U.S. patent application number 13/055214 was filed with the patent office on 2011-06-09 for mg-based alloy cold worked member.
This patent application is currently assigned to NATIONAL INSTITUTE FOR MATERIALS SCIENCE. Invention is credited to Akira Kato, Toshiji Mukai, Tetsuya Shoji, Hidetoshi Somekawa.
Application Number | 20110135532 13/055214 |
Document ID | / |
Family ID | 41570420 |
Filed Date | 2011-06-09 |
United States Patent
Application |
20110135532 |
Kind Code |
A1 |
Mukai; Toshiji ; et
al. |
June 9, 2011 |
MG-BASED ALLOY COLD WORKED MEMBER
Abstract
The present invention has as its object to provide an Mg-based
alloy cold worked member which can remarkably lower the load weight
required for cold plastic working and enables practical usage of
the same. The present invention is an Mg-based alloy cold worked
member obtained by cold working an Mg-based alloy to form it into a
predetermined shape, characterized by having a microstructure which
includes crystal grains divided and made finer by cold working.
Inventors: |
Mukai; Toshiji; (Ibaraki,
JP) ; Somekawa; Hidetoshi; (Ibaraki, JP) ;
Shoji; Tetsuya; (Aichi, JP) ; Kato; Akira;
(Aichi, JP) |
Assignee: |
NATIONAL INSTITUTE FOR MATERIALS
SCIENCE
Ibaraki
JP
|
Family ID: |
41570420 |
Appl. No.: |
13/055214 |
Filed: |
July 22, 2009 |
PCT Filed: |
July 22, 2009 |
PCT NO: |
PCT/JP2009/063452 |
371 Date: |
February 19, 2011 |
Current U.S.
Class: |
420/405 ;
420/402 |
Current CPC
Class: |
C22C 23/06 20130101;
C22F 1/00 20130101; C22F 1/06 20130101 |
Class at
Publication: |
420/405 ;
420/402 |
International
Class: |
C22C 23/06 20060101
C22C023/06; C22C 23/00 20060101 C22C023/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 22, 2008 |
JP |
2008-188397 |
Claims
1. An Mg-based alloy cold worked member obtained by cold working an
Mg-based alloy to form it into a predetermined shape, characterized
by having a microstructure which includes crystal grains divided
and made finer by cold working.
2. An Mg-based alloy worked member as set forth in claim 1,
characterized in that the Mg-based alloy forming the member has one
or more types of lanthanoid type rare earth elements added to
it.
3. An Mg-based alloy worked member as set forth in claim 1,
characterized by having an average value of its crystal grain size
of 30 .mu.m or less.
4. An Mg-based alloy worked member as set forth in claim 1,
characterized by being given an equivalent strain by cold working
at room temperature or other temperature of 150.degree. C. or less
of, by absolute value, 0.17 (by nominal compressive strain, 0.15)
or more.
Description
TECHNICAL FIELD
[0001] The present invention relates to an Mg-based alloy to which
yttrium or another lanthanoid-series rare earth element has been
added and relates to an Mg-based alloy which can be easily
plastically worked.
BACKGROUND ART
[0002] In a conventional Mg-based alloy of this type, the plastic
workability in the cold working temperature (room temperature or so
temperature) region was difficult, so while utilization for light
weight materials for structural use etc. has been desired,
realization has been difficult.
SUMMARY OF INVENTION
[0003] The present invention has as its object to provide an
Mg-based alloy cold worked member which can remarkably lower the
load weight required for cold plastic working and enables practical
usage of the same.
[0004] The present invention provides an Mg-based alloy cold worked
member obtained by cold working an Mg-based alloy to form it into a
predetermined shape, characterized by having a microstructure which
includes crystal grains divided and made finer by cold working.
[0005] In the Mg-based alloy worked member of the present
invention, preferably the Mg-based alloy forming the member has one
or more types of lanthanoid-series rare earth elements added to
it.
[0006] Further, in the Mg-based alloy worked member of the present
invention, preferably the average value of the crystal grain size
is 30 .mu.m or less.
[0007] Due to the above such characteristic internal structure, the
anisotropy of deformation normally observed in conventional wrought
alloys such as the AZ31 alloy is eliminated and the demerit that,
for example, the yield stress when a tensile load acts, that is,
the plastic deformation starting stress, having to be 1.2 to 1.4
times the plastic deformation starting stress when a compressive
load acts is eliminated.
