U.S. patent application number 13/638267 was filed with the patent office on 2013-06-06 for magnesium alloy sheet material.
The applicant listed for this patent is Yoshihito Kawamura, Masafumi Noda, Hiroshi Sakurai. Invention is credited to Yoshihito Kawamura, Masafumi Noda, Hiroshi Sakurai.
Application Number | 20130142689 13/638267 |
Document ID | / |
Family ID | 44762825 |
Filed Date | 2013-06-06 |
United States Patent
Application |
20130142689 |
Kind Code |
A1 |
Kawamura; Yoshihito ; et
al. |
June 6, 2013 |
MAGNESIUM ALLOY SHEET MATERIAL
Abstract
Disclosed is a magnesium alloy material having excellent tensile
strength and favorable ductility. Therefore, the magnesium alloy
sheet material formed by rolling a magnesium alloy having a long
period stacking order phase crystallized at the time of casting
includes in a case where a sheet-thickness traverse section of an
alloy structure is observed at a substantially right angle to the
longitudinal direction by a scanning electron microscope, a
structure mainly composed of the long period stacking order phase,
in which at least two or more .alpha.Mg phases having thickness in
the observed section of 0.5 .mu.m or less are laminated in a
layered manner with the sheet-shape long period stacking order
phase.
Inventors: |
Kawamura; Yoshihito;
(Kumamoto, JP) ; Noda; Masafumi; (Kumamoto,
JP) ; Sakurai; Hiroshi; (Kanagawa, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
Kawamura; Yoshihito
Noda; Masafumi
Sakurai; Hiroshi |
Kumamoto
Kumamoto
Kanagawa |
|
JP
JP
JP |
|
|
Family ID: |
44762825 |
Appl. No.: |
13/638267 |
Filed: |
March 31, 2011 |
PCT Filed: |
March 31, 2011 |
PCT NO: |
PCT/JP2011/058305 |
371 Date: |
December 10, 2012 |
Current U.S.
Class: |
420/411 ;
420/580 |
Current CPC
Class: |
C22C 23/04 20130101;
C22C 30/06 20130101; C22F 1/06 20130101; C22C 23/06 20130101; C22F
1/00 20130101; C22C 23/00 20130101 |
Class at
Publication: |
420/411 ;
420/580 |
International
Class: |
C22C 23/00 20060101
C22C023/00; C22C 30/06 20060101 C22C030/06 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2010 |
JP |
2010-084516 |
Claims
1. A magnesium alloy sheet material formed by rolling a magnesium
alloy having a long period stacking order phase crystallized at the
time of casting, comprising: in a case where a sheet-thickness
traverse section of an alloy structure is observed at a
substantially right angle to the longitudinal direction by a
scanning electro microscope, a structure mainly composed of the
long period stacking order phase, in which at least two or more
.alpha.Mg phases having thickness in the observed section of 0.5
.mu.m or less are laminated in a layered manner with the
sheet-shape long period stacking order phase.
2.-7. (canceled)
8. The magnesium alloy sheet material according to claim 1,
comprising 2 atom % of Zn, 2 atom % of Y, and the remaining part
including Mg and unavoidable impurities.
9. The magnesium alloy sheet material according to claim 1, wherein
regarding the structure mainly composed of the long period stacking
order phase, in a case where a section cut in the sheet thickness
direction along the rolling direction of the magnesium alloy sheet
material is observed from the perpendicular direction of the
section, a range of an area ratio of the long period stacking order
phase is 36% or more.
10. The magnesium alloy sheet material according to claim 1,
wherein the long period stacking order phase in the laminated
structure has maximum thickness in the observed section of 9 .mu.m
or less.
11. The magnesium alloy sheet material according to claim 1,
wherein in the laminated structure, the sheet-shape long period
stacking order phase and the .alpha.Mg phases having
smaller-thickness in the observed section than the long period
stacking order phase are laminated in a layered manner.
12. The magnesium alloy sheet material according to claim 1,
wherein the sheet-shape long period stacking order phase in the
laminated structure has minimum thickness in the observed section
of 0.25 .mu.m or more.
13. The magnesium alloy sheet material according to claim 1,
wherein the laminated structure includes an intermetallic
compound.
14. The magnesium alloy sheet material according to claim 1,
wherein at least part of the laminated structure is shear-deformed
or compression-deformed.
15. The magnesium alloy sheet material according to claim 1,
wherein at least part of the laminated structure is curved or bent.
