WO1981000977A1

WO1981000977A1 - Process for producing dispersion strengthened precious metal alloys

Info

Publication number: WO1981000977A1
Application number: PCT/US1980/001061
Authority: WO
Inventors: F Roehrig
Original assignee: Owens Corning Fiberglass Corp
Priority date: 1979-10-04
Filing date: 1980-08-18
Publication date: 1981-04-16
Also published as: GB2075553A; JPS56501456A; CA1178828A; SE8103480L

Abstract

Process for producing dispersion-strengthened precious metal alloys having superior creep resistance. According to this invention precious metal powders and dispersoids are mechanically alloyed together.

Description

D E S C R I P T I O N

PROCESS FOR PRODUCING DISPERSION STRENGTHENED PRECIOUS METΛL ALLOYS

TECHNICAL FIELD This invention relates to a process for producing dispersion strengthened precious metal alloys. The present invention can provide alloys containing platinum, palladium, rhodium and gold which are useful in the production of glass fibers.

BACKGROUND ART One of the most exacting applications of platinum is in the proαuction of glass fibers. Molten glass often at temperatures ranging from 1200 to 1600°C passes through a series of orifices in a Lushing. Advances in glass fiber production a r e demanding both larger bushings and higher operating temperatures. Structural components such as these at elevated temperatures 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 mi crostr ucture 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 di spersoi ds) to strengthen the microstructure of the component. These systems have become to be known as dispersion-strengthened metals and alloys a nG the disperse ids used a r e usually oxides. A recent development in di spersion-strengthening is called mechanical alloying. Generally, the process uses a high energy ball mill to achieve the intimate mechanical mixing typical of the process. An attritor mill or vibratory mill also can be used. While mechanical alloying has bean applied to some of the transition metals, no actual work has been reported on precious metals such as piatinum.

DISCLOSURE OF THE INVENTION The present invention provides a process for producing dispersion-strengthened precious metal alloys having creep resistance superior to known dispersi on- strengthened platinum alloys.

According to the process of this invention, precious metal pov.der end cispεrsoids ere mechanically alloyed together. The mechanical alloying uses a high energy ball mill to achieve the intimate mechanical mixing of this process. The oxide particles are forged into the precious metal matrix powder particle to form a composite povider particle.

BRIEF DESCRIPTION OF DRAWINGS FIG. 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.

BEST MODE OF CARRYING OUT INVENTION There are several high- energy ball mills commercially available either using a stirrer to induce the deformation events or vibratory motion. FIG. I shows an overall view of the attritor mill . The stainless steel bearings or grinding media and the powder charge go into the cylinαrical container of the mill . The high-energy impacts are effected by the rotating impeller. FIG. 1 also illustrates the internal arrangement in the attritor mill, impact events occur in the dynamic interstices of the media created by the impeller during stirring. Dispersion strengthened precious metals are known in the art and are commercially available. One such material is that available from Johnson, Matt hey & Co. Limited, under their designation ZGS. The above indicated ZGS material consists essentially of platinum in which the disperoid is zirconia; the latter is present in an amount of about 0.5% by volume.

The dispersion strengthened precious metals of this invention generally comprise a precious metal, or precious metal alloy, preferably platinum, as the dispersing medium, or matrix, and a disperse id of a metal oxide, metal carbide, metal suicide, metal nitride, metal sulfic'a or a metal boride which dispersoid is present in effective dispersion strengthening amounts. Usually such amounts will be between about C.l percent to about 5.0 percent by volume. Preferably the dispersoid will be an oxide. Exemplary of metal compounds which may be employed as the dispersoid are compounds of metals of Group IIA, IIIA, 111 B (including non-hazardous metals of the Actinide and Lanthanide classes), I VB , VS , VI B and VI IB. More specifically exemplary of suitable metals a r e the following: Be, Hg, Ca, Ba, Y, La, Ti , Zr, Hf, Mo, W, Ce, Nd , Gd , and Th as well as Al .