[0008] The present alloy has isotropy of deformation. Equal
deformation in all directions is exhibited for a constant load. At
the same time, the load which is required for deformation work does
not depend on the load either and is equal.
BRIEF DESCRIPTION OF DRAWINGS
[0009] FIG. 1 is a high resolution transmission type electron
microscope photograph of the internal microstructure forming the
present alloy (Example 4).
[0010] FIG. 2 is a high resolution transmission type electron
microscope photograph by the Z contrast method of the internal
microstructure forming the present alloy (Example 4).
[0011] FIG. 3 includes a top figure of a photograph which shows the
locations of presence of yttrium atoms, observed by a 3D atom
probe, by dots for the present alloy (Example 4). The bottom figure
is a schematic view based on the distribution of the top figure and
shows the regions where yttrium atoms are segregated at a high
concentration by gray color contour figures.
[0012] FIG. 4 is a graph showing the compressive nominal
stress-nominal strain relationship of the present alloy for the
Mg-0.6 at % Y alloy of Example 4.
[0013] FIG. 5 is a graph for the case where obtaining a test piece
from a member, in the compression test shown in FIG. 4, given a
nominal strain of 0.4 in a direction parallel to extrusion, that
is, compressive deformation until 60% of the initial height, and
performing a static compression test in the same way as the case of
FIG. 4.
[0014] FIG. 6 is a longitudinal cross-sectional view of a die set
for evaluation of cold workability.
[0015] FIG. 7 is a graph using a jig shown in FIG. 6 for evaluating
the cold workability. It shows the results for the materials shown
in Example 1, Example 2, Example 4, and Comparative Example 1.
[0016] FIG. 8 is a photograph of the cross-section of a sample
after shaping. It shows the results of an extrusion speed of 0.0003
mm/sec and a 4.5 ton load.
[0017] FIG. 9 is a photograph of the cross-section of a sample
after shaping. Here, it shows the results of an extrusion speed of
0.03 mm/sec and a 4.5 ton load. The top figure shows the example of
an AZ31 alloy, while the bottom figure shows the example of an
Mg-0.6 at % Y alloy of the present alloy.
[0018] FIG. 10 shows the changes in distribution of orientation of
crystal grains before and after compressive deformation of a
material of Mg-0.6 at % Y extruded at 425.degree. C. and held at
400.degree. C. for 24 hours and the average crystal grain size (d).
Here, it shows the internal microstructure formed before shaping
and after 4% (nominal strain 0.04), 15% (nominal strain 0.15), and
25% (nominal strain 0.25) deformation.
[0019] FIG. 11 shows the internal microstructure formed after
deformation of a material similar to FIG. 10 by 45% (nominal strain
0.45) and average crystal grain size (d).
[0020] FIG. 12 is an enlarged view of the internal microstructure
formed after deformation of a material similar to FIG. 10 by 15%
(nominal strain 0.15).
[0021] FIG. 13 shows the change in microstructure formed inside a
material after cold working of a comparative material, obtained by
extruding the conventional material of an AZ31 alloy at 250.degree.
C., then holding it at 400.degree. C. for 24 hours, by the method
shown in FIG. 6 by a top die at a speed of 0.0003 mm/sec.
[0022] FIG. 14 shows the change in microstructure formed inside a
material after cold working of a material, obtained by extruding
Mg-0.6 at % Y at 320.degree. C., then holding it at 400.degree. C.
for 24 hours, by the method shown in FIG. 6 by a top die at a speed
of 0.0003 mm/sec.
[0023] FIG. 15 shows the change in microstructure formed inside a
material after cold working of a material, obtained by extruding
Mg-0.1 at % Y at 290.degree. C., then holding it at 400.degree. C.
for 24 hours, by the method shown in FIG. 6 by a top die at a speed
of 0.0003 mm/sec.
[0024] FIG. 16 shows, as an example of cold working, the internal
microstructure of a boss-shaped protrusion formed after cold
working a material, obtained by extruding Mg-0.1 at % Y at
290.degree. C. and holding it at 400.degree. C. for 24 hours, and a
material, obtained by extruding Mg-0.3 at % Y at 300.degree. C. and
holding it at 400.degree. C. for 24 hours, by the method shown in
FIG. 6 by a top die at a speed of 0.0003 mm/sec and 3.0 mm/sec.