Description
TECHNICAL FIELD
[0001] The present invention relates to a magnesium alloy sheet
material. In detail, the present invention relates to a magnesium
alloy sheet material having high tensile strength and high
ductility.
BACKGROUND ART
[0002] In general, a magnesium alloy has the lowest density and the
lightest weight and also has high tensile strength among
practically utilized alloys. Thus, magnesium alloy is increasingly
applied to a casing of an electric product, a wheel, a suspension,
and parts around an engine of an automobile, and the like.
[0003] Particularly, high mechanical properties are required for
parts used in relation to automobiles. Thus, as a magnesium alloy
to which elements such as Gd and Zn are added, a material of a
specific form is manufactured by a single roll method and a rapid
solidification method (for example, refer to Patent Document 1 and
Patent Document 2).
[0004] However, regarding the magnesium alloy described above,
although high mechanical properties are obtained with a specific
manufacturing method there is a problem that special facilities are
required in order to realize the specific manufacturing method and
moreover, productivity is low. Furthermore, there is a problem that
applicable members are limited.
[0005] Conventionally, there is a proposed technique that in a case
of manufacturing a magnesium alloy, even when highly-productive
normal melting and casting and then plastic working (extrusion) are
performed without using the special facilities or processes as
described in Patent Document 1 and Patent Document 2 above,
practically useful mechanical properties are obtained (for example,
refer to Patent Document 3).
CITATION LIST
[0006] Patent Document [0007] Patent Document 1: Japanese Published
Unexamined Patent Application No. H6-41701 [0008] Patent Document
2: Japanese Published Unexamined Patent Application No. 2002-256370
[0009] Patent Document 3: Japanese Published Unexamined Patent
Application No. 2006-97037
SUMMARY OF THE INVENTION
Problem to be Solved by the Invention
[0010] A magnesium alloy having a long period stacking order phase
(hereinafter, referred to as the "LPSO" phase) disclosed in Patent
Document 3 is excellent in balance between tensile strength and
ductility. Although a cast material does not have very high tensile
strength, by performing plastic working such as extrusion,
improvement in tensile strength can be realized without lowering
ductility very much. That is, even when plastic working of a large
working ratio such as extrusion is performed, sufficient ductility
can be obtained.
[0011] However, when tensile strength is to be improved with
plastic working at the time of manufacturing a sheet material or a
rod material as a material, ductility is consequently lowered.
[0012] For example, FIG. 6 shows yield strength, tensile strength,
and elongation of a cast material of a Mg.sub.96ZnY.sub.3 alloy and
hot-rolled materials (R1, P2). It is found that the hot-rolled
material (P2) has higher yield strength and higher tensile strength
but smaller elongation than the hot-rolled material (R1). It should
be noted that FIG. 6 is described in Non-patent Document (R. G. Li,
D. Q. Fang, J. An, Y. Lu, Z. Y. Cao, Y. B. Liu, MATERIALS
CHARACTERIZATION 60 (2009) 470-475).
[0013] FIG. 7 shows mechanical properties of various materials.
When the mechanical properties of the same alloys of different
processes are compared, it is found that alloys realizing high
yield strength and high tensile strength have small elongation. It
should be noted that FIG. 7 is described in Non-patent Document (T.
Itoi et al./Scripta Materialia 59 (2008) 1155-1158).
[0014] As described above, both the characteristics of tensile
strength and ductility are not easily improved at the same
time.
[0015] The present invention has been made in view of these
circumstances, and an object thereof is to provide a magnesium
alloy sheet material capable of realizing improvement in tensile
strength and at the same time, also realizing improvement in
ductility.
Means for Solving the Problem
[0016] In order to achieve the above object, a magnesium alloy
sheet material of the present invention is a magnesium alloy sheet
material formed by rolling a magnesium alloy having a long period
stacking order phase crystallized at the time of casting,
including, in a case where a sheet-thickness traverse section of an
alloy structure is observed at a substantially right angle to the
longitudinal direction by a scanning electron microscope, a
structure mainly composed of the long period stacking order phase,
in which at least two or more .alpha.Mg phases having thickness in
the observed section of 0.5 .mu.m or less are laminated in a
layered manner with the sheet-shape long period stacking order
phase.
