U.S. patent application number 14/390833 was filed with the patent office on 2015-03-26 for magnesium alloy, magnesium alloy member and method for manufacturing same, and method for using magnesium alloy.
The applicant listed for this patent is NATIONAL INSTITUTE FOR MATERIALS SCIENCE. Invention is credited to Akira Kato, Toshiji Mukai, Yoshiaki Osawa, Alok Singh, Hidetoshi Somekawa, Kota Washio.
Application Number | 20150083285 14/390833 |
Document ID | / |
Family ID | 49673314 |
Filed Date | 2015-03-26 |
United States Patent
Application |
20150083285 |
Kind Code |
A1 |
Somekawa; Hidetoshi ; et
al. |
March 26, 2015 |
MAGNESIUM ALLOY, MAGNESIUM ALLOY MEMBER AND METHOD FOR
MANUFACTURING SAME, AND METHOD FOR USING MAGNESIUM ALLOY
Abstract
A magnesium alloy of the present invention has the chemical
composition that contains 0.02 mol % or more and less than 0.1 mol
% of at least one element selected from yttrium, scandium, and
lanthanoid rare earth elements, and magnesium and unavoidable
impurities accounting for the remainder. A magnesium alloy member
of the present invention is produced by hot plastic working of the
magnesium alloy in a temperature range of 200.degree. C. to
550.degree. C., followed by an isothermal heat treatment performed
in a temperature range of 300.degree. C. to 600.degree. C. The
magnesium alloy is preferred for use in applications such as in
automobiles, railcars, and aerospace flying objects. The magnesium
alloy and the magnesium alloy member can overcome the yielding
stress anisotropy problem, and are less vulnerable to the rising
price of rare earth elements.
Inventors: |
Somekawa; Hidetoshi;
(Ibaraki, JP) ; Osawa; Yoshiaki; (Ibaraki, JP)
; Mukai; Toshiji; (Ibaraki, JP) ; Singh; Alok;
(Ibaraki, JP) ; Washio; Kota; (Aichi, JP) ;
Kato; Akira; (Aichi, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NATIONAL INSTITUTE FOR MATERIALS SCIENCE |
Ibaraki |
|
JP |
|
|
Family ID: |
49673314 |
Appl. No.: |
14/390833 |
Filed: |
May 28, 2013 |
PCT Filed: |
May 28, 2013 |
PCT NO: |
PCT/JP2013/064755 |
371 Date: |
October 6, 2014 |
Current U.S.
Class: |
148/667 ;
148/420; 420/402; 420/405 |
Current CPC
Class: |
C22C 1/02 20130101; C22F
1/06 20130101; C22C 23/00 20130101; C22C 23/06 20130101; C22F 1/00
20130101 |
Class at
Publication: |
148/667 ;
420/402; 420/405; 148/420 |
International
Class: |
C22C 23/00 20060101
C22C023/00; C22C 23/06 20060101 C22C023/06; C22F 1/06 20060101
C22F001/06 |
Foreign Application Data
Date |
Code |
Application Number |
May 31, 2012 |
JP |
2012-124127 |
Claims
1. A magnesium alloy comprising 0.02 mol % or more and less than
0.1 mol % of at least one element selected from yttrium, scandium,
and lanthanoid rare earth elements, and Mg and unavoidable
impurities accounting for the remainder.
2. A magnesium alloy member comprising the magnesium alloy of the
chemical composition of claim 1, wherein the crystal structure of
the member is an equiaxial grain structure with no texture.
3. (canceled)
4. The magnesium alloy member according to claim 2, wherein the
average grain size is 10 .mu.m or more.
5. The magnesium alloy member according to claim 2, wherein a
compressional nominal strain of 0.4 or more is applied by cold
working performed in a temperature range of from room temperature
to 150.degree. C.
6. The magnesium alloy member according to claim 2, wherein the
average grain size of the magnesium alloy after cold working
performed in a temperature range of from room temperature to
150.degree. C. is 80% or less of the average grain size of an
unworked magnesium alloy.
