U.S. patent application number 12/375691 was filed with the patent office on 2009-12-31 for high strength magnesium alloy and method of production of the same.
Invention is credited to Akira Kato, Satoshi Ohhashi, An Pang Tsai.
Application Number | 20090320967 12/375691 |
Document ID | / |
Family ID | 39183902 |
Filed Date | 2009-12-31 |
United States Patent
Application |
20090320967 |
Kind Code |
A1 |
Tsai; An Pang ; et
al. |
December 31, 2009 |
HIGH STRENGTH MAGNESIUM ALLOY AND METHOD OF PRODUCTION OF THE
SAME
Abstract
A high strength magnesium alloy improving the high temperature
strength while not using any expensive rare earth elements and
thereby reducing the cost and its method of production are
provided, that is, a high strength magnesium alloy of a composition
of the formula Mg.sub.100-(a,b)Zn.sub.aX.sub.b wherein X is one or
more elements selected from Zr, Ti, and Hf, a and b are the
contents of Zn and X expressed by at %, and the following
relationships (1), (2), and (3): (1) a/28.ltoreq.b.ltoreq.a/9, (2)
2<a<10, (3) 0.05<b<1.0 are satisfied and comprising an
Mg matrix phase in which Mg--Zn--X-based quasi-crystals and their
approximant crystals are dispersed in the form of fine particles
and a method of production of a high strength magnesium alloy
comprising melting Mg in an inert atmosphere to form an Mg melt,
adding Mg--Zn--X-based quasi-crystals (X being at least one of Zr,
Ti, and Hf) into the Mg melt to form an alloy melt, casting the
alloy melt, and heat treating the obtained casting to make the
quasi-crystal and their approximant crystals precipitate in an Mg
matrix phase.
Inventors: |
Tsai; An Pang; (Miyagi,
JP) ; Ohhashi; Satoshi; (Miyagi, JP) ; Kato;
Akira; (Shizuoka, JP) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Family ID: |
39183902 |
Appl. No.: |
12/375691 |
Filed: |
September 12, 2007 |
PCT Filed: |
September 12, 2007 |
PCT NO: |
PCT/JP2007/068214 |
371 Date: |
January 30, 2009 |
Current U.S.
Class: |
148/538 ;
148/406 |
Current CPC
Class: |
C22C 23/04 20130101;
C22F 1/06 20130101; C22C 1/0483 20130101 |
Class at
Publication: |
148/538 ;
148/406 |
International
Class: |
C22F 1/06 20060101
C22F001/06; C22C 23/04 20060101 C22C023/04 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 15, 2006 |
JP |
2006-251434 |
Claims
1. A high strength magnesium alloy of a composition of the formula
Mg.sub.100-(a+b)Zn.sub.aX.sub.b wherein X is one or more elements
selected from Zr, Ti, and Hf, a and b are the contents of Zn and X
expressed by at %, and the following relationships (1), (2), and
(3): a/28.ltoreq.b.ltoreq.a/9 (1) 2<a<10 (2) 0.05<b<1.0
(3) are satisfied and comprising an Mg matrix phase in which
Mg--Zn--X-based quasi-crystals and their approximant crystals are
dispersed in the form of fine particles.
2. A high strength magnesium alloy as set forth in claim 1, wherein
said quasi-crystals are at least one type selected from
Mg.sub.11Zn.sub.83Zr.sub.6, Mg.sub.11Zn.sub.83Ti.sub.6, and
Mg.sub.11Zn.sub.83Hf.sub.6.
3. A method of production of a high strength magnesium alloy as set
forth in claim 1, including: a step for melting Mg in an inert
atmosphere to form an Mg melt, a step of adding Mg--Zn--X-based
quasi-crystals (X being at least one of Zr, Ti, and Hf) into said
Mg melt to form an alloy melt, a step of casting said alloy melt,
and a step of heat treating the obtained casting to make said
quasi-crystals and their approximant crystals precipitate in an Mg
matrix phase.
