The present invention relates to a magnesium alloy having
superior high-temperature strength. More particularly, the invention relates to
a particle-dispersed magnesium alloy having superior high-temperature strength.
Magnesium has the specific gravity of 1.74 and is the lightest
among the metal materials for industrial purposes. Its mechanical property is comparable
to that of aluminum alloy, and for that reason it has drawn attention as a material
suitable for aircraft and automobiles, particularly as a material contributing to
light weight and improved mileage.
For example, magnesium alloy has already been used as the
material for automotive wheels or engine head covers. There is currently a growing
demand for making components of all kinds more lightweight, and the range of application
of magnesium alloy is becoming wider. Applications of magnesium alloy under consideration
include structural components, such as engine blocks, and even functional components
such as pistons, that experience high temperature. If the piston is made of magnesium
alloy instead of aluminum alloy, not only the piston becomes lighter in weight but
also other components can be made lighter because of the decrease in inertia weight
or the like.
Magnesium alloy products are usually made of cast products
including die-cast products.
Among the conventional magnesium alloys, Mg-Al alloys (ASTM
standards - AM60B, AM50A, AM20A, for example) contain 2 to 12% Al, to which small
amounts of Mn are added. The Mg component consists of eutectic crystal of &agr;-Mg
solid solution and &bgr;-Mg17Al12 compound, in which age
hardening is caused by the precipitation of a Mg17Al12 mesophase
upon heat treatment. Strength and toughness also improve by solution heat treatment.
In the Mg-Al-Zn alloys (ASTM standards-AZ91D, for example)
in which 5 to 10% Al and 1 to 3% Zn are contained, there is a wide &agr;-solid
solution region on the Mg side, where a Mg-Al-Zn compound crystallizes. While they
are strong and highly anticorrosive in the as-cast condition, their mechanical property
can be improved by aging heat treatment, and a pearlite-like compound phase is precipitated
at the grain boundary by hardening and tempering.
In Mg-Zn alloys, the maximum strength and elongation can
be obtained in the as-cast condition when 2% Zn is added to Mg. In order to improve
castability and obtain a robust cast product, greater amounts of Zn are added. The
as-cast Mg-6%Zn alloy has a tensile strength on the order of 17 kg/mm2,
which, although it can be improved by the T6 treatment, is much inferior to that
of Mg-Al alloys. One example of such Mg-Zn alloys is ZCM630A (Mg-6%Zn-3%Cu-0.2Mn).
Meanwhile, efforts have been made to search for a magnesium
alloy that has superior heat resistance and is suitable for use at high temperatures.
As a result, it has been found that an alloy to which a rare earth element (R.E.)
is added provides a mechanical property that, although somewhat inferior to that
of aluminum alloys in room temperature, is comparable to that of aluminum alloys
at high temperatures from 250 to 300°C. Examples of alloys that contain R.E.
that have been put to practical use include EK30A alloy (2.5 to 4% R.E.-0.2%Zr)
which contains no Zn, and ZE41A alloy (1%R.E.-2.0%Zn-0.6%Zr) that contains Zn.
In such Mg alloys, strength is improved by the following
SUMMARY OF THE INVENTION
- (1) In
JP Patent Publication (Kokai) No. 2002-309332 A
, after casting an Mg-Zn-Y alloy, a quasicrystal phase that forms eutetic
crystal with &agr;-Mg is uniformly and finely dispersed in the microstructure
by hot forming. The quasicrystal is a quasicrystal-phase-reinforced magnesium alloy
that is much harder than a crystalline compound with an approximate composition
and that has superior strength and elongation property. The composition is limited
to Mg, 1-10 at.%Zn, 0.1-3 at.%Y. In the as-cast microstructure of Mg-Zn-Y alloy,
an eutetic crystal microstructure of quasicrystal is formed at the &agr;-Mg crystal
grain boundary. By hot forming the eutetic crystal microstructure, the quasicrystal
can be finely and uniformly dispersed so as to achieve enhanced strength.
- (2) In sand casting Mg alloys such as AZ91C and ZE41, after the casting of an
alloy, a predetermined strength is obtained by heat treatment such as T6 or T5.
Such alloys are precipitation hardening alloys and that is why they require heat
treatment such as T6 or T5 in order to adjust them to a predetermined strength and
obtain long-term stability in their characteristics. If exposed to temperatures
above room temperature (generally 50°C or higher) for a long time, aging precipitation
of dissolved elements might occur, resulting in a gradual change in alloy microstructure
- (3) In Mg alloys for forging, such as AZ61A and AZ31B, the crystal grain is
made finer by recrystallization caused by intense processing such as rolling and
extrusion, thereby enhancing strength. The major reinforcing mechanism for such
alloys is the refinement of crystal grains. Refinement of crystal grains, however,
triggers a decrease in strength at high temperatures of 1000°C and above where
a strong grain boundary sliding unique to Mg occurs. Furthermore, grain growth occurs
at high temperatures, so that such alloys, once exposed to high temperature, would
potentially not be able to regain their original strength even after the temperature
The Mg-Zn-Y alloy cast material disclosed in
JP Patent Publication (Kokai) No. 2002-309332
is a general eutetic crystal alloy, and it has a strength comparable to
that of commercially available alloys with a similar composition, such as ZE41.