[0025] FIG. 17 shows, as an example of cold working, the internal
microstructure of a boss-shaped protrusion formed after cold
working a material, obtained by extruding Mg-0.1 at % Y at
290.degree. C. and holding it at 400.degree. C. for 24 hours, and a
material, obtained by extruding Mg-0.3 at % Y at 300.degree. C. and
holding it at 400.degree. C. for 24 hours, by the method shown in
FIG. 6 by a top die at a speed of 3.0 mm/sec.
[0026] FIG. 18 shows, as an example of cold working, the internal
microstructure of a boss-shaped protrusion formed after cold
working a material, obtained by extruding Mg-0.1 at % Y at
290.degree. C. and holding it at 400.degree. C. for 24 hours, by
the method shown in FIG. 6 by a top die at a speed of 0.0003
mm/sec.
[0027] FIG. 19 shows the hardness of a protrusion formed after
shaping by the cold working method described in FIG. 6 in
comparison with parts with little amounts of deformation.
[0028] FIG. 20 shows, as a comparative example, a nominal
stress-nominal strain curve (top figure) obtained when causing
compressive deformation of a material obtained by extruding pure
magnesium at 328.degree. C. and holding it at 400.degree. C. for 24
hours and a nominal stress-nominal strain curve obtained when
causing compressive deformation of a compression test piece,
obtained by stopping deformation at a nominal strain of 0.14, then
machining again, in parallel and perpendicular directions to the
extrusion.
[0029] FIG. 21 shows, as an example, a nominal stress-nominal
strain curve (top figure) obtained when causing compressive
deformation of a material obtained by extruding Mg-0.3 at % Y at
300.degree. C. and holding it at 400.degree. C. for 24 hours and a
nominal stress-nominal strain curve obtained when causing
compressive deformation of a compression test piece, obtained by
stopping deformation at a nominal strain of 0.4, then machining
again, in parallel and perpendicular directions to the
extrusion.
[0030] FIG. 22 shows, as an example, a nominal stress-nominal
strain curve (top figure) obtained when causing compressive
deformation of a material obtained by extruding Mg-1.0 at % Y at
425.degree. C. and holding it at 400.degree. C. for 24 hours and a
nominal stress-nominal strain curve obtained when causing
compressive deformation of a compression test piece, obtained by
stopping deformation at a nominal strain of 0.4, then machining
again, in parallel and perpendicular directions to the
extrusion.
[0031] FIG. 23 shows, as an example, a nominal stress-nominal
strain curve (top figure) obtained when causing compressive
deformation of a material obtained by extruding Mg-0.3 at % Yb at
300.degree. C. and holding it at 450.degree. C. for 24 hours and a
nominal stress-nominal strain curve obtained when causing
compressive deformation of a compression test piece, obtained by
stopping deformation at a nominal strain of 0.4, then machining
again, in parallel and perpendicular directions to the
extrusion.
[0032] FIG. 24 shows, as an example, a nominal stress-nominal
strain curve (top figure) obtained when causing compressive
deformation of a material obtained by extruding Mg-0.3 at % Gd at
300.degree. C. and holding it at 450.degree. C. for 24 hours and a
nominal stress-nominal strain curve obtained when causing
compressive deformation of a compression test piece, obtained by
stopping deformation at a nominal strain of 0.35, then machining
again, in parallel and perpendicular directions to the
extrusion.
[0033] FIG. 25 shows, as a comparative example, the crystal grain
structure of a material obtained by extruding Mg-0.6 at % Y at an
extrusion ratio of 25:1 and a temperature of 320.degree. C. The
black lines in the figure show the boundaries of a crystal
orientation difference of 5.degree. or more as crystal grain
boundaries.
[0034] FIG. 26 shows the results when taking test pieces in the
parallel and perpendicular directions of extrusion from the
material obtained by extrusion of Mg-0.6 at % Y at an extrusion
ratio of 25:1 and temperature of 320.degree. C. shown as a
comparative example in FIG. 25 and testing them by a compression
test at room temperature.
DESCRIPTION OF EMBODIMENTS
[0035] In a preferred embodiment of the present invention, the
Mg-based alloy has an alloy microstructure which is homogeneous as
a whole in 1 .mu.m.sup.3 units and has high Y concentration parts
of average diameters of 2 to 50 nm dispersed irregularly in 1
.mu.m.sup.3.
[0036] In a still more preferable embodiment of the present
invention, the Mg-based alloy has high Y concentration parts of
high concentrations of 1.5 times or more the Y concentration in 1
.mu.m.sup.3 units.
