Phase transitions driven by solute concentration, temperature, and pressure in uranium-6wt % niobium alloy

TL;DR

This study develops a new angular-dependent potential for U-Nb alloys to accurately predict phase transitions, elastic properties, and high-pressure behavior, revealing a twinning-coupled alpha-prime to gamma transition and resolving shear stress relaxation discrepancies.

Contribution

The paper introduces a novel ADP and EAM potential for U-Nb, enabling precise modeling of phase transitions and high-pressure phenomena in the alloy.

Findings

Accurately predicts phase transitions driven by solute, temperature, and pressure.

Reproduces melting points and lattice expansion of U-Nb alloys.

Identifies a twinning-coupled alpha-prime to gamma transition at high pressure.

Abstract

An angular-dependent potential for the U-Nb system is developed based on an existing ADP for U and a new EAM potential for Nb through fitting flexible cross-interaction functions and alloy parameters to experimental and first-principles data, enabling accurate prediction of phase transitions (alpha to gamma driven by solute concentration; alpha-prime to gamma under temperature in U-6Nb alloy), elastic properties, defect energetics, and mixed enthalpy. The potential reliably reproduces melting points of U-Nb solid solutions and captures lattice parameter expansion of gamma U-6Nb. Notably, it correctly predicts Hugoniot relations and equations of state up to about 90 GPa and resolves the alpha-prime to gamma transition under static high pressure. Combined with atomic simulations, we reveal a twinning-coupled alpha-prime to gamma transition of U-6Nb under high pressures: {112}gamma twins form via nanosecond-scale twinning precursors generated during the transient adiabatic compressions. The static phase transition pressure is predicted to be 54.5 GPa, comparable to 67.2 GPa by first-principles calculations. Besides, our result suggests that U-6Nb single-crystal would experience a nonlinear elastic relaxation before yielding plastically at 3.1 GPa (shear stress: 0.9 GPa). The results in this work help resolve long-standing discrepancies in understanding the abnormal shear stress relaxation mechanisms under high-pressure shock loading.