Metal powder sintering process

Abstract

A powder metallurgical process for making sintered articles and shaped parts from nickel-base so-called superalloys of the types characterized as normally having carbide and gamma-prime strengthening. The superalloy in the form of a powder of controlled particle size is loosely packed in a mold cavity or, alternatively, is molded into a shape-retaining blank with the aid of a volatile organic binder, whereafter it is subjected to a carefully controlled two-stage vacuum sintering process producing an integral porous preform having superior physical properties. The resultant sintered preform is thereafter further densified, if desired, by hot or cold pressing or hot forging and is subjected to a heat treating step in order to achieve optimum physical properties consistent with the intended end use of the sintered component.

Description

BACKGROUND OF THE INVENTION

An ever-increasing number of alloys are being developed and made available for commercial and experimental use which are uniformly characterized as possessing excellent oxidation resistance and excellent physical properties when subjected to elevated temperatures of a magnitude such as encountered in the combustion chamber and turbine section of high-performance gas turbine engines and the like. The continuing development of such new alloys has at least in part been stimulated by the requirements of aerospace technology for providing still further improvements in the durability, performance and efficiency of gas turbine engines capable of operating at higher temperatures.

The continued improvement in the high temperature physical properties of such so-called superalloys including improvements in their tensile strength, creep resistance, thermal fatigue and corrosion resistance has been achieved by a careful control of the complex alloy chemistry which involves the use of a comparatively large number of different alloying constituents. The use of such a large number of alloying constituents to achieve a desired alloy microstructure has resulted in a corresponding increased difficulty in the working and shaping of such alloys into articles and components which are of uniform composition and grain structure and of excellent physical properties. When such superalloys are cast into ingots or into castings of a prescribed configuration, the complex chemistry of the alloys usually results in castings which are characterized as having a non-uniform grain structure and a lack of homogeneity which is caused primarily by the segregation of massive carbides and intermetallic phases. A further problem arises when cast ingots of such super-alloys must be post-worked or shaped into final parts due to the extreme difficulty in effecting an appreciable degree of deformation of such superalloy blanks even when heated to comparatively high temperatures.

The aforementioned problems associated with cast ingots and cast components comprised of superalloys has at least in part been overcome by employing powder metallurgical techniques in which the superalloy is first reduced to a finely-divided powder state and is thereafter consolidated while confined within a deformable sheath such as by hot pressing, forging and/or extrusion into a mass approaching 100% theoretical density. Such consolidated billets of superalloy powders are characterized as being devoid of the conventional voids, blow-holes or pockets ordinarily associated with billets produced by casting the same alloys and wherein the microstructure of the densified powder billets is of a uniform and fine-sized grain structure. Billets and components produced employing the aforementioned powder metallurgical techniques are further characterized as possessing a wrought grain structure and having excellent high temperature physical properties. Unfortunately, such densified billets of superalloy powders, as well as shaped components thereof, are comparatively expensive due to the large number of steps involved, as well as the care and trained personnel required, in addition to relatively expensive equipment employed.

Attempts to produce sintered masses of superalloy powders which in spite of some porosity are of adequate high temperature strength have been generally unsuccessful due to the relatively low strength of the sintered bond or neck between powder particles. Attempts to improve the strength of such sintered masses by special heat treatments and further compaction or densifying processes have for the most part failed to sufficiently improve the physical properties to a magnitude necessary to meet the requirements for most high temperature uses. In accordance with the present process, a twostage vacuum sintering technique is utilized which has provided for an unexpected increase in the strength of the sintered bond resulting in a sintered mass which is possessed of comparatively high physical strength properties in comparison to cast as well as conventionally sintered masses of the same alloy composition. The present process further enables the fabrication of sintered parts which closely approxmate the final shape and dimensions of the finished component, thereby eliminating or minimizing final finishing operations. Further improvements in the physical properties of the final components can be achieved by effecting a further compaction of the sintered mass as well as a heat treatment thereof in order to achieve optimum properties consistent with the intended end use of the component.

