Atomic mechanism for elastic–plastic behaviors of uranium-6 wt. % niobium single crystal alloy under shock compressions

The absence of an elastic precursor (EP) in shock-loaded U-6 wt. % Nb alloys has long been attributed to the rapid shear stress relaxation due to phase transformation or twinning. As a first step toward uncovering such issues, we investigate the shock responses of uranium-6 wt. % niobium single-crystal alloys through large-scale non-equilibrium molecular dynamics simulations combined with first-principles calculations. Our results suggest that no clear EP is observed for shocks along the b and c axes, while an EP emerges along the a axis under certain conditions. The EP disappearance stems from a slower elastic-wave velocity compared to the plastic wave velocity—a direct consequence of enhanced compression modulus during shock compression. The modulus enhancement in the α″ phase is attributed to the first stage of a two-stage plastic mechanism: picosecond-scale stress decay via coordinated (001)-plane shuffling forms a transient intermediate phase, followed by a much longer timescale γ-phase transformation through [100]/[1¯00] shear on (010) planes, generating {112}〈111〉 twins. This mechanism underscores the critical role of transient intermediate phases under rapid compression, challenging prior interpretations and providing atomic-scale insights for the shock responses of uranium alloys.