2011 Volume 52 Issue 3 Pages 358-363
The analysis of the influence of precipitation processes as well as dislocation structure on the transformation course and its characteristic parameters in NiTi shape memory alloys was carried out. In order to describe structural changes caused by thermo-mechanical treatment, transmission electron microscopy technique was applied; the study included in situ observations during cooling and heating the specimen in the microscope. The structural changes were related to the evolution of the martensitic transformation determined from the differential scanning calorimetry (DSC) measurements. It was found that the non-homogeneity of stress fields caused by presence of coherent precipitates or by specific dislocation structure results in a multi-stage martensitic transformation. The transformation is preceded by the R-phase transition. Also this transformation can occur in many stages. A thermodynamical model of the multi-stage martensitic transformations occurring in the two-component NiTi alloys was elaborated, which allows anticipation of the transformation sequences in these alloys.