[0037] The internal structure of the material of the present
invention is characterized in that regions in which the yttrium
atoms are present in a concentration higher by 50% or more of the
average concentration in the material, that is, a 1.5 times or
higher concentration, form sizes of average diameters of 2 nm to 50
nm and, furthermore, these high concentration regions are
distributed in the crystal grains of the material at intervals of 2
nm to 50 nm.
[0038] Further, the yttrium atoms distributed in a high
concentration do not form intermetallic compounds with the matrix
of the magnesium atoms, that is, a regular structure, but form a
high concentration, but random distribution.
[0039] The material of the present invention is characterized in
that, by being cold worked at a nominal strain of 0.15 or more (as
absolute value of equivalent strain, 0.17 or more), its internal
crystal microstructure is divided and made finer whereby it is
given crystal grain sizes of average values of 30 .mu.m or
less.
[0040] The Mg-based alloy of the present invention can be used to
produce any long bars, sheet materials, or block materials. It
becomes possible to secure cold workability of magnesium, which had
been considered difficult in the past. It is expected to contribute
much to all sorts of applications as a light weight structural
material.
Examples
Preparation of Alloy
[0041] Yttrium (Y) and pure magnesium (Mg) (purity 99.95%) were
completely melted in an argon atmosphere and cast in iron casting
molds to prepare nine types of Mg--Y alloys having Y contents of
0.1 at %, 0.3 at %, 0.6 at %, 1.0 at %, 1.2 at %, 1.5 at %, 2.0 at
%, 2.2 at %, and 3.0 at %. Table 1 shows these as Examples 1 to 18
and Comparative Example 1.
[0042] The obtained cast alloys were held at a temperature of
500.degree. C. for 24 hours in a furnace (air atmosphere), then
water cooled for solution treatment.
[0043] After this, they were machined to columnar materials of
diameters of 40 mm and lengths of 70 mm.
[0044] These columnar materials were held in containers (in the
air) held at the extrusion temperatures shown in Table 1 for 30
minutes, then extruded at an extrusion ratio of 25:1 as strong
strain hot working. The average equivalent plastic strain, found
from the rate of reduction of cross-section, was 3.7.
[0045] The extruded materials were held isothermally in furnaces of
a temperature of 300 to 550.degree. C. for 24 hours, then were air
cooled outside the furnaces. For the extrusion temperatures, the
temperatures shown in Table 1 were used. The average recrystallized
grain size (.mu.m), tensile yield stress (A), compressive yield
stress (B), yield stress ratio (B/A), and compressive strain at
break were measured. The results are shown together in Table 1.
TABLE-US-00001 TABLE 1 Compressive Average Tensile yield Yield
Extrusion recrystallized yield stress stress Compressive Sample
temp. grain size stress (A) (B) ratio strain No. (.degree. C.)
(.mu.m) (MPa) (MPa) (B/A) at break Alloy (at %) 1 Mg--0.1Y 310 80
87 56 0.64 0.49 2 Mg--0.3Y 310 50 88.2 59 0.67 0.5 3 Mg--0.3Y 310
264 53 44 0.83 0.5 4 Mg--0.6Y 425 44 86 77 0.9 0.51 5 Mg--0.67Y 320
17 97 95 0.98 0.5 6 Mg--0.67Y 320 49 89 76 0.85 0.5 7 Mg--0.67Y 320
174 64 52 0.81 0.48 8 Mg--1.2Y 340 17 119 115 0.97 0.51 9 Mg--1.2Y
340 29 88 87 0.99 0.5 10 Mg--1.2Y 340 193 78 70 0.9 0.41 11
Mg--1.5Y 360 33 100 101 1.01 0.47 12 Mg--1.5Y 360 164 94 91 0.97
0.35 13 Mg--2.0Y 420 37 152.8 144 0.94 0.37 14 Mg--2.2Y 425 240 117
118 1.01 0.32 15 Mg--3.0Y 450 148 156 154 0.99 0.28 Comp. ex. 1
AZ31 250 40 115 0.12 alloy
[0046] FIG. 1 is a high resolution transmission type electron
microscope photograph of the internal microstructure forming the
present alloy. The fine dots forming FIG. 1 show the positions of
present of the component atoms. This photograph is taken from a
direction parallel to a certain crystal plane of the present alloy
(Example 4), so that majority of the points, that is, atoms, are
arranged on a certain line in the structure.