SUMMARY OF THE INVENTION

The benefits and advantages of the present invention are achieved by a process which comprises the steps of providing a mass of a superalloy powder of the general type characterized as being of a nickel base and as normally having carbide and gamma-prime strengthening. The mass of powder is formed into a three-dimensional shape of a desired configuration, whereafter the shaped mass is heated in an atmosphere approaching a substantially perfect vacuum to a first or transformation temperature at which the chemical equilibrium is conducive toward, and preferably, which optimizes a conversion of primary metal carbides to secondary or complex carbides. The mass is maintained at the first temperature for a period of time sufficient to effect an appreciable conversion of the primary carbides to the secondary or complex carbides accompanied by a migration of carbides from the surface to the interior of the powder particles and an initiation of the diffusion bonding of the powder particles to each other at their points of contact. Thereafter, the mass, while still in an evacuated atmosphere, is heated to a second or sintering temperature which is above the carbide transformation temperature and may range up to a level at which incipient melting of the superalloy powder particles occurs. The mass is maintained at the second temperature for a period of time sufficient to form an integral porous sintered preform in which the powder particles are securely bonded to each other by necks which bridge or interconnect adjoining powder particles at their initial points of contact. In accordance with a preferred practice of the present invention, the resultant sintered mass is further densified to effect a reduction in the porosity thereof and is subjected to a heat treatment, whereby a further enhancement and optimization in the physical properties thereof are achieved.

Additional benefits and advantages of the present invention will become apparent upon a reading of the description of the preferred embodiments taken in conjunction with the accompanying drawing and the specific examples provided.

BRIEF DESCRIPTION OF THE DRAWING

The drawing comprises a flow diagram illustrating the sequence of the important steps of the process in accordance with the preferred practice of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The compositions of the various high temperature alloys and mixtures of metal powders are described in the specification and subjoined claims in terms of percentages by weight, unless clearly indicated to the contrary. It will also be understood that while the process is herein described in terms of producing sintered powder metal billets and components of substantially uniform composition throughout, it is also contemplated within the scope of the present invention that composite articles can be produced comprising wrought or cast sections to which a layer of sintered metal powder is tenaciously bonded, such as by brazing, diffusion bonding, etc., over all or a portion of the surface of the solid section.

The billets and shaped parts produced in accordance with the process of the present invention are comprised of so-called "nickel-based superalloys" of the types which normally have carbide strengthening and gamma-prime strengthening in their cast and wrought forms. Characteristically, such superalloys contain comparatively large amounts of second-phase gamma-prime and complex carbides in a gamma matrix which contribute appreciably to their high temperature physical properties including tensile strength, creep resistance, thermal fatigue and corrosion resistance at elevated temperatures. These excellent physical properties enable the use of components fabricated from such superalloys at elevated temperatures generally above 1400.degree.F and frequently as high as 1800.degree.F or higher. Temperatures of the foregoing magnitude are usually encountered in sections of gas turbines, such as, for example, by the turbine blades, stator vanes, combustion chambers, etc.

Typical of superalloy compositions which can be reduced to powder form and sintered to provide for high strength sintered billets and shaped components are those as set forth in Table 1. It will be appreciated that the specific compositions enumerated in Table 1 are merely illustrative of those which have been developed for experimental and commercial applications and the list provided is accordingly not intended as being restrictive of other superalloy compositions falling within the general definition as hereinbefore set forth, which also can be subjected to the two-phase sintering process, achieving thereby unexpected improvements in their high temperature physical properties.

