Interaction of deformation twins with α 2 plates in γ-TiAl base alloy deformed at room temperature

Abstract

Intersections of deformation twins with α plates in a two phase γ TiAl alloy deformed at room temperature have been investigated in detail by transmission electron microscopy (TEM). It has been found that two types of configurations, depending on the orientation of the intersection line, should be distinguished. For a type-I intersection with the intersection line parallel to 〈110] y, the deformation twins are blocked by the α 2 plate. At these intersections ordinary glide dislocations were emitted back into the γ matrix on cube slip planes, together with the production of twinning in γ parallel to the γ/ α 2 interface. No shear transfer across the γ/ α 2 interface was observed in this case. For a type-II intersection with the intersection line parallel to 〈011] γ, the incident twinning shear is usually accommodated by a-type slip on {11¯00} planes in the α 2 plate. In one particular case, {22¯01} twinning was observed in the α 2 plate at the intersection. The observations were analysed by taking into account all possible dissociations of a twinning partial (or a number of them) into two glissile dislocations, or into one glissile dislocation (glissile in matrix or plate) and a residual interface dislocation. This was done by considering a number of conditions for dissociation of twinning partials at the intersection and subsequent relaxation by slip. These conditions are similar as reported earlier for twin—twin intersections in the γ phase, and for slip transfer at grain boundaries in metals and alloys. The analysis showed that for a particular dissociation not all conditions could be fulfilled at the same time. The combination of the analysis and the observations showed the following order of priority of conditions: (i) a common line of intersection with the γ/ α 2 interface of the incoming twinning plane and the relaxation slip planes; (ii) the slip directions of the glissile product dislocations are consistent with the applied load, which was checked by taking into account the sign of the Schmid factor S; (iii) a zero or small residual interface dislocation, and (iv) a high value of ‖ S‖ (sometimes S was low).