Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
2~3
This invention relates to a process for producing dis-
persion strengthened precious metal alloys. The present inven-
tion can provide alloys containing platinum, palladium, rhodium
and gold which are useful in the production o~ glass fibers.
One of the most exacting applications of platinum is
in the production of glass fibers. Molten glass often at tem-
peratures ranging from 1200 to 1600C passes through a series of
orifices in a bushing. Advances in glass fiber production are
demanding both larger bushings and higher operating temperatures.
Structural components such as these at elevated tem-
peratures under constant loads experience continuous dimensional
changes or creep during their lives. This creep behavior depends
upon the interaction between the external conditions (load,
temperature) and the microstructure of the component. In recent
times, increased resistance to creep of material systems has
been accomplished by using a dispersion of very small, hard
particles (called dispersoids) to strengthen the microstructure
of the component. These systems have become to be known as dis-
persion-strengthened metals and alloys and the dispersoids used
are usually oxides.
A recent development in dispersion-strengthening is
called mechanical alloying. Generally, the process uses a high
energy ball mill to achieve the intimate mechanical mixing typi-
cal of the process. An attritor mill or vibratory mill also can
be used. While mechanical alloying has been applied to some of
the transition metals, no actual work has been reported on pre-
cious metals such as platinum.
The present invention provides a process for producing
dispersion-strengthened precious metal alloys having creep re-
sistance superior to known dispersion-strengthened platinum
- 1 -
1~7~i8~8
alloys.
According to the process of this invention, a process
for producing dispersion strengthened precious metal alloys com-
prises the step of mechanically alloying precious metal powder
and at least one dispersoid together wherein the dispersoid is
present in effective dispersion strengthening amounts.
The mechanical alloying is preferably carried out by
a high energy ball mill to achieve the intimate mechanical mixing
of this process. The oxide particles are forged into the pre-
cious metal matrix powder particle to form a composite powderparticle.
In the drawing,
Figure 1, illustrates the internal arrangement in an
attritor mill showing the impeller, grinding media and external
cooling jacket. Impact events occur in the dynamic interstices
of the media created by the impeller during stirring.
There are several high-energy ball mills commercially
available either using a stirrer to induce the deformation events
or vibratory motion. Figure 1 shows an overall view of the
attritor mill 10. The stainless steel bearings or grinding
media 12 and the powder charge go into the cylindrical ~ontainer
of the mill. The high-energy impacts are effected by the ro-
tating impeller 14. Figure 1 also illustrates the internal
arrangement in the attritor mill, impact events occur in the dy-
namic interstices of the media created by the impeller during
stirring.
1~06~ 7~
1 Dispersion strenyt~ened precious metals are kno~n
in the art dnd are commercially available. One such
m2terial is that available from Johnson, ~dtthey & Co.
Limited, under their designation ZCS. The above indicated
5 ZGS rnateridl consists essentially of platinum in ~hich the
disperoid is zirconia; the latter is present in an amount
of about 0.5~. by volume.
Tne dispersion strengthened precious metals of
this invention generally comprise a precious metal, or
10 precious metal alloy, preferably platinum, as the
dispersing medium, or matrix, and a dispersoid of a metal
oxide, metal carbide, metal silicide, metal nitride, metal
sulfide or d metal boride which dispersoid is present in
effe~ctive dispersion strengthening amounts. Usually such
15 amounts will be between about U.l percent to about 5.0
percent by volurne. Preferably the dispersoid ~Jill be an
oxide. Exemplary of metal compounds which may be employed
as the dispersoid are compounds of metals of Group IIA,
IIIA, IIIB (including non-hazardous metals of the Actinide
20 and Lanthanide classes), IVB, VB, VIB and VIIB. More
specifically exemplary of sui~able metals are the
following: Be, Mg, Ca, Ba, Y, La, Ti, Zr, Hf, Mo, W, Ce,
Nd, Gd, and Th as well as Al.