TABLE 1 __________________________________________________________________________ Nominal Composition, percent Alloy C Cr Ni Co Mo W Cb Ti Al B Zr Ta Other __________________________________________________________________________ Inconel X-750 0.04 15.0 73.0 -- -- -- 0.85 2.5 0.8 -- -- -- 6.75 Fe M-252 0.15 19.0 Bal. 10.0 10.0 -- -- 2.5 1.0 0.005 -- -- -- Waspaloy 0.07 19.5 Bal. 13.5 4.3 -- -- 3.0 1.4 0.006 0.09 -- -- Rene 41 0.09 19.0 Bal. 11.0 10.0 -- -- 3.1 1.5 0.008 -- -- -- Inconel 700 0.12 15.0 46.0 28.5 3.75 -- -- 2.2 3.0 -- -- -- -- Udimet 500 0.08 19.0 Bal. 19.5 4.0 -- -- 2.9 2.9 0.01 -- -- -- GMR-235 D 0.15 15.5 Bal. -- 5.0 -- -- 2.5 3.5 0.05 -- -- -- Udimet 700 0.10 15.0 Bal. 18.5 5.2 -- -- 3.5 4.25 0.02 -- -- -- Alloy 713C 0.12 12.5 Bal. -- 4.2 -- 2.0 0.8 6.1 0.012 0.10 -- -- Alloy 713LC 0.05 12.0 Bal. -- 4.5 -- 2.0 0.6 5.9 0.01 0.10 -- -- MAR-M 200 0.15 9.0 Bal. 10.0 -- 12.5 1.0 2.0 5.0 0.015 0.05 -- -- MAR-M 211 0.15 9.0 Bal. 10.0 2.5 5.5 2.75 2.0 5.0 0.015 0.05 -- -- Nimonic 80A 0.10 19.5 Bal. -- -- -- -- 2.3 1.35 .030 -- -- 0.5 Fe Nimonic 105 0.15 14.9 Bal. 20.0 5.0 -- -- 1.50 5.25 0.03 -- -- -- Nimonic 108 0.14 14.9 Bal. 20.0 5.25 -- -- 1.25 5.0 0.03 -- -- -- Nimonic 118 0.14 15.0 Bal. 15.0 4.0 -- -- 4.0 5.0 0.03 -- -- -- IN-100 0.15 10.0 Bal. 15.0 3.0 -- -- 4.75 5.5 0.015 0.05 -- 1.0 V B-1900 0.10 8.0 Bal. 10.0 6.0 -- -- 1.0 6.0 0.015 0.08 4.3 -- X-40, HS 31 0.50 25.0 10.0 Bal. -- 7.5 -- -- -- -- -- -- -- Rene 80 0.17 14.0 Bal. 9.5 4.0 4.0 -- 5.0 3.0 0.015 0.03 -- -- MAR-M-421 0.15 15.5 Bal. 10.0 1.75 3.0 1.75 1.75 4.25 0.015 0.05 -- -- AiResist 213 0.18 19 -- Bal. -- 4.7 -- -- 3.5 -- 0.15 6.5 0.1 Y AiResist 215 0.35 19 -- Bal. -- 4.5 -- -- 4.3 -- 0.13 7.5 0.17 Y B-1910 0.10 10 Bal. 10 3.0 -- -- 1.0 6.0 0.015 0.10 7.0 -- Haynes Alloy 188 0.08 22 22 Bal. -- 14 -- -- -- -- -- -- 0.08 La, 1.5 Fe IN-738 0.17 16 Bal. 8.5 1.75 2.6 0.9 3.4 3.4 0.01 0.10 1.75 Fe, Mn, Si, S* IN-792 0.21 12.7 Bal. 9.0 2.0 3.9 -- 4.2 3.2 0.02 0.10 3.9 -- LDA-204 0.80 25.5 10.5 Bal. -- 7.5 -- -- -- -- -- 4.0 -- MAR-M 432 0.15 15.5 Bal. 20 -- 3.0 2.0 4.3 2.8 0.015 0.05 2.0 -- MAR-M 905 0.05 20 20 Bal. -- -- -- 0.5 -- -- 0.10 7.5 -- MP 35N -- 20 35 35 10 -- -- -- -- -- -- -- -- Rene 80 0.17 14 Bal. 9.5

