Effect of the precipitates on the thermodynamics of the martensitic transformations in Ti-rich Ni-Ti-Co thin films

Abstract

The wide-spread multiple transformation in Ti-rich Ni-Ti shape memory alloys produced by Ti2Ni nano-precipitates was investigated. For this purpose, a series of amorphous Ti-Ni-Co thin films with the same composition were subjected to different annealing treatments. The characteristic temperatures and heat of the transformations were measured by four-point probe resistivity and differential scanning calorimetry. A critical review of the thermodynamics of thermally-induced thermoelastic martensitic transformations is presented. The values of the frictional work and the stored elastic energy associated with the thermoelastic transformations were assessed from the transformation temperatures, and related with the transformation heats measured in the samples annealed at different temperatures. It was observed that the stored elastic energy decreases as the annealing temperature rises due to a change in the distribution of the Ti2Ni precipitates, in concordance with an increase of the transformation heat measured by the calorimeter. Nevertheless, the values of the frictional work were independent of the annealing temperature. The influence of the dissipation of the stored elastic energy on the transformation temperatures and thermal hysteresis is discussed. Finally, a better understanding of the wide-spread multiple transformation, as an effect of the local stresses introduced by Ti2Ni nano-precipitates in the matrix, is proposed.