Atomistic Principles of Martensitic and Ordering Phenomena

The increasing computational power and improved theories allow nowadays to model realistic material systems, and consequently to apply so-called theory guided materials design. In particular, state-of-the-art Density Functional Theory based predictions are typically in excellent agreement with experimental observations. However, many materials for real-life applications rely on complex chemistry, complicated hierarchical microstructures and/or finite temperature effects which at present still remain challenging tasks. Therefore, this proposal focuses on developing strategies for studying such phenomena starting from the electronic level. Phase transformations are essence of materials science. Two kinds of phase transformations will be in the central focus of this project. On the one hand, martensitic phase transformations that do not involve any diffusional processes and therefore can proceed extremely fast. On the other hand, transformations where the chemical species in an alloy order via diffusion and hence are typically slower. We will study a TiAl+Mo model system, in which both kinds of transformations have been recently reported. It belongs to intermetallics, a novel and widely applied materials class of compounds and alloys consisting of two or more metal elements. We will start with obtaining important characteristics of the chemical interactions in the Ti-Al-Mo system (effective interactions, heat of formation) which will allow for a construction of structural models of this alloys system. This will subsequently lead to estimation of their thermodynamical properties. Temperature-dependent elastic properties will serve as inputs for modelling of martensitic (diffusionless) transformations. Simultaneously, diffusion mechanisms will be theoretically characterised and used in kinetic Monte Carlo simulations to study the order-disorder transitions. As a conclusive result, a complex model capable of quantitative predictions of martensitic and ordering transitions in Ti-Al-Mo model system is expected as an outcome. The theoretical predictions will be verified with synchrotron experiments revealing corresponding material characteristics (e.g., transformation temperatures). The project shall provide detailed insights into the working of the intermetallic TiAl+Mo system, which is of great interest on its own for developments of novel light-weight alloys, but also to contribute to the quest of bottom-up approach for materials design by proving the feasibility of predicting the interplay between chemistry and finite temperature effects in materials.

The MOTIF project (Atomistic Principles of Martensitic and Ordering Transformations at Finite Temperatures) employed advanced quantum-mechanical calculations to elucidate processes that happen in TiAl-based light-weight intermetallic alloyed when rapidly cooled. Their microstructures contain (meta)stable "frozen-in" phases that do not exist under equilibrium conditions and are difficult to characterise. We focused on the -TiAl-based system alloyed with Mo, a known stabiliser of the /o phase, which helps to tune the mechanical properties of the system. The fundamental interest was laid on developing a methodology that extracts structural and mechanical properties of (meta)stable chemically complex phases. Our calculations proved the beneficial impact of Mo on stabilisation of the /o while destabilising (avoiding) other structures. Mo was also shown to improve the ductility of and hexagonal Ti0.5Al0.5 phases. Several dedicated experimental measurements corroborated these theoretical predictions. The structural models and extracted effective interactions between species were further used to study martensitic (displacive) and ordering transformations. It has been concluded that the displacive transformations between ordered phases proceed mostly without any transformation barrier, i.e. in a spontaneous manner, thereby excluding an equilibrium co-existence of o and phases with the same composition. Chemical disorder, on the contrary, introduces a small transformation barrier, hence effectively stabilising the -TiAl phase with a small amount of Mo with respect to the -TiAl or 2'/B19-TiAl phases in equilibrium. Lastly, an increase in Mo content was shown to lead to higher ordering temperatures, i.e. effectively stabilising the ordered phases to higher temperatures. Therefore, to obtain the martensitic microstructure, it is desirable to balance the amount of Mo since higher Mo concentration, on the one hand, stabilises the o phase but on the other hand results in a too-large thermodynamic driving force for ordering. The latter is related to short-scale diffusion and hence leading to a phase separation in equilibrium conditions (co-existence of phases with a different composition, i.e. not a martensitic microstructure).