Phase-field simulation of twin boundary fractions in fully lamellar TiAl alloys

The phenomenon of high twin boundary fractions found experimentally for neighboring γ lamellae in fully lamellar TiAl alloys was investigated by phase-field simulations. The nucleation and growth processes during α2′→α2+γ transformation that leads to the lamellar structure were simulated. In particular, the effects of coherency stress and the interfacial energy difference among different types of interfaces and undercooling on the nucleation mechanism were analyzed. It is found that the twin boundary fraction increases with increasing coherency elastic strain energy and increasing interfacial energy difference among different types of interfaces, and with decreasing undercooling. Depending on the relative contributions of these factors, the nucleation events simulated by the Langevin noises could be collective, correlated or independent. These findings could shed light on the control of twin boundary fraction within lamellar structures by adjusting alloy composition and cooling route.