Calorimetric and Volumetric Studies of Dislocations During Martensitic Transformations in Shape Memory TiNi Alloy

Abstract

Based on the use of appropriate approaches, methods and results of the analysis of a number of topical problems of physical materials science, an in-depth analysis has been done of calorimetric and volumetric data for direct, reverse and deformation martensitic transformations in a nanostructured Ti49.3Ni50.7 shape memory alloy obtained by cold rolling with simultaneous action of pulsed high-density current. For the first time, by applying a new technique for processing calorimetric spectra (peaks), the staging and “kinetics” of changes in heat content, as well as thermal effects (enthalpies of individual stages) during direct and reverse martensitic transformations during cooling or heating of samples at a constant rate (3 × K/ min) in the range 170–370 K has been done. It is shown, by processing volumetric data, using theoretical values of the dislocation density and elements of the classical theory of dislocations, that in the Ti49.3Ni50.7 shape memory alloy subjected to cold deformation accompanied by the action of a pulsed current, a deformation martensitic transformation occurs, leading to a positive volume effect (∆V/V) ≈ 3 × 10–3), which can be largely due to dislocations. It is shown, by applying the theoretical values of the dislocation density and elements of the classical theory of dislocations, that the possible contributions of dislocations to the enthalpies of direct and reverse martensitic transformations (in the Ti49.3Ni50.7 alloy) can and should be significantly lower in absolute value, but opposite in sign to the observed enthalpies of direct and reverse martensitic transformation in a given alloy. It is shown that the physics of the processes under consideration is contained to a certain extent in scientific discoveries No. 239 “The phenomenon of thermoelastic equilibrium during phase transformations of the martensitic type – the Kurdyumov effect” and No. 339 “The phenomenon of the formation of non-equilibrium grain boundaries in polycrystals when they absorb lattice dislocations”.