4.0 4.0 -- 5.0 3.0 0.015 0.03 -- -- Rene 85 0.27 9.3 Bal. 15 3.25 5.35 -- 3.3 5.3 0.015 0.03 -- -- Rene 95 0.15 14 Bal. 8.0 3.5 3.5 3.5 2.5 3.5 0.01 0.05 -- -- TAZ 8B 0.125 6 Bal. 5.0 4.0 4.0 1.5 -- 6.0 0.004 1.0 8 -- TD-NiCr -- 20 Bal. -- -- -- -- -- -- -- -- -- 2.0 ThO.sub.2 TD-NiMo 0.5 -- Bal. -- 20 -- -- -- -- -- 0.3 -- 3.0 ThO.sub.2 TRW VI A 0.13 6 Bal. 7.5 2.0 5.8 0.5 1.0 5.4 0.02 0.13 9 0.5 Re, 0.43 Hf Udimet 710 0.07 18 Bal. 15 3.0 1.5 -- 5.0 2.5 0.02 -- -- -- Unitemp AF2-1DA 0.35 12 Bal. 10 3.0 6.0 -- 3.0 4.6 0.015 0.10 1.5 -- __________________________________________________________________________ *Low as possible

In accordance with the process sequence as illustrated in the drawing, the first stage comprises a conversion of the super-alloy of the desired composition to a finely-particulated metal powder generally having a particle size of less than about 425 microns and preferably of a particle size ranging from about 175 microns to about 10 microns. In accordance with a preferred practice, the powder particles are distributed over the range of 175 microns to 10 microns, providing for a greater degree of packing of the loose powder, achieving thereby a sintered preform of lower porosity. It is a characteristic of superalloy powders that the particles are generally spherical in configuration when such powders are formed by microcasting techniques, including gas atomization, airless spraying and centrifugal techniques for effecting a fragmentation of a molten mass of the alloy. Typical of a gas microcasting technique is that described in U.S. Pat. No. 3,253,783, which is assigned to the same assignee as the present invention and wherein a nozzle arrangement is disclosed for effecting an atomization of a molten mass of the metal into particles of controlled size.

As will be noted in Table 1, the superalloy compositions generally contain a large variety of alloying constituents, many of which have an affinity for oxygen at temperatures corresponding to those at which the alloys are heated to effect an atomization thereof. While oxygen contents in the metal powder of up to about 300 parts per million (ppm) do not have any appreciable adverse effect on the high temperature mechanical properties of the resultant sintered components, it is usually preferred that such powders have oxygen contents of less than about 100 ppm. The production of metal powders containing oxygen contents of less than about 100 ppm can readily be achieved by employing an inert gas, such as argon or helium, for example, to effect an atomization of the molten mass, as well as in providing an inert atmosphere in the chamber in which the molten particles are cooled and collected.

Regardless of the particular technique employed for providing the superalloy powder, a powder of the prescribed composition and average particle size range is shaped into a desired three-dimensional configuration, whereafter it is subjected to a controlled two-stage sintering operation under a vacuum atmosphere. As illustrated in the flow diagram comprising the drawing, the sintering of the powder to form a preform can be achieved by placing the powder in a mold cavity of the desired configuration or, alternatively, mixing the powder with a volatile binder and cold-pressing the powder in a die cavity of the desired configuration to form a three-dimensional briquette possessing sufficient green strength to retain its shape during the sintering step. When employing a mold, it is usually preferred to subject the mold to sonic or supersonic vibratory frequencies to effect optimum packing thereof to a density usually ranging from about 60% up to about 70% of a theoretical 100% density. Alternatively, when an organic binder is employed, the cold compaction of the metallic powder-binder mixture produces a green briquette of a density similarly ranging from about 60% to about 70% of 100% theoretical density. In this latter regard, any one of a variety of well known organic binder materials can be employed in amounts usually ranging from about 2% up to about 5% of the powder-binder mixture provided that the binder is sufficiently volatile so as to substantially completely decompose without leaving any detrimental residue during the sintering operation. Binders suitable for this purpose include acrylic resins, paraffin wax, phenol formaldehyde resin, polyvinylchloride, polyvinyl alcohol, and the like, of which paraffin wax constitutes a preferred material when employed in amounts of from about 1% to about 3% based on the total binder-powder blend.

The green cold-pressed briquettes are prepared in accordance with known techniques wherein a uniform mixture of the powder and particulated organic binder or a solution of the binder in a volatile solvent is placed in a die cavity of the desired configuration and the resultant powder mixture is cold compacted at unit pressures of about 30,000 psi up to about 100,000 psi or even higher, depending upon equipment limitations.

