CA2151143A1 - A process for densifying powdered ceramics and cermets at temperatures above 1400°c - Google Patents

Abstract

Densify powdered materials, either ceramic or metal or both, at temperatures of 1400 °C or higher using a fluid die fabricated from a mixture of alumina and a calcium aluminate cement. The fluid die may be separated from a fluid pressure-transmitting medium by a barrier material such as graphite foil.

Description

21S11~3 A PROCESS FOR DENSIFYING POWDERED CERAMICS

Technical Field The present invention generally concerns consolidating metallic and nonmetallic powders or combinations thereof to form a consolidated body of predetermined density. The present invention more particularly concerns consolidating such powders at temperatures in excess of 1400 Centigrade (C). The present invention also concerns a monolithic die material suitable for use in consolidating the powders at such temperatures.
10 Backqround Art Consolidated, or high density as a percent of theoretical density, ceramic-containing bodies are useful in applications such as cutting, drilling and shaping hard materials. Hard materials include rock, metalsand metal alloys.
Powdered materials, such as ceramic powders, are consolidated by conventional 15 procedures. The procedures typically begi n by cold pressing the powdered material into a preform. As an alternative, the powdered material is hermetically sealed in a can. The powdered material, either as a preform or as the contents of a hermetically sealed can, is then subjected to consolidation pressure. Pressure may be applied either by mechanical means, such as a forging press, or by gaseous means, such as a gas at superatmospheric pressure.
U .S.-A 4,428,906 discloses a pressure-transmitting medium prepared from a mixture of borosilicate glass and a refractory powder. The refractory powder consists of magnesium oxide (MgO), ammonium dihydrogen phosphate and silica (SiO2) powder in the form of quartz and cristobalite. The borosilicate glass and refractory powders are mixed with watertoformaslurrythatiscastintotheshapeofadie. Therefractorypowderandwater 25 reactatambienttemperaturestoformmagnesiumammoniumphosphatehexahydratewhich acts as a cement to bind the glass and SiO2 powders. The phosphate decomposes to an amorphous phase upon heating to about 250C during drying. Further heating of the die to about 1100~C during preheating results in conversion of the amorphous phase to magnesium pyrophosphate or, if excess MgO is present, magnesium orthophosphate.

