Out-of-plane ordering in quaternary MAX alloys: an alloy theoretic perspective

This letter is motivated by an apparent paradox, in that some quaternary systems (particularly in the Ti–Zr–Al–C system) have been shown to exhibit M-site out-of-plane ordering, while prior work and calculations by the present authors suggest endothermic interactions between Zr and Ti. In this letter we provide a resolution to this issue and provide a more extended analysis on the out-of-plane and in-plane ordering in the M sites of quaternary MAX alloys. The results provide further insights to develop criteria to predict potential out-of-plane ordering tendencies in other MAX systems.fx1

IMPACT STATEMENT In this work, we resolve a recently evident contradiction between theoretical predictions for phase separation in the Ti–Zr–Al–C 312 MAX system and experimental observations that indicate out-of-plane ordering.