Several rnechanical alloying experiments were
25 performed using the attritor mill to generate a composite
powder for consolidation. Wash heats intended to coat a
thin layer of platinum on the internal workings surfaces of
the attritor mill were carried out. This "conditioning"
treatment was intended to prevent iron contamination of
subsequent milling experiments, but several washes were
re4uired before the iron contamination was reduced to what
was believed to be an acceptable level.
The samples then are consolidated by vacuum hot
pressing ~VHP) at elevated temperatures and pressures. In
35 the alternative, the samples can be consolidated by first
cold pressing at elevated pressures followed by sintering
dt elevated temperatures. VHP generally is carried out at
,
.
.
.
:
-4--
1 a temperaturc ranying fron 1300 to 1700C under a pressure
ranging from 500 to 10,C00 psi for a time ranging from 10
to 30 minutes. Preferably, the temperature ranges from
1400 to 15U0C under a pressure of 3,000 ~o 6,Q0~ psi for a
5 time of 15 to 25 minutes. Generally, the cold pressing is
carried out at 3 pressure ranging from ~,Q00 to 10,000 psi
for up to 5 minutes followed by sintering at a tPmperature
ranginy from 1200 to 1700C for 2 to 6 hours.
EXA~PLE I
Approximately one kgm of -325 mesh (-44 micron)
platinum sponge from Englehard WdS blended with an amount
of yttria (Y203) to giv~ nominally 0.65 volume percent
(0.15 weight percent3 oxide loading in the final compact.
The yttria was 200-600 angstrom in size. The platinum
15 matrix starting powder for the experiment consisted of very
fine, near spherical particles or chdined aggregates. Most
of the particles below 2 microns appeared to be single
crystals. The starting powder had a fairly high specific
surface area.
The powder mixture was charged into the
container of the attritor mill while it was running. The
grinding media had ~een previously loaded to give a volume
ratio of rnedia to powder of about 20:1. The grinding media
used was a hardened 400 series stainless steel bearing
25 nominally 3/8 inch (0.953 cm) diameter. The impeller
rotational speed was selected at 130 rpm.
Samples of powder were removed at various times
to obtain information on the changes in particle nlorphology
and specific surface area with milling time. The first
30 sample WdS taken after one hour of milling and indicated
that flake generation was in progress.
After milling for three hours, another powder
sample was taken for metallographic characterization.
~hile rnore flakes were yenerated, the extent of plastic
deformation seemed to have increased. Flake cold welding
dppeared to have taken place as well. Tne composite flake
appeared to have three or four component flakes cold welded
togethex. No edge cracking appeared in the composite flake sug-
gesting that work hardening saturation had not been reached at
this point.
After milling for 23 hours, the composite flakes appear-
ed to thicken. This clearly demonstrates the cold welding aspect
of the milling action. Along with cold welding, the flake diame-
ter appeared to increase.
The experiment was continued for 71 hours then terminat-
ed, and the powder was removed for further processing.
There appeared to be a ~airly high initial surface a-
rea generation rate. The iron contamination in the milled pow-
der was greatly reduced compared to the previous experiments and
reflects the coating action that appeared to minimize wear debris
generation during milling. The maximum iron contamination level
in the powder was approximatel~ 300 wppm. The milled powder was
consolidated by vacuum hot pressing and thermomechanically pro-
cessing into sheet for creep testing, the details are to follow.
EXAMPLE II
Example I produced a powder of relatively low iron con-
tamination. Since this experiment resulted in small powder lots(nominally 80 gms) taken at various times during the milling ex-
periment, each sample was individually consolidated by vacuum hot
pressing (VHP) at 1,450 C under 5,000 psi (34.5 MN/m ) for twenty
minutes. The resultant compacts were nominally 1 inch (2.54 cm)
in diameter.
Relative density of specimens are listed.
Specimen Milling Time (hr.) Relative Density (~)
A 0 95.2
B 1 98.2
30 C 2.5 99.8
D 6 99.8
. -- 5 --
"'~-."'
