On the influence of thermomechanical treatments on the microstructure and phase transformation behavior of Ni–Ti–Fe shape memory alloys

The present work shows how the microstructure and the phase transformation behavior of as-cast Ti-48 at.% Ni-2 at.% Fe shape memory alloys respond to different thermomechanical treatments. The applied procedures involve homogenization, swaging at different temperatures and recrystallization annealing. The as-cast material is characterized by a coarse-grained microstructure. During repeated hot working in combination with recrystallization treatments, a fine-grained microstructure evolves in the surface volume of the swaged Ni–Ti–Fe rod. In contrast, its center remains almost unaffected by recrystallization (because less dislocations are introduced during swaging and recovery processes are more intense). This type of microstructural heterogeneity can be avoided by cold work at the end of the thermomechanical procedure. Swaging at low temperatures results in higher dislocation densities and thus in a more homogeneous microstructural evolution during recrystallization. In the cold-worked state, the phase transition R → B19′ is strongly impeded by the presence of dislocations, whereas the transition characteristics of the recrystallized and of the non-deformed original material state almost coincide.