Synthesis of an ultrafine grained TiAl based alloy by subzero temperature milling and HIP, its microstructure and mechanical properties

An ultrafine grained TiAl based alloy with a composition of Ti–45Al–2Cr–2Nb–1B–0.5Ta (at%) was synthesized by a combination of subzero temperature (ST) (−140 to −5 °C) milling of a mixture of elemental powders and hot isostatic pressing (HIP). The size, morphology and microstructure of the Ti/Al based composite powder particles produced by ST milling and room temperature milling, respectively, were also compared, and it is demonstrated that ST milling significantly enhances the effectiveness of milling in forming and refining the composite structure. The bulk ultrafine grained TiAl based alloy samples produced using the ST milled Ti/Al composite powder exhibited a near-gamma two-phase microstructure with grain sizes in the ranges of 50–200 and 100–400 nm corresponding to HIP temperature of 800 and 1000 °C respectively. These samples showed relatively low tensile strength at room temperature and 800 °C but high tensile elongation of 70–130% at 800 °C. The reasons for the effect of ST milling and the mechanical behaviour of the consolidated samples are discussed.