7. The magnesium alloy member according to claim 2, wherein the
strength and hardness of the member after applying nominal strain
by cold working performed in a temperature range of from room
temperature to 150.degree. C. are 15% greater than strength and
hardness of the undeformed ones.
8. A method for producing a magnesium alloy member, the method
comprising: hot plastic working of a magnesium alloy that contains
0.02 mol % or more and less than 0.1 mol % of at least one element
selected from yttrium, scandium, and lanthanoid rare earth
elements, and Mg and unavoidable impurities accounting for the
remainder, the hot plastic working being performed in a temperature
range of 200.degree. C. to 550.degree. C.; and an isothermal heat
treatment of the magnesium alloy in a temperature range of
300.degree. C. to 600.degree. C. after the hot plastic working.
9. A method for using a magnesium alloy that contains 0.02 mol % or
more and less than 0.1 mol % of at least one element selected from
yttrium, scandium, and lanthanoid rare earth elements, and Mg and
unavoidable impurities accounting for the remainder, the method
using the magnesium alloy as a wrought magnesium member after hot
plastic working performed in a temperature range of 200.degree. C.
to 550.degree. C., and a subsequent isothermal heat treatment
performed in a temperature range of 300.degree. C. to 600.degree.
C.
Description
TECHNICAL FIELD
[0001] The present invention relates to magnesium alloys that
contain trace amounts of yttrium, scandium, and lanthanoid rare
earth elements, and to magnesium alloy members that allow for easy
plastic working in cold and room temperature ranges.
[0002] The present invention also relates to a method for
manufacturing a magnesium alloy member that allows for easy cold
working, and that is preferred for use in applications such as in
automobiles, railcars, aerospace flying objects, and housings of
electronic devices.
BACKGROUND ART
[0003] These types of magnesium alloys are desired in applications
where light structural members are needed. Examples of such
structural member applications include automobiles, railcars,
aerospace flying objects, and housings of electronic devices.
However, use of magnesium alloys as structural members has not been
realized because of the considerable difficulties involved in the
plastic working in cold and room temperature ranges. Wrought
magnesium alloys produced by processes such as press-rolling and
extrusion are also problematic in terms of yielding stress
anisotropy, because the basal plane {0001} crystal orientation
becomes in line with the working direction, and creates a large
difference between the tensile and compression yielding stresses.
As used herein, "cold temperature" means ordinary temperature or a
temperature below the recrystallization temperature of the
material. The cold working temperatures of magnesium alloys are
typically 200.degree. C. or less.
[0004] PTL 1 and PTL 2 disclose wrought magnesium alloys that
contain 0.1 to 1.5 mol % of yttrium. These wrought magnesium alloys
advantageously overcome the yielding stress anisotropy problem, and
have excellent cold workability. A problem, however, is that these
materials contain yttrium, and are vulnerable to the rising price
of yttrium.
[0005] PTL 3 and PTL 4 disclose rolled magnesium alloys that
contain 0.01 to 0.5 mol % of yttrium. The advantage of these rolled
magnesium alloys is the low yttrium content. However, the basal
plane is in line with the press-roll direction (PTL 4, FIG. 1), and
it is not difficult to imagine that a large difference occurs
between the tensile and compression yielding stresses.
[0006] PTL 5 and PTL 6 disclose rolled magnesium alloys that
contain only trace amounts of yttrium for easy workability. These
rolled magnesium alloys contain 6 to 16 mass % of lithium, and the
.beta. phase of the BCC (body-centered cubic lattice) structure is
dispersed in the a phase of the HCP (hexagonal close-packed)
structure to improve workability. However, the use of the active
element lithium severely impairs the corrosion resistance of the
material, and poses a safety problem.
