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[ EN ]
This invention relates to thermomechanical processing of dispersion-strengthened precious metal alloys The present invention can provide alloys containing platinum, palladium, rhodium and gold which are useful tn the production of glass fibers.
One of the most exacting applications of platinum is in the production of glass fibers. Molten glass often at temperatures 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 temperatures under constant loads experience continuous dimensional changes or creep during their lives. This creep behavior depends upon the interaction between the external conditions Cload, 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 dispersion-strengthened metals and alloys and the dispersoids used are usually oxides. A recent development in dispersion-strengthening is mechanical alloying which 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.
Accordingly, the present invention provides a process for producing sheets of a dispersion-strengthened precious
- metal alloy which includes (1) platinum or a platinum alloy and (2) at least one metal oxide, comprising the step of thermomechanically processing the compacted disperslonstrengthened precious metal alloy.
Preferably a series of mechanical deformation and annealing cycles are used to help develop a creep resistant microstructure. Specifically, this may be achieved by rolling and annealing a powder compact of dispersion-strengthened precious metal. The material may be cross-rolled as well as longitudinally rolled or just longitudinally rolled.
Figure 1 is a schematic drawing of the rolling operation.
In a preferred embodiment of this invention, theprocedure used to thermomechanically process the compact was to roll the compact for a 10 percent reduction in area, then anneal the rolled specimen. The reduction in area is carried out under a pressure that elongates the rolled specimen without substantially widening it. Generally, the annealing is carried out for a period of time and at a temperature sufficient to develop a specimen with a minimum creep rate. Preferably the annealing is carried out five minutes at l,900F (1,040C) before further rolling. The total extent of deformation ranges from 50 to 90 percent reduction in area and generally is approximately an 85 percent reduction in area. This roll/anneal cycle was selected to help develop a creep resistance microstructure. The roll~anneal cycles are continued until the 85 percent reduction in area is accomplished.
There are several high-energy ball mills commercially available either using a stirrer or vibration 1 to induce mechanical alloying. Stainless steel bearings or grinding media and the powder charge go into the cylindrical container of the mill. The high-energy impacts are affected by the rotating impeller. In the internal 5 arrangemellt of the attritor mill, ilnpact 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 10 material is that available from Johnson, Matthey ~ Co.
Limited, under their designation ZGS. The above indicated
ZGS material consists essentially of platinum in which the dispersoid is zirconia; the latter is present in an amount of about 0.50 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 dispersoid of a metal oxide, metal carbide, metal silicide, metal nitride, metal 20 sulfide or a metal boride which dispersoid is present in effective dispersion-strengthening amounts. Usually such amounts will be between about 0.1 percent to about 5.0 percent by volume. Preferably the dispersoid will be an oxide. Exemplary of metal compounds which may be employed 25 as the dispersoid are compounds of metals of Group IIA,
IIIA, IIIB (including non-hazardous metals of the Actinide and Lanthanide classes), IVB, VB, VIB and VIIB. store specifically exemplary of suitable metals are the following: Be, Mg, Ca, Ba, Y, La, Ti, Ir, 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 working surfaces of 35 the attritor mill were carried out. This "conditioning" treatment was intended to prevent iron contamination of subsequent milling experiments, but several washes were 1 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 and pressures. In 5 the alternative, the samples can be consolidated by first cold pressing at elevated pressures followed by sintering at elevated temperatures. VHP generally is carried out at a temperature ranging from 1300 to 1700C under a pressure ranging from 500 to lU,000 psi for a time ranging from 10 10 to 30 minutes. Preferably, -the temperature ranges from 1400 to 1500C under a pressure of 3,00Q to 6,000 psi for a time of 15 to 25 minutes. Generally, the cold pressing is carried out at a pressure ranging from 2,000 to 10,000 psi for up to 5 minutes followed by sintering at a temperature 15 ranging from 1200 to 1700C for 2 to 6 hours.
Approximately one kgm of -325 mesh (-44 micron) platinum sponge from Englehard was blended with an amount of yttria (Y203) to give nominally 0.65 volume percent 20 (0-15 weight percent) oxide loading in the final compact.
The yttria d was nominally 20G-600 angstrom in size. The platinum matrix starting powder for the experirnent consisted of very fine, near spherical particles or chained aggregates. Most of the particles below 2 microns appeared 25 to be single crystals. The starting powder had a fairly high specific surface area.
