Ab initio modeling of TWIP and TRIP effects in $β$-Ti alloys

Abstract

Transformations in bcc-$β$, hcp-$α$, and the $ω$ phases of Ti alloys are studied using Density Functional Theory for pure Ti and Ti alloyed with Al, Si, V, Cr, Fe, Cu, Nb, Mo, and Sn. The $β$-stabilization caused by alloying Si, Fe, Cr, and Mo was observed, but the most stable phase appears between the $β$ and the $α$ phases, corresponding to the martensitic $α''$ phase. Next, the $\{112\}\langle11\bar1\rangle$ bcc twins are separated by a positive barrier, which further increases by alloying w.r.t. pure Ti. The $\{332\}\langle11\bar3\rangle$ twinning yields negative barriers for all species but Mo and Fe. This is because the transition state is structurally similar to the $α$ phase, which is preferred over the $β$ phase for the majority of alloying elements. Lastly, the impact of alloying on twin boundary energies is discussed. These results may serve as design guidelines for novel Ti-based alloys with specific application areas.