[0007] PTL 7 discloses a magnesium alloy in which quasicrystal
grains are dispersed in the magnesium matrix in order to reduce
yielding stress anisotropy. However, this magnesium alloy is a
Mg--Zn--Re alloy, containing rare earth elements in a content of
0.2 to 1.5 mol %. A problem, then, is that the material is
vulnerable to the rising price of rare earths. There is indeed a
need to reduce the rare earth content.
[0008] PTL 8 discloses a wrought magnesium alloy that contains 0.03
to 0.54 mol % of yttrium. This wrought magnesium alloy has an
average magnesium crystal grain diameter of 1.5 .mu.m or less, and
a high concentration of yttrium is segregated in the vicinity of
the grain boundary to improve material strength. The solute element
remains at high concentration in the vicinity of the grain boundary
when the size of matrix is fine and the percentage volume of the
grain boundary is high. However, the solute element exists in a
solid solution state not in the vicinity of the crystal grain
boundary but inside the size of matrix in applications where the
crystals have coarse grain diameters (for example, 10 .mu.m or
more). The material cannot have high strength in this case.
CITATION LIST
Patent Literature
[0009] PTL 1: WO2010/010965
[0010] PTL 2: WO2008/117890
[0011] PTL 3: JP-A-2010-13725
[0012] PTL 4: JP-A-2008-214668
[0013] PTL 5: JP-A-2003-226929
[0014] PTL 6: JP-A-9-41066
[0015] PTL 7: JP-A-2010-222645
[0016] PTL 8: Japanese Patent No. 4840751
SUMMARY OF INVENTION
Technical Problem
[0017] In magnesium alloys, strength and ductility are improved by
making fine grains using press-rolling, extrusion, and other
processes that apply strain, as with the case of other metallic
materials. However, the basal plane {0001} becomes in line with the
working direction, specifically a basal plane texture is formed
during the hot working for reasons attributed to the magnesium
crystal structure. For example, the crystal orientation of the
basal plane of press-rolled or extruded magnesium aligns parallel
to the press-roll or extrusion direction. This is problematic in
terms of yielding stress anisotropy, because the compression
yielding stress is only 50% to 60% of the tensile yielding stress.
There have been attempts to overcome this problem by dispersing
quasicrystal grains (PTL 7) or producing alloys (PTLs 1 to 6).
However, all of these techniques involve addition of 0.1 mol % or
more of rare earth elements, and are vulnerable to the rising price
of rare earths.
Solution to Problem
[0018] According to a first aspect of the present invention, there
is provided a magnesium alloy that comprises 0.02 mol % or more and
less than 0.1 mol % of yttrium, scandium, or lanthanoid rare earth
elements, and Mg and unavoidable impurities accounting for the
remainder. The magnesium alloy has a homogenous composition, and a
homogenous crystal structure with an average grain size of several
micrometers and several ten micrometers.
[0019] According to a second aspect of the present invention, there
is provided a magnesium alloy member that is produced by hot
plastic working of the magnesium alloy of the chemical composition
of the first aspect of the invention in a temperature range of
200.degree. C. to 550.degree. C., followed by an isothermal heat
treatment performed in a temperature range of 300.degree. C. to
600.degree. C. The isothermal heat treatment is a process by which
a magnesium alloy sample is placed in a maintained constant
temperature bath, maintained for a predetermined time period, and
slowly cooled in air outside of the bath. The magnesium alloy
member may be a wrought magnesium member such as a plate member, a
rod member, and a pipe member.
[0020] A third aspect of the present invention is the magnesium
alloy member according to the second aspect in which the crystal
structure of the member is an equiaxial grain structure with no
texture. Equiaxial grain means a three-dimensionally isotropic
crystal grain structure that does not stretch or flatten
unidirectionally. Texture, or crystal texture as it is also called,
refers to a distribution state of the crystal lattice orientation
(crystal orientation) of each crystal grain present in a
polycrystalline material such as metal. For example, solidifying a
cubical crystal metal forms a preferred orientation [100]. In the
case of magnesium, the basal plane {0001} tends to align in the
strain applying direction, as noted above.
