Dislocation slip and deformation twinning interplay during high temperature deformation in γ-TiAl base intermetallics

Interrupted compression creep tests were performed at 973 K and 350 MPa to study strain dependent microstructural changes in near-gamma equiaxed Ti-48Al-2Cr-2Nb-B alloy. Dislocation densities and volume fraction of deformation twins were measured in the range of strain where sharp creep rate minima occur. The steep decrease of creep rate during initial 1% of strain is associated with a considerable increase in density of ordinary dislocations and superdislocations. Twinning processes are rather limited during this primary stage. On the other hand. in the strain range of minimum creep rate the volume fraction of deformation twins grows due to the increasing number of twinned grains and decreasing spacing between individual twins. Based on these results it is suggested that, in the pure- and near-gamma TiAl equiaxed grain microstructures, the increase of creep rate after minimum and its new gradual saturation reflects a contribution of deformation twins to the creep strain accumulation kinetics. The contribution is twofold: (i) deformation twins relax incompatibility stresses set in during primary creep; and (ii) twinning supplies up to 30% of the overall strain in advanced stages of creep. (C) 2001 Elsevier Science B.V. All rights reserved.