~,796~ 21511~3 Dies prepared from the mixture of borosilicate glass and refractory powder provide satisfactory results at temperatures of from 1 1 00C up to 1 400C. At higher temperatures, the magnesium phosphates react with the SiO2 and glass to form magnesium silicates and volatile phosphorus oxides. Volatilization of the phosphorus oxides leaves behind 5 a weakened, porous structure that is prone to premature collapse during handling. In addition, condensation of the phosphorus oxides on comparatively cooler surfaces within a heating furnace can lead to corrosion of furnace parts.
Some ceramic materials and cermets must be heated to a temperature that exceeds 1400C, such as 1600-1975C, in orderto attain densities approaching theoretical 10 density. Assuch,itwouldbedesirableiftherewereapressure-transmittingmediumorfluid die material that would allow use of such higher temperatures.
Disclosure of Invention The present invention is a process for preparing dense, consolidated bodies of predetermined density by subjecting, in a forging press, an isostatic die assembly to densification conditions of a temperature within a range of from greater than 1 400C to 1800C, time and pressure sufficient to form a dense, consolidated body of desired shape, the die assembly comprising a powder body surrounded by a fluid pressure-transmitting medium, the medium and the preform being surrounded by a shell fabricated from a monolithic material that is an admixture of calcium aluminate cement and alumina, the fluid pressure-20 transmitti ng medium being substantially nonreactive with the monolithic material at saiddensification conditions.
The temperature range may be extended to an upper limit of 1975C by placing at least one layer of a barrier material between the shell and the fluid pressure-transmitting medium. The barrier material substantially precludes any reaction between the shell and the 25 fluid pressure-transmitting medium.
Definitions The term "powder body", as used herein, refers to a preform as well as to a quantity of powdered material hermetically sealed in a can. When the can is formed from a metal, care must be taken to ensure that the metal does not melt or react with the powdered 30 material at a tem peratu re less than or equal to that empl oyed to consol idate or densify the powdered material.
The term "monolithic", as used herein, refers to an article that is cast as a single piece or is formed without joi nts or seams.
Detailed Description of the Invention The present invention is an improved process for consolidating powder. The powder can be one or more metals, one or more nonmetals, or mixtures of one or more metals with one or more nonmetals. The powder need not be pure or even substantial Iy pure. It can include other materials such as stabilizers or constituent elements. One such constituent AMENDED SH~ET
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element is carbon in the case of a refractory carbide. The powder is desirably a ceramic-containing powder or a mixture of ceramic-containing powders.
Ceramic materials suitable for consolidation by the process of the present invention include particulate materials having refractory characteristics. Typical refractory 5 ceramic materials include refractory oxides, carbides, nitrides, phosphides, silicides, borides, sulfides and mixtures thereof. Other suitable refractory ceramic materials include mixed crystals such as sialons. Preferred refractory ceramic materials include alumina, zirconia, magnesia, mullite, zircon, thoria, beryllia, urania, spinels, tungsten carbide, tantalum carbide, titanium carbide, niobium carbide, silicon carbide, aluminum nitride, titanium nitride, 10 zirconium nitride, tantalum nitride, hafnium nitride, niobium nitride, boron nitride, silicon nitride, titanium boride, chromium boride, niobium boride, zirconium boride, tantalum boride, molybdenum boride, tungsten boride, cerium sulfide, molybdenum sulfide, cadmium sulfide, zinc sulfide, titanium sulfide, magnesium sulfide, zirconium sulfide and mixtures thereof. More preferred ceramic materials include alumina, silicon nitride, silicon carbide, mullite, cordierite, spinel, zirconia, titanium carbide and mixtures thereof. Ceramic materials include ceramic composites such as a composite of silicon carbide whiskers and alumina.
Metallic materials that can be employed alone or with ceramic materials include metals, metalloids, alloys and mixturesthereof. Typical metallic materials include cobalt, nickel, iron, tungsten, rhenium, steel, stainless steel, superalloys, molybdenum, tantalum, 20 titanium, chromium, niobium, boron, zirconium, vanadium, palladium, hafnium, aluminum, copper, alloys thereof and mixtures thereof. Suitable metallic materials include cobalt, nickel, titanium, chromium, niobium, boron, palladium, hafnium, tantalum, molybdenum, zirconium, vanadium, aluminum, copper, alloys thereof and mixtures thereof. Additional suitable materials include magnesium, titanium aluminide, nickel aluminide, silicon, beryllium, 25 germanium and mixtures and alloys of these. Desirable metallic materials include cobalt, chromium, nickel, titanium, niobium, palladium, hafnium, tantalum, aluminum, copper, molybdenum, tungsten, rhenium, vanadium and mixtures and alloys of these with each other and other elements and compounds.