The refractory mold filled with the superalloy powder or the cold-pressed green briquettes in accordance with the process sequence illustrated in the drawing is thereafter placed in a furnace chamber capable of being evacuated to produce a substantially complete vacuum under which the powder is heated to a first transformation temperature and thereafter to a second sintering temperature in a manner and for the purposes as hereinafter described.

It has been found important in accordance with the practice of the process comprising the present invention to effect the two-stage sintering operation in a substantially evacuated environment in order to effect a degassing of the sintered part and also to prevent any reaction and particularly oxidation of the metal alloy at the elevated temperatures encountered. The entrapment of gases within the enclosed pores of the resultant sintered matrix has been found to cause pore swelling and premature failure of the components when subsequently heated to elevated temperatures during use. While vacuums of a magnitude as high as about 1000 microns (microns of mercury absolute) have been found satisfactory in many instances, it is usually preferred that vacuums of less than about 100 microns and especially less than about 10 microns assure repetitive high strength mechanical properties of the resultant sintered component. It is also usually preferred to initially purge the furnace chamber of air by filling with an inert gas, such as argon for example, and thereafter drawing a vacuum such that any residual gases remaining comprise inert constituents which are not reactive with the alloying constituents of the superalloy powder.

After the appropriate vacuum has been attained, or concurrently therewith, the refractory mold filled with metal powder or green briquettes is progressively heated to a first temperature which may more aptly be described as a carbide transformation temperature and at which the chemical equilibrium favors a conversion of primary carbides to complex carbides in accordance with the following equation:

MC .revreaction. M.sub.23 C.sub.6 + gamma-prime

wherein:

M comprises a metal such as titanium, chromium, molybdenum, etc., depending upon the specific alloy employed forming a carbide;

MC comprises a primary carbide such as (Ti 0.6; Cr 0.2; Mo 0.2)C; and

M.sub.23 C.sub.6 comprises a secondary or complex carbide.

Generally the carbide transformation temperature for superalloys of the type to which the present invention is applicable is within a relatively narrow band located somewhere between about 1600.degree. up to about 2000.degree.F. The specific transformation temperature to which the powder is heated during the first stage sintering operation will vary depending upon the chemistry of the alloy and is selected so as to optimize the conversion of primary carbides to complex carbides plus gamma-prime such that at the conclusion of the first stage sintering step, the secondary or complex carbides are in abundance, while the primary carbides are present in substantially small quantities. The duration of the first stage sintering step will vary depending upon the specific alloy composition employed and is controlled so as to effect an appreciable transformation of primary to secondary carbides and a migration of the carbides from the surfaces of the powder particles to their interiors. Generally sintering times of from about 10 hours up to about 100 hours will enable the attainment of a conversion of the primary to the secondary carbides at the specific transformation temperature employed. First stage sintering periods greater than about 100 hours have not been found to provide for any appreciable benefits and are usually commercially undesirable for economic considerations.

In addition to effecting a transformation of primary to complex carbides, an initial diffusion bonding or sintering of the particles at their points of contact also occurs during the first sintering stage forming a so-called "neck," which progressively grows, particularly during the second sintering stage, forming an integrally-bonded three-dimensional matrix of increased density.

At the completion of the first sintering stage, the presintered matrix is heated to a second or sintering temperature which is conventionally selected as one slightly below or at about the incipient melting temperature of the alloy to promote a more rapid atomic diffusion and neck growth in order to complete the sintering step. While temperatures at or slightly above the transformation temperature employed in the first sintering stage can be used in the second sintering stage, the rate of diffusion and neck growth is generally too slow from a commercial standpoint, and it is for this reason that temperatures at or about the incipient melting point of the alloy are used. The incipient melting point for most superalloys generally ranges from about 2100.degree.F up to about 2350.degree.F, at which optimum atomic mobility is achieved to promote the diffusion reaction and neck growth.

