Internal friction in two-stage martensitic transformation of Ni-Ti alloy

Transformation sequences in a NiTi shape memory alloy after deformation and subsequent annealing were studied. For specimens deformed by rolling between 5 and 10% and then annealed in the range between 673 K and 823 K, the internal friction curve exhibits a broad peak in the range of martensitic transformation which is the result of the overlapping of two peaks. Similar effects were also observed using the DTA method. The Granato-Lucke analysis shows that the two peaks may be correlated with the difference in the dislocation densities existing in the specimens. It is suggested that the specific arrangements of dislocations cause non-homogeneous internal stresses which are responsible for the two-stage martensitic transformation.