[0021] A fourth aspect of the present invention is the magnesium
alloy member according to the second or third aspect in which the
average grain size is 10 .mu.m or more.
[0022] A fifth aspect of the present invention is the magnesium
alloy members according to any one of the second to fourth aspect
in which a compressional nominal strain of 0.4 or more is applied
by cold working performed in a temperature range of from room
temperature (here and below, room temperature means 15.degree. C.
to 35.degree. C.) to 150.degree. C.
[0023] A sixth aspect of the present invention is the magnesium
alloy members according to any one of the second to fifth aspect in
which the average grain size of the magnesium alloy after cold
working performed in a temperature range of from room temperature
to 150.degree. C. is 80% or less of the initial average grain size
(undeformed magnesium alloy).
[0024] A seventh aspect of the present invention is the magnesium
alloy member according to the third aspect in which the strength
and hardness of the member after applying nominal strain by cold
working performed in a temperature range of from room temperature
to 150.degree. C. are 15% greater than strength and hardness of the
undeformed ones.
[0025] A first method of the present invention is a method for
producing a magnesium alloy member, the method comprising: hot
plastic working of a magnesium alloy that contains 0.02 mol % or
more and less than 0.1 mol % of at least one element selected from
yttrium, scandium, and lanthanoid rare earth elements, and Mg and
unavoidable impurities accounting for the remainder, the hot
plastic working being performed in a temperature range of
200.degree. C. to 550.degree. C.; and an isothermal heat treatment
of the magnesium alloy in a temperature range of 300.degree. C. to
600.degree. C. after the hot plastic working.
[0026] A second method of the present invention is a method for
using a magnesium alloy that contains 0.02 mol % or more and less
than 0.1 mol % of at least one element selected from yttrium,
scandium, and lanthanoid rare earth elements, and Mg and
unavoidable impurities accounting for the remainder, the method
comprising using the magnesium alloy as a wrought magnesium member
after hot plastic working performed in a temperature range of
200.degree. C. to 550.degree. C., and a subsequent isothermal heat
treatment performed in a temperature range of 300.degree. C. to
600.degree. C.
Advantageous Effects of Invention
[0027] The present invention induces room temperature
recrystallization (grain refining) by controlling the dispersion
state of one or more elements selected from yttrium, scandium, and
lanthanoid rare earth elements in a magnesium alloy. This makes it
possible to develop excellent compressional deformation
characteristics. The magnesium alloy member of the present
invention overcomes the yielding stress anisotropy problem with its
random crystal orientation distribution (after working), and has
the same yielding stress for the tensile and compressional
deformation with the maintained high strength. Further, the
magnesium alloy member of the present invention does not break even
under a large applied compressional strain in excess of 50%, and
has excellent deformability. Because of the considerably low
yttrium, scandium, and lanthanoid rare earth element content, the
magnesium alloy of the present invention is less vulnerable to the
material price of yttrium, scandium, and lanthanoid rare earth
elements as compared to conventional rare earth-containing
magnesium alloys.
BRIEF DESCRIPTION OF DRAWINGS
[0028] FIG. 1 is a photographic representation showing the
appearance of a material when the hot working temperature is in the
appropriate range.
[0029] FIG. 2 represents the nominal stress-nominal strain curve
obtained after the room-temperature tensile and compression testing
of extruded material Mg-0.05Y and extruded and heat-treated
material Mg-0.05Y.
[0030] FIG. 3 represents the nominal stress-nominal strain curve
obtained after the room-temperature compression testing of extruded
material Mg--Y alloy.
[0031] FIG. 4 represents the nominal stress-nominal strain curve
obtained after the room-temperature compression testing of extruded
and heat-treated material Mg--Y alloy.
[0032] FIG. 5 represents the nominal stress-nominal strain curve
obtained after the room-temperature compression testing of cast and
heat-treated material Mg-1 mol % Y.