The densification or consolidation process beneficially applies mechanically 30 induced pressure to a pressure-transmitting medium, such as glass, that is liquid at consolidation conditionsto omnidirectionally consolidate materials. The mechanically induced pressure is desirably supplied by a forging press. U.S.-A 4,428,906 and U.S.-A 4,389,362 illustrate t the application of mechanically induced pressure to a pressure transmitting medium to consolidate powdered materials.
U.S.-A 4,428,906 describes, at column 1, line 52 through column 2, line 49, a method for densifying a material using, as a pressure-transmitting medium, a rigid interconnected skeleton structure that contains a fluidizing means and is collapsible in response to a predetermined force. The predetermined force is supplied by a press, the WO 94/13419 2 1 5 ~ I ~ 3 PCT/US92110865 operation of which is described at column 3, lines 42-57 and at column 4, line 6l through column 6, line l9. In essence, the pressure-transmitting medium encapsulates a material to be densified. The pressure-transmitting medium and the encapsulated material are preheated to a compaction temperature, placed in a pot die and subjected the downward movement of a 5 ram in a forging press. The applied pressure collapses the skeleton structure into fragments that disperse in the fluidizing means. The force of the ram is transmitted hydrostatically and omnidirectionally to the encapsulated material through the fluidizing means. After full compaction, the ram is retracted and the cooled and solidified pressure-transmitting medium is removed from the pot die. The compacted part is then recovered by conventional means.
U.S.-A 4,389,362 describes a process whereby a press is used to transmit a compressive force through a powder medium to an outer capsule. The outer capsule contains an inner capsule and a pressure-transmitting medium that is viscous at the compacting temperature. The inner capsule has a shape corresponding to the final desired shape and contai ns metal powder.
Refractory calcium aluminate cements suitable for use in the process of the present invention may be prepared in accordance with the example shown at columns 5 and 6 of U.S.-A 3,963,508. Refractory cements, like portland cements, are made by admixing selected raw materials and then heating them to a temperature at which they react to form clinker with phases such as CaAI2O4, CaAI4O7 and Ca12AI14O33. The clinker is then ground to a suitable size.
20 Calcium aluminate cements are suitably finely ground admixtures of alumina (Al2O3) and a clinker having CaAI2O4 as a major constituent. One such cement, commercially available from Aluminum Company of America underthe trade designation CA-25C ~casting grade), constitutes on a composition weight basis, 36 weight percent (wt-% ) (Al2O3), 43 wt-%
CaAI2O4, 5wt-% CaAI4O7, 6wt-% Ca12AI14O33and l0wt-% unspecified material.
At temperatures below 1900 C, the form of Al2O3 is not critical and suitable results are obtained with tabular Al2O3. At temperatures sreater than or equal to l900 C, a superfine grade of Al2O3, such as ALCOA A-l 000, provides optimal results. If desired, the superfine grade may be used at lower temperatures. AlzO3 begins to react with calcium aluminate cement at a temperature of about 1000C. As the temperature increases, the 30 reaction proceeds at a faster rate and higher melting point reaction products begin to form. At a temperature of 1975C, substantiallyall reactions of interest for purposes of the present invention may be regarded as complete.
The calcium aluminate cement and the Al203 are suitably present in proportions .
sufficient to provide a refractory article or shell that meets three criteria. First, the article must 35 deform while retaining substantially all of the fluid pressure-transmitting medium contained therein under densification conditions used to form dense, consolidated bodies. Second, the article must have sufficient structural integrity at densification temperatures to allow the article to be transferred from a furnace to a forging press. Third, the article should, subsequent WO 94/13419 21 51 1 ~ ~ lPCT/US92/10865 to appiication and release of densification pressure, fracture or fail in such a manner as to help recover the dense, consolidated bodies contained therein.
A refractory article cast from a water mixture of a calcium aluminate cement such as CA-25C would be expected, upon heating to temperatures in excess of 1400C, to soften slightly at about 1450C as that is near Ca12AI 14O33's melting point of 1455C.A greater amount of softeni ng is expected at about 1 600C as that is near CaAI2O4's melting point of 1 602C. If an amount of sufficiently reactive Al2O3 is present at temperatures in excess of 1 600C, the Al2O3 will react with CaAI2O4 to form more complex calcium-aluminum-oxygen compounds with higher melting points. High melting pointcompounds include CaAI4O7 (melting point of about 1762C) and CaAI12O19 (melting point of about 1 830C). The higher melting point compounds, if present in sufficient amounts, effectively increase the softening temperature of refractory articles. The increased softening temperatures, in turn, increase the temperature at which the refractory article can be used in the process of the present invention.
As projected use temperatures increase, mixtures of calcium aluminate cement and Al2O3 desirably have increasing fractions of Al2O3. Table I provides general guidelines for mixtures in view of projected use temperatures. By varying the amount of Al2O3 in the mixtures, it is possible to produce a fluid die that meets the three criteria for a satisfactory refractory article or shell.
Table I

