Prediction of the REM–Fe–Co phase diagrams in melting-crystallization region

Abstract

The crystal structures of compounds and phase diagrams of the binary REM–Fe and REM–Co systems and ternary Fe–Co–REM system were analyzed and systematically compared. This allowed the nature of physicochemical interaction of components in unstudied systems to be predicted and the reliability of existing experimental data to be evaluated. Based on regular changes in the REM–Fe and REM–Co phase diagrams, typical features that define their liquidus configuration were formulated. According to this characteristic, the REM–Fe and REM–Co systems were divided into six and five types, respectively. Each type was characterized by the presence, number, and composition of congruent compounds. Linear correlations between the temperatures of the same-type invariant equilibria and the melting temperatures of REMs allowed estimating the temperatures of both eutectic equilibria and congruent melting of intermetallics in the unstudied Pm–Fe and {Pm, Tb, Tm, Lu}–Co systems. Based on linear correlations of the standard enthalpies of formation for R₂Co₁₇, RCo₅, and RCo₃ intermetallics with REM atomic numbers, more reliable values were identified among the great number of scattered published data and missing values were estimated. The most typical features of the ternary phase diagrams were determined using a comparative analysis of the phase diagrams for a series of binary REM–Fe and REM–Со systems and limited published data on phase relations in the ternary systems. The common features of the REM–Fe–Co phase diagrams enabled prediction of the solidus projections for the unstudied systems. Phase regions of the solidus surfaces observed in all systems, except for La–Fe–Co, were established. The solidus temperatures of these regions were estimated for the unstudied systems and experimentally verified on systems with samarium and terbium. The experimental results showed good agreement with the predicted values. Based on the solidus projections, liquidus projections were proposed.