Deformation mechanisms and anomalous hardening at high temperature in γ-titanium aluminides

Gamma titanium aluminides exhibit anomalous hardening at high temperatures similar to L1(2)Ni(3)Al and Ni3Ga. Nevertheless, deformation and hardening mechanisms in TiAl alloys are much more complex than those of Ni3Al due to the simultaneous operation of multi-slip systems and different cross slip modes. Both superdislocations and ordinary dislocations equally contribute to the deformation and anomalous hardening from room temperature to 1073K. There are two different cross slip modes of superdislocations with b=1/2<101], i.e., i) the octahedral-to-octahedral cross slip occurring from room to 650K and ii) the octahedral-to-cube cross-slip occurring between 650K and 1073K. Two different cross-slip modes appear to be associated with two different activation energies for the anomalous hardening. In addition, the CRSS values were found to be deformation orientation dependent. The TEM study shows that each superdislocation core dissociate into two superpartials, and cross-slips onto either neighboring octahedral planes or cube planes depending on temperature. The transition from the octahedral cross-slip mode to the cube cross-slip occurs near 650 K. On the other hand, ordinary dislocation slip also causes anomalous hardening at high temperatures. The resulting CRSS values depend on both temperature and deformation orientation. For the anomalous hardening by ordinary dislocations was found to have a single activation energy and appears to be governed by a single mechanism.