Developing a Kinetic Law for High-Pressure Martensitic Phase Transformations

Phase transformations are governed by the interplay of thermodynamics and kinetics. While the thermodynamics of materials at high pressures has been a subject of significant study, experimental limitations have restricted the exploration of kinetic effects in pressure-induced phase transformations. The development of piezo-electrically driven dynamic diamond anvil (dDAC) cells, allowing access to a wide range of compression rates, paired with advances in X-ray detector technology, enabling ms-scale time-resolved diffraction measurements, has enabled the exploration of overpressure and compression rate effects on structural transitions at high pressure. Understanding kinetic effects requires not just the experimental capability to measure the evolution of phase fraction, but an underlying model that closely matches the evolution of the material being studied. In order to assess if the Avrami Equation, a widely used kinetic law at ambient pressure, is suitable to describe the behavior of pressure-induced phase transformations, dDAC experiments were performed on CdSe and Fe, which experience a B4-B1 transition at 3 GPa and the BCC-HCP transition at 13 GPa, respectively. Both materials show evidence of kinetic behavior that cannot be reconciled with the underlying assumptions of nucleation, growth and impingement that constitute the Avrami formalism. A new kinetic law is developed that is better able to explain and reproduce the observed behaviors in CdSe and Fe. The proposed law, which can be expressed as a modified version of the first order rate law, considers microstructural effects that are significant in determining the behavior of martensitic transformations.