SUPERPLASTIC DEFORMATION IN NI3(SI,TI) ALLOYS

Superplastic deformation in fine-grained Ni3(Si,Ti) alloys was observed over a relativelv limited temperature range, from about 1073 to 1173 K, and at strain rates below about 5 x 10(-4)/s. Superplastic deformation was very sensitive to the grain size with the elongation-to-failure increasing with a decrease in grain size- Partially recrystallized alloys showed higher elongation values rather than fully recrystallized alloys. Phenomenological feature was characterized in the constitutive equation expressed by epsilon = Asigma(n) exp(-Q/RT); in the superplastic range, where the stress exponent, n, is about 2.3, the activation energy for the boron-doped alloy was 3 1 8 kJ/mol and 273 kJ/mol for the alloy without boron. It was shown by OM, TEM and SEM observations that several processes, including maintenance of the equiaxed grain structure, extensive grain boundary sliding, dislocation motion near grain boundaries and triple points. nucleation-type and bulged-type recrystallizations and cavity formation, were occurring during the superplastic deformation. It was qualitatively discussed that the grain boundary sliding-based mechanism with long range accommodation such as generation of dislocations or nucleations of new grains at prior grain boundaries was responsible for the superplastic deformation in Ni3(Si,Ti) alloys.