Tem peratu re (C) Al 23 1800 30:70 251850 15: 85 1900 10:90 1975 5:95 Admixtures of calcium aluminate cement and Al2O3 are present in proportions sufficient to provide a shell that deforms while retaining substantially all of the fluid pressure-transmitting medium under densification conditions. The proportions are desirably from 5 to 80 parts by weight of calcium aluminate cement and, conversely, from 95 to 20 parts by weight of Al2O3. The proportions are preferably from 5 to 40 parts by weight of calcium aluminate 35 cement and, conversely, from 95 to 60 parts by weight of Al2O3. The proportions are selected to total 100 parts by weight.

WO94/13419 2~1 14~ PCT/US9211086~

If desired, projected use temperatures below 1600C may be obtai ned by admixing an amount of SiO2 with the calcium aluminate and Ai2O3. The actual amount may be determined without undue experimentation.
The pressure-transmitting medium in which a powder body is embedded can be 5 any material or mixture of materials that is liquid or fl uid at the consolidation conditions.
Several of these are known in the art. Typical media include certain glasses and salts, with glasses being preferred. Boron-containing glass is more preferred. The teachings of U.S.-A
4,446,100; U.S.-A 3,469,976; and U.S.-A 3,455,682 disclose glasses, salts, and other pressure-transmittingmedia. U.S.-A4,446,100discloses,atcolumn4,1ines34-65,anumberofB203-10 containing glasses. These glasses include Pyrex~ glass and Vycor~ glass. U.S.-A 3,469,976 discloses borosil icate 91 ass contai ning 83-97.7 wt-% SiO2 and 0.3- 17 wt-% B2O3. U .S.-A
3,455,682 discloses, at column 2, lines 15-22, mixtures consisting essentially of 5-40 wt-% of an alkali or alkaline earth metal chloride, fluoride, silicate or a mixture thereof with 60-95 wt-% of a second component selected from SiO2, Al2O3, ZrO2, MgO, CaO, spinels, mullite, anhydrous aluminosilicates and mixtures thereof. Pyrex3 glass and Corning Vycor~ glass, Corning 7931 borosilicate glass and sand (employed at temperatures above its melting point) provide satisfactory results.
Caicium aluminate and Al2O3 can react with SiO2 to form calcium aluminosilicate compounds such as anorthite (CaAI2Si2O3) and gehlenite (Ca2AI2SiO7), both of which melt at 20 temperatures below 1600C. If a calcium aluminate-AI2O3 fluid die containing either glass or sand as the pressure-transmitting medi um is held at a sufficiently high pre-heat temperature, calcium aluminosilicates begin to form where the die and pressure-transmitting medium interface. As the preheat temperature or time at that temperature or both increase, calci um aluminosilicate concentration also increases. Fluid die strength decreases concurrent with 25 increases in calcium aluminosilicate content. Eventually, the fluid die softens and collapses in the absence of external pressure. Preheattemperatures in excess of 1800C particularly favor formation of calcium aluminosilicate compounds when Vycor~ glass is in contact with calcium aluminate-AI2O3 compositions.
Reactions also occur between Pyrex~ glass and calcium aluminate-AI2O3 fluid dies.
30 The reactions occur at temperatures as low as 1500C. In addition, reactions occur between Corning 7931 borosilicate glass and calcium aluminate-AI2O3 fluid dies at temperatures in excess of 1700C. Sand is used as a pressure-transmitting medium at temperatures above its melting point. At these temperatures, sand also reacts with the calcium aluminate-AI2O3 fluid dies.
One means of minimizing, if not eliminating, formation of calcium aluminosilicate compounds or other undesirable reaction products requires placing a barrier between i nner surfaces of a fl uid die and the pressure-transmitti ng mediu m. One such barrier material is a graphite foil such as that commercially available from Union Carbide under the WO 94/13419 21~ 3 PCT/US92/10865 trade designation Grafoil7M. A foil thickness of 0.020 inch (0.51 mm) provides particularly satisfactory results. Multiple plies of thinner foils provide satisfactory results, but present handling difficulties.
Any combination of pressure, temperature and time under which the desired 5 consolidation takes place can be employed. The actual pressure, temperature and time required to achieve the desired results depend upon the particular material being densified as well as the apparatus used to effect consolidation. U.S.-A 4,744,943 discloses, at column 5, lines 34-39, illustrative temperatures of 400C to 2900C, pressures of 68.9 to 3,450 MPa and times of from 20 seconds or less up to one hour or more. Those skilled in the art can choose the l O satisfactory pressures, temperatures and times based upon well-known criteria without undue experimentation.
Powders being consolidated via the process of the present invention are beneficially converted to preforms prior to densification. Preforms are readily prepared using technologyfamiliartothepowdermetalsorceramicsindustry. U.S.-A4,446,100describes,at column l, li nes 10-33, various procedures for preparing preforms.
General Procedure for Preparinq a Fluid Die The following powders are added to the mixing bowl of a Hobart mixer to prepare a fluid die: 1.73 pounds (0.78 kg) calcium aluminate cement, commercially available from Aluminum Company of America underthe trade designation CA-25C; 2.01 pounds (0.91 20 kg) -48 mesh (Tyler Sieve Series) (300 llm maximum particle size) tabular A1203; and 2.01 pounds (0.91 kg) - 14 + 28 mesh (Tyler Sieve Series) (600 lum to 1.18 m m particle si ze) tabul ar Al203. If theAI203isasuperfinegrade,suchasALCOAA-1000,theAI203andthecalcium aluminatecementshould beadded last. Afterdryblendingthe powdersfortwo minutes, 315 cc of warm 100F (37.8C) water are added to the mixing bowl. Mixing continues for a period of 25 l .5 minutes. The mixer is stopped and the contents of the mixing bowl are manually stirred to incorporate dry powders from the bottom of the bowl that are not affected by the mixer beaters. The mixer is restarted and the contents are blended for an additional minute. After stopping the mixer once again for manual stirring, the mixer is restarted for a final minute of blendi ng.
The mixture or cement slurry is poured into a fluid die mold situated on a vibrating table. The vibration promotes removal of entrapped air pockets. The filled mold is placed in a humidity controlled oven set at 90F (32.2C) and 90% relative humidity for a period of 24 hours to harden the cement. The hardened fluid die is removed from the mold and calcined in a furnace programmed to heat from ambient temperature to a temperature of 35 1470F (798.8C) at a rate of about 4.5F/m i nute (2.5C/m i nute) over a period of 5 hou rs and remain at that temperature for an additional 3 hours. Upon cooling to ambient temperature, the fluid die is ready for use.