[0017] Here, in a case where the sheet-thickness traverse section
of the alloy structure is observed at a substantially right angle
to the longitudinal direction by the scanning electron microscope,
the structure mainly composed of the long period stacking order
phase, in which at least two or more .alpha.Mg phases having
thickness in the observed section of 005 .mu.m or less are
laminated in a layered mariner with the sheet-shape long period
stacking order phase is provided, improvement in tensile strength
can be realized and at the same time, improvement in ductility can
also be realized, so that excellent tensile strength and favorable
ductility can be realized.
[0018] That is, the LPSO phase is formed in a sheet shape (plate
shape). Thus, when comparing with a case where the LPSO phase is
formed in a block shape, at least part of the LPSO phase is brought
into a structure state that the part is easily shear-deformed or
compression-deformed in accordance with rolling. In addition, since
at least part of the LPSO phase is in the structure state that the
part is easily shear-deformed or compression-deformed, a kink band
is easily introduced into the LPSO phase, and as a result,
excellent tensile strength can be realized. In addition, since at
least part of the LPSO phase is in the structure state that the
part is easily shear-deformed or compression-deformed, favorable
ductility can also be realized.
[0019] In a case where maximum sheet thickness of the LPSO phase in
the laminated structure is 9 .mu.m or less, generally 10% or more
elongation can be realized.
[0020] Furthermore, in a case where the laminated structure
(specifically, the LPSO phase or the .alpha.Mg phases) includes an
intermetallic compound (such as Mg.sub.3Zn.sub.3Y.sub.2), the
structure state is such that the intermetallic compound is
sandwiched by the sheet-shape (plate-shape) LPSO phase. Since the
intermetallic compound easily facilitates deformation of the LPSO
phase, such a structure state is a state that the LPSO phase is
easily deformed. Therefore, the kink band is easily introduced into
the LPSO phase, so that excellent tensile strength can be
realized.
[0021] When at least part of the laminated structure is
shear-deformed or compression-deformed, at least part of the
laminated structure is curved or bent. Such a curved or bent
structure can be a cause for realizing excellent tensile
strength.
[0022] Here, the "sheet-shape LPSO phase in a case where the
sheet-thickness traverse section of the alloy structure is observed
at a substantially right angle to the longitudinal direction by the
scanning electron microscope" indicates a structure as shown in
FIG. 8, for example. A light gray point in FIG. 8 indicates the
LPSO phase. It should be noted that FIG. 8(a) is a scanning
electron micrograph of a magnification of 150.times., FIG. 8(b) is
a scanning electron micrograph of a magnification of 2,500.times.,
and FIG. 8(c) is a scanning electron micrograph of a magnification
of 3,000.times..
[0023] The "sheet-thickness traverse section" indicates a section
whose thickness is reduced by rolling, the section which is
substantially parallel to the forward direction of the sheet
material at the time of rolling (section at a substantially right
angle to a mill roll). Furthermore, the "longitudinal direction of
the sheet-thickness traverse section" indicates the direction which
is substantially parallel to the forward direction of the sheet
material at the time of rolling (direction at a substantially right
angle to the rolling roll). The "substantially right angle to the
longitudinal direction of the sheet-thickness traverse section"
indicates the thickness direction of the sheet-thickness traverse
section.
[0024] That is, the "sheet-thickness traverse section is observed
at a substantially right angle to the longitudinal direction"
indicates that the "`section whose thickness is reduced by rolling,
the section which is substantially parallel to the forward
direction of the sheet material at the time of rolling` is observed
in the `thickness direction of the section` at the substantially
right angle to the `direction which is substantially parallel to
the forward direction of the sheet material at the time of
rolling.`"
[0025] The "magnesium alloy in which the LPSO phase is crystallized
at the time of casting" includes Mg--Zn-RE (RE=Y, Dy, Ho, Er, Tm),
Mg--Cu-RE (RE=Y, Gd, Tb, Dy, Ho, Er, Tm), Mg--Ni-RE (RE=Y, Sm, Gd,
Tb, Dy, Ho, Er), Mg--Co-RE (RE=Y, Dy, Ho, Er, Tm), and Mg--Al--Gd.
It should be noted that RE indicates a rare-earth element.
[0026] Furthermore, the "magnesium alloy in which the LPSO phase is
crystallized at the time of casting" is not necessarily limited to
a three-component system as exemplified above but may be a
four-component system in which another additive element is added to
the magnesium alloy described above or a larger component
system.
