The Prediction of HIP Parameters for Intermetallic Prealloyed Ni — Al Powder

Rapid solidification processes have received considerable attention as a suitable method to produce spherical powders with fine grain size, increased chemical homogeneity and extended solid solubility. Depending on the type of process the material’s response to rapid heat extraction differs widely. Microstructures of high-grade homogeneity can be obtained if process control during atomization allows a high undercooling of the particles prior to solidification. Undercooling controls the solidification rate. The solidification front velocity must exceed the diffusivity of atoms in the melt at solidification temperature if segregation is to be avoided. High pressure Ar gas atomization is one of the preferred processes for producing polycrystalline, narrow size distribution spherical powder particles with inner grain sizes of only a few micrometers. These attributes are beneficial for HIP consolidation. Transferred to the production of fine grained intermetallic compounds these attributes offer promising properties for technological applications. In a previous paper experiments were described where Ni50Al50, Ni75Al25 and nonstoichiometric prealloyed powders were produced by Ar-gas atomization (1). The consolidation of these fine grained intermetallics with ordered lattice structure by plastic deformation was limited to the very beginning of the densification. Grain boundary and volume diffusion were also limited by the ordered structure. Full density material was achieved only if HIP cycles reach approx. 0.8 Tm. Grain growth decreased the ductility (2) and, hence, cycle times should be optimized to a minimum length to obtain full density and fine grained HIPed material. The purpose of this paper is to demonstrate the collection of data from an actually used material tuning the sensitive parameters in HIP diagram calculations (3).