~.17~
The thermomechanical processing (TMP) schedule used
on the compact consisted of several roll/anneal cycles. The
basic operation involved rolling a sheet specimen and cropping
pieces after various rolling passes
- 5a -
I~vo3A -6-
1 for rnicrostructural characterization. The procedure used
was to roll the compact for d 10 percent reduction in area
then anneal the rolled specimen for five minutes a~
nominal'ly 1,040C before further rolling.
Specimen D was the most responsive to the TMP
cycles. After the 10th rolling pass, the grain structure
was fairly elongated. The lack of oxide clusters during
optica1 metdlloyraphic eXamindtiOn suggested that the
milling action had worked the yttria into the platinum
10 Illatrix- A me~tdlloyraphic analysis of the same region
showed the development of d moderate grain aspect ratio
(grain length to thickness ratio in the viewing plane). As
the number of roll/anneal cycles increased, the grain
aspect ratio (GAR) increased. At this stage a moderate GAR
15 also had been developed in a transverse direction. The
significance of this observation is that the grains took on
the shape of a pancake structure thin in a direction
perpendiculdr to the sheet yet extended in the other two
directions. Since a GAR seems to extend in two directions
2~ in the rolled sheet and the state of stress in a bushing
tip plate is biaxial, this tr~rsverse GAR development may
be very benef'icial for good creep resistance in bushing
applications.
After the 16th rolling pdSS, the elongation of
25 the grains had increased significantly. A higher
m~gnifio~tion view of the same region revealed the de3ree
of grain elongation and fineness of the grain size. The
transverse GAR had a'lso significantly increased. These
elongated grain morphologies are desirable microstructures
30 for good creep resistance.
INDUSTRIAL APPLICABILITY
EXAMPLE III
Creep Testing
All the creep testing was done in air using
constant load machines, the elongation ~as measured by an
LVDT connected to a multi-point recorder and a precision
digital voltmeter. Specimen temperature was monitored with
1 ~,0~
-7--
1 a calibrated Pt/Pt-Rh thermocouple attached so that the
bead was adjacent to the gage section of the creep
specimen. The creep specimen WdS a flat plate type with a
gage lenyth of approximately 2.25 inch (5.72 crn). The
5 tensile stress was applied parallel to the rolling
direction (longitudinal direction). The yeneral procedure
was to hang the specimen in the furance to reach thermal
equilibrium then start the rig timer upon application of
the load. Periodic temperature and extension measurements
10 were made eitner until the specimen failed or the test was
terminated (specimen rerlloval or furnace burn-out).
Creep results were obtained from specimens that
were processed according to Example II except that these
specimens were milled 10 hours and received the above
15 thermomecnanical processing treatment of 10% reduction in
area per pass with an intermediate anneal at nominally
1040C for 5 minutes. The extent of deformation was
nominally an ~5% reduction in area. The first specimen had
a varied creep history that started by applying a tensile
20 stress of 1,C00 psi (6.~9 Mn/m2) at 2,400F (1,316C). The
resultant secondary creep rate was too low to adequately
measure; therefore, the temperature was increased to
2,600F (1,427C) and a secondary creep rate of 4.5x10-6
hr 1 was observed. After approximately 118 hours the
25 stress was increased to 1,400 psi (9.65 Mn/m2) and a new
secondary creep rate of nominally 3x10 5 hr 1 was recorded.
These creep rates are two orders of magnitude less than
that for the previously indicated ZGS under the same
testing conditions. The ~GS material will have a stress
rupture life of at least 48 hours when tested at l400C and
lO00 psi in the rolling direction of the sheet.
The general microstructure of the crept specirnen
indicated that the yrains were highly elongated in the
rolliny direction ~creep stress direction also) and the
grain boundries were ragged. There appeared to be evidence
of subgrains in the structure as well. The microstructure
observed in this specimen ~!as typical of that of a good
3~ 7
-8-
1 creep resistant material as evidenced by the exceptionally
good creep properties.
;
: .
,
