NiAl powder alloys: I. Production of NiAl powders

The influence of five methods of production of Ni₅₀Al₅₀ powder alloys on the processes occurring during reactive alloy formation of nickel monoaluminide during heating is considered. It is shown that, when powder mixtures obtained by agitation in ball mills and cladded composite powders with a low level of internal stresses are used, it is possible to produce a material with a nearly equilibrium phase composition in the course of reactive sintering due to an exothermic effect with the participation of a liquid phase (aluminum melt) in the reaction. The sintered material is porous and has an island structure. Mechanical alloying in a high-energy ball mill (attritor) results in the formation of layered Ni/Al granules with a developed interface and a high level of internal stresses and defects, which makes it possible to decrease the temperatures of initiation of reactive interaction by ∼300°C. This interaction develops in the solid phase according to a slow diffusive mechanism leading to the formation of intermediate nickel aluminides and hindering the achievement of equilibrium phase composition. The microingot granules (∼80 wt % particles 100–400 μm in size) produced by melt spraying by gases (N, Ar) has the composition of the melt, but grain boundaries are depleted of aluminum in comparison with the volume. The NiAl powders (∼90 wt % particles <40 μm in size) produced by combined hydride-calcium reduction are characterized by a highly homogeneous nickel and aluminum distribution, and their composition is close to equilibrium. These two types of powders are selected as the initial material for investigating the compacting and production of NiAl-based alloys.