Ab initio modeling of TWIP and TRIP in Ti alloys

Advanced manufacturing methods, such as additive manufacturing, introduce new challenges like suboptimal microstructures and inhomogeneities. The deliberate engineering of a materials mechanical properties—strength, elasticity, hardness and toughness to name but a few—can help to alleviate these shortcomings. In this contribution, we used Density Functional Theory to present a fundamental study on the impact of nine of the most common alloying elements—namely Al, Cr, Cu, Fe, Mo, Nb, Si, Sn and V—on transformation energies in bcc-β, hcp-α and the ω phases of Ti alloys compared to pure Ti. We employed ab-initio methods to calculate the potential energy surface for a β ↔ α and β ↔ ω transformation as a model for transformation-induced plasticity (TRIP), as well as stacking fault energies together with their respective barriers for different twinning mechanisms as a measure of the twinning-induced plasticity (TWIP). Regarding the TRIP effect, alloying with Cr, Fe, Mo and Si lead to a general stabilization of the β phase, although the even more stable phase occurred in the region between β and α, corresponding to the martensitic α′′ phase. We compared the super-positioning of the results of multiple binary alloys with the results of fully simulated real world alloys, and report reasonable success. Although the twins in the {112}⟨111¯⟩ bcc system were separated by a positive energy barrier which increased with additional alloying, the twins in the {332}⟨113¯⟩ bcc system exhibited negative energy barriers for all alloying elements except Fe and Mo, albeit always smaller in size than that of pure Ti. Finally, the effect of changes in the proximity of alloying atoms to the twin boundaries were examined. These findings have shown promise as guidelines for designing novel Ti-based alloys or adopting existing ones for new production methods, reducing the reliance on the semi-empirical methods in use so far.