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

Submicron-grained intertmetallic compounds based on gamma -TiAl were prepar ed by high-energy milling and hot isostatic pressing. At temperatures above 500 degreesC, the flow stress is strongly reduced and work-hardening compl etely disappears. Compression tests performed at temperatures between 500 a nd 900 degreesC reveal a marked strain rate sensitivity of the flow stress, suggesting superplasticity to occur. This could be confirmed by tensile st raining 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 ty pe Ti-5(Si,Al)(3), embedded in the grain boundaries of the gamma -TiAl matr ix, impede a coarsening of the microstructure. However, at strain rates abo ve (epsilon )over dot = 10(-3) s(-1), these dispersoids are suggested to pr omote the formation of voids and to reduce the overall deformability. At 80 0 degreesC, an apparent activation energy of Q(app) = 351 kJ/mol can be der ived. Superplastic behavior at 800 degreesC is accomplished by grain bounda ry sliding accommodated by diffusional processes inside the gamma -TiAl pha se. Thus, the high temperature deformation mode is similar to the mechanism s found for more conventionally grained TiAl alloys at deformation temperat ures greater than or equal to 1000 degreesC. (C) 2001 Elsevier Science Ltd. All rights reserved.