Author: Duane Johnson - University of Illinois, Urbana-Champaign

Title: Predicting Solid-Solid Transformations in Metals

Abstract:

Thermomechanically induced transformations in metals impact numerous engineering and technological applications, whether it be under standard or extreme conditions (e.g., high-pressure or high-strain rates). Hence, quantitative prediction of the onset stresses for the transformation and the transformation pathways are important, which also have a directly relation to characterization experiments. I will discuss recent approaches and results for predicting quantitatively (1) the onset twinning stresses in metals and alloys, (2) the transformation pathways for metals in high-pressure experiments, and (3) the effects of chemical short-range order in metals, affecting; e.g., the high-temperature yield-strength behavior. These three areas are connected in that we use Density Functional Theory (DFT) to determine potential energy surface (PES) information needed to predict quantitatively the key physics. Together these results show that we can accurately predict a wide-range of phenomena using DFT methods, when the correct materials physics/chemistry effects are included. In particular, I will show our analytically-derived expression.(combining heterogeneous nucleation and mesoscale dislocation mechanics) and show results that accurately predict onset twinning stresses for bulk fcc metals and alloys, when chemistry effects, such as the Suzuki phenomenon, are included. In addition, I will discuss the pressure-induced bcc-to-hcp transformation path for iron, quantitatively explaining both diamond-anvil and shock data ranging 10-30 GPa, in contrast to 150-300 GPa in previous DFT-based results. Lastly, I will briefly discuss predicting chemical short-range order in alloys via a new density-functional theory that has direct application to high-temperature yield-strength anomalies in alloys, e.g., used in nuclear reactors.