Impurities Block the Alpha to Omega Martensitic Transformation in Titanium

Impurities control phase stability and phase transformations in nature: from shape memory alloys to steel to planetary cores. Experiments and empirical databases are still central to tuning the impurity effects. Missing is a broad theoretical underpinning. Consider, for example, the titanium martensitic transformations: diffusionless structural transformations proceeding near the speed of sound. Pure Ti transforms from ductile $\alpha$ to brittle $\omega$ at 9~GPa creating serious technological problems for $\beta$-stabilized Ti alloys. Impurities in the Ti alloys A-70 and Ti-6Al-4V suppress the transformation up to at least 35 GPa enhancing their technological utility as lightweight material in aerospace applications. These and other empirical breakthroughs in technological materials call for broad theoretical understanding. Impurities pose two theoretical challenges: The effect on \emph{the relative phase stability} and \emph{the energy barrier} of the transformation. {\it Ab initio} nudged-elastic band methods calculate both changes due to impurities. We show that interstitial O, N, and C retard the transformation while substitutional Al and V influence the transformation by changing the d-electron concentration. The resulting microscopic picture explains the suppression of the transformation in commercial A-70 and Ti-6Al-4V alloys. In general, the effect of impurities on relative energies and energy barriers is central to understanding structural phase transformations.