Phase transformation and dislocation evolution in the fusion zone of plasma arc welded TiAl joints during heat treatment

Well-welded TiAl joints without solid-state cracks can be obtained using plasma arc with heating at 300 °C, but post-welding heat treatment (HT) is still needed to relieve the residual stress and to regulate the microstructure. In this work, the microstructure evolution and the involved phase transformations, as well as the dislocation evolution in the fusion zone of TiAl joints at 1240 °C are researched. Pearlite-like structures induced by the cellular reaction quickly occur at the beginning of HT, but have low stability at 1240 °C and soon disappear within 5 min. The discontinuous coarsening of the γ lamellae in the β_o phase is the initial stage of the cellular reaction. Owing to the inherited orientation relationship of the γ phase from the β_o phase, the coarsening process of the γ lamellae occurs on preferred plane, which is parallel to the lamellae interface, due to the lowest included angle and misfit degree between the {110}β_o and {111}γ. Both coarsening and decomposition of the γ lamellae happen during HT, which are driven by the decrease of interfacial energy and coherent strain energy, and by the decrease of chemical energy, respectively. The transformation of the γ lamellae is beneficial for the activation of dislocations both in the γ and α₂ lamellae. When the α₂ lamellae are enough wide, dislocation tangles occur and form the low angle grain boundaries. However, further dislocation accumulation is not enough for static recrystallization.