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

Submicron-grained intertmetallic compounds based on gamma -TiAl were prepared by high-energy milling and hot isostatic pressing. At temperatures above 500 degreesC, the flow stress is strongly reduced and work-hardening completely disappears. Compression tests performed at temperatures between 500 and 900 degreesC 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 degreesC. Small silicide particles (d approximate to 100 nm) of the type Ti-5(Si,Al)(3), embedded in the grain boundaries of the gamma -TiAl matrix, impede a coarsening of the microstructure. However, at strain rates above (epsilon )over dot = 10(-3) s(-1), these dispersoids are suggested to promote the formation of voids and to reduce the overall deformability. At 800 degreesC, an apparent activation energy of Q(app) = 351 kJ/mol can be derived. Superplastic behavior at 800 degreesC is accomplished by grain boundary sliding accommodated by diffusional processes inside the gamma -TiAl phase. Thus, the high temperature deformation mode is similar to the mechanisms found for more conventionally grained TiAl alloys at deformation temperatures greater than or equal to 1000 degreesC. (C) 2001 Elsevier Science Ltd. All rights reserved.