[0033] FIG. 6 is a photographic representation of the observed
scanning electron micrograph/electron backscatter diffraction image
of extruded and heat-treated material Mg-0.03Y.
[0034] FIG. 7 shows a pole figure image of the region observed in
FIG. 5, in which ED and TD are directions parallel to and
perpendicular to extrusion direction, respectively.
[0035] FIG. 8 is a photographic representation of extruded and
heat-treated material Mg-0.03Y as observed by scanning electron
microscopy/electron backscatter diffraction after applying 20%
compressional strain.
[0036] FIG. 9 is a photographic representation of extruded and
heat-treated material Mg-0.03Y as observed by scanning electron
microscopy/electron backscatter diffraction after applying 50%
compressional strain.
[0037] FIG. 10 is a photographic representation showing the
appearance of a material at a low hot working temperature.
DESCRIPTION OF EMBODIMENTS
[0038] The magnesium alloy of the present invention contains at
least one element selected from yttrium, scandium, and lanthanoid
rare earth elements, as mentioned above. A magnesium alloy
containing yttrium, and an alloy member of such an alloy will be
described below as embodiments of the magnesium alloy and the
magnesium alloy member of the present invention.
[0039] For the magnesium alloy member of the present invention to
exhibit effect, hot plastic working (hereinafter, also referred to
as "hot working") of a magnesium alloy is required to segregate
yttrium to a grain boundary, and the yttrium needs to be diffused
in the grain interior by isothermal heat treatment. The procedures
are as follows.
[0040] For the magnesium alloy member of the present invention to
exhibit effect, the magnesium alloy contains yttrium in 0.02 mol %
or more and less than 0.1 mol %, and magnesium and unavoidable
impurities accounting for the reminder. The yttrium content is
preferably 0.025 mol % or more and less than 0.1 mol %, more
preferably 0.025 mol % or more and less than 0.05 mol %. When the
yttrium content is 0.02 mol %, the yttrium exists at
19.5.times.10.sup.-10 m radius intervals. This value corresponds to
the magnitude about three times the Burgers vector of magnesium,
and represents a value that limits the interactions of lattice
defects such as dislocations in terms of atomic binding theory. The
grain size that allows the yttrium to homogenously segregate to a
grain boundary by the hot working becomes coarser as the yttrium
content decreases. It is, however, difficult to obtain the effect
because the estimated average grain size after the hot working is
10 .mu.m or more. Here, the Burgers vector represents the distorted
direction of atoms around a dislocation line introduced as a
crystallographic linear crystal defect. In edge dislocations, the
dislocation line and the Burgers vector are perpendicular to each
other, whereas these are parallel to each other in screw
dislocations.
[0041] The hot plastic working temperature is preferably
200.degree. C. to 550.degree. C., more preferably 250.degree. C. to
350.degree. C. When the working temperature is below 200.degree.
C., the low working temperature makes dynamic recrystallization
less likely to occur. FIG. 1 is a photographic representation
showing the appearance of a material when the hot working
temperature is in the appropriate range. FIG. 10 is a photographic
representation showing the exterior of a material at a low hot
working temperature. By comparing FIG. 1 and FIG. 10, it can be
seen that an appropriate magnesium alloy member can be produced by
setting the hot working temperature in the appropriate temperature
range. Above 550.degree. C., the high working temperature makes it
difficult to produce an average grain size of 10 .mu.m or less.
There is also a potential problem in mold lifetime such as in
extrusion. The hot working is typically extrusion, forging,
press-rolling, or drawing. However, any plastic working may be
used, as long as strain can be applied. The equivalent plastic
strain during strain application is 1.5 or more, preferably 2.0 or
more. When the equivalent plastic strain is less than 1.5, a
sufficient strain cannot be applied, and a mixed structure of
coarse grains and fine grains appears, making it difficult to
homogenously segregate the yttrium in the vicinity of the grain
boundary. With the isothermal heat treatment of a cast material
alone without the hot working, the yttrium does not homogenously
diffuse and disperse in the grain interior, and fracture occurs at
a nominal strain of about 0.3 as shown in FIG. 5. That is, the
effects of the present invention cannot be obtained.