4. A method of production of a high strength magnesium alloy as set
forth in claim 3, further comprising preparing said Mg--Zn--X-based
quasi-crystals by: a step of weighing the Mg, Zn, and X materials
to give a quasi-crystal composition, a step of charging said
weighed materials in a crucible and melting them in an inert
atmosphere to form a melt, a step of lowering a temperature of said
melt and holding it at a single phase region where only said
quasi-crystals are present, and a step of cooling from the
temperature of said single phase region to room temperature.
Description
TECHNICAL FIELD
[0001] The present invention relates to a high strength magnesium
alloy, more particularly a magnesium alloy improved in high
temperature strength and a method of production of the same.
BACKGROUND ART
[0002] Magnesium alloys are being used for various structural
members due to their light weight. In particular, if used for
automobiles, they are effective for improvement of the fuel economy
and protection of resources and the environment.
[0003] As commercially available materials, as magnesium alloys for
sand mold casting, ASTM AZ91C (standard composition [wt %]:
Mg-8.7Al-0.7Zn-0.13Mn), ZE41A (same: Mg-4.2Zn-1.2RE-0.7Zr), etc.
are in general use, while as wrought magnesium alloys, AZ61A (same:
Mg-6.4Al-1.0Zn-0.28Mn), AZ31B (same: Mg-3.0Al-1.0Zn-0.15Mn), etc.
are in general use.
[0004] Among these, the sand mold casting use alloys AZ91C and
ZE41A are precipitation effect type alloys. By applying T6
(solution treatment+aging) or T5 (only aging) to the cast material,
it is adjusted to the required strength. However, if exposed to
room temperature or more, in particular 50.degree. C. or more, for
a long period of time, aging precipitation of the solid solution
elements occurs, the alloy structure gradually changes, and, along
with this, changes occur in the characteristics along with aging.
As result, there was the defect that the thermal stability of the
structure and characteristics was low and a stable, better high
temperature strength could not be obtained.
[0005] Further, the wrought alloys AZ61A and AZ31B utilize crystal
grain refinement through working and recrystallization at the time
of rolling, extrusion, etc. as the reinforcement mechanism.
However, if becoming a 100.degree. C. or more high temperature,
remarkable grain boundary slip distinctive to Mg occurs, so crystal
grain refinement conversely becomes a cause of a drop in strength
due to the increase in the grain boundary slip sites. Further, if
exposed to a high temperature, the crystal grains grow and the
effect of refinement is lost. This becomes a cause of a drop in the
room temperature strength. As result, there was the defect that not
only could not a high temperature strength be secured, but also the
room temperature strength became thermally unstable.
[0006] To counter the defects of the conventional commercially
available materials and secure a better high temperature strength,
Japanese Patent Publication (A) No. 2002-309332 discloses a Mg-1 to
10 at % Zn-0.1 to 3 at % Y alloy with a solid solution matrix
strengthened by dispersion of quasi-crystal grains. The cast
structure is one where eutectic structures of quasi-crystals are
formed at the .alpha.-Mg crystal grain boundaries and where the
quasi-crystals are finely and uniformly dispersed by hot working.
Quasi-crystals are far higher in hardness than similar compositions
of crystalline compounds, so magnesium superior in strength and
ductility can be obtained. However, while the thermal stability was
high, the strength itself became an extent equal to the
commercially available alloy of a similar composition like ZE41.
There was the limit that a further high temperature strength could
not be obtained.
[0007] As opposed to this, Japanese Patent Publication (A) No.
2005-113235 discloses, as an alloy improving the high temperature
strength, a magnesium alloy of a composition of
Mg.sub.100-(a+b)Zn.sub.aY.sub.b satisfying a/12.ltoreq.b.ltoreq.a/3
and 1.5.ltoreq.a.ltoreq.10 and comprising an Mg matrix phase in
which Mg.sub.3Zn.sub.6Y.sub.1 quasi-crystals and their approximant
crystals (complex structural phases derived from quasi-crystals)
are dispersed in the form of fine particles.