[0047] However, there are partial locations where the arrangement
is disturbed. This is because yttrium atoms with relatively large
atomic radii are dispersed among the magnesium matrix atoms and
distort the array units, that is, the lattice.
[0048] Furthermore, due to the plurality of yttrium atoms being
irregularly concentrated, the lattice distortion becomes
remarkable. As a result, regions of remarkable lattice distortion,
such as shown by the white broken line circles or the white broken
line ellipses in the figure, are formed.
[0049] Therefore, the present alloy is characterized in that the
yttrium atoms do not form regular structures with the magnesium
matrix atoms, that is, so-called "intermetallic compounds", but
form high concentration regions of yttrium.
[0050] The sizes of the lattice distorted regions can be measured
from an electron micrograph such as shown in the illustration.
Based on the results of measurement, it was confirmed that the
lattice distorted regions had an average diameter size of 2 to 50
nm and dispersion interval of 2 to 50 nm. However, in some regions,
formation of unavoidable intermetallic compounds was observed, so
the formation of high concentration regions where over half of the
yttrium atoms are distributed at random is made the characterizing
feature of the present alloy.
[0051] Note that the concentration of yttrium can be made a range
of 0.1 at % to 3.0 at %.
[0052] The method of production of the material comprises
production of an alloy having a predetermined yttrium concentration
by casting etc., extrusion etc. to impart hot an equivalent plastic
strain of 1 or more to the material, then holding it isothermally
at 300 to 550.degree. C. in range.
[0053] FIG. 2 is a high resolution transmission type electron
microscope photograph of the internal microstructure forming the
present alloy (Example 4). This photograph uses the Z contrast
method to show the yttrium atoms, which are heavier atoms than
magnesium matrix atoms, as dots of different contrasts. For
example, the white broken line circles or white broken line
ellipses in the figure show regions where large numbers of yttrium
atoms are concentrated.
[0054] The sizes and dispersion intervals of these regions match
those of the lattice distorted regions shown in FIG. 1, so it is
clear that the characterizing structure of the present alloy is
formed by the yttrium atoms being concentrated at a high
concentration and at random.
[0055] The top figure of FIG. 3 is a photograph which shows the
locations of presence of yttrium atoms observed by a 3D atom probe
by dots for the present alloy (Example 4).
[0056] The bottom figure is a schematic view based on the
distribution of the top figure and shows the regions where yttrium
atoms are segregated at a high concentration by gray color contour
figures.
[0057] Here, the results are shown relating to an alloy containing
0.6 at % of yttrium as an example of the alloy.
[0058] The average concentration of the yttrium in the illustrated
alloy is 0.6 at %. Here, the size and dispersion interval of
regions of 1.0 at % or more concentration regions, that is, regions
where the concentrations are 1.67 times or more of the average
concentration, are shown.
[0059] The sizes of the concentration regions are 5 to 15 nm. The
intervals between regions are also 5 to 15 nm. Results similar to
the strain regions of FIG. 1 and the high concentration regions of
FIG. 2 are shown.
[0060] FIG. 4 is a graph showing the compressive nominal
stress-nominal strain relationship of the present alloy for the
Mg-0.6 at % Y alloy of Example 4. As the directions of the
compression tests, three are selected: one parallel to the
extrusion direction of the extruded material (180.degree.), one
perpendicular to it (90.degree.), and one between the two
(45.degree.).
[0061] The yield stress, that is, the plastic deformation start
stress, is about 60 MPa. By the later work hardening and,
furthermore, by a strain of about 0.12, the gradient of work
hardening becomes gentler. Up to a nominal strain of 0.43 to 0.5 or
so, a state free from breakage is maintained. As clear from a
comparison of the results of deformation in three directions, it is
learned that there is almost no dependency of deformation on
direction.
[0062] The non-directional dependent compressive deformation
performance such as shown above is a property not exhibited by the
conventional wrought materials of AZ31 alloy etc.
[0063] FIG. 5 is a graph for the case where obtaining a test piece
from a member, in the compression test shown in FIG. 4, given a
nominal strain of 0.4 in a direction parallel to extrusion, that
is, compressive deformation until 60% of the initial height, and
performing a static compression test in the same way as the case of
FIG. 4.
