Modelling of Structural Phase Transformations in Intermetallic and Related Alloys

Atomistic quantum mechanical calculations provide insights into the functioning of materials beyond experimental resolution and can be used as an unbiased tool for the design of novel alloys. In this talk, I will use our recent activities to demonstrate their application to the study of phase transformations in several Ti-containing systems.<br/><br/>NiTi is a class of shape memory alloys. The transformation temperature can be effectively shifted by exposing the material to hydrogen-rich environments. Through a combination of transmission electron microscopy and synchrotron diffraction experiments, we have been able to demonstrate the uptake of hydrogen into the material in the B2 austenite phase. The changes in lattice constant measured in this way were compared with DFT predictions, allowing the H content in the material to be quantified. The experimental work led to a proposal for the existence of a hydride phase. We then used Monte Carlo simulations to demonstrate that there is a thermodynamic driving force for hydride formation, rather than uniformly distributed H interstitials.<br/><br/>In another case study, we used Monte Carlo to study the effect of Mo alloying in an intermetallic TiAl alloy on its ordering temperature. While the low-temperature variant consists of an ordered cubic B2 or hexagonal B19 phase, increasing the temperature leads to disordering of the system to a bcc or hcp alloy, respectively. Using DFT and MC calculations, we have shown that alloying Mo at the expense of Al (Ti-rich compositions) dramatically increases the ordering temperature by almost 1000 K for 15 at.% Mo, whereas replacing Ti (Ti-lean compositions) has a negligible effect.<br/><br/>The B2-TiAl phase is unstable with respect to spontaneous transformation into both gamma-TiAl (Bain's transformation pathway) and B19 phase. By calculating the potential energy surfaces for increasing Mo content, we show that a small barrier appears between the two phases before the B2 phase finally becomes the most stable. Thus, Mo acts as a stabiliser for both structural and ordering transformations in the model TiAl+Mo intermetallic alloy.<br/><br/>As a final example, I will discuss the influence of several alloying elements on transformation barriers for TWIP (twinning-induced plasticity) and TRIP (transformation-induced plasticity) effects in binary beta-Ti alloys. Here Mo and Cr are proposed to stabilise the system with respect to the TRIP effect, while Mo and Al show stabilisation with respect to {331}&lt;11-3&gt; twinning.