Effects of the Invention
[0027] With the magnesium alloy sheet material of the present
invention, the improvement in tensile strength can be realized and
at the same time, the improvement in ductility can also be
realized.
BRIEF DESCRIPTION OF THE DRAWINGS
[0028] FIG. 1A (a) is a micrograph (1) showing a crystalline
structure of a Mg.sub.96Zn.sub.2Y.sub.2 alloy serving as a
magnesium alloy sheet material of the present invention;
[0029] FIG. 1A(b) is a micrograph (2) showing the crystalline
structure of the Mg.sub.96Zn.sub.2Y.sub.2 alloy serving as the
magnesium alloy sheet material of the present invention;
[0030] FIG. 1A(c) is a micrograph (3) showing the crystalline
structure of the Mg.sub.96Zn.sub.2Y.sub.2 alloy serving as the
magnesium alloy sheet material of the present invention;
[0031] FIG. 1B(a) is a micrograph (4) showing the crystalline
structure of the Mg.sub.96Zn.sub.2Y.sub.2 alloy serving as the
magnesium alloy sheet material of the present invention;
[0032] FIG. 1B(b) is a micrograph (5) showing the crystalline
structure of the Mg.sub.96Zn.sub.2Y.sub.2 alloy serving as the
magnesium alloy sheet material of the present invention;
[0033] FIG. 1B(c) is a micrograph (6) showing the crystalline
structure of the Mg.sub.96Zn.sub.2Y.sub.2 alloy serving as the
magnesium alloy sheet material of the present invention;
[0034] FIG. 2 is a flowchart for illustrating a manufacturing
method of the magnesium alloy sheet material;
[0035] FIG. 3 is a micrograph for illustrating an intermetallic
compound Mg.sub.3Zn.sub.3Y.sub.2;
[0036] FIG. 4A is a micrograph (1) showing a crystalline structure
of the magnesium alloy material formed by performing rolling S4 on
a plastically-worked item to which no heating step is
performed;
[0037] FIG. 4B(a) is a micrograph (2) showing the crystalline
structure of the magnesium alloy material formed by performing the
rolling S4 on the plastically-worked item to which no heating step
is performed;
[0038] FIG. 41B(b) is a micrograph (3) showing the crystalline
structure of the magnesium alloy material formed by performing the
rolling S4 on the plastically-worked item to which no heating step
is performed;
[0039] FIG. 4B (c) is a micrograph (4) showing the crystalline
structure of the magnesium alloy material formed by performing the
rolling S4 on the plastically-worked item to which no heating step
is performed;
[0040] FIG. 5 is a graph showing 0.2% yield strength, tensile
strength, and elongation of Example and Comparative Example;
[0041] FIG. 6 is a graph showing yield strength, tensile strength,
and elongation of a cast material of a Mg.sub.96ZnY.sub.3 alloy and
hot-rolled materials (R1, R2);
[0042] FIG. 7 is a table showing mechanical properties of various
materials;
[0043] FIG. 8(a) is a micrograph (1) for illustrating one example
of a sheet-shape structure;
[0044] FIG. 8(b) is a micrograph (2) for illustrating one example
of the sheet-shape structure;
[0045] FIG. 8(c) is a micrograph (3) for illustrating one example
of the sheet-shape structure;
[0046] FIG. 9 is a graph showing a relationship between a heating
time and tensile yield strength and a relationship between the
heating time and room temperature elongation;
[0047] FIG. 10(a) is a diagram (1) for illustrating a relationship
between maximum thickness of an LPSO phase in a lamellar structure
and elongation of the magnesium alloy sheet material;
[0048] FIG. 10(b) is a diagram (2) for illustrating the
relationship between the maximum thickness of the LPSO phase in the
lamellar structure and elongation of the magnesium alloy sheet
material;
[0049] FIG. 11A(a) is a scanning electron micrograph (1) of the
magnesium alloy sheet material formed by rolling an excessively
heated material;
[0050] FIG. 11A(b) is a scanning electron micrograph (2) of the
magnesium alloy sheet material formed by rolling the excessively
heated material;
[0051] FIG. 11B (a) is a scanning electron micrograph (3) of the
magnesium alloy sheet material formed by rolling the excessively
heated material; and
[0052] FIG. 11B (b) is a scanning electron micrograph (4) of the
magnesium alloy sheet material formed by rolling the excessively
heated material.
MODE FOR CARRYING OUT THE INVENTION
[0053] Hereinafter, an embodiment of the present invention will be
described with reference to the drawings for understanding of the
present invention.