The powder mixture Wd1 charged into the container of the attritor mill while it was running. The grinding media had been previously loaded to sive a volume 30 ratio of media to powder of 20:1. The grinding media used was a harderled 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 35 to obtain information on the changes in particle morphology and specific surface area with milling time. The first 1 sample was taken after one hour of milling and indicated that flake generation was in progress.
After milling For three hours, another pow(ler sample was taken for metallo-Jrdphic characterization. 5 awhile more flakes were generated, the extent o~ plastic defornlaticn seemed to have increased. Flake cold welding appeared to have taken place as well. The composite flake appeared to tlaYe three or four component flakes cold welded together. No edge cracking appeared in the composite flake 10 suggesting that work hardening Datura-tion hac 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 15 welding, the flake diameter appeared to increase.
The experin,ent was continued for 71 hours then terminated, and the powder was removed for further processing.
There appeared to be a fairly high initial 20 surface area generation rate. The iron contamination in the milled holder 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 25 approximately 000 wppm. The milled powder was consolidated by vacuum hot pressing and thermomechanically processing into sheet for creep testing, the details are to follow.
Example I produced a powder of relatively low 30 iron contamination. Since this experiment resulted in small powder lots (nominally ~0 gms) taken at various times during the milling experiment, each sample was individually
- consolidated by vacuum hot pressing (VHP) at 1,450C under 5,000 psi (34.5 ~lN/m2) for twenty minutes. The resultant compacts were nominally 1 inch (2.54 cm) in diameter.
Relative density of specimerls are listed.
1 Specilllen filling Time (hr.) Relative density (Ulm)
The thermomechanical processing (TMP) 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 10 characterization. Tle procedure used has to roll the compact for a 10 percent reduction in area then anneal the rolled specimen for five minutes at nominally 1,040C before further rolling.
Specimen D was the most responsive to the TMP 15 cycles. After the 10th rolling pass, the grain structure was fairly elongated. The lack of oxide clusters during optical metallographic examination suggested that the milling action had worked the yttria into the platinum matrix. A metallographic analysis of the same region 20 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 had been developed in a transverse direction. The 25 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 GhR seems to extend in thio directions in the rolled sheet and the state of stress in a bushing 30 tip plate is biaxial, this transverse GAR development may be very beneficial for good creep resistance in bushing applications.
After the 16th rolling pass, the elongation of the grains had increased significantly. A higher 35 magnification view of the same region revealed the degree !~ of drain elongation and fineness of the grain size. The transverse GAR had also significantly increased. These 1 elongated grain morphologies are desirable microstructures for good creep resistance.
EXAMPLE III 5 Creep Testing ll thc creep testing was done 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 10 a calibrated Pt/Pt-Rh thermocouple attached so that the bead was adjacent to the gage section of the creep specimen. The creep specimen was a flat plate type wi-th a gage length of approximately 2.25 inch (5.72 cm). The tensile stress was applied parallel to the rolling 15 direction (longitudinal direction). The general procedure was to hang the specimen in the furnace 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 tne test was 20 terminated (specimen removal or furnace burn-out).
Creep results were obtained from specimens that were processed according to Example II except that these specirnens ~lere milled 10 hours and received the above thermomechanical processing treatment of 10% reduction in 25 area per pass with an intermediate anneal at nominally 1040C 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 ~In/m2) at 2 400F (1 316C). The 30 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 ~;.5xlO 6 hr~l was observed. After approximately 118 hours ths i stress ~las increased to 1 400 psi (9.65 Mn/nl2) and a new secondary creep rate of nominally 3xlO 5 hr 1 was recorded.
These creep rates dre thio orders of magnitude less than that for the previously indicated VGS under the same 1 testing conditions. The ZnS rnaterial will have a stress rupture life of at least 4~ hours when tested at 140U C and 1000 psi in the rolling direction of the sheet.
The general microstructure of the crept sp~cimfn
S indicated that the grains were highly elongated in the rolling direction (creep stress direction also) and the grain boundries were ragged. There appeared -to be evidence o~ subgrains in the structure as well. The microstructure observed in this specimen was typical of that of a good 10 creep resistant material as evidenced by the exceptionally good creep properties.