Conventionally, the second stage sintering step is carried out for a period of time to achieve maximum densification and pore shrinkage of the powdered mass. For most superalloys time periods of from about 1 hour up to about 20 hours when heated to a temperature slightly below or at the incipient melting point of the alloy are satisfactory for achieving optimum mechanical properties of the resultant sintered matrix. The resultant sintered mass, depending upon the specific powder particles employed and the duration of the second stage sintering operation, will have porosities usually ranging from about 20% to about 10% by volume.

The unexpected improvements in the high temperature physical properties of the resultant sintered matrix is not fully understood at the present time due to the complexity of the transformation reactions and the diffusion mechanism by which a bonding and neck growth is effected during the first and second sintering steps. As a possible explanation, it is theorized that during the first-stage sintering operation, the primary carbides predominantly present on the surfaces of the powder particles are converted to secondary or complex carbides which migrate toward the interior of the powder particles such that the necks formed through the atomic diffusion mechanism are substantially devoid of deleterious primary carbide phases, imperfections and brittle phases which apparently seriously detract from the physical properties of the final sintered matrix. In accordance with the present process, the conversion of primary carbides to complex carbides and gamma-prime results in a bonding neck formation during the first sintering stage which is substantially "clean" and devoid of brittle carbide phases and the rapid neck growth during the final sintering stage prevents any appreciable reconversion of complex carbides to primary carbides. The resultant sintered matrix is, accordingly, possessed of unexpectedly high mechanical properties. It will be understood, however, that the foregoing theory does not comprise any part of the present invention and is merely offered as a possible explanation of the unexpected results obtained.

At the conclusion of the final sintering operation, the sintered matrix is removed from the furnace and conventionally is of a density ranging from about 80% to about 90% of theoretical density. In accordance with the preferred practice, as illustrated in the flow diagram comprising the drawing, the sintered porous preform can be further compacted or densified such as by cold or hot coining and cold or hot pressing to provide for a more accurate sizing and shaping of the preform and to effect a further densification thereof from about 90% up to about 100% theoretical density. Alternatively, the sintered preform can be subjected to cold or hot forging in which a comparatively high deformation thereof is effected, producing forged components or parts of a desired shape and of densities approaching 100% theoretical density.

The sintered preform with or without further densification is also preferably subjected to a heat treatment to optimize and further enhance the physical properties thereof consistent with the intended end use of the component. Typical heat treatments include a heating of the sintered preform to a temperature above the gamma-prime solvus to effect dissolving of the gamma-prime whereafter the preform is quenched. The resultant structure having a very fine-sized and uniform gamma-prime can thereafter be aged to grow the gamma-prime phase to a size and morphology consistent with the properties desired at the ultimate operating temperatures. In lieu of the foregoing, any conventional heat treatment cycle can be employed to achieve a desired modification of the properties of the preform consistent with its intended end use.

In order to further illustrate the benefits of the process comprising the present invention, the following example is provided. It will be understood that the example is provided for illustrative purposes and is not intended to be limiting of the scope of this invention as herein described and as set forth in the subjoined claims.

EXAMPLE 1

A quantity of a superalloy powder having a composition corresponding to the alloy IN-100 as set forth in Table 1 and of an average particle size of 75 microns was mixed with 2% by weight of paraffin wax as a binder and placed in a steel die cavity shaped as a dog-bone tensile specimen and compacted under a pressure of 60,000 psi. The green compact was thereafter removed from the die and placed in a vacuum furnace at 1800.degree.F for a period of 15 hours at a vacuum of about 1 micron. At the completion of the first stage sintering operation, the furnace was increased in temperature to 2250.degree.F and the pre-sintered matrix was sintered for an additional 24 hour period, after which it was removed.

For comparative purposes, green compacts of the same material were sintered in a one-stage sintering operation at 2250.degree.F for 24 hours in a vacuum atmosphere and thereafter removed. Comparative test data of the ultimate tensile strength, the yield strength and the percent elongation of test specimens prepared from the two-stage sintering step (Sample A) in accordance with the practice of the present invention, the one-stage sintering step (Sample B) and from a cast ingot (Sample C) of the IN-100 alloy, are set forth in Table 2.

