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 α2-Ti3Al and γ-TiAl. For both alloys, a larger volume fraction of α and α2 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 α2 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 α2/γ grains also present.