Early stage phase separation of AlCoCr0.75Cu0.5FeNi high-entropy powder at the nanoscale

Nicolas J Peter,Maria J Duarte,Christian H Liebscher,Vikas C Srivastava,Volker Uhlenwinkel,Eric A Jägle,Gerhard Dehm

doi:10.1016/j.jallcom.2019.153149

Abstract

High entropy alloys are generally considered to be single phase material. This state is, however, typically a non-equilibrium state after fabrication at high cooling rates. Phase constitution after fabrication or heat treatment is mostly known for isothermal annealing only and for casts as well as rapidly quenched alloys. Knowledge on early phase separation stages of high entropy alloys and their mechanisms are missing so far. Here, we present results on phase separation at intermediate cooling rates, by characterization of gas atomized powder of the AlCoCr0.75Cu0.5FeNi alloy. Although investigation by X-ray diffraction and Electron Backscatter Diffraction indicates a single-phase nature of the powder particles, aberration-corrected scanning transmission electron microscopy and atom probe tomography reveal a nanoscale phase separation into Ni–Al-rich B2 and Fe–Cr-rich A2 regions as well as a high number density of 3.1 × 1024 Cu-rich clusters per m3 in the B2 matrix. The observed phase separation and cluster formation are linked to spinodal decomposition and nucleation processes, respectively. The study highlights that adequate characterization