[0054] FIGS. 1A and 1B are scanning electron micrographs showing a
crystalline structure of a Mg.sub.96Zn.sub.2Y.sub.2 alloy serving
as a magnesium alloy sheet material of the present invention. In
FIGS. 1A and 1B, a .alpha.Mg phase is black, an LPSO phase is gray,
and a Mg.sub.3Zn.sub.3Y.sub.2 is white.
[0055] It should be noted that in the present embodiment,
description will be given taking the Mg.sub.96Zn.sub.2Y.sub.2 alloy
as an example. However, the present invention is not limited to
such an alloy composition. For example, another three-component
system or a four-component system in which another additive element
is added may be adopted.
[0056] As clear from FIGS. 1A and 1B, the magnesium alloy sheet
material to which the present invention is applied has an LPSO
phase and .alpha.Mg phases, and the LPSO phase and the .alpha.Mg
phases are formed in a lamellar manner. However, not all the
structures are lamellar structures but a region shown by reference
sign X in FIG. 1A(c) is not the lamellar structure.
[0057] It should be noted that the LPSO phase is a precipitate
precipitated in a grain and a grain boundary of a magnesium alloy,
which is a structural phase that sequence of bottom surface atomic
layers in an HCP structure is repeated in the bottom surface normal
direction with a long period order, that is, a long period stacking
order phase. By precipitation of this LPSO phase, mechanical
properties of the magnesium alloy sheet material (tensile strength,
0.2% yield strength, and elongation) are improved.
[0058] The LPSO phase has a sheet-shape (plate-shape) structure
(regions shown by reference sign S in FIG. 15(b)). The .alpha.Mg
phase is placed in a gap between the sheet-shape (plate-shape)
structure. That is, the sheet-shape (plate-shape) structure is
laminated as multiple layers in the LPSO phase.
[0059] Specifically, the lamellar structure described above in the
magnesium alloy sheet material to which the present invention is
applied (refer to reference sign S in FIG. 1B (b) is mainly
composed of the LPSO phase, and in a case where a sheet-thickness
traverse section is observed at a substantially right angle to the
longitudinal direction by a scanning electron microscope, the
plurality of .alpha.Mg phases having thickness in the observed
section of 0.5 .mu.m or less and the sheet-shape (plate-shape) LPSO
phase are laminated in a layered manner. It should be noted that in
a case where the sheet-thickness traverse section observed at a
substantially right angle to the longitudinal direction by the
scanning electron microscope, the sheet-shape (plate-shape) LPSO
phase has thickness of 0.25 .mu.m or more in the observed
section.
[0060] Regarding the lamellar structure described above (refer to
reference sign S in FIG. 1B(b)), by appropriately heating a
material thereof (such as an extrusion material) before rolling,
the structure of the LPSO phase can be controlled to have a desired
sheet shape (plate shape).
[0061] FIG. 9(a) shows a "relationship between a heating time and
tensile yield strength," and FIG. 9(b) shows a "relationship
between the heating time and room temperature elongation." It
should be noted that a heating temperature is 480.degree. C. As
clear from the "relationship between the heating time and the room
temperature elongation" shown in FIG. 9(b), the elongation is not
improved by simply heating but there is a need for appropriately
heating in such a manner that a thin sheet material after rolling
can realize large elongation.
[0062] FIG. 10(a) shows a "relationship between maximum thickness
of the LPSO phase in the lamellar structure and elongation of the
magnesium alloy sheet material." As clear from FIG. 10(a), in a
case where the structure is refined so that the maximum thickness
in the observed section of the LPSO phase in the lamellar structure
is 9 .mu.m or less, generally 10% or more elongation can be
obtained.
[0063] That is, by appropriately heating before rolling, it is
extremely important technically that the maximum thickness in the
observed section of the LPSO phase in the lamellar structure after
rolling is 9 .mu.m or less.
[0064] It should be noted that the "thickness in the observed
section of the LPSO phase" indicates length in the perpendicular
direction to the longitudinal direction of the sheet-shape
(plate-shape) LPSO phase (direction of arrow shown in FIG.
10(b)).
