What is Wrong with the Clapeyron Equation in Martensitic Transformations?

Abstract

Recent research has concentrated on martensitic transformations due to their potential for the utilization of green energy in applications. A comprehension of the thermodynamic characteristics of these transformations, particularly the entropy differences ( ), is of paramount importance for the development of advanced materials. The Clapeyron equation is frequently employed to examine the stress-temperature relationships associated with martensitic transformations, with experiments conducted at constant temperatures serving as a common evaluation method. However, it is also important to consider cases where the temperature is varied while maintaining constant stress, condition typical of actuator applications. It is frequently assumed that these results are unique, thereby overlooking the potential for phase evolution and its impact on thermodynamic stability and equilibrium. This study presents the results of experiments conducted on two representative materials: Cu–Zn-Al single crystals and ultrafine-grained Ni–Ti wires. It is demonstrated that the calculated values can be significantly altered under testing conditions that, at first glance, appear to be appropriate. To address these inconsistencies, a framework is proposed for establishing an experimental protocol that ensures consistent results across various transformation paths. These findings enhance the understanding of stress-temperature dependencies in martensitic transformations and contribute to their optimization for applications.