[0042] The temperature of the isothermal heat treatment is
preferably equal to or greater than the hot working temperature, so
that the yttrium segregated at the grain boundary can diffuse in
the grain interior. Specifically, temperature of the isothermal
heat treatment is preferably 300.degree. C. to 600.degree. C., more
preferably 350.degree. C. to 450.degree. C. A heat treatment
temperature above 600.degree. C. may cause the material to burn
during the heat treatment. The retention time, which varies with
the heat treatment temperature, is preferably 3 minutes to 24
hours. A retention time longer than 24 hours has the possibility of
causing abnormal grain growth during the heat treatment.
[0043] The magnesium alloy member having an equiaxial grain
structure with no texture can be obtained in this manner. It is
also possible to obtain a magnesium alloy member having an average
grain size of 10 .mu.m or more, for example 30 .mu.m to 50
.mu.m.
[0044] The magnesium alloy member may be subjected to cold plastic
working. (hereinafter, also referred to as "cold working") in a
temperature range of from room temperature to 150.degree. C. For
example, a compressional nominal strain of 0.4 or more can be
applied. The upper limit is 1.5. The cold working refines the
crystal grains of the magnesium alloy member. For example, the size
of matrix can be refined to 80% or less of the average grain size
of the magnesium alloy member after the cold working. The lower
limit is 5%, though it is not particularly limited.
[0045] Refining of the crystal grains by cold working can increase
the hardness and strength of the magnesium alloy member. For
example, the hardness of the magnesium alloy member can be
increased 15% or more after the cold working. The strength of the
magnesium alloy member also can be increased 15% or more after the
cold working.
[0046] As described above, in the present embodiment, the
dispersion state of yttrium is controlled by hot working the
magnesium alloy to segregate yttrium at a grain boundary, and
performing an isothermal heat treatment to diffuse the yttrium in
the grain interior. The subsequent cold working refines the grains,
and improves the hardness and strength of the magnesium alloy
member.
[0047] Controlling the dispersion state of yttrium induces room
temperature recrystallization (grain refining), and excellent
compressional deformation characteristics can be developed.
[0048] The foregoing embodiments described the yttrium-containing
magnesium alloy, and the alloy member of such a magnesium alloy.
However, the, present invention is not limited to these
embodiments. The present invention also encompasses a magnesium
alloy and an alloy member in which some of or all of the yttrium
are substituted with scandium or lanthanoid rare earth elements
such as lanthanum and cerium, or with scandium and lanthanoid rare
earth elements. Scandium, and lanthanoid elements such as lanthanum
and cerium belong to the same group as yttrium, and are located
above and below yttrium in the periodic table. These elements thus
have many similarities in chemical and physical properties, and the
present invention can sufficiently exhibit effect even when some of
or all of the yttrium are substituted with these elements. The
desired effect of the present invention can be more effectively
obtained with the yttrium-containing magnesium alloy, and the alloy
member of such a magnesium alloy.
EXAMPLES
[0049] Yttrium (Y) and pure magnesium (Mg; purity 99.95%) were
completely melted in an argon atmosphere, and cast into an iron
mold to fabricate five types of Mg--Y alloy cast materials with the
target Y contents of 0.01 mol %, 0.02 mol %, 0.03 mol %, 0.04 mol
%, and 0.05 mol %. The target Y contents of 0.03 mol %, 0.04 mol %,
and 0.05 mol % fall within the range of the present invention
(Examples). The target Y contents of 0.01 mol % and 0.02 mol % fall
outside of the range of the present invention (Comparative
Examples). The Y content, and the concentrations of other
composition elements were evaluated by ICP atomic emission
spectrometry after a 2-hour solution treatment of the cast material
at 500.degree. C. The results of the composition analysis are
presented in Table 1. The five alloys were produced by using the
following procedures under the following conditions.
