Multiple potential phase-separation paths in multi-principal element alloys

It is now well established that multi-principal element alloys (MPEAs) offer ample opportunities for exploring new compositions beyond those accessed previously by conventional alloys. However, there is one more realm of possibility presented by MPEAs that has not been touch upon thus far. Here we show that, different from conventional alloys based on a single host element, a given starting MPEA solid solution on its way towards equilibrium can take a rich variety of potential decomposition pathways via multi-stage phase separation, offering a wide range of composition destinations. If/when some of them are reached, assuming kinetically allowed, the multiple phase separation reactions one after another would lead to domains that are compositionally complex and spatially localized. This hypothetical scenario is demonstrated in this paper using a model that mimics Cr-CoNi MPEA, showing a preponderance of multiplicity even when assuming only fcc-based phases can form. The complex chemical heterogeneities created as such are expected to be an additional knob to turn for tuning spatially variable composition and chemical order and therefore mechanical properties. Our results thus advocate multiple phase separation possibilities with many potential paths and terminal chemical heterogeneities as yet another important characteristic that distinguishes MPEAs from conventional alloys.