[0064] As a result, it was learned that a material given a nominal
compressive strain of 0.4 as an initial strain increases in
deformation start stress to about 200 MPa. Further, as clear from
the test results, the magnitude of the deformation start stress and
the ratio of the subsequent work hardening are similar regardless
of the compression test direction.
[0065] Further, in a material made to deform in the same direction
as the test direction of FIG. 4, it was learned that the nominal
strain at break of a large value of 0.37 was obtained.
[0066] From the findings shown in the above FIG. 4 and FIG. 5, it
was suggested that the present alloy has plastic workability at
room temperature, that is, cold workability, and, furthermore, the
mechanical properties after plastic working are excellent as
well.
[0067] To clarify the cold workability of the present alloy, the
die set such as shown in FIG. 6 was used to evaluate the plastic
workability.
[0068] A test piece before deformation was made a columnar shape of
a diameter of 8 mm and a height of 6 mm. This was set at a bottom
die of tool steel having a columnar cross-section hole of the same
diameter. After that, the top die, which has a columnar
cross-section hole of a diameter of 3 mm at the center axis and an
R part of a radius of 1.5 mm at the shoulders, was brought into
contact with the top surface of the test piece. The top die was
made to move from the top to the bottom of the figure so as to make
the test piece constrained by the bottom die plastically flow along
the center hole provided in the top die to thereby confirm the
shapeability. Note that the surfaces of the test piece and die were
coated with a lubricant of silicone grease.
[0069] In the process of the shaping test, the load weight required
for shaping and the amount of extrusion by the top die were
measured and used as indicators of shapeability.
[0070] If the shapeability of the material is sufficient, a boss of
the same diameter as the top die such as shown in FIG. 6 is formed
into a protruding shape, so from the observation of the
cross-section after working, it is possible to directly confirm the
shapeability and any cracking along with working.
[0071] FIG. 7 is a graph using a jig shown in FIG. 6 for evaluating
the cold workability.
[0072] The results are shown using, as the test pieces, as examples
of the present alloy, the Mg-0.1 at % Y of Example 1, the Mg-0.3 at
% Y of Example 2, the Mg-0.6 at % Y of Example 4, and, as
Comparative Example 1, an AZ31 alloy under the same conditions.
[0073] Here, as the extrusion speeds of the top die, 0.0003 and
0.03 mm/sec were selected.
[0074] Note that, in the present test, the maximum load given was
made 4.5 ton (45 kN).
[0075] As clear from the relationship of the load and the amount of
extrusion deformation, it is learned that the load required for
shaping the present alloy to obtain the same shaping height is
about 20 to 40% lower compared with the case of the conventional
material of AZ31.
[0076] FIG. 8 is a photograph of the cross-section of a sample
after shaping. It shows the results of an extrusion speed of 0.0003
mm/sec and a 4.5 ton load. The top figure shows the case of the
AZ31 alloy shown in Comparative Example 1. The shaping height
including the R part was 1.8 mm.
[0077] The bottom figure shows an example of the Mg-0.6 at % Y
alloy of the present alloy (Example 4). The shaping height was 3.7
mm. A shaping height of at least 2 times that of an AZ31 alloy was
obtained. The shapeability of the present alloy was therefore
confirmed.
[0078] FIG. 9 is a photograph of the cross-section of a sample
after shaping. Here, it shows the results of an extrusion speed of
0.03 mm/sec and a 4.5 ton load. The top figure shows the case of
the AZ31 alloy shown in Comparative Example 1. The shaping height,
including the R part, was 1.4 mm. The bottom figure shows the
Mg-0.6 at % Y alloy of the present alloy (Example 4). The shaping
height was 2.9 mm. A shaping height of at least 2 times that of an
AZ31 alloy was obtained. The shapeability of the present alloy was
therefore confirmed.
[0079] Below, further data showing deformation of the crystals will
be used to clarify the state of the cold working and the reason why
this increases the strength. Experiments will be used to clearly
indicate the numerical limits at which these phenomena occur.
Further, it is shown that similar phenomena occur for rare earth
elements other than Y.
[0080] FIG. 10 shows the changes in distribution of orientation of
crystal grains before and after compressive deformation of a
material of Mg-0.6 at % Y extruded at 425.degree. C. and held at
400.degree. C. for 24 hours and the average crystal grain size.
Here, it shows the internal microstructure formed before shaping
and after 4% (nominal strain 0.04), 15% (nominal strain 0.15), and
25% (nominal strain 0.25) deformation. In a material giving
deformation of a nominal strain of 0.15 or more, compared with the
material before deformation, the average crystal grain size becomes
30 .mu.m or less, that is, becomes finer.