[0008] Furthermore, Japanese Patent Publication (A) No. 2006-89772
discloses a magnesium alloy of a composition of
Mg.sub.100-(a+b+c)Zn.sub.aZr.sub.bY.sub.c satisfying
a/12.ltoreq.(b+c).ltoreq.a/3 and 1.5.ltoreq.a.ltoreq.10 and
0.05<b<0.25c and comprising an Mg matrix phase in which fine
particles of approximant crystals are dispersed.
[0009] Further, Japanese Patent Publication (A) No. 2005-113234
discloses a magnesium alloy of a composition of
Mg.sub.100-(a+b+c)Zn.sub.aAl.sub.bY.sub.c satisfying
(a+b)/12.ltoreq.c.ltoreq.(a+b)/3 and 1.5.ltoreq.a.ltoreq.10 and
0.05a<b<0.25a and comprising an Mg matrix phase in which
Mg.sub.3Zn.sub.6Y.sub.1 quasi-crystals and their approximant
crystals (complex structural phases derived from quasi-crystals)
are dispersed in the form of fine particles.
[0010] The magnesium alloys of the above prior arts all realize
better high temperature strengths by dispersing quasi-crystals and
their approximant crystals as fine reinforcing particles in the Mg
matrix phase, but had the problem that a rise in cost was
unavoidable since a rare earth element (Y) was an essential
ingredient.
DISCLOSURE OF THE INVENTION
[0011] The present invention has as its object the provision of a
high strength magnesium alloy improving the high temperature
strength while not using any expensive rare earth elements and
thereby reducing the cost and its method of production.
[0012] To achieve that object, the high strength magnesium alloy of
the first aspect of the invention has a composition of the formula
Mg.sub.100-(a+b)Zn.sub.aX.sub.b wherein X is one or more elements
selected from Zr, Ti, and Hf, a and b are the contents of Zn and X
expressed by at %, and the following relationships (1), (2), and
(3):
a/28.ltoreq.b.ltoreq.a/9 (1)
2<a<10 (2)
0.05<b<1.0 (3)
are satisfied and
[0013] comprising an Mg matrix phase in which Mg--Zn--X-based
quasi-crystals and their approximant crystals are dispersed in the
form of fine particles.
[0014] A method of the second aspect of the invention for producing
the high strength magnesium alloy of the first aspect of the
invention includes:
[0015] a step for melting Mg in an inert atmosphere to form an Mg
melt,
[0016] a step of adding Mg--Zn--X-based quasi-crystals (X being at
least one of Zr, Ti, and Hf) into the Mg melt to form an alloy
melt,
[0017] a step of casting the alloy melt, and
[0018] a step of heat treating the obtained casting to make the
quasi-crystals and their approximant crystals precipitate in an Mg
matrix phase.
[0019] The high strength magnesium alloy of the present invention
is comprised of a Mg matrix phase in which fine particle-shaped
Mg--Zn--X-based quasi-crystals and their approximant crystals are
dispersed as reinforcing particles whereby high strength, in
particular high temperature strength equal to an alloy in which
quasi-crystals and approximant crystals are dispersed using
conventional rare earth elements, can be achieved.
BRIEF DESCRIPTION OF THE DRAWINGS
[0020] FIG. 1 is a photo of an electron beam diffraction pattern of
a Mg.sub.11Zn.sub.83Zr.sub.6 quasi-crystal prepared by the present
invention.
[0021] FIG. 2 is a transmission electron micrograph showing the
metal structure of an Mg alloy prepared by the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0022] The high strength magnesium alloy of the present invention,
by setting the above range of composition, can be given a structure
comprised of an Mg matrix phase in which quasi-crystals comprised
of Mg--Zn--X (X=one or more of Zr, Ti, and Hf) and their
approximant crystals are dispersed.
