Time-Resolved Transformation Kinetics of the BCC to HCP Transition in Iron Using Piezo-driven Compression
John Copley, Princeton University
Understanding the effect of kinetics on the behavior of phase transformations at high pressure is critical to interpreting experimental results but has historically been difficult to investigate due to limitations in compression rates and detector technologies. Current detector technologies, paired with the relatively rapid compression achievable by use of dynamic diamond anvil cells, have enabled time-resolved diffraction experiments that can probe the effect of compression rate and overpressure on the phase transition kinetics.
Iron, which experiences a BCC→HCP pressure-induced structural transition at approximately 13 GPa, has been extensively studied by both shock and static compression techniques. We examine the effects of fixed overpressure in traditional Avrami style experiments, as well as the effects of compression rate, with rates on the order of 10-3 to 102 GPa/s. In addition to overpressure and compression rate effects, we investigate the effects of stress state and particle size by performing continuous compression experiments on cells loaded without pressure media and with a smaller mean particle diameter.
We examine the results of these experiments in an Avrami framework, where phase evolution is determined by the interplay of nucleation, growth, and impingement. We assess the suitability of several kinetic models, including the JMAK equation and Czandera’s model for rapidly onsetting transitions, in terms of both fit to the data and the interpretation of the mechanics governing the shape of the phase evolution curve. We additionally develop a third model, where non-standard impingement processes can significantly alter the phase evolution, that allows us to accommodate initially high transformation rates observed in pressure induced transformations while maintaining a physical interpretation consistent with classical nucleation theory. With the development of a kinetic law for high pressure transformations, future studies may more directly access interesting kinetic parameters, such as nucleation rate, that are currently difficult to assess experimentally.
Abstract Author(s): J.A. Copley, I.K. Ocampo, S. Han, J.S. Smith, G. Shen, T.S. Duffy