Design of self-stable nanocrystalline high-entropy alloy

Abstract

Nanocrystalline (NC) materials are prone to grain-coarsening at low temperatures—requiring extra solute-element addition for stability. While this approach is established mainly in simple-binary-alloys, it is adjudged “complex” for multicomponent-alloys due to complex-interactions among constituent-elements. We report for the first time that nanograins in multicomponent-high entropy alloy (HEA) stabilize themselves without requiring additional solute if constituent-HEA-elements with highest mixing enthalpy and melting point preferentially segregate to grain boundaries (GBs); a process we term self-stabilization effect in HEAs. Using in-situ X-ray diffraction, scanning/transmission electron microscopy, and atom-probe-tomography (APT), we show that Cr and Fe in NC-AlCoCrFe-HEA (9 nm grain-size) segregate at GBs by site-competition to stabilize it at 0.5Tm (Tm–melting temperature). At 0.6Tm, GB-desegregation is established to be precursor to phase decomposition, and it competes with nanograin stability; this culminates in the onset of grain coarsening at this temperature. Compared with the literature (e.g., NC-AlCoCrFeNi), NC-AlCoCrFe HEA shows exceptional nanograin-stability at high homologous-temperatures; this suggests possible breakdown of the cocktail and sluggish-effects in HEAs, since more elements do not necessarily improve nanograin stability. To the authors’ knowledge, this is the finest stable-NC-HEA produced—it paves a new way of engineering NC-HEAs without coarsening in scalable solid-state processes that require substantially-high temperatures.