Elemental partitions and deformation mechanisms of L1₂-type multicomponent intermetallics

Multicomponent high-entropy alloys (HEAs) with compositionally disordered elemental arrangement have attracted extensive attention because of their excellent mechanical properties. However, these alloys usually exhibit low strength at elevated temperatures. Recent works suggest that HEAs strengthened by L1₂ ordered intermetallics of the Ni₃Al type possess a superior synergy of strength and ductility. The performance of such ordered-intermetallics-strengthened HEAs depends critically on the composition and deformation mechanism of the strengthening component, which will inevitably suffer partial disorder in HEAs. In this work, we study the elemental partitions and deformation mechanisms in typical multicomponent intermetallics (MCIs) based on density functional theory (DFT) calculations. Informed by experiments, systems considered in this work include L1₂-type (Ni,Co,Fe)₃(Al,Ti,Fe) and its six derived subsystems. We first determine the site preference of different elements in MCIs using Monte-Carlo simulations with the energies obtained from DFT calculations (DFT-MC). The results confirm that Ti prefers Al sites. In contrast, most Co and Fe atoms tend to occupy Ni sublattice. With the established elemental occupations, we build quasirandom structures with partial disorders in either or both sublattices of the L1₂ structures. DFT results show that incorporation of Ti in the Al sublattice dominates the increase of stable planar fault energies (γ _SPF) in the considered MCIs. In contrast, Co in the Ni sublattice decreases the energy cost for planar fault formation. We further uncover quantitative relations between ϒ _SPF and geometrical structures as well as electronic structures in MCIs. The analytical results show that the incorporation of Ti decreases the adaptability of (Ni,Co,Fe)₃(Al,Ti,Fe) both geometrically and electronically. Specifically, we propose that ϒ _SPF can be described by the ability of the alloy to accommodate planar faults, which is measured by the interlayer spacing changes between {111} planes [Δd(111)] and charge density redistribution (Δρ) at critical points (cps). We address the fundamental understandings of the deformation behavior of MCIs by pinpointing the contributions of different species to their local geometrical and electronic structures, which pave the way for rationally designing ordered-precipitate-strengthened HEAs.