Metastable–solid phase diagrams derived from polymorphic solidification kinetics

Significance

Kinetic stabilization of metastable phases in rapidly cooled metals and alloys has been established in experiments for decades. However, atomistic theories that can quantitatively predict the solidification conditions that produce nonequilibrium phases are still in their infancy. Recent advances in pulsed power/laser technologies, as well as in situ characterization, have brought to bear unprecedented understanding of matter at extreme temperatures and pressures. However, accurate predictions of kinetic stabilization of metastable phases that are necessary for physical interpretation of these experiments are lacking. This work provides a blueprint for development of kinetic phase maps of materials undergoing rapid solidification from first principles. Through atomistic simulations, the phases dominating nucleation are identified, and their kinetic stabilities during the growth stage are characterized.

Abstract

Nonequilibrium processes during solidification can lead to kinetic stabilization of metastable crystal phases. A general framework for predicting the solidification conditions that lead to metastable-phase growth is developed and applied to a model face-centered cubic (fcc) metal that undergoes phase transitions to the body-centered cubic (bcc) as well as the hexagonal close-packed phases at high temperatures and pressures. Large-scale molecular dynamics simulations of ultrarapid freezing show that bcc nucleates and grows well outside of the region of its thermodynamic stability. An extensive study of crystal–liquid equilibria confirms that at any given pressure, there is a multitude of metastable solid phases that can coexist with the liquid phase. We define for every crystal phase, a solid cluster in liquid (SCL) basin, which contains all solid clusters of that phase coexisting with the liquid. A rigorous methodology is developed that allows for practical calculations of nucleation rates into arbitrary SCL basins from the undercooled melt. It is demonstrated that at large undercoolings, phase selections made during the nucleation stage can be undone by kinetic instabilities amid the growth stage. On these bases, a solidification–kinetic phase diagram is drawn for the model fcc system that delimits the conditions for macroscopic grains of metastable bcc phase to grow from the melt. We conclude with a study of unconventional interfacial kinetics at special interfaces, which can bring about heterogeneous multiphase crystal growth. A first-order interfacial phase transformation accompanied by a growth-mode transition is examined.