[0023] "Quasi-crystals" are compounds of a structure with a regular
structure in a short range (typically a fivefold symmetry), but
without the translational symmetry characterizing ordinary
crystals. As compositions forming quasi-crystals, up until now
Al--Pd--Mn, Al--Cu--Fe, Cd--Yb, Mg--Zn--Y, etc. have been known.
Being special structures, compared with crystalline intermetallic
compounds of similar compositions, they have various special
properties such as high hardness, high melting point, low
coefficients of friction, etc.
[0024] "Approximant crystals" are crystalline compounds having
complex structures derived from quasi-crystals and partially having
structures similar to those quasi-crystals. They have properties
similar to the original quasi-crystals.
[0025] The Mg--Zn--X-based quasi-crystals have compositions of
preferably, by at % ratio of Zn and X, a:b=83:6 and are one or more
crystals selected from Mg.sub.11Zn.sub.83Zr.sub.6,
Mg.sub.11Zn.sub.83Ti.sub.6, and Mg.sub.11Zn.sub.83Hf.sub.6.
However; the invention does not have to be limited to these.
Quasi-crystals of compositions allowed within the above prescribed
range of alloy composition and their approximant crystals are also
allowed. The fine particles have sizes of several tens of nm to
several hundreds of nm.
[0026] Quasi-crystals and their approximant crystals are extremely
hard and remain stable without breaking down up to about
230.degree. C., so strongly interact with the dislocations when
dispersing as fine particles in the Mg matrix phase, exhibit an
extremely high dispersion reinforcing action, and improve the
ordinary temperature and high temperature strength. In particular,
the fine particles present at the .alpha.-Mg crystal grain
boundaries suppress the grain boundary slip at a high temperature
and contribute to better high temperature strength.
[0027] The alloy of the present invention uses Zr, Ti, or Hf
instead of the rare earths of the conventional alloys as a
component ingredient of the quasi-crystals and their approximant
crystals. These elements have a hard time dissolving in the melt of
the main ingredient Mg of the alloy, so it is not possible to
produce the alloy by directly producing the alloy melt of the final
composition like with the conventional alloys including rare earth
elements. That is, even if weighing out the materials of the
ingredients of the alloy in accordance with the final alloy
composition, charging them together in a melting furnace, and
heating them to form the melt, the high melting point Zr, Ti, or Hf
will not dissolve in the melt and will end up remaining as a solid.
These high melting point elements have melting points of extremely
high temperatures or higher than even the melting point of Mg.
[0028] The Y and other rare earth elements used in conventional
alloys have far higher melting points than Mg, but when contacting
the Mg melt formed earlier at a low temperature at the time of
melting, they form alloys and become lower in melting point so
easily dissolve in the Mg melt. Therefore, conventional alloys
could be obtained by directly producing melts of the final
compositions.
[0029] The alloy of the present invention, in the above way, cannot
be obtained by directly producing an alloy melt of the final
composition, so in the method of the present invention, only Mg is
melted to form an Mg melt, then the quasi-crystals are added to
this to enable the formation of the alloy melt. The final alloy
composition is adjusted to by the amount of the Mg melt and the
composition and amounts of addition of the quasi-crystals. The
alloy melt is sufficiently stirred to obtain a uniform melt.
[0030] The obtained alloy melt is cast by an ordinary method. The
obtained casting is heat treated to make the quasi-crystals and
their approximant crystals precipitate as fine particles in the Mg
matrix phase.
[0031] Due to this, finally, a structure of an Mg matrix phase in
which fine particles of quasi-crystals and their approximant
crystals are dispersed is obtained.