[0065] A heating condition before rolling is appropriately
selected. Then, even with the structure in which the thickness in
the observed section of the LPSO phase in the lamellar structure
looks large, in a case where confirmation is performed with a
magnification of the scanning electron microscope being increased,
the .alpha.Mg phases of thin films of 0.1 .mu.m or less than 0.1
.mu.m form a laminated structure together with the LPSO phase. That
is, a multilayer structure in which the LPSO phase of a thin film
and the .alpha.Mg phases having smaller thickness in the observed
section than the LPSO phase are laminated can be confirmed.
[0066] Meanwhile, by insufficient heating, the sheet-shape
(plate-shape) LPSO phase cannot sufficiently be formed. By
excessive heating such as a long heating time, the thickness in the
observed section of the sheet-shape (plate-shape) LPSO phase is
increased, so that a formation frequency of the layer structure
with the thin .alpha.Mg phases is lowered (refer to FIGS. 11A and
11B).
[0067] FIGS. 11A and 11B show scanning electron micrographs of the
magnesium alloy sheet material formed by rolling an excessively
heated material. It should be noted that in order to improve
convenience in visual recognition, FIGS. 11A(a) and 11B(a) show
states in which a contrast of the LPSO phase is enhanced and FIGS.
11A(b) and 11B(b) show states in which a contrast of the compound
is enhanced.
[0068] In the magnesium alloy sheet material to which the present
invention is applied, by appropriately heating the material thereof
before rolling as in a manufacturing method described below, the
structure is controlled so that the thickness in the observed
section of the LPSO phase in the lamellar structure, in other
words, the thickness in the observed section of the LPSO phase not
sandwiching the .alpha.Mg phase of a thin film of 0.5 .mu.m or less
is 8 .mu.m at maximum.
[0069] The LPSO phase has the sheet shape (plate-shape) structure.
Thus, when comparing with an LPSO phase having a block shape
structure, at least part of the LPSO phase is easily shear-deformed
or compression-deformed in accordance with rolling. It should be
noted that the fact that at least part of the LPSO phase is easily
shear-deformed or compression-deformed in accordance with rolling
is clear from the fact that part of the lamellar structure of the
LPSO phase and .alpha.Mg phases is curved or bent as described
below.
[0070] Since at least part of the LPSO phase is in a structure
state that the part is easily shear-deformed or
compression-deformed in accordance with rolling, a kink band is
easily introduced into the LPSO phase as a result, so that
excellent tensile strength can be realized. Since at least part of
the LPSO phase is in the structure state that the part is easily
shear-deformed or compression-deformed, in accordance with rolling,
favorable ductility can also be realized.
[0071] It should be noted that the LPSO phase not only has the
sheet-shape (plate-shape) structure but also sometimes has a
block-shape structure as in a region shown by reference sign Y in
FIG. 1A(b), for example. That is, a structure shape of the LPSO
phase is a sheet shape (plate-shape) or a mixture of a sheet shape
(plate-shape) and a block shape.
[0072] It is found that in both the LPSO phase and the .alpha.Mg
phases of the lamellar structure, the structure is totally curved.
This is thought to be because the structure or part of the
structure is curved or bent due to shear-deformation or
compression-deformation of the sheet-shape (plate-shape) LPSO phase
and the .alpha.Mg phases sandwiched by such a sheet-shape
(plate-shape) LPSO phase (region shown by reference sign T in FIG.
1B(b)). It should be noted that curving or bending of the lamellar
structure can be a cause for realizing excellent tensile
strength.
[0073] Furthermore, Mg.sub.3Zn.sub.3Y.sub.2 is minutely spread in
the LPSO phase or the .alpha.Mg phases (regions shown by reference
sign Z in FIGS. 1A(b) and 1A(c) and regions shown by reference sign
T and reference sign U in FIG. 1B(c)).
[0074] The intermetallic compound Mg.sub.3Zn.sub.3Y.sub.2 is in a
structure state that the compound is sandwiched by the LPSO phase.
The LPSO phase has the sheet-shape (plate-shape) structure.
Therefore, the intermetallic compound Mg.sub.3Zn.sub.3Y.sub.2
facilitates deformation of the LPSO phase. Thus, as a result of
facilitation of the deformation of the LPSO phase, the kink band is
easily introduced into the LPSO phase, so that excellent tensile
strength can be realized.
[0075] As described above, in the magnesium alloy sheet material of
the present invention, the IPSO phase has the sheet-shape
(plate-shape) structure and is in the structure state that the LPSO
phase is easily shear-deformed or compression-deformed in
accordance with rolling, and the intermetallic compound
Mg.sub.3Zn.sub.3Y.sub.2 facilitates the deformation of the LPSO
phase. Thus, improvement in tensile strength can be realized and at
the same time, improvement in ductility can also be realized.