TABLE-US-00001 TABLE 1 Y Fe Si Mn Cu Mg--0.05Y 0.16 (=0.044) 0.002
0.002 0.003 <0.001 Mg--0.04Y 0.13 (=0.036) 0.002 0.002 0.004
<0.001 Mg--0.03Y 0.09 (=0.025) 0.002 0.002 0.004 <0.001
Mg--0.02Y 0.06 (=0.016) 0.002 0.002 0.003 <0.001 Mg--0.01Y 0.02
(=0.005) 0.002 0.002 0.003 <0.001 Figures in parentheses are mol
%. Other figures are mass %.
[0050] The cast material was maintained in a furnace at a
temperature of 500.degree. C. for 2 hours, and then water cooled as
a solution treatment. The product was then machined to produce a
columnar extrusion billet measuring 40 mm in diameter and 70 mm in
height. The same unit billet was maintained for 30 minutes in a
container maintained at the extrusion temperature shown in Table 2,
and subjected to a hot strain applying process, which was performed
by extruding the material at an extrusion ratio of 25:1. The
resulting product will be called "extruded material." The average
equivalent plastic strain was 3.7 as determined from the percentage
reduction of a cross section. The extruded material was
isothermally maintained in a 400.degree. C. furnace for 15 minutes,
and allowed to cool in air to prepare a sample. This product will
be called "extruded and heat-treated material."
TABLE-US-00002 TABLE 2 Y concentration, Extrusion Heat treatment
Heat treatment grain size, Fracture mol % temperature, degrees
temperature, degrees time, min .mu.m Tys, MPa Cys, MPa strain
Mg--0.05Y 0.044 306 -- -- 5 278 140 0.15 0.044 306 400 15 32 91 65
>0.50 Mg--0.04Y 0.036 302 -- -- 5 252 148 0.15 0.036 302 400 15
38 91 65 >0.50 Mg--0.03Y 0.025 315 -- -- 5 207 134 0.14 0.025
315 400 15 40 85 65 >0.50 Mg--0.02Y 0.016 304 -- -- 75 94 68
0.17 0.016 304 400 15 94 84 34 0.31 0.016 212 -- -- 5 116 103 0.38
0.016 212 400 15 44 100 47 0.36 Mg--0.01Y 0.005 317 -- -- 75 92 47
0.27 0.005 317 400 15 >100 32 0.32 0.005 238 -- -- 30 97 65 0.35
0.005 238 400 15 65 80 34 0.33 Tys: Tensile yielding stress, Cys:
Compressional yielding stress
[0051] Mg--Y alloy samples collected from the extruded materials
and the extruded and heat-treated materials were subjected to a
room-temperature tensile and compression test at a strain rate of
1.times.10.sup.-3 s.sup.-1. All test samples were collected in a
direction parallel to the extrusion direction. FIGS. 2 to 4
represent the nominal stress-nominal strain curves obtained after
the room-temperature tensile and compression test. It can be seen
that fracture occurs in the extruded materials in the nominal
strain range of 0.2 to 0.3, irrespective of the amounts of yttrium
added. Here, "fracture" is defined as at least 20% reduction in
stress, and denoted as BK in the figures. A fracture occurred in
the extruded and heat-treated materials Mg-0.01Y and Mg-0.02Y in
the nominal strain range of 0.2 to 0.3 as in the extruded
materials. However, no fracture occurred in the extruded and
heat-treated materials Mg-0.03Y, Mg-0.04Y, and Mg-0.05Y even under
the applied nominal strain of 0.5. These results suggest that the
extruded and heat-treated materials Mg-0.03Y, Mg-0.04Y, and
Mg-0.05Y are highly suited for cold working. As Comparative
Example, a Mg-1 mol % Y alloy was fabricated by casting, and
subjected to a room-temperature compression test after a solution
treatment, without performing hot working. The result is shown in
FIG. 5. It can be seen that fracture occurs at a nominal strain as
low as about 0.3, despite the high yttrium content. The
post-casting hot strain applying process can thus be said as
essential for the present invention to exhibit effect.