Several mechanical alloying experiments were 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 required 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 ana pressures. In the alternative, the samples can be consolidated by first coIo pressing at elevated pressures followed by sintering at elevated temperatures. VHP generally is carried out at a temperature ranging from 1300 to 1700°C under a pressure ranging from 500 to 10,000 psi for a time ranging from 10 to 30 minutes. Preferably, the temperature ranges from 1400 to 1500°C under a pressure of 3,000 to 6,000 psi for a time of 15 to 25 minutes. Generally, the cold pressing is carried cut at a pressure ranging from 2,000 to 10,000 psi for up to 5 minutes followed by sintering at a temperature ranging from 1200 to 1700°C for 2 to 6 hours.

EXAMPLE I

Approximately one kgm of -325 mesh (-44 micron) platinum sponge from Englεhard was blended with an amount of yttria (Y₂O₃ ) to give nominally 0.65 volume percent (0.15 weight percent) oxide loading in the final compact. The yttria was 200-600 angstrom in size. The platinum matrix starting powder for the experiment consisted of \/ ery fine, near spherical particles or chained aggregates. Most of the particles below 2 microns appeared to be single crystals. Tne 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 been previously loaded to give a volume ratio of media to powder of about 20:1. The grinding media useo was s hardened 400 series stainless steel bearing 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 morphology and specific surface area with milling time. The first sample was 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 metal 1 ographic characterization. While more flakes were generated, the extent of plastic deformation seemed to have increased. Flake cold welding appeared to h a v e taken place as well . The composite flake appeared to have three or four component flakes cold welded together. No edge cracking appeared in the composite flake suggesting that work hardening saturation had not been reached at this point.

After milling for 23 hours, the composite flakes appeared to thicken. This clearly demonstrates the cold welding aspect of the milling action. Along with cold welding, the flake diameter appeared to increase.

The experiment was continued for 71 hours then terminated, and the powαer was removed for further processing.

There appeared to be a fairly high initial surface area generation rate. The iron contamination in the milled powder was greatly reduced compared to the previous experiments and reflects the coating action that appeared to minimize wear debris generation curing milling. The maximum iron contamination level in the powder was approximately 300 wppm. The milled powder was consolidated by vacuum hot pressing and thermomechanically processing into sheet for creep testing, the details are to follow.

EXAMPLE II

Example I produced a powder of relatively low iron contamination. Since this experiment resulted in small powder lots (nominally 20 gms) taken at various times during the milling experiment, each sample was individually consolidated by vacuum hot pressing (VHP) at 1,450°C under 5,G0C psi (34.5 MN/m²) for twenty minutes. The resultant compacts were nominally 1 inch (2.54 cm) in diameter.

Relative densit of s eciments are listed.

The thermomechanical processing (TMP) scheduled 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 for microstructural characterization. The procedure used was to roll the compact for a 10 percent reduction in area then anneal the rolled specimen for five minutes at nominally 1,040°C 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 optical metal 1 ographi c examination suggested that the milling action had worked the yttria into the platinum matrix. A metallogrephic analysis of the same region showed the development of a 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 also hάα 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 perpendicular to the sheet yet extended in the other two directions. Since a GAR seems to extend in two directions in the rolled sheet and the state of stress in a bushing tip plate is biaxial, this transverse GAR development may be v ery beneficial for good creep resistance in bushing applications.

After the 16th rolling pass, the elongation of the grains had increased significantly. A higher magnification view of the same region revealed the degree of grain elongation and fineness of the grain size. The transverse GAR had also significantly increased. These elongated grain morphologies are desirable microstructures for good creep resistance.

INDUSTRIAL APPLICABILITY EXAMPLE III

Creep Testing

All the creep testing was cone in air using constant load machines, the elongation was measured by an LVDT connected to a multi-point recorder and a precision digital voltmeter. Specimen temperature was monitored with a calibrated Pt/Pt-Rh thermocouple attached so that the bead was adjacent to the gεge section of the creep specimen. The creep specimen was a flat plate type with a gage length of approximately 2.25 inch (5.72 cm) . The tensile stress was applied parallel to the rolling direction (longitudinal direction) . The general 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 were made either until the specimen failed or the test was terminated (specimen removal or furnace burn-out).