[0076] In the magnesium alloy sheet material of the present
invention, the LPSO phase is minutely spread by appropriate heating
in order to obtain large elongation, and without destroying the
LPSO phase by strong shear-deformation or compression-deformation
by rolling serving as the following step, distortion, that is, kink
deformation is effectively given to the LPSO phase. Thus, a
reinforcing mechanism of the LPSO phase can sufficiently be
activated. Therefore, the magnesium alloy sheet material with the
same working ratio of rolling but having larger elongation can be
obtained.
[0077] Hereinafter, the manufacturing method of the magnesium alloy
sheet material of the present invention will be described.
[0078] FIG. 2 is a flowchart for illustrating the manufacturing
method of the magnesium alloy sheet material of the present
invention. As shown in FIG. 2, in the manufacturing method of the
magnesium alloy sheet material, of the present invention, casting
is first performed in a casting step S1. In the casting step S1, a
Mg--Zn--Y alloy containing Zn and Y, and the remaining part
including Mg and unavoidable impurities is cast, so as to form a
cast material containing the LPSO phase and the .alpha.Mg
phases.
[0079] It should be noted that a forming method of the cast
material may be any method such as a method of high-frequency
induction melting in an Ar gas atmosphere (refer to Example 1 of
International Publication No, 2007/111342) and a method for melting
a magnesium alloy while making a CO.sub.2 gas flow into an iron
crucible using an electric furnace, and charging the alloy into an
iron casting mold (refer to Example 3 of International Publication
No, 2007/111342).
[0080] It is found that in a case where the Mg % Zn.sub.2Y.sub.2
alloy is cast, the intermetallic compound Mg.sub.3Zn.sub.3Y.sub.2
of approximately 0.5 .mu.m to 2.0 .mu.m is formed at a time of
casting. It should be noted that FIG. 3(a) is a scanning electron
micrograph showing a crystalline structure of an annealed material
of the Mg.sub.96Zn.sub.2Y.sub.2 alloy at 400.degree. C. for one
hour, FIG. 3(b) is a scanning electron micrograph showing a
crystalline structure of the annealed material of the
Mg.sub.9Zn.sub.2Y.sub.2 alloy at 450.degree. C. for one hour, FIG.
3(c) is a scanning electron micrograph showing a crystalline
structure of the annealed material of the Mg.sub.96Zn.sub.2Y.sub.2
alloy at 500.degree. C. for one hour, and it is found that the
intermetallic compound Mg.sub.2Zn.sub.3Y.sub.2 is formed. It should
be noted that the points indicated by reference signs a in the
micrographs shown in FIGS. 3(a) to 3(c) indicate intermetallic
compounds Mg.sub.3Zn.sub.3Y.sub.2.
[0081] Next, a plastic working step S2 is performed on the cast
material. Plastic working of this plastic working step S2 is, for
example, extrusion, casting, rolling, drawing, or the like. In a
plastically-worked item obtained by performing plastic working on
the cast material containing the LPSO phase, tensile strength, 0.2%
yield strength, and elongation are improved in comparison to before
plastic working.
[0082] Successively, by performing a heating step S3 of heating the
plastically-worked item, the LPSO phase is formed in a sheet shape
(plate shape). As one example, heating is performed within a
temperature range of 400.degree. C. or more and 500.degree. C. or
leis and within a time range of 0.5 hours or more and 10 hours or
less, for example.
[0083] It should be noted that the LPSO phase is formed in a sheet
shape (plate shape) by the heating step S3. However, it is only
necessary to form the LPSO phase in a sheet shape (plate shape)
prior to a rolling step S4 described below in order to realize the
crystalline structure shown in FIGS. 1A and 1B. Therefore, as long
as the LPSO phase can be formed in a sheet shape (plate shape), the
heating step S3 is not always required but any method may be used.
Similarly, since it is only necessary to form the LPSO phase in a
sheet shape (plate shape), the present invention is not limited to
the temperature range and the time range exemplified above.
[0084] Thereafter, by performing the rolling S4 on the
plastically-worked item heated so as to form the LPSO phase in a
sheet shape (plate shape), the magnesium alloy sheet material of
the present invention as shown in FIGS. 1A and 1B can be
obtained.