[0081] FIG. 11 shows the internal microstructure formed after
deformation of a material similar to FIG. 10 by 45% (nominal strain
0.45). The boundary lines shown by the black lines in the figure
show parts where the crystal orientation difference is 5 degrees or
more as the crystal grain boundaries. Compared with the
microstructure before deformation shown in FIG. 10, it is learned
that the crystal grain size is made a finder 1/5 size.
[0082] FIG. 12 is an enlarged view of the internal microstructure
formed after deformation of a material similar to FIG. 10 by 15%
(nominal strain 0.15). The changes in crystal orientation on the
lines shown by L and T in the figure are shown by the solid lines
in the right figures. The locations where the density changes, for
example, the parts shown by the white arrows in the figure, clearly
show an increase of the orientation angle by 5 degrees from the
bottom right graph. That is, the cold worked member of the present
invention, by being cold worked, changes in orientation inside
certain crystal grains. Along with the increase of the strain
imparted, the difference in crystal orientation becomes greater.
Finally, crystal grain boundaries are formed, whereby the crystal
grains are divided and the average crystal grain size inside the
material becomes finer, it is shown.
[0083] FIG. 13 shows the change in microstructure formed inside a
material after cold working of a comparative material, obtained by
extruding the conventional material of an AZ31 alloy at 250.degree.
C., then holding it at 400.degree. C. for 24 hours, by the method
shown in FIG. 6 by a top die at a speed of 0.0003 mm/sec. At the
center part D of the worked material, no division of the crystal
grains is seen. Deformation twins are formed as a banded structure
in an oblique direction.
[0084] FIG. 14 shows the change in microstructure formed inside a
material after cold working of a material, obtained by extruding
Mg-0.6 at % Y at 320.degree. C., then holding it at 400.degree. C.
for 24 hours, by the method shown in FIG. 6 by a top die at a speed
of 0.0003 mm/sec. At the center part D of the worked material,
there is no formation of a banded structure in any specific
direction such as deformation twins. New grain boundaries are
formed in random directions, it is learned.
[0085] FIG. 15 shows the change in microstructure formed inside a
material after cold working of a material, obtained by extruding
Mg-0.1 at % Y at 290.degree. C., then holding it at 400.degree. C.
for 24 hours, by the method shown in FIG. 6 by a top die at a speed
of 0.0003 mm/sec. At the center part D of the worked material,
there is no formation of a banded structure in any specific
direction such as deformation twins. New grain boundaries are
formed in random directions, it is learned.
[0086] FIG. 16 shows, as an example of cold working, the internal
microstructure of a boss-shaped protrusion formed after cold
working a material, obtained by extruding Mg-0.1 at % Y at
290.degree. C. and holding it at 400.degree. C. for 24 hours, and a
material, obtained by extruding Mg-0.3 at % Y at 300.degree. C. and
holding it at 400.degree. C. for 24 hours, by the method shown in
FIG. 6 by a top die at a speed of 0.0003 mm/sec and 3.0 mm/sec.
[0087] FIG. 17 shows, as an example of cold working, the internal
microstructure of a boss-shaped protrusion formed after cold
working a material, obtained by extruding Mg-0.1 at % Y at
290.degree. C. and holding it at 400.degree. C. for 24 hours, and a
material, obtained by extruding Mg-0.3 at % Y at 300.degree. C. and
holding it at 400.degree. C. for 24 hours, by the method shown in
FIG. 6 by a top die at a speed of 3.0 mm/sec. In the schematic view
of deformation of FIG. 13, the microstructure of the part shown by
D is shown. It is learned that by a working speed of a speed of 3.0
mm/sec, similar crystal grain refinement occurs.
[0088] FIG. 18 shows, as an example of cold working, the internal
microstructure of a boss-shaped protrusion formed after cold
working a material, obtained by extruding Mg-0.1 at % Y at
290.degree. C. and holding it at 400.degree. C. for 24 hours, by
the method shown in FIG. 6 by a top die at a speed of 0.0003
mm/sec. From the distribution chart of orientation of the
microstructure after deformation, it is learned that the crystal
grain structure becomes finer.
[0089] FIG. 19 shows that the hardness of a protrusion formed after
shaping by the cold working method described in FIG. 6 increases in
comparison with parts with little amounts of deformation. It is
learned that refinement of the crystal grains due to cold working
increases the strength.
