EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Extensive modelling efforts have been devoted to the understanding of thermodynamics and diffusion in the fcc Fe-Ni system, which has a complicated magnetochemical phase diagram. However, the effects of magnetic excitations and transitions on these properties are unclear and usually neglected. Furthermore, a close interplay between the magnetic and chemical degrees of freedom is also expected, as the Fe-Ni system undergoes successively the chemical and magnetic transitions with increasing temperatures.

In this work, we first perform a systematic ab initio study of energetic and vibrational properties of Fe-Ni alloys [1]. Based on these results, magnetic effective interaction models (EIMs) for fcc Fe-Ni alloys are constructed, and implemented for Monte Carlo simulations enabling simultaneous chemical and magnetic evolutions with temperatures. The calculated phase diagrams are in good agreement with available experimental results, and are confronted to the recent CALPHAD prediction at low temperatures. Our thermodynamic results reveal an important interplay between the magnetic and chemical orders in fcc Fe-Ni alloys.

The magnetic EIMs are subsequently extended to include the presence of vacancy and self-interstitial atom. The temperature-dependent evolutions of defect formation and diffusion properties are investigated over the whole concentration range. Effects of transversal and longitudinal spin fluctuations on these properties are discussed. It is found that the impacts of magnetic excitations and transitions are more important in alloys than in the constituent pure phases. In alloys, the extent of such impacts is closely related to the chemical orders. Both magnetic and chemical transitions show important influences on defect formation and diffusion properties in concentrated alloys around 50% and 75% Ni. Finally, we also discuss the relative importance of magnetic and vibrational excitations on defect equilibrium concentrations and diffusion coefficients.

[1] K. Li, C.-C. Fu, Phys. Rev. Mater. 4, 023606 (2020).