[0052] FIG. 6 shows an example of the observed scanning electron
micrograph/electron backscatter diffraction image of the extruded
and heat-treated material Mg-0.03Y. The symbols. ED and TD
represent directions parallel and perpendicular to the extrusion
direction, respectively. It can be seen that the material does not
tensile in the extrusion direction: ED, and has an equiaxial
structure. The average diameter of grains with 15.degree. or
greater misorientation was 40 .mu.m. FIG. 7 shows a pole figure
image of the region observed in FIG. 6. Each point corresponds to
the crystal orientation of the measured crystal grain. It can be
seen that the material has a random texture without accumulation of
basal plane in the specific direction (extrusion direction).
[0053] FIG. 8 shows an example of the microstructure of the
extruded and heat-treated material Mg-0.03Y observed by scanning
electron microscopy/electron backscatter diffraction after applying
20% compressional nominal strain (=0.20). In contrast to the
undeformed material of FIG. 6, refining of the grains can be
observed. The average diameter of the grains with 15.degree. or
greater misorientation was 30 .mu.m, 75% of the initial grain
diameter before the room temperature recrystallization. In the
figure, the symbol "LG" represents a low-angle grain boundary with
less than 15.degree. misorientation. This is considered to be
largely due to the room temperature recrystallization forming a
low-angle boundary of about 5.degree. in the grains. FIG. 9 shows
an example of the microstructure of the extruded and heat-treated
material Mg-0.03Y observed by scanning electron microscopy/electron
backscatter diffraction after applying 50% compressional nominal
strain (=0.50). In contrast to the undeformed material of FIG. 6,
refining of the grains can be observed. The average diameter of the
crystal grains with 15.degree. or greater misorientation was 11
.mu.m, 25% of the initial grain diameter.
[0054] Hardness measurement was performed for the extruded and
heat-treated material Mg-0.03Y (undeformed sample) and the sample
to which 50% compressional nominal strain (=0.50) was applied.
Hardness was 30.5 Hv for the undeformed sample, and 36.5 Hv for the
50% deformed sample. The improved hardness over the undeformed
material is attributed to the finer grain size imparted after the
room temperature working. It can be seen from these results that
the material of the present invention improves hardness and
strength after the room-temperature plastic working.
COMPARATIVE EXAMPLE
[0055] When the working temperature is below 200.degree. C., the
low working temperature makes dynamic recrystallization less likely
to occur. FIG. 10 is a photographic representation showing the
appearance of the material of Comparative Example, representing the
situation where dynamic recrystallization is limited by low working
temperature. The limited dynamic recrystallization makes the
material of Comparative Example less usable in producing a good
material.
INDUSTRIAL APPLICABILITY
[0056] The present invention induces room temperature
recrystallization (grain refining), and develops excellent
compressional deformation characteristics by controlling the
dispersion state of one or more elements selected from yttrium,
scandium, and lanthanoid rare earth elements in a magnesium alloy.
The magnesium alloy member of the present invention has a random
crystal orientation distribution (after working), and the same
yielding stress for the tensile and compressional deformation with
the maintained high strength. The magnesium alloy member of the
present invention can thus be used as a wrought magnesium member
such as a plate member, a rod member, and a pipe member. In a
three-dimensional structure using such a wrought magnesium member,
any external force acting on the structure deforms the magnesium
alloy member near isotropically, and the strengths against the
locally acting tensile and compressional loads become essentially
the same. Further, the magnesium alloy member of the present
invention does not break even under a large applied compressional
strain in excess of 50%, and has excellent deformability. The
magnesium alloy member of the present invention can thus be used as
a structural member or a shock absorbing material in applications
such as automobiles, railcars, aerospace flying objects, and
portable electronic devices.
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