TABLE 2 ______________________________________ Ultimate Tensile 0.2% Yield Percent Sample Strength (psi) Strength (psi) Elongation ______________________________________ A 157,000 125,000 13 B 119,600 108,300 7 C 147,000 125,000 7 ______________________________________

It is apparent from an examination of the comparative yield strengths, ultimate tensile strengths and percentages elongation of the three samples that the sintered superalloy component produced in accordance with the two-phase sintering process comprising the present invention is substantially superior in comparison to the single phase sintered material in all three categories and significantly superior than the as-cast alloy with respect to both ultimate tensile strength and percentage elongation.

EXAMPLE 2

Sintered preforms corresponding to Sample A of Example 1 are subjected to a further compaction step by cold coining the specimens at 100,000 psi, effecting an increase in their density of from about 90% to about 97% of 100% theoretical density. The cold compacted preforms are thereafter annealed at 2250.degree.F for a period of 24 hours in vacuum. The resultant test specimens have an ultimate tensile strength of about 168,000 psi, a 0.2% yield strength of about 140,000 psi and a percent elongation of about 17%.

EXAMPLE 3

A quantity of superalloy powder identical to that employed in Example 1 is placed in a refractory mold cavity and sintered in a vacuum of one micron at 1800.degree. for a period of 15 hours followed by a second phase sintering step at 2250.degree.F for 24 hours. The resultant sintered preform is removed from the mold cavity and has a density of about 80% of 100% theoretical density. The preform, after a correction of cross sectional area to compensate for density variations, has physical properties comparable to those obtained on Sample A of Example 1.

While it will be apparent that the invention herein disclosed is well calculated to achieve the benefits and advantages hereinabove set forth, it will be appreciated that the invention is susceptible to modification, variation and change without departing from the spirit thereof.

Claims

What is claimed is:

1. A process for making sintered articles which comprises the steps of providing a mass of nickel-base superalloy powder having an oxygen content of less than about 300 ppm and of the type characterized as normally having carbide and gamma-prime strengthening, forming said mass of powder into a three-dimensional shape of the desired configuration, heating the shaped said mass of powder in the substantial absence of a surrounding atmosphere to a first sintering and transformation temperature ranging from about 1600.degree. to about 2000.degree.F at which the chemical equilibrium is conducive for effecting a conversion of primary carbides to complex carbides, maintaining said mass at said first sintering temperature for a period of time to effect an appreciable conversion of said primary carbides to said complex carbides under the prevailing equilibrium conditions and to initiate a diffusion bonding and neck formation between the powder particles at their points of contact, heating said mass of powder to a second sintering temperature above said first temperature up to the incipient melting point of said powder particles for a period of time sufficient to effect growth of said neck and the formation of an integral porous sintered preform, and thereafter cooling said preform.

2. The process as defined in claim 1, including the further step of applying pressure to said preform to effect a reduction in the porosity thereof.

3. The process as defined in claim 1, including the further step of heat treating said preform to improve the physical properties thereof.

4. The process as defined in claim 1, wherein said transformation temperature is selected at the level at which the chemical equilibrium maximizes the conversion of primary to complex carbides.

5. The process as defined in claim 1, wherein said superalloy powder is of an average particle size less than about 425 microns.

6. The process as defined in claim 1, wherein said superalloy powder is of an average particle size ranging from about 175 microns to about 10 microns.

7. The process as defined in claim 1, wherein the heating of the shaped said mass at said first temperature is performed in a vacuum of less than about 1,000 microns.

8. The process as defined in claim 1, wherein the heating of the shaped said mass of powder at said first temperature is performed in a vacuum of less than about 10 microns.

9. The process as defined in claim 1, wherein the forming of said mass of powder into a three-dimensional shape is achieved by loosely confining said powder in a mold cavity of said desired configuration.

10. The process as defined in claim 1, wherein the forming of said mass of powder into a three-dimensional shape is achieved by admixing an organic binder with said powder and thereafter compacting the powdered mixture in a die cavity of said desired configuration under pressure.

11. The process as defined in claim 1, wherein said first sintering and transformation temperature is about 1800.degree.F.