U.S. patent application number 09/734174 was filed with the patent office on 2002-01-10 for process for making oxide dispersion-strengthened tungsten heavy alloy by mechanical alloying.
Invention is credited to Hong, Soon Hyung, Ryu, Ho Jin.
Application Number | 20020002879 09/734174 |
Document ID | / |
Family ID | 19676970 |
Filed Date | 2002-01-10 |
United States Patent
Application |
20020002879 |
Kind Code |
A1 |
Hong, Soon Hyung ; et
al. |
January 10, 2002 |
Process for making oxide dispersion-strengthened tungsten heavy
alloy by mechanical alloying
Abstract
Disclosed is a process for making an oxide
dispersion-strengthened tungsten heavy alloy by mechanical alloying
that includes the steps of: adding 0.1 to 5 wt. % of Y.sub.2O.sub.3
powder to a mixed powder comprising more than 90 wt. % of tungsten
powder, and nickel and iron powders for the rest; and subjecting
the resulting mixture to a mechanical alloying to prepare an oxide
dispersion-strengthened tungsten heavy alloy powder. The oxide
dispersion-strengthened tungsten heavy alloy prepared by the
mechanical alloying is characterized in that fine Y.sub.2O.sub.3
particles are uniformly dispersed in the matrix which are stable at
high temperatures results in enhanced high-temperature strength and
a reduction of the shearing strain of the fracture during
high-speed shear deformation.
Inventors: |
Hong, Soon Hyung; (Taejon
Kwangyeok-si, KR) ; Ryu, Ho Jin; (Taejon
Kwangyeok-si, KR) |
Correspondence
Address: |
ROSENBERG, KLEIN & LEE
3458 ELLICOTT CENTER DRIVE-SUITE 101
ELLICOTT CITY
MD
21043
US
|
Family ID: |
19676970 |
Appl. No.: |
09/734174 |
Filed: |
December 12, 2000 |
Current U.S.
Class: |
75/235 ; 419/20;
419/32; 75/248 |
Current CPC
Class: |
F42B 12/74 20130101;
B22F 2998/10 20130101; C22C 32/0031 20130101; B22F 2009/041
20130101; B22F 2998/00 20130101; B22F 2998/00 20130101; B22F 1/07
20220101; B22F 2998/10 20130101; B22F 9/04 20130101; B22F 3/02
20130101; B22F 3/10 20130101; B22F 2998/00 20130101; B22F 1/07
20220101 |
Class at
Publication: |
75/235 ; 419/20;
419/32; 75/248 |
International
Class: |
C22C 029/12; B22F
001/00; C22C 032/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jul 8, 2000 |
KR |
2000-39100 |
Claims
What is claimed is:
1. A process for making an oxide dispersion-strengthened tungsten
heavy alloy by mechanical alloying, comprising the steps of: adding
0.1 to 5 wt. % of Y.sub.2O.sub.3 powder to a mixed powder
comprising more than 90 wt. % of tungsten powder, and nickel and
iron powders for the rest; subjecting the resulting mixture to a
mechanical alloying to prepare an oxide dispersion-strengthened
tungsten heavy alloy powder; compacting the tungsten heavy alloy
powder into a compressed powder using a press; and sintering the
compressed powder at a temperature in the range of 1400 to
1600.degree. C.
2. The process as claimed in claim 1, wherein the sintering step of
the compressed powder is performed under the hydrogen
atmosphere.
3. The process as claimed in claim 2, further comprising the step
of subjecting the sintered alloy to a heat treatment under the
nitrogen atmosphere to remove residual hydrogen.
4. The process as claimed in claim 1, wherein the oxide
dispersion-strengthened tungsten heavy alloy powder obtained by the
mechanical alloying has a tungsten grain size of less than 100 nm
and a lamella distance of less than 0.5 .mu.m.
5. An oxide dispersion-strengthened tungsten heavy alloy prepared
according to claim 1.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a process for making an
oxide dispersion-strengthened tungsten heavy alloy mainly used for
an amour-plate-destroying warhead by mechanical alloying.
[0003] 2. Description of the Related Art
[0004] Tungsten heavy alloy is mainly comprised of 90 to 98 wt. %
of tungsten to use the high-density property of the tungsten, and
nickel and iron for the rest at a weight ratio of 7:3 to 8:2 to
enhance the sintering property and processability. Such a tungsten
heavy alloy has a fine structure in which body-centered cubic (BCC)
spherical tungsten grains with a particle size of 30 to 40 .mu.m
are dispersed in the face-centered cubic (FCC) W--Ni--Fe matrix.
