The role of ledges in creep of TiAl alloys with fine lamellar structures

Creep experiments have been conducted on two powder-metallurgy TiAl alloys with fine grains, fine lamellar structures, and compositions of Ti-47Al + 2Cr + 2Nb and Ti-47Al + 2Cr + 1Nb + 1Ta. Results show that the stress exponent n and the activation energy Q decrease with decreasing 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, zero 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 stresses with cross-twinning being a microstructure exclusive at high stresses. To understand the underlying deformation processes, a ledge creep mechanism is proposed. The mechanism involves bunching and escaping of unit-height ledges into or from multiple-height ledges at interfacial boundaries (alpha(2)/gamma and gamma/gamma). It is assumed that the ledge motion, which is diffusion-controlled, causes phase transformation and thus deformation of the sample. Using this mechanism, the experimental results including the stress exponents, activation energies and transient creep behaviors after stress reduction at different stresses are reasonably interpreted. (C) 1998 Beta Metallurgica Inc.