WO 94/13419 2151 1~ 3 PCT/US92/1086!i The foregoing procedure yields a 30/0 calcium aluminate cemenV70% Al2O3 fluid die. Other suitable fiuid dies result from appropriate adjustments to the amounts of calcium al uminate cement and tabular Al2O3 powders.
General Procedure for Usinq the Fluid Die to Densify a Ceramic Preform The article or powder body to be densified is embedded in a material that acts as a viscous liquid at process temperature and serves to isostatically densify the article when both are confined and compressed in a closed cavity with a ram. The material that acts as a pressure-transmitting medium is suitably SiO2 sand or a glass having a viscosity at densification temperature that is high enough to minimize, if not eliminate, penetration by the glass into 10 the embedded article or powder body during densification. One such glass is Corning Vycor glass.
After embedding the article or powder body in the material to be used as a pressure-transmitting medium, the fluid die is capped with a cover or lid made from the same composition as the fluid die. The lid isolates the pressure-transmitting medium from furnace conditions, such as those present in a graphite furnace containing an oxygen-free atmosphere, that favor chemical reduction reactions. Without a lid, exposure of SiO2 contained in the glass or sand used as a pressure-transmitting medium produces silicon suboxides that tend to condense on furnace surfaces and react with the graphite. The reaction with graphite results in erosion of the furnace insulation.
The capped and filled fluid die is placed in a furnace and heated to 1100C at arate of 20C/minute and held there for 15 minutes. An alternative procedure involves heating to 1000C at a rate of 5 to 30Clminute. The die is then heated to a temperature selected for consolidation or compaction, such as 1800C, at a rate of 10Clminute and held there for 15 to 30 minutes. The heated die is then transferred to a forging press where it is compressed with a 25 ram to a pressure of from 10,000 psi (689 MPa) up to 120,000 psi (830 MPa) for a period of time sufficientto attain a predetermined density. The predetermined density mayvary from as low as 50% of theoretical density to a density approximating theoretical density. The predetermined density more typically is 95 % of theoretical density or higher. The predetermined density is preferably equivalent to theoretical density. The period of time may 30 be as short as 0.5 second or as long as 2 or 3 hours. Typical periods of time are much shorter than 3 hours. Satisfactory results are obtained with periods of time varying from as little as 0.5 second to as long as 60 seconds. The pressing procedure is described in more detail in U.S.-A
4,744,943 and U.S.-A 4,428,906 at the locations cited above. U.S.-A 4,656,002 discloses, particularly at column 4, lines 1 through column 5, line 5, an assembly known as a self-sealing 35 fluid die and its operation. The fluid die is cooled in air and the resultant densified article is recovered using conventional technology and sand blasted. The die itself tends to crack severely upon cooling. As such, it is readily removed.