[0032] The mechanism for the production of the fine particles of
the quasi-crystals and their approximant crystals has not been
fully elucidated at the present point of time, but is believed to
be as follows:
[0033] An alloy melt comprised of an Mg melt into which
quasi-crystals have been added and sufficiently stirred may appear
visually to be uniform, but microscopically there are variations in
composition in the melt. Fine regions where specific elements are
segregated are distributed across the entire region of the melt. In
the cooling process at the time of casting, quasi-crystals and
approximant crystals or their precursor compositions finely
precipitate around these fine segregated regions. The solidified
casting is comprised of an Mg matrix phase in which the component
elements Zn and X (at least one of Zr, Ti, and Hf) of-the
quasi-crystals or approximant crystals are dissolved in a
supersaturated state. The quasi-crystals or approximant crystals
precipitate as fine particles around the fine precipitates due to
heat treatment.
[0034] That is, in the metal structure of the finally obtained
alloy, fine particles of the quasi-crystals and approximant
crystals formed in the cooling process at the time of casting and
fine particles of the quasi-crystals and approximant crystals
precipitating in the subsequent heat treatment are present. The two
both contribute to improvement of the strength by dispersion
strengthening.
Examples
Example 1
[0035] According to the present invention, Mg--Zn--Zr magnesium
alloys of compositions giving the final compositions of the alloys
as a whole as shown in Table 1 were prepared and examined for metal
structure and subjected to tensile tests.
[0036] (1) Preparation of Quasi-Crystals
[0037] Metals of pure Mg (99.9%), pure Zn (99.99%), and pure Zr
(99.5%) (figures in parenthesis indicate purity) were weighed out
to give a quasi-crystal composition, by at % ratio, of
Mg.sub.11Zn.sub.83Zr.sub.6. A total amount of 5 g was set in an
alumina Tammann tube (.phi.12 mm.times.10 mm) which was then sealed
in a quartz tube. The inside of the quartz tube was replaced with
high purity argon.
[0038] This was raised in temperature from room temperature to
700.degree. C. over 5 hours, was held there for 12 hours, then was
lowered in temperature to 500.degree. C. and was held there for 48
hours. Due to this, Mg.sub.11Zn.sub.83Zr.sub.6 quasi-crystals were
obtained. As shown by the electron beam diffraction pattern in FIG.
1, typical fivefold symmetry was confirmed.
[0039] The obtained clumps of quasi-crystals were crushed to obtain
particles of a size of several tens of .mu.m which were then used
for the following.
[0040] (2) Production of Alloy
[0041] Pure Mg (purity 99.99%) was charged into a graphite crucible
and raised in temperature to 700.degree. C. in a high frequency
melting furnace held in an argon atmosphere so as to melt it and
form an Mg melt.
[0042] Into the Mg melt, the quasi-crystals prepared in the above
(1) were added in an amount adjusted to give a final alloy
composition of the Mg--Zn--Zr composition shown in Table 1, then
the melt was stirred until the crystals completely dissolved and
the melt became uniform while holding the melt temperature at
700.degree. C., then was held in that state for 2 hours.
[0043] (3) Casting
[0044] The obtained alloy melt was held at 700.degree. C. and cast
into a cast iron JIS No. 4 boat type casting mold (70 mm.times.70
mm.times.300 mm) preheated to about 100.degree. C.
[0045] (4) Heat Treatment
[0046] The obtained casting was heat treated in an argon atmosphere
at 500.degree. C. for 48 hours.
[0047] <Observation of Metal Structure>
[0048] The heat treated sample was observed for metal structure by
a transmission electron microscope (TEM).
[0049] As result, as shown in FIG. 2, fine precipitates of several
tens of nm size were observed in white spots in the .alpha.-Mg
crystal grains. The precipitates were confirmed to be
Mg.sub.11Zn.sub.83Zr.sub.6 quasi-crystals and their approximant
crystals.
[0050] <Tensile Test>
[0051] A rod-shaped tensile test piece with parallel parts of
.phi.5.times.25 mm was taken from the heat treated sample and
subjected to a tensile test at room temperature, 150.degree. C.,
and 200.degree. C. The test was conducted using an AG-250 kND
tensile tester made by Shimadzu at a tensile rate of 0.8
mm/min.