Creep results were obtained from specimens that we r e processed according to Example II except that these specimens were milled 10 hours and received the above thermomechani cal processing treatment of 10% reduction in area per pass with an intermediate anneal at nominally 1040°C for 5 minutes. The extent of deformation was nominally an 85% reduction in area. The first specimen had a varied creep history that started by applying a tensile stress of 1,000 psi (6.89 Mn/m²) at 2,400°F (1,316°C) . The resultant secondary creep rate was toe lew to adequately measure; therefore, the temperature was increased to 2,600°F (1,427°C) and a secondary creep rate of 4.5x10^-6 h r^-1 was Observed. After approximately US hours the stress was increased to 1,400 psi (9.65 Mn/m² ) and a new secondary creep rate of nominally 3x10^-5 h r^-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 ZGS material will have a stress rupture life of at least 48 hours when tested at 1400ºC and 1000 psi in the rolling direction of the sheet.

The general microstructure of the crept specimen indicated that the grains were highly elongated in the rolling direction (creep stress direction also) end the grain bouncries were ragged. There appeared to be evidence of subgrains in the structure as well . The microstructure observed in this specimen was typical of that of a good creep resistant material as evidenced by the exceptionally good creep properties.

Claims

C L A I MS

1. A process for producing dispersion strengthened precious metal alleys comprising the step of mechanically alloying precious metal powder and at least one dispersoid together wherein the dispersoid is present in effective dispersion strengthening amounts.

2. A process for producing dispersion strengthened precious metal alloys comprising the steps of:

(1) mechanically alloying precious metal powder and at least one dispersoid together wherein the dispersoid is present in effective dispersion strengthening amounts; and (2) consolidating the resulting powder.

3. A process according to claim 2 wherein the consolidating is carried out by vacuum hot pressing at elevated temperature and pressures.

4. A process according to claim 2 wherein the consolidating is carried out by first cold pressing at elevated pressures ana then sintering at elevated temperatures.

5. A process according to claims 1 or 2 wherein the precious metal powder is platinum or a platinum alloy.

6. A process according to claims 1 or 2 wherein the disperoids include a metal oxide.

7. A process according to claims 1 or 2 wherein the precious metal powder is platinum and the disperoids include yttria (Y₂O₃).

C. A process according to claims 1 or 2 wherein high energy ball milling is used to achieve the mechanical alloying.

9. A process for producing dispersion strengthened precious metal alloys comprising the steps of :

(1) mechanically alloying platinum powder and yttria (Y₂O₃) together wherein the yttria is present in effective dispersion strengthening amounts; and

(2) consolidating the resulting powder by vacuum hot pressing at elevated temperatures and pressures .

10. A process according to claim 9 wherein the amount of yttria ranges between 0.1 and 5.0 percent by voIume.

11. A process according to claim 9 wherein the amount of yttria is 0.65 percent by volume (0.15 percent by weight) .

12. A process according to claim S wherein the vacuum hot pressing is carried out at a temperature ranging from 1300 to 1700ºC uncer a pressure ranging from 500 to 10,000 psi for a time ranging from 10 to 30 minutes.

13. A process according to claim 9 wherein the vacuum hot pressing is carried out at a temperature ranging from 1400 to 1500 ºC under a pressure ranging from 3,000 to 6,000 psi for a time ranging from 15 to 25 minutes.

14. A process according to claim 9 wherein the vacuum hot pressing is carried out at a temperature of 1,450ºC unαer a pressure of 5,000 psi for a time of twenty minutes.

15. A process according to claim 9 wherein high energy bell milling is used to achieve the mechanical alloying.

16. A process for producing dispersion strengthened precious metal alloys comprising the steps of:

(2) consolicating the resulting powder by first cole pressing at a pressure ranging from 2,000 to

10,000 psi for up to 5 minutes and then sintering at a temperature ranging from 1200 to 1700°C for 2 to 6 hours