Two-Stage Martensitic Transformation in NiTi Alloys Caused by Stress Fields

It has been proved that the two-stage martensitic transformation in the NiTi alloys preceded by the R-phase transition my be induced by : low temperature annealing of the deformed alloy, thermal cycling and precipitation caused by ageing. The TEM observations of samples with two stage transformation have shown that the dislocations are distributed inhomogeneously and form a small-angle subgrain boundaries. The Granato-Lucke analysis of the internal friction peaks related to the two-stage transformation confirmed the difference in dislocation density released in both stages. Thus the two stage martensitic transformation is caused by the inhomogenity in the stress field which results from the dislocation configuration during the previous mentioned processes