Time-Resolved Transformation Kinetics of the BCC to HCP Transition in Iron Using Piezo- Driven Compression

Understanding of the effect of kinetics on pressure induced phase transformations is important in interpreting experiments, but has historically been challenging due to the need for relatively high compression rates and time resolution. Use of dynamic diamond anvil cells (dDAC) and fast X-ray detectors allows us to overcome these historical limitations in a study of the BCC to HCP transition in iron. This transformation near 13 GPa has been extensively studied by both dynamic and static compression techniques. We examine the effects of compression rate, from 10^-3 to 10² GPa/s. In addition, we investigate the effects of stress state and particle size by performing experiments on cells loaded without pressure media and on iron samples with a smaller mean particle diameter.



The results are examined in an Avrami framework, where phase evolution is determined by the interplay of nucleation, growth, and impingement. We assess the suitability of the Avrami equation for the high-pressure phase transformation in iron, discuss what differentiates high-pressure experiments from more conventional studies, and develop a new model which allows for non-standard impingement processes that is also more suited to high-pressure experiments. With the development of a kinetic law for high-pressure transformations, future studies may more directly constrain kinetic parameters, such as nucleation rate, that are currently difficult to assess experimentally.