Enhancing the stability of the ω phase of zirconium alloys via local interlayer twists

The addition of solute provides an effective means to alter the stability of phases in metallic materials. This is particularly true for the metastable t\ensuremath{\omega} phase in Ti- and Zr-based alloys; however, the underlying mechanism remains inconclusive. In the present work, we show that the $\ensuremath{\omega}\ensuremath{\rightarrow}\ensuremath{\beta}$ phase-transformation process can be hindered to stabilize the \ensuremath{\omega} phase over a wide temperature and pressure range. This is demonstrated by molecular dynamics simulations of Zr-Nb alloys. In particular, we show that Nb dopants lead to the formation of a defect structure in the Zr-\ensuremath{\omega} phase, i.e., there is a corotation of six atoms in the {0002} plane by 30 \ifmmode^\circ\else\textdegree\fi{} along the $c$ axis of the \ensuremath{\omega} lattice. We refer to this as a local interlayer twist (LIT). The LITs are energetically preferred within Nb-rich regions and can interlock \ensuremath{\omega} lattices, thus retarding the $\ensuremath{\omega}\ensuremath{\rightarrow}\ensuremath{\beta}$ phase transformation. Furthermore, our density-functional theory calculations suggest that this mechanism should work in Zr- and Ti-based alloys with small solute atoms. The findings enrich our understanding of complex phase-transformation kinetics in alloys.