Deformation of a 45 degrees <(3)over-bar21 > oriented polysynthetically twinned TiAl crystal at high strain rate and high temperature

Dislocation microstructures in individual domains in a 45 degrees(<(3)over bar 21>) oriented polysynthetically twinned TiAl crystal deformed in compre ssion at a strain rate of 3000 s(-1) and a temperature of 800 degrees C wer e characterized. Ordinary dislocation slip was found to be the dominant def ormation mode for the deformation of domains [II], [V] and [VI] where the t ransmission electron microscopy (TEM) foil normals were close to [0(1) over bar 1], [01(1) over bar] and [<(1)over bar 01>], respectively; mechanical twinning and superdislocation slip were found to be the complementary defor mation modes to accommodate the deformation of these domains. Domain [I], w here the TEM foil normals were close to [<(1)over bar 10>], was observed to be minimally (or barely) deformed under the deformation conditions imposed . Although mechanical twinning dominated the deformation of domains [III] a nd [IV] where the TEM foil normals were close to [10(1) over bar] and [1(1) over bar 0], respectively, dislocation slip within untwinned matrices was also observed. The majority of the dislocations observed in domains [III] a nd [IV] were the 1/2[110] ordinary dislocations. A correlation of the domin ant deformation modes in the individual domains with the mechanical respons e of the crystal suggests that the yield stress of Ti-rich TiAl alloys is d etermined either by ordinary dislocation slip or by mechanical twinning or by both, dependent upon the sample orientation.