[0052] Furthermore, the heat treated, then extruded sample was also
similarly subjected to a tensile test. The extrusion conditions
were an extrusion temperature of 250.degree. C. and an extrusion
ratio of 10:1.
[0053] Further, the comparative examples with compositions outside
the scope of the present invention were also similarly subjected to
a tensile test.
Example 2
[0054] According to the present invention, Mg--Zn--Ti magnesium
alloys of compositions giving final compositions of the alloys as a
whole as shown in Table 1 were prepared and examined for metal
structure and subjected to tensile tests.
[0055] Except for preparing, as quasi-crystals,
Mg.sub.11Zn.sub.83Ti.sub.6 quasi-crystals and adding them to a pure
Mg melt, the same procedure was used as in Example 1 for
preparation.
[0056] After heat treatment, using a transmission electron
microscope, fine precipitates of several tens of nm size were
observed in white spots in the .alpha.-Mg crystal grains. The
precipitates were confirmed to be Mg.sub.11Zn.sub.83Ti.sub.6
quasi-crystals and their approximant crystals.
[0057] The heat treated samples and samples extruded under the same
conditions as in Example 1 were subjected to tensile tests under
the same conditions as in Example 1.
[0058] Further, the comparative materials with compositions outside
the scope of the present invention were also subjected to similar
tensile tests.
[0059] Furthermore, the conventional materials were also subjected
to similar tensile tests.
[0060] The above test results are shown together in Table 1.
TABLE-US-00001 TABLE 1 Tensile strength (MPa) Elongation (%) Method
of Room Room Class production Composition (at %) temperature
150.degree. C. 200.degree. C. temperature 150.degree. C.
200.degree. C. Mg Zn Zr Present Casting 94.59 5.05 0.36 200 183 156
4 22 26 invention 96.82 3.02 0.16 206 189 161 3 14 22 (Example 1)
91.24 8.04 0.72 201 193 163 3 11 18 Extrusion 97.67 2.24 0.09 225
180 150 6 25 27 Comparative Casting 95.35 4.52 0.13 187 135 108 5
23 32 material Mg Zn Ti Present Casting 94.56 5.07 0.37 203 193 152
4 21 24 invention 96.79 3.04 0.17 204 185 163 3 15 21 (Example 2)
91.23 8.03 0.74 209 195 165 2 12 16 Extrusion 91.23 8.03 0.74 320
258 235 21 24 30 Comparative Casting 91.62 8.15 0.23 190 152 126 2
15 27 material Conventional Casting AZ91C-T6 275 185 140 5 31 33
material ZE41A-T5 205 165 130 5 15 29 Extrusion AZ61A-F 325 225 150
16 40 42 AZ31B-F 275 170 110 12 39 42
[0061] The present invention materials are particularly superior in
tensile strength at 150.degree. C. compared with the conventional
materials. Further, the drop in strength accompanying a rise in
temperature from room temperature to 150.degree. C. is extremely
small. This is because the fine particles comprised of the
quasi-crystals and their approximant crystals precipitated and
dispersed in the .alpha.-Mg crystal grains have extremely high
thermal stabilities, so strongly interact with the dislocations
even under a high temperature of 150.degree. C. and effectively
function as barriers to movement of the dislocations. The
comparative materials of the compositions outside the scope of the
present invention either do not form quasi-crystals and their
approximant crystals or even if forming them form them in extremely
fine amounts, so almost no dispersion strengthening action is
obtained due to these formed phases and a high strength cannot be
obtained.
INDUSTRIAL APPLICABILITY
[0062] According to the present invention, there are provided a
high strength magnesium alloy improving the high temperature
strength while not using any expensive rare earth elements and
thereby reducing the cost and its method of production.
* * * * *