INVITED: Understanding phase stability and diffusion kinetics in mechanically unstable but dynamically stabilized phases from first-principles

The phase diagram of numerous materials of technological importance features high-symmetry high temperature phases that exhibit phonon instabilities. Leading examples include shape-memory alloys, as well as ferroelectric, refractory, and structural materials. In this talk I will introduce a new thermodynamic model for free energy calculation in these phases from first-principles[1]. This model efficiently explores the system’s ab-initio energy surface by partitioning it into piecewise polynomials around local minima, which is combined with a continuous yet constrained sampling in the vicinity of these local minima. I present the application of this model to the bcc phase of titanium as well as the austenite and martensite phases in NiTi and PtTi shape memory alloys, in which we illustrate that constant anharmonicity-driven hopping between local low-symmetry distortions stabilizes the system to maintain a high-symmetry time-averaged structure[2]. In addition, I will try to shed light on diffusion kinetics in dynamically-stabilized phases based on a first-principles approach within the transition state theory. Finally, I introduce the implementation of the model as an open-access and fully automated software toolkit called the Piecewise Polynomial Potential Partitioning (P4), which can be integrated into the Alloy Theoretic Automated Toolkit (ATAT)[3].

References

[1] S. Kadkhodaei, Q.-J. Hong, and A. van de Walle, “Free energy calculation of mechanically unstable but dynamically stabilized bcc titanium,” Phys. Rev. B, vol. 95, no. 6, p. 064101, Feb. 2017.

[2] S. Kadkhodaei and A. van de Walle, “First-principles calculations of thermal properties of the mechanically unstable phases of the PtTi and NiTi shape memory alloys,” Acta Mater., vol. 147, pp. 296–303, Apr. 2018.

[3] S. Kadkhodaei and A. van de Walle, “Software tools for thermodynamic calculation of mechanically unstable phases from first-principles data,” Comput. Phys. Commun., vol. 246, p. 106712, Jan. 2020.