Interfacial dislocations and deformation twinning in fully lamellar TiAl

Deformation twinning, which takes place abnormally within lamellar TiAl subjected to creep deformation at strain rates as low as 10−7 s−1, has been found to be intimately related to the motion, pileup and dissociation of interfacial (Shockley partial) dislocations. Since the interfacial (Shockley partial) dislocations are energetically unfavorable to undergo cross-slip or climb, under normal conditions, they can only move conservatively along interfaces. Consequently, the pileup configuration once generated cannot be easily dissipated and thus remains in place even at elevated temperatures. The dislocation pileup eventually leads to the emission of deformation twins from the interfaces into γ lamellae when a local stress concentration due to the dislocation pileup becomes sufficiently large. Deformation twinning of {111}〈112〉 and {112}〈111〉 types (both generate Σ3 twin boundaries) has been observed. Both types of twinning can be rationalized by dislocation mechanisms involving the core dissociation of interfacial dislocations: 1/6[121](111)→1/6[011](100)+1/6[112]111 and 1/6[121](111)→1/2[010](001)+1/6[111](112), and the emission of 1/6〈112〉 and 1/6〈111〉 twinning dislocations into γ lamellae to form the (111)[112]- and (112)[111]-type twins, respectively. The critical shear stress for the {111}〈112〉-type twinning is evaluated using the Peach–Koehler formula based upon the pileup configuration of interfacial dislocations.