[0085] FIGS. 4A and 4B are micrographs showing a crystalline
structure of the magnesium alloy sheet material formed by
performing the rolling S4 on the plastically-worked item to which
no heating step is performed. In FIGS. 4A and 4B, the .alpha.Mg
phase is black, the LPSO phase is gray, and Mg.sub.3Zn.sub.3Y.sub.2
is white.
[0086] As clear from FIGS. 4A and 4B, regarding the magnesium alloy
sheet material formed by performing the rolling S4 on the
plastically-worked item to which no heating step S3 is performed
and in which the LPSO phase is not formed in a sheet shape (plate
shape), the LPSO phase and the .alpha.Mg phases are formed in a
lamellar manner.
[0087] However, as clear from FIGS. 4A(b) and 4A(c), regarding the
sheet-shape structure of the magnesium alloy material formed by
performing the rolling S4 on the plastically-worked item to which
no heating step S3 is performed and in which the LPSO phase is not
formed in a sheet shape (plate shape), the LPSO phase is formed in
a block shape, and the LPSO phase minutely spread in the .alpha.Mg
phases is extremely small. As clear from FIGS. 4E (b) and 4E (c),
the LPSO phase is straight and no curved or bent part is found.
[0088] It should be noted that the manufacturing method of the
magnesium alloy sheet material described above is only one example,
and the magnesium alloy sheet material may be manufactured by
various other manufacturing methods as a matter of course. The
magnesium alloy of the present invention is not limited to the
alloy obtained by the manufacturing method described above.
EXAMPLE
[0089] Hereinafter, an example and a comparative example of the
present invention will be described. It should be noted that the
example shown below is only one example and does not limit the
present invention.
Example
[0090] First, as a magnesium alloy sheet material of the example of
the present invention, a Mg--Zn--Y alloy containing 2 atom % of Zn,
2 atom % of Y, and the remaining part including Mg and unavoidable
impurities was melted in a high-frequency melting furnace. Next,
the heated and melted material was cast by a mold, so that an ingot
(cast material) of .phi.69 mm.times.L200 mm was produced.
Furthermore, plastic working (extrusion) was performed at an
extrusion temperature of 350.degree. C. at an extrusion ratio of
10, so that the ingot was made into a sheet form. Successively,
one-hour heating (annealing) was performed at a heating temperature
of 100.degree. C. to 500.degree. C., so that an LPSO phase was
formed in a sheet shape (plate shape). Thereafter, rolling was
performed, so that a test piece was produced.
[0091] A result of a tensile test performed on the magnesium alloy
sheet material obtained in such a way at a room temperature and an
evaluation of mechanical properties is shown in FIG. 5(b). It
should be noted that reference sign A in FIG. 5 indicates 0.2%
yield strength, reference sign B in FIG. 5 indicates tensile
strength, and reference sign C in FIG. 5 indicates ductility.
Comparative Example
[0092] Next, as a magnesium alloy sheet material of the comparative
example, a Mg--Zn--Y alloy containing 2 atom % of Zn, 2 atom % of
1, and the remaining part including Mg and unavoidable impurities
was melted in a high-frequency melting furnace. Next, the heated
and melted material was cast by a mold, so that an ingot (cast
material) of (.phi.69 mm.times.L200 mm was produced. Furthermore,
plastic working (extrusion) was performed at an extrusion
temperature of 350.degree. C. at an extrusion ratio of 10, so that
the ingot was made into a sheet form. Thereafter, without forming
an LPSO phase in a sheet shape (plate shape), rolling was
performed, so that a test piece was produced.
[0093] A result of a tensile test performed on the magnesium alloy
sheet material obtained in such a way at the room temperature and
an evaluation of mechanical properties is shown in FIG. 5(a). It
should be noted that reference sign A in FIG. 5 indicates 0.2%
yield strength, reference sign B in FIG. 5 indicates tensile
strength, and reference sign C in FIG. 5 indicates ductility.
[0094] As clear from FIG. 5, it is found that in the magnesium
alloy sheet material of the example of the present invention, both
0.2% yield strength and tensile strength are improved in comparison
to the magnesium alloy sheet material of the comparative example.
It is found that ductility is also improved. That is, with the
magnesium alloy sheet material of the example of the present
invention, tensile strength and ductility are improved at the same
time without changing an alloy composition in the magnesium alloy
sheet material containing the LPSO phase.
* * * * *