-WO 94/13419 ~ 3 PCT/US92/10865 The following examples illustrate the invention and do not, either explicitly or by implication, limit its scope. All parts and percentages are by weight unless otherwise indicated.
Metric units given in parentheses are approximate equivalents of English units.
Example 1 A starting powder mixture of 48~/o tantalum nitride, 32% zirconium diboride and 20% tungsten carbide was intensely mixed, in the presence of heptane, in an attritor containing tungsten carbide-cobalt balls for 8 hours. Paraffin wax, 2-3% based on powder mixture weight, was added as a binder during the last 30 minutes of attritor mixing. The resultant mixture was dried and screened through a 20 mesh screen. A greenware part was made by cold-pressing the mixture that passed through the screen in steel tooling at 34,000 psi (234MPa). Thecold-pressedpartwasthencoldisostaticallypressedat30,000psi(210MPa).The resultant part was dewaxed under vacuum at 380C.
The dewaxed part was spray coated with 3 layers of boron nitride and wrapped in boron nitride coated graphite foil. The wrapped part was placed into a 30% calcium aluminate/70/O Al2O3 fluid die prepared as described herein and embedded in Vycor~ brand glass. The die, after being capped with a lid as described herein, was preheated to a temperature of 1659C and pressed at 120,000 psi (830 MPa) for 30 seconds. The fluid die was cooled in air. The partwasthen recovered and sand blasted. The part had a density of 69.4%
of theoretical density.
20 Example 2 The procedure of Example 1 was duplicated for a starting powder mixture of 4.8% niobium diboride and 95.2% titanium diboride. The recovered part has a density of 79 of theoretical density.
Examples 3 and 4 The procedure of Example 1 was replicated save for using a ball mill instead of an attritor for two powder mixtures. One mixture was 97.1 % titanium diboride and 2.9%
tungsten boride. The other mixture was 94% zirconium diboride and 6% tungsten. The recovered parts had respective densities of 77 and 67.6% of theoretical density.Examples 5 and 6 The procedure of Example 1 was replicated save for lining a fluid die with a 0.020 inch (0.51 mm) layer of graphite foil and increasing the preheat temperature to 1800C for 3 powder compositions. One composition was 100% molybdenum disilicide. A second composition was 61.2% tungsten disilicide and 38.8% molybdenum disilicide. A third composition was 88.7% molybdenum disilicide and 11.3% silicon carbide. The recovered parts 35 had respective densities of 93.1 %,98.4% and 96.1 % of theoretical density.

