AN INSITU STUDY OF TWIN PROPAGATION IN TIAL

An in situ study of twinning in a TiAl alloy has been performed in order to determine twin properties and to discuss the processes controlling twin propagation. This study shows that twinning is due to a/6[112] partial dislocations gliding in octahedral planes and that twinning is an intensive mode of deformation for some specimen orientations. Twins move slowly at an approximately constant velocity by a collective movement of partial dislocations which are separated by a constant distance during the motion. Twins in the TiAl alloy are perfect. A polar twin source is analysed. The occurrence of twinning with respect to other possible deformation modes is discussed and interpreted using the Schmid law. It is shown that the twin propagation is controlled by thermally activated frictional forces acting on partial dislocations and by the dragging of a stacking fault.