A Combined Powder Metallurgical Approach to Process Gamma-TiAl with Composite Structure

Gamma-TiAl (> 99 pct) sample with composite structural design was fabricated by a combined powder metallurgical approach of the SEBM capsule and HIP. Selective electron beam melting (SEBM) was used to create the pre-sintered powder bed, composite structure and the capsule, followed by hot isostatic pressing (HIP) at 1250 °C. A mixture of plasma rotating electrode (PREP) and gas atomization (AA) processed powders, with the respective higher (49.66 at. pct) and lower Al (47.61 at. pct) concentrations, was employed successfully to track the transition from powder to bulk sample, providing enriched information to elucidate the microstructure formation mechanism. The selective melting created composite structure consisted of the fine equiaxed γ-grains, while the rest of the powder bed that had been subjected to preheat only and then HIP was characterized by the triple microstructure. Formation mechanisms of such unique microstructure, consisting of primarily the coarse γ-grains and fine γ-grains with the previous particle boundaries (PPBs), were correlated to the Al concentration. The Al-rich powder was transformed into the coarse γ-grains, while the Al-depleted powder was responsible for the fine γ-grains with the PPBs. This finding suggests that the detrimental PPBs can be eliminated by increasing the Al concentration in the nascent powder. In the post-heat treated condition (1350 °C), no PPBs was found, suggesting that heat treatment is beneficial by eliminating the PPBs. For the designed composite structures, there was evidence to suggest desired distribution of the hard and soft regions, with the fine γ-grains associated with the composite structure being responsible for the high hardness region.[Figure not available: see fulltext.].