The tungsten heavy alloy possesses excellent mechanical properties,
for example, high density of 16 to 18.5 g/cm.sup.2, high tensile
strength of 800 to 1200 MPa and elongation percentage of 20 to 30%
and is broadly used as a material for the balanced supports of
aircrafts, radiation shielding devices, vibration attenuators, and
the piercers of amour-plate-destroying kinetic energy piece. The
conventional tungsten heavy alloy is manufactured by a powder
metallurgy process that involves sintering a mixed powder of
tungsten, nickel and iron at a temperature above 1460.degree.
C.
[0005] The amour-plate-destroying piercers using a high kinetic
energy to pierce the amour plate are made from depleted uranium or
a tungsten heavy alloy that have high density, high strength and
high impact energy. The amour-plate-destroying piercer made from
depleted uranium is excellent in the piercing ability but currently
limited in its use because it is disadvantageously poor in
processability and abrasion resistance and, especially, incurs
radioactive contamination. The amour-plate-destroying piercer made
from a tungsten heavy alloy has a smaller piercing depth per
density than the amour-plate-destroying piercer made from depleted
uranium in piercing the amour plate. For that reason, many studied
have been made on a material for the amour-plate-destroying piercer
replacing the depleted uranium with improved mechanical properties
of the tungsten heavy alloy.
[0006] The oxide dispersion strengthening method involves
dispersion of a thermodynamically stable fine oxide in a metallic
matrix without reaction with the base phase to fabricate a
high-temperature material, which is advantageously excellent in
high-temperature mechanical properties and usable at a high
temperature approximately up to the melting point.
[0007] The known oxide dispersion-strengthened alloys are as
follows. The first commercially available oxide
dispersion-strengthened alloy was ThO.sub.2 dispersion-strengthened
tungsten (GE, U.S.A., 1910) and developed as TD-Nickel (Du Pont,
U.S.A., 1962) containing 2 vol % of ThO.sub.2 dispersed in the Ni
matrix as a structural material. The earnest applications of the
oxide dispersion-strengthened alloys started from the manufacture
of Y.sub.2O.sub.3 dispersion-strengthened Ni-based superalloy using
the mechanical alloying technique by Benjamin at Inco Co. (U.S.A)
in 1970. The oxide dispersion-strengthening technique has been
applied in the manufacture of dispersion-strengthened Cu alloy,
dispersion-strengthened Al alloy and dispersion-strengthened W
alloy as well as Ni-based superalloy, which are used as a material
for aircraft engine, welding electrodes, electrical contact
material, and so forth.
[0008] No document has been found that suggests the process for
making an oxide dispersion-strengthened tungsten heavy alloy by
mechanical alloying and the use of the oxide
dispersion-strengthened tungsten heavy alloy for
amour-plate-destroying piercers. Especially, it is preferable for
the amour-plate-destroying piercers to be susceptible to local
fracture with concentrated stress under the shearing strain around
the oxide dispersed in the matrix in order to have an enhanced
piercing performance, since the piercers need the self-sharpening
effect by which the edge of the piercers is fractured to make the
piercers sharpened. However, there is still no report on the
systematic research on the process for making the oxide
dispersion-strengthened tungsten heavy metal, in particular,
enhancing the self-sharpening by the oxide dispersion.
[0009] Accordingly, the inventors have studied on the subject and
found out that an oxide dispersion-strengthened tungsten heavy
alloy containing a uniform Y.sub.2O.sub.3 dispersion in the
tungsten heavy alloy can be manufactured from a composition
including a Y.sub.2O.sub.3 powder and a tungsten heavy alloy powder
by mechanical alloying and liquid-phase sintering.
SUMMARY OF THE INVENTION
[0010] It is, therefore, an object of the present invention to
provide a process for making an oxide dispersion-strengthened
tungsten heavy alloy with enhanced high-temperature compression
strength and susceptible to fracture during shear deformation by
dispersing an oxide in the matrix uniformly and minutely by a
mechanical alloying.
[0011] To achieve the above object of the present invention, there
is provided a process for making an oxide dispersion-strengthened
tungsten heavy alloy by mechanical alloying, the process including
the steps of: adding 0.1 to 5 wt. % of Y.sub.2O.sub.3 powder to a
mixed powder comprising more than 90 wt. % of tungsten powder, and
nickel and iron powders for the rest; subjecting the resulting
mixture to a mechanical alloying to prepare an oxide
dispersion-strengthened tungsten heavy alloy powder; compacting the
tungsten heavy alloy powder into a compressed powder using a press;
and sintering the compressed powder at a temperature in the range
of 1400 to 1600.degree. C.
