THE ROLE OF LEDGES IN CREEP OF TIAL ALLOYS WITH FINE LAMELLAR STRUCTURES

Creep experiments have been conducted on two powder-metallurgy TiAl al loys with fine grains, fine lamellar structures, and compositions of T i-47Al + 2Cr + 2Nb and Ti-47Al + 2Cr + 1Nb + 1Ta. Results show that th e stress exponent n and the activation energy Q decrease with decreasi ng stress (n similar to 7-9 to 1.5, Q similar to 300 kJ/mol to similar to 150 kJ/mol). Stress reduction led to the occurrence of negative, z ero and positive creep at low stresses but only positive creep at high stresses. Thinning and dissolving of alpha(2) lamellae and continuous coarsening of gamma lamellae were observed at both low and high stres ses with cross-twinning being a microstructure exclusive at high stres ses. To understand the underlying deformation processes, a ledge creep mechanism is proposed. The mechanism involves bunching and escaping o f unit-height ledges into or from multiple-height ledges at interfacia l boundaries (alpha(2)/gamma and gamma/gamma). It is assumed that the ledge motion, which is diffusion-controlled, causes phase transformati on and thus deformation of the sample. Using this mechanism, the exper imental results including the stress exponents, activation energies an d transient creep behaviors after stress reduction at different stress es are reasonably interpreted. (C) 1998 Beta Metallurgica Inc.