Influence of a Tensile Stress on the Thermodynamics of Martensitic Transformations: Frictional Work and Stored Elastic Energy Calculations in Polycrystalline Alloys

The influence of a tensile stress on the thermodynamics of martensitic transformations in polycrystalline samples has been studied. The presentation of a thermodynamic framework from the principles of thermodynamics allowed the calculation of frictional work and stored elastic energy as functions of applied stress. The framework was used to analyze the martensitic transformations of a sputter-deposited Ni \(_{46.3}\) $_{46.3}^{}$ Ti \(_{51.5}\) $_{51.5}^{}$ Co \(_{2.2}\) $_{2.2}^{}$ (at. pct) thin film, with a microstructure characterized by Guinier–Preston zones and oriented Ti \(_2\) $_2^{}$ Ni precipitates inside grains. From experimental results of thermal cycles at different constant stresses, frictional work and stored elastic energy were calculated. The frictional work showed a constant behavior with applied stress in both B2 \(\leftrightarrow \) $\leftrightarrow$ R and R \(\leftrightarrow \) $\leftrightarrow$ B19’ transformations in the analyzed stress range. The stored elastic energy showed a decreasing behavior with applied stress in the B2 \(\leftrightarrow \) $\leftrightarrow$ R transformation, and a constant behavior in the R \(\leftrightarrow \) $\leftrightarrow$ B19’ transformation. It was found that the temperature interval of martensitic transformations is no longer an indicator of the stored elastic energy magnitude when a tensile stress is applied. The observed decrease of the thermal hysteresis as the applied stress increases was analyzed in terms of the stored elastic energy dissipation. Complementary, the utilization of a Clausius–Clapeyron-like equation in polycrystalline alloys has been discussed.