Mechanical behavior of submicron-grained γ-TiAl-based alloys at elevated temperatures

Submicron-grained intermetallic compounds based on γ-TiAl were prepared by high-energy milling and hot isostatic pressing. At temperatures above 500°C, the flow stress is strongly reduced and work-hardening completely disappears. Compression tests performed at temperatures between 500 and 900°C reveal a marked strain rate sensitivity of the flow stress, suggesting superplasticity to occur. This could be confirmed by tensile straining of Ti–45Al–2.4Si samples, resulting in elongations of up to 175% at 800°C. Small silicide particles (d≈100 nm) of the type Ti5(Si,Al)3, embedded in the grain boundaries of the γ-TiAl matrix, impede a coarsening of the microstructure. However, at strain rates above ε̇=10−3 s−1, these dispersoids are suggested to promote the formation of voids and to reduce the overall deformability. At 800°C, an apparent activation energy of Qapp=351 kJ/mol can be derived. Superplastic behavior at 800°C is accomplished by grain boundary sliding accommodated by diffusional processes inside the γ-TiAl phase. Thus, the high temperature deformation mode is similar to the mechanisms found for more conventionally grained TiAl alloys at deformation temperatures ⩾1000°C.