[0090] FIG. 20 shows, as a comparative example, a nominal
stress-nominal strain curve (top figure) obtained when causing
compressive deformation of a material obtained by extruding pure
magnesium at 328.degree. C. and holding it at 400.degree. C. for 24
hours and a nominal stress-nominal strain curve obtained when
causing compressive deformation of a compression test piece,
obtained by stopping deformation at a nominal strain of 0.14, then
machining again, in parallel and perpendicular directions to the
extrusion. The yield strength greatly differs and anisotropy of
deformation can be confirmed.
[0091] FIG. 21 shows, as an example, a nominal stress-nominal
strain curve (top figure) obtained when causing compressive
deformation of a material obtained by extruding Mg-0.3 at % Y at
300.degree. C. and holding it at 400.degree. C. for 24 hours and a
nominal stress-nominal strain curve obtained when causing
compressive deformation of a compression test piece, obtained by
stopping deformation at a nominal strain of 0.4, then machining
again, in parallel and perpendicular directions to the extrusion.
It can be confirmed that the anisotropy of the yield strength is
reduced.
[0092] FIG. 22 shows, as an example, a nominal stress-nominal
strain curve (top figure) obtained when causing compressive
deformation of a material obtained by extruding Mg-1.0 at % Y at
425.degree. C. and holding it at 400.degree. C. for 24 hours and a
nominal stress-nominal strain curve obtained when causing
compressive deformation of a compression test piece, obtained by
stopping deformation at a nominal strain of 0.4, then machining
again, in parallel and perpendicular directions to the extrusion.
It can be confirmed that the anisotropy of the yield strength is
reduced.
[0093] FIG. 23 shows, as an example, a nominal stress-nominal
strain curve (top figure) obtained when causing compressive
deformation of a material obtained by extruding Mg-0.3 at % Yb at
300.degree. C. and holding it at 450.degree. C. for 24 hours and a
nominal stress-nominal strain curve obtained when causing
compressive deformation of a compression test piece, obtained by
stopping deformation at a nominal strain of 0.4, then machining
again, in parallel and perpendicular directions to the extrusion.
It can be confirmed that the anisotropy of the yield strength and
rate of work hardening is reduced.
[0094] FIG. 24 shows, as an example, a nominal stress-nominal
strain curve (top figure) obtained when causing compressive
deformation of a material obtained by extruding Mg-0.3 at % Gd at
300.degree. C. and holding it at 450.degree. C. for 24 hours and a
nominal stress-nominal strain curve obtained when causing
compressive deformation of a compression test piece, obtained by
stopping deformation at a nominal strain of 0.35, then machining
again, in parallel and perpendicular directions to the extrusion.
It can be confirmed that the anisotropy of the yield strength and
rate of work hardening is reduced.
[0095] FIG. 25 shows, as a comparative example, the crystal grain
structure of a material obtained by extruding Mg-0.6 at % Y at an
extrusion ratio of 25:1 and a temperature of 320.degree. C. The
black line in the figure shows the interface of a crystal
orientation difference of 5.degree. or more as a crystal grain
boundary. Compared with the example shown in FIG. 11, in the cold
worked microstructure of this comparative example, it is learned
that the division is insufficient at the left part and the center
part in the figure and that coarse crystal grain structures
remain.
[0096] FIG. 26 shows the results when taking test pieces in the
parallel and perpendicular directions of extrusion from the
material obtained by extrusion of Mg-0.6 at % Y at an extrusion
ratio of 25:1 and temperature of 320.degree. C. shown as a
comparative example in FIG. 25 and testing them by a compression
test at room temperature. It is learned that the nominal strain at
the time of break is 0.13 or less and that, while having a
composition similar to the alloys of the working examples shown in
FIG. 4 and FIG. 5, the cold workability is lower. Further, the rate
of work hardening after yielding greatly differs depending on the
direction in which the sample is taken. It is learned that the
nominal stress right before breakage at the time of a compression
test in a direction parallel to extrusion becomes a value close to
two times that in the direction perpendicular to extrusion, so the
anisotropy of deformation is strong.
INDUSTRIAL APPLICABILITY
[0097] According to the present invention, an Mg-based alloy cold
worked member which can remarkably lower the load weight required
for cold plastic working is provided and can be practically
used.
* * * * *