Ab initio phase stabilities of rare-earth lean Nd-based hard magnets

Abstract

Due to the resource criticality of rare earth (RE) elements on the world market, combined with an increasing demand for high-performance hard magnets for applications like wind turbines or electric vehicles, new research activities are going on in recent years. The goal is to find new hard-magnetic compounds with comparable performance to Nd2Fe14B but with significantly fewer critical elements. We have reported theoretical investigations of finite temperature phase stabilities of (Nd,X)-Fe-Ti (X = Y and Ce) hard magnets. Inexpensive and less resource-critical elements Y and Ce have been considered as possible candidates to reduce the critical Nd amount. To explore the impact of such substitution on ThMn12-type Nd-based hard magnets, systematic ab initio thermodynamics modeling has been performed for possible binaries and ternaries. Special attention has been paid to the important 1:12, 2:17 and 3:29 (RE:Fe) phases. Ternary compounds have been determined by examining Ti solubilities in these three phases. Employing state-of-the-art approaches for vibrational, electronic, and magnetic entropy contributions, the Helmholtz free energy, F (T,V), has been calculated for all hard magnetic and relevant competing phases. Competition energy formalism has been developed for each ternary and quaternary system to understand the relative finite temperature phase stabilities. Our ab initio based free energy calculations have revealed that Ce is a strong candidate to compensate Nd with the composition of NdCeFe22Ti2, which have appeared as stable quaternary at all considered temperatures. Nevertheless, Y substituted quaternary NdYFe23Ti and NdYFe22Ti2 have been found to be less stable than main NdFe11Ti.