FLOW BEHAVIOR AND MICROSTRUCTURE OF CO3TI INTERMETALLIC ALLOY DURING SUPERPLASTIC DEFORMATION

The superplastic deformation of the Lit-type Co3Ti alloy was observed as functions of temperature, strain rate and initial grain size, and t hen characterized by the flow behavior, the constitutive equation and the deformation microstructure. In the region of higher strain rate (a lso lower temperature and larger initial grain size), the flow curve e xhibited a stress peak followed by a rapid stress decrease, and dynami c recrystallization (DRX) occurred. The activation energy of deformati on in this region was estimated to be Q = 158 kJ/mol, similar to that for the bulk diffusion. It is suggested that the mechanism associated with DRX is responsible for the deformation in this region. In the reg ion of lower strain rate, (also higher temperature and smaller initial grain size), the flow curve exhibited continuous work hardening until fracture, and the concomitant motion of grain boundary sliding and gr ain growth occurred. The activation energy of deformation in this regi on was estimated to be Q = 80 kJ/mol suggestive of that of grain bound ary diffusion. Large tensile elongation, i.e. superplastic deformation beyond 200% was observed in the latter region. It is suggested that t he grain boundary sliding-based mechanism is responsible for the defor mation in this region. (C) 1998 Acta Metallurgica Inc.