Microstructural evaluation of as-solidified and heat-treated γ-TiAl based powders

Abstract

Powders with nominal compositions (in atomic percent) Ti48Al and Ti48Al2Nb2Cr were prepared by the plasma rotating electrode process (PREP) and gas atomization (GA) techniques. As-solidified and heat-treated (1000°C per 3 h) powder samples were examined by metallography, scanning electron microscopy, X-ray diffraction and transmission electron microscopy. The microstructures of the powders were characterized as a function of atomization technique, alloy content, powder particle size (solidification rate) and thermal history. All of the as-solidified powders were comprised of disordered α, and ordered α 2-Ti 3Al and γ-TiAl. For both alloys, a larger volume fraction of α and α 2 was observed in the PREP powders relative to GA powders of comparable size. Additionally, for both alloys and both atomization techniques, the volume fraction of α 2 was observed to increase with decreasing powder particle size. In general, the PREP powders appeared to solidify more rapidly than the GA powders. The heat-treated powders were comprised of equiaxed γ grains with limited amounts of lamellar α 2/γ grains also present.