Crystal chemistry and Calphad modelling of the χ phase

Abstract

This article is a comprehensive review devoted to the phase diagram and crystal structure properties of the χ phase intermetallic compound. An extensive study of the available literature has been performed and completed by key experimental measurements performed in the following systems: Fe–Re, Mn–Mo, Mn–Re, Mn–Ru, Mn–V, Mo–Re, Nb–Os, Nb–Re, Re–Ti, Re–Zr. The χ phase is an important Frank–Kasper (topologically close-packed) phase whose presence in certain systems has implications in the process of industrial materials such as Ni-based super-alloys. As a binary phase, it exists mainly in transition metal systems of Tc, Re and Os with elements of groups 3–6, rare earths-Mg systems and (αMn) solid solutions. It has the peculiarity to exist in three ordered variants corresponding to the different prototypes αMn, Ti 5Re 24 and Mg 17Al 12. Particular attention has been paid to the way the composition of the binary compounds is accommodated by atom mixing on the different sites. In our experimental work, occupancy parameters on the four crystallographic sites have been obtained by Rietveld analysis of X-ray powder diffraction data. The investigation has been made as a function of composition when possible. In addition to the study of the intermetallic compounds, particular emphasis has been placed on the study of the ordering in (αMn) solid solutions. This study has implications on the modelling of the χ phase with the Calphad method. Different models are reviewed and recommendations are made for future thermodynamic assessments.