[0012] The oxide dispersion-strengthened tungsten heavy alloy
prepared by the mechanical alloying is characterized in that the
fine Y.sub.2O.sub.3 particles are uniformly dispersed in the
matrix. The addition of the Y.sub.2O.sub.3 particles stable at high
temperatures results in enhanced high-temperature strength and a
reduction of the shearing strain of the fracture during high-speed
shear deformation, based on which fact, a technique has been
developed to control the mechanical properties of the oxide
dispersion-strengthened tungsten heavy alloy by varying the added
amount of the Y.sub.2O.sub.3.
[0013] The tungsten heavy alloy uses the high density (19.3
g/cm.sup.3) of tungsten and its chief application, e.g., the
amour-plate-destroying kinetic energy piercer has the piercing
performance enhanced with an increase in the density. The depleted
uranium alloy used for the kinetic energy piercer has a high
density, for example, 17.2 g/cm.sup.3 for DU-8Mo and 17.3
g/cm.sup.3 for DU-6Nb. For that reason, the tungsten heavy alloy
suitable for the kinetic energy piercer must have a high density of
more than 17 g/cm.sup.3 so that it has to contain more than 90 wt.
% of tungsten.
[0014] The conventional ball milling method using a low energy
simply mixes the powers without a reduction of the grain size or
the lamella distance. The most important thing in the process for
making the oxide dispersion-strengthened tungsten heavy alloy is
uniformly dispersing the fine oxide particles in the matrix. So,
the present invention utilizes the mechanical alloying method to
obtain a tungsten heavy alloy powder having a reduced grain size of
less than 100 nm and a reduced lamella distance of less than 0.5
.mu.m.
[0015] The added amount of the Y.sub.2O.sub.3 is preferably in the
range from 0.1 to 5 wt. % based on the total weight of the tungsten
heavy alloy. The amount of the Y.sub.2O.sub.3 less than 0.1 wt. %
hardly contributes to the reduction of the tungsten grain size or
an increase in the high-temperature mechanical properties, whereas
the amount of the Y.sub.2O.sub.3 more than 5 wt. % rather results
in a deterioration of the mechanical properties and mechanical
processability.
[0016] The tungsten alloy powder obtained by the mechanical
alloying method of the present invention is then compacted under a
pressure into a compressed powder and then subjected to a sintering
in the temperature range of 1400 to 1600.degree. C. The sintering
temperature below 1400.degree. C. hardly achieves sufficient
sintering and reduces the relative density of the final product
with a deterioration of the mechanical properties, whereas the
sintering temperature above 1600.degree. C. excessively increases
the liquid phase fraction and the diffusion velocity to result in a
rapid growth of the tungsten grains with a deterioration of the
mechanical properties.
[0017] The sintering step is preferably performed under the
reducing atmosphere, for example, hydrogen atmosphere in order to
reduce the oxide that is contained in the metal powder and inhibits
the densification. As such, a second heat treatment is preferably
performed under the nitrogen atmosphere so as to remove the
residual hydrogen.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a diagram showing a process for making the oxide
dispersion-strengthened tungsten heavy alloy in accordance with the
present invention;
[0019] FIG. 2 shows the variations of the particle size of tungsten
grains based on the Y.sub.2O.sub.3 content after sintered at
1485.degree. C. for one hour;
[0020] FIG. 3 is a curve showing the compression strength of the
oxide dispersion-strengthened tungsten heavy alloy of the present
invention subjected to high-temperature compression at 800.degree.
C.;
[0021] FIG. 4 shows the variations of the shape ratio of tungsten
grains based on the distance from the fracture surface after a
shearing test that involves high-speed compression of the
conventional tungsten heavy alloy and the oxide
dispersion-strengthened tungsten heavy alloy of the present
invention; and
[0022] FIG. 5 shows the piercing depth per density of a kinetic
energy piercer made from the conventional tungsten heavy alloy and
one made from the oxide dispersion-strengthened tungsten heavy
alloy of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0023] Hereinafter, the present invention will be described in
further detail with reference to the following example.
EXAMPLE
[0024] 0.1 to 5 wt. % of Y.sub.2O.sub.3 powder was added to a
tungsten heavy alloy comprising 93 wt. % of W, 5.6 wt % of Ni and
1.4 wt. % of Fe and the mixture was uniformly mixed by a mechanical
alloying method. The W powder had a purity of 99.9% and a particle
size of 2.5 .mu.m, the Ni powder having a purity of 99.7% and a
particle size of 2.5 .mu.m, the Fe powder having a purity of 99.6%
and a particle size of 3.5 .mu.m, the Y.sub.2O.sub.3 powder having
a purity of 99.9% and a particle size of 2 .mu.m. The mechanical
alloying method using ball milling was carried out under the
conditions that the milling speed was 75 rpm, the ball-to-powder
ratio 20:1, the ball packing percentage 15% and the milling time 72
hours.
