The use of a powder-metallurgy approach to modern superalloys for high temperature applications are discussed in terms of better homogeneity and formability. The use of inert-atmosphere atomized powder results in dense alloy products with low interstitial content and with mechanical properties equivalent to or better than the cast or wrought form of the alloy.
The advantages and disadvantages of the various densification processes are presented, with direct-powder extrusion or direct-powder forging to the dense product being the most attractive. Experimental results are given showing two extreme structural conditions obtained for direct-extruded P/M superalloys: the first is an ultra-fine grain structure which exhibits superplastic behavior; the second arises from a thermo-mechanical treatment which results in extremely large grain sizes, in some cases - single crystal structures which exceed the stress-rupture properties of the cast and wrought alloys.
As a result of the high alloying element content and the combination of high-and-low melting-point constituents in most nickel-base superalloys, segregation and banding of the cast ingots is often a severe problem. A P/M product made from atomized powder is inherently superior to a large casting since each powder particle is in essence an individual ingot. Cooling from the molten state is much faster for a powder particle than for a casting, thereby further minimizing inhomogeneity. If there is any segregation at all in the dense P/M product, it can only be micro-inhomogeneity whereas a casting can and often does exhibit macro-inhomogeneity. The homogenization processing for the P/M product usually can be accomplished during densification whereas the processing steps for the casting are often extremely difficult and lengthy involving complex thermo-mechanical processing. The P/M product, all other things being equal, should offer a considerable advantage over the cast product in terms of reproducibility of properties.
The second advantage of a P/M process for nickel-base superalloys arises from the difficulty encountered in working the as-cast materials. Since these alloys are used because of their superior high-temperature strength owing to the complex carbides and the high resolutioning temperature of the gamma-prime phase which is very close to the incipient melting temperature of the alloy, even hot working is often impossible without causing severe cracking. In a P/M process the powder can be densified to shape and require no post-densification deformation processing. A P/M process is, therefore, not limited to alloy compositions used for casting i.e., more “brittle” or hard alloy compositions can be used. Therefore, alloys which could not be cast to shape because of inhomogeneities, or could not be wrought subsequent to casting because brittleness, can be utilized in powder form. Powders can be direct extruded, forged, hot die-pressed, cold pressed and spark sintered, or hot isostatically compacted to a shape which will be homogeneous and, in some instances, superior in properties to the cast or wrought form of the alloy.
In a P/M process perhaps the most vital requirement for successful application, is high purity of the pre-alloyed powder, in particular, the powder particle surfaces. The surfaces must be free from oxides or interstitial contamination; or when the powder is compacted, continuous prior boundaries will persist. In the particular case of nickel-based superalloys, these continuous boundaries act as sites for carbide formation, which when continuous, is deletereous to the mechanical properties and causes the alloy to behave in a brittle manner. Furthermore, continuous prior boundaries inhibit grain growth thereby limiting the grain size and minimizing the high-temperature strength which is the forte of these nickel-base superalloys. Use of an alloy powder with low-interstitial and oxide-surface contamination results in non-existent or discontinuous prior powder boundaries which exhibit normal properties and permit grain growth past them. These requirements for high powder purity are met with inert-gas atomized powders of the nickel-base superalloys.