WO 94/13419 2 ~ 3 PCT/US92/10865 Example 7 The procedure of Example 1 was duplicated save for increasing the preheat temperature to 1 800C for a powder mixture of 94/0 tungsten carbide and 6% tungsten metal .
The recovered part had a density of 99.1/0 of theoretical density.
5 Example 8 A powder mixture of 65.4% aluminum oxide and 34.6% titanium carbide was converted to a dewaxed part via the procedure of Example 1. The dewaxed part was wrapped in 2 overlapping layers of graphite foil. The wrapped part was placed into a 15% calcium aluminate/85% Al2O3 fluid die prepared as described herein and embedded in Vycor~ brand glass as in Example 1. The die, after being capped with a lid as in Example 1, was preheated to a temperature of 1 850C and pressed at 120,000 psi (830 MPa) for 60 seconds. The recovered part had a density of 100% of theoretical density, as calculated based on a linear rule of mixtures.
The foregoing examples demonstrate the suitability of calcium aluminate/AI2O3 mixtures for use in fabricating fluid dies. Similar results are expected with other compositions 15 and operating conditions, all of which are disclosed herein.

Claims (10)

1. A process for preparing dense, consolidated bodies of predetermined density by subjecting, in a forging press, an isostatic die assembly to densification conditions of a temperature within a range of from greater than 1400°C to 1800°C, time and pressure sufficient to form a dense, consolidated body of desired shape, the die assembly comprising a powder body surrounded by a fluid pressure-transmitting medium, the medium and the preform being surrounded by a shell fabricated from a monolithic material that is an admixture of calcium aluminate cement and alumina, the fluid pressure-transmitting medium being substantially nonreactive with the monolithic material at said densification conditions.

2. A process for preparing dense, consolidated bodies of predetermined density by subjecting, in a forging press, an isostatic die assembly to densification conditions of a temperature within a range of from greater than 1400°C to 1975°C, time and pressure sufficient to form a dense, consolidated body of desired shape, the die assembly comprising a powder body surrounded by a fluid pressure-transmitting medium, the medium and the preform being surrounded by a shell fabricated from a monolithic material that is an admixture of calcium aluminate cement and alumina, the shell being separated from the fluid pressure-transmitting medium by at least one layer of a barrier material that substantially precludes any reaction between the shell and the fluid pressure-transmitting medium.

3. A process as claimed i n Claim 1 or Claim 2 wherein the calcium aluminate cement and the alumina are present in proportions sufficient to provide a shell that deforms while retaining substantially all of the fluid pressure-transmitting medium under the densification conditions.

4. A process as claimed in Claim 3 wherein the proportions are from 5 to 80 parts by weight of calcium aluminate cement and from 95 to 20 parts by weight of alumina.

5. A process as claimed in Claim 3 wherein the proportions are from 5 to 40 parts by weight of calcium aluminate cement and from 95 to 60 parts by weight of alumina.

6. A process as claimed in Claim 1 wherein the fluid pressure-transmitting medium is separated from the die by at least one layer of a barrier material.

7. A process as claimed in Claim 2 or Claim 6 wherein the barrier material is a graphite foil.

8. A process as claimed in Claim l or Claim 2 wherein the predetermined density is from 50 to 100% of theoretical density.

9. A process as claimed in Claim 8 wherein the predetermined density is greater than 95% of theoretical density.

10. A process as claimed in Claim l or Claim 2 wherein the time is from 0.5 second to 3 hours and the pressure is from 68.9 MPa to 830 MPa.