[0025] The oxide dispersion-strengthened tungsten heavy alloy thus
obtained by the mechanical alloying was compacted into a compressed
powder under the pressure of 100 MPa using a press. The compacted
compressed powder was then sintered under the hydrogen atmosphere
at 1485.degree. C. for one hour. To remove the tungsten heavy alloy
of the residual hydrogen that deteriorates the mechanical
properties, the sintered powder was subjected to a heat treatment
under the nitrogen atmosphere at 1150.degree. C. for one hour and
then cooled in water.
[0026] FIG. 1 is a diagram showing a process for making the oxide
dispersion-strengthened tungsten heavy alloy in accordance with the
present invention.
[0027] An observation of the fine structure of the tungsten heavy
alloy thus obtained shows that Y.sub.2O.sub.3 almost appears in the
interface between the tungsten and the matrix and not in the
tungsten grains. The oxide, i.e., Y.sub.2O.sub.3 existing in the
interface between the tungsten and the matrix inhibits the growth
of the tungsten grains. When the tungsten grains were sintered with
varying the amount of the oxide, the particle size of the tungsten
grains was decreased from 30 .mu.m to 10 .mu.m with an increase in
the amount of the Y.sub.2O.sub.3 from 0.1 wt. % to 5.0 wt. %, as
shown in FIG. 2. This demonstrates that the addition of the oxide
is effective in the refinement of the tungsten grains.
[0028] Meanwhile, a high-temperature compression test was carried
out at a testing temperature of 800.degree. C. with a hot working
simulator (Thermecmaster) that realized the strain rate of
10.sup.1/s in order to examine the high-temperature compacting
property of the oxide dispersion-strengthened tungsten heavy alloy
of the present invention. The results are presented in FIG. 3.
[0029] As shown in FIG. 3, the oxide dispersion-strengthened
tungsten heavy alloy had a high-temperature compression strength
gradually increased with an increase in the added amount of the
Y.sub.2O.sub.3 from 0.1 wt. % to 1.0 wt. %. This means that the
dispersion strengthening of the oxide stable at high temperatures
inhibits the deformation of the tungsten heavy alloy and that the
addition of the oxide effectively enhances the high-temperature
compression strength.
[0030] FIG. 4 shows the variations of the shape ratio of the
tungsten grains based on the distance from the fracture surface
after a shearing test that involves high-speed compression of the
conventional tungsten heavy alloy and the oxide
dispersion-strengthened tungsten heavy alloy of the present
invention. As is apparent from FIG. 4, a Hopkinson bar test was
carried out on the samples designed to have the same shearing
strain to examine the change of the sharing strain of the samples
fractured at a strain rate of 10.sup.4/s. As the distance from the
fracture surface decreased, the conventional tungsten heavy alloy
had a sharp increase in the shape ratio of the tungsten grains,
whereas the oxide dispersion-strengthened tungsten heavy alloy of
the present invention showed a gradual increase in the shape ratio
of the tungsten grains. This implies that the oxide
dispersion-strengthened tungsten heavy alloy is susceptible to
local fracture under the shearing strain.
[0031] On the other hand, a 7 mm-diameter kinetic energy piercer
composed of the tungsten heavy alloy was subjected to a piercing
test and the residual piercer was obtained. As a result, the oxide
dispersion-strengthened tungsten heavy alloy was more susceptible
to fracture at the edge portions under the shearing strain than the
conventional tungsten heavy alloy and showed self-sharpening.
According to the results of the experiments carried out by the
inventors, after the piercing test, the conventional tungsten heavy
alloy had a diameter of 11.8 mm with a diameter increment of 69%
and the oxide dispersion-strengthened tungsten heavy alloy had a
diameter of 8.8 mm with a diameter increment of no more than
25%.
[0032] FIG. 5 shows the piercing depth per density of the
conventional tungsten heavy alloy and the oxide
dispersion-strengthened tungsten heavy alloy of the present
invention, in which the latter has a larger piercing depth per
density than the former. This presumably results from the
self-sharpening effect of the oxide dispersion-strengthened
tungsten heavy alloy, which is caused by the increased
high-temperature compression strength and the reduced elongation
percentage.
[0033] Compared to the conventional tungsten heavy alloy, the oxide
dispersion-strengthened tungsten heavy alloy manufacture according
to the present invention maintains refinement of the tungsten
grains with enhanced high-temperature compression strength and
readily fractures under shearing strain to have the self-sharpening
effect necessary to an excellent piercing property. Due to these
features, the oxide dispersion-strengthened tungsten heavy alloy of
the present invention is useful as a material for kinetic energy
piercers.
[0034] It is to be noted that like reference numerals denote the
same components in the drawings, and a detailed description of
generally known function and structure of the present invention
will be avoided lest it should obscure the subject matter of the
present invention.
* * * * *