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 FIG. 2 and Table 1. It is
found that the magnesium alloy in Example 3 exhibits a 59% higher
tensile strength than that in the below-mentioned Comparative
Example 3. Further, the magnesium alloy in Example 3 exhibits a 7%
higher tensile strength than that in Comparative Example 4 which
has subgrains without dispersion of fine particles. This indicates
that the presence of the fine particles contributes to a further
increase of the strength.
Example 4
[0046] 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.degree.
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., Mg.sub.2Zn, which have incoherent interface
between the particle and the matrix (magnesium).
Example 5
[0047] 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.degree. 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.
Example 6
[0048] 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.degree. C., and
the groove rolling was repeated 15 times, which corresponds to the
total cross-section reduction ratio of 92%. FIG. 5 is an example of
the microstructural observation using a SEM/EBSD. In FIG. 5, RD
indicates the direction parallel to the groove rolling and TD
indicates the direction perpendicular to the groove rolling. FIG. 5
shows a crystal orientation analysis using an EBSD and the grain
which is surrounded by high-angle brain boundary, i.e.,
misorientation angle of more than 15.degree. is indicated by a
group of black curves. In FIG. 5, G is one grain, which is
surrounded by high-angle grain boundary. These grains had an
average size of 1.7 .mu.m. The grains, which are surrounded by high
angle grain boundary, are not a mixture of fine and coarse sizes.
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 6
which has dispersion of fine quasicrystal phase particles improves
the yield anisotropy, as compared to that in Comparative Example 5
which does not include the fine particles but is produced by the
same total cross-sectional reduction in Example 6. The dispersion
of the quasicrystal particles in the crystalsubgrains was confirmed
by observation using a TEM.
Example 7
[0049] 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.degree. 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
(Mg.sub.2Zn) which is an incoherent interface between the particle
and magnesium matrix.
Example 8
[0050] 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.degree. 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. FIGS. 8 and 9
are the microstructural observation of the magnesium alloy in
Example 8 using a TEM and a SEM/EBSD, respectively. In FIG. 8, the
fine particle, marked by P, with an average size of 100 nm is
dispersed into the matrix. The fine particles are second phase
particles (Mg.sub.17Al.sub.12) which have an incoherent interface
between the particle and magnesium. In FIG. 9, the grain, which is
surrounded by the high-angle grain boundary and marked by G, is an
average of 2.2 .mu.m. These grains are uniformly distributed.
Further, the magnesium alloy in Example 8 exhibits a 15% higher
tensile strength and a 30% or more higher compressive strength than
those in Comparative Example 5, which indicates that the presence
of the fine particles contributes to a further increase of the
strength.
Comparative Example 1
[0051] 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.degree.
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 FIG. 2
and Table 1.
Comparative Example 2
[0052] 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 FIG. 2 and Table 1.
Comparative Example 3
[0053] 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 FIG. 2 and Table 1.
Comparative Example 4
[0054] 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.degree.
C. FIG. 6 is microstructural observation of the microstructure of
the rolled material using a TEM. The microstructure in this alloy
is formed from subgrains, which is the same as that in the
magnesium alloy in Example 1, but fine particles are not dispersed
into the subgrains. With respect to the rolled alloy obtained in
Comparative Example 4, tensile and compression tests were performed
under the same conditions as those in Example 1. The results are
shown in Table 1.
Comparative Example 5
[0055] 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.degree. 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.
TABLE-US-00001 TABLE 1 Crystal control Metal element added (mass %)
Processing High angle Trade Cross-section temperature grain
boundary Zn Y Al Zr name Method reduction ratio (%) (.degree. C.)
(.mu.m) Example 1 7.5 1.7 Groove rolling 95 350 2 Example 2 8 4
Groove rolling 95 200 1.5 Example 3 6 3 Groove rolling 95 200 1.5
Example 4 6 0.5 ZK60 Groove rolling 95 200 2 Example 5 8 4 Groove
rolling 92 300 1.8 Example 6 6 3 Groove rolling 92 300 1.7 Example
7 6 0.5 ZK60 Groove rolling 92 200 3 Example 8 1 6 AZ61 Groove
rolling 92 200 2.2 Comparative 7.5 1.7 Extrusion 94 230 1 Example 1
Comparative 8 4 Extrusion 94 230 1.5 Example 2 Comparative 6 3
Extrusion 94 230 1.5 Example 3 Comparative 1 3 AZ31 Groove rolling
95 200 2.5 Example 4 Comparative 1 3 AZ31 Groove rolling 92 200
<2 Example 5 Fine particles Maximum Average tensile Elongation-
Yield stress (MPa) Subgrains particle Fraction strength to-fail-
Compres- Anisotropy (.mu.m) diameter (nm) (%) (MPa) ure (%) Tensile
sion ratio* Example 1 1 100 6.5 420 9.2 405 352 0.87 Example 2 0.5
50 9 452 7.0 434 403 0.93 Example 3 0.5 50 6 460 9.5 438 401 0.92
Example 4 0.5 75 6.5 440 12.4 429 385 0.9 Example 5 0.5 50 9 410
10.8 375 329 0.88 Example 6 0.5 50 6 405 11.5 370 320 0.86 Example
7 1 75 6.5 412 14.6 392 352 0.9 Example 8 1 50 5 438 9.4 423 395
0.93 Comparative None** 75 6.5 337 17.9 303 Example 1 Comparative
None** 100 9 370 15.3 316 Example 2 Comparative None** 100 6 347
20.1 276 Example 3 Comparative 0.3 None** None** 422 11.0 409 337
0.82 Example 4 Comparative 0.4 None** None** 386 11.5 369 289 0.78
Example 5 *Anisotropy ratio: Compressive yield stress/Tensile yield
stressN **"None" does not mean that the absence of grains or
particles was confirmed but means that the presence of grains or
particles could not be confirmed by the analysis with the level of
sensitivity at the time of filing of the present application.
INDUSTRIAL APPLICABILITY
[0056] 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.
* * * * *