Microstructural evolution associated with creep rate minima in pure- and near-γ TiAl intermetallics

Interrupted tensile and compression creep tests were performed at 1100K and 100MPa to study strain dependent microstructural changes in pure-gamma Ti-52Al. A mean twin spacing (MTS) and dislocation densities were investigated in the range of strain where sharp creep rate minima occur. It has been found that the overall dislocation density approaches a stationary value at strains close to 1%. On the other hand, strains as large as 7% are not sufficient to establish a dynamic equilibrium for deformation twinning. Based on these results it is suggested that, in the pure- and near-gamma equiaxed grain microstructures, the increase of creep rate after minimum and its new gradual saturation reflects a sluggish nucleation of deformation twins. A simple model has been proposed which addresses competitive effects of strain hardening and strain softening associated with the twinning process.