In sand casting Mg alloys such as AZ91C and ZE41, the thermal stability of precipitates
is so low that aging proceeds continuously at room temperature or above. Furthermore,
Mg alloys for forging, such as AZ61A and AZ31B, have no mechanism for pinning the
grain boundary or controlling grain growth at high temperatures.
The high-strength magnesium alloy of the invention has
been made in view of the aforementioned problems, and it is an object of the invention
to improve the strength, particularly high-temperature strength, of a Mg-Zn-RE alloy.
The invention is based on the inventors' realization that
by substituting a part of RE in an Mg-Zn-RE alloy with a particular element, a high-strength
magnesium alloy can be obtained that has such a microstructure that nanoparticles
having a complex structure deriving from a quasicrystal are dispersed in the crystalline
magnesium parent phase.
The invention provides a high-strength magnesium alloy
which comprises 2.0 to 10 at.% zinc, 0.05 to 0.2 at.% zirconium, 0.2 to 1.50 at.%
rare earth element, and the balance being magnesium and unavoidable impurities.
Preferably, the rare earth element (RE) is yttrium (Y).
Preferably, the magnesium alloy of the invention is expressed
by the following general formula:
where RE is a rare earth element, and a, b, and c are atomic percentages of zinc
(Zn), zirconium (Zr), and rare earth element (RE), respectively, where the following
relationship is satisfied:
The magnesium alloy of the invention having the above composition
has the following characteristics:
- (1) The &agr;-Mg crystal grains occupy 50% or more in volume, and the alloy
contains nanoparticles having a complicated structure, such as quasicrystal or approximate
crystal, at the &agr;-Mg crystal grain boundary. The quasicrystal herein refers
to a new ordered structure having no translational symmetry but having fivefold
or tenfold symmetry and quasiperiodicity, which are not crystallographically allowed.
Alloys known to produce quasicrystal include Al-Pd-Mn, Al-Cu-Fe, Cd-Yb, and Mg-Zn-Y,
for example. Because of its specific structure, the quasicrystal has specific characteristics,
such as high degree of hardness, high melting point, and low µ, as compared
with ordinary crystals.
- (2) Fine precipitates (1 µm or smaller) are uniformly dispersed within
the &agr;-Mg crystal grains. Such fine precipitates enhance the strength of the
magnesium alloy of the invention.
- (3) The major fine precipitates are approximate crystals and MgY intermetallic
compounds. The approximate crystal, which is related to quasicrystal, herein refers
to an intermetallic compound having a structure and composition similar to those
of the quasicrystal (Mg3Zn6Y1).
- (4) Upon solution heat treatment, the quasicrystal or approximate crystal phase
of the &agr;-Mg crystal grain boundary pins the shifting of the crystal grain
boundary. Therefore, growth of crystal grain is controlled, and the decrease in
strength due to the coarsening of the crystal grain does not occur even at high
temperature of 300°C or above.
- (5) Due to aging after solution heat treatment, approximate crystals or the
like having grain diameter of 100 nm or smaller are precipitated at high number
density. As a result, precipitates having grain diameter of several tens to hundreds
of nm are dispersed at high concentration within the &agr;-Mg grains, together
with products crystallized upon casting. Such precipitates highly interact with
dislocation and do not become dissolved until nearly 230°C. The quasicrystal
and approximate crystal that exist in the &agr;-Mg crystal grain boundary control
the grain boundary sliding at high temperature. Their synergistic effect provides
very high-temperature strength.
In the magnesium alloy of the invention having the above
composition, the &agr;-Mg phase occupies 50% or more of the volume, and quasicrystal
or approximate crystal particles exist in the &agr;-Mg crystal grain boundary.
These particles pins the shifting of the crystal grain boundary, so that the growth
of crystal grain can be controlled. Thus, no decrease in strength due to the coarsening
of the crystal occurs even at high temperature. Further, fine crystals are also
precipitated within the grains.
BRIEF DESCRIPTION OF THE DRAWINGS
The major fine precipitates are approximate crystals and Mg-Y intermetallic compounds.
BEST MODES FOR CARRYING OUT THE INVENTION
- Fig. 1A shows an SEM microstructure image of Example 1. Fig. 1B shows an SEM
microstructure image of Comparative Example. Fig. 2 shows enlarged images of the
inside of a grain of a Mg-6Zn-0.1Zr-0.9Y(at.%) cast material according to Example
1. Fig. 3 shows enlarged images of the grain boundary (more strictly, an eutetic
crystal-like portion) of a Mg-6Zn-0.1Zr-0.9Y(at.%) cast material according to Example
The magnesium alloy of the invention is manufactured by
adding all predetermined additive elements in molten Mg, mixing them uniformly,
and casting the mixture in a casting mold. The casting method is not particularly
limited and a variety of methods, such as gravity casting, die-casting, or rheocast,
may be employed.
Preferably, the magnesium alloy of the invention is not
just cast but subjected to heating process after casting, or to hot working and
heating process after casting, so as to improve strength.
Examples of the rare earth element of which the magnesium
alloy of the invention is composed include scandium (Sc), yttrium (Y), lanthanum
(La), cerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm), samarium
(Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho),
erbium (Er), thulium (Tm), ytterbium (Yb), and lutetium (Lu), of which yttrium (Y)
In the following, examples and a comparative example of
the invention will be described.
An alloy of Mg-6Zn-0.1 Zr-0.9Y(at.%) cast material was
manufactured by the following steps.
Pure Mg (99.9%):
Pure Zn (99.99%):
Pure Zr (99.9%):
Pure Y (99.9%):
Pure Mg was dissolved in an iron crucible, and molten metal was maintained at 700°C.
Other constituent materials were added in the molten metal, which was stirred while
its temperature was matainained at approximately 700°C until all the materials
were uniformly dissolved. The order of addition of the constituent materials in
the molten metal does not affect characteristics and is therefore not specified.
The alloy molten metal whose temperature was maintained at approximately 700°C
was cast in a JIS 4 boat-shaped mold which had been preheated to about 100°C.
Mg-3Zn-0.5Y, which is a conventional material, was cast
in the same way as in Example 1 except that the following materials were used.
[Microstructural comparison between Example 1 and Comparative
Pure Mg (99.9%):
Pure Zn (99.99%):
Pure Y (99.9%):
Fig. 1A shows an SEM microstructure image of Example 1,
and Fig. 1B shows an SEM microstructure image of Comparative Example. Example 1
and Comparative Example have similar exterior, having an eutetic crystal structure
of approximate crystal (Example 1) or Mg3Zn6Y1
quasicrystal (Comparative Example) at the &agr;-Mg crystal grain boundary. However,
the shape of the eutetic crystal structure is different between Example 1 and Comparative
Example; in Example 1, the eutetic crystal structure is generally finer and more
Fig. 2 shows an enlarged image of the inside of a grain
of the Mg-6Zn-0.1Zr-0.9Y(at.%) cast material of Example 1. The image shows the &agr;-Mg
phase, a MgY intermetallic compound that could be either Mg24Y5
or Mg12Y, and an unidentifiable phase.
Fig. 3 shows an enlarged image of the grain boundary (or,
to be more precise, the eutetic crystal-like portion) of the Mg-6Zn-0.1Zr-0.9Y(at.%)
cast material of Example 1. The image shows the W phase (cubic crystal ≈
Zn3Mg3Y2), a Zn6Y4 binary
compound, a hexagonal compound, and an unidentifiable phase.
[Strength comparison between Example and Comparative Example]
From ingots of the above JIS 4 boat-shaped mold according
to Example 1 (Mg-6Zn-0.1Zr-0.9Y) and Comparative Example (Mg-3Zn-0.5Y), cylindrical
tensile specimens measuring ϕ5×25 mm at the parallel portion were acquired
and subjected to tensile test at room temperature and 150°C. Similar tensile
tests were conducted on Examples 2 to 4 with various composition ratios and on AZ91C-T6
and ZE41A-T5, which are conventional materials. The tests were conducted using AG-250kND
manufactured by Shimadzu Corporation as a tensile tester, at the pulling rate of
0.8 mm/min. The results are shown in Table 1 below.
The results in Table 1 show that the cast materials of
Examples 1 to 4 are superior to the conventional cast materials such as Comparative
Example in terms of tensile strength at 150°C. Further, Examples 1 to 4 show
much lower decrease in strength associated with the temperature increase from room
temperature to 150°C. One cause for these results is believed to be an increase
in the fine precipitates in the &agr;-Mg crystal grains. Since fine precipitates,
such as approximate crystals and MgY intermetallic compounds, have high thermal
stability, they are supposedly functioning as an effective dislocation barrier even
In the magnesium alloy of the invention, nanoparticles
deriving from quasicrystal are present at the Mg crystal grain boundary, and fine
crystals are precipitated even within the grains. As a result, there is no decrease
in strength due to the coarsening of crystals at high temperature. Thus, high strength
can be maintained at high temperature.
Normally, high-temperature strength can be increased by
increasing the content of rare earth elemen. This, nevertheless, results in an increased
cost. For example, WE54 can exhibit high strength by increasing the rare earth content
to nearly 10% and carrying out T6 heat treatment, although at very high cost. In
accordance with the invention, high-temperature strength comparable to the strength
of conventional heat-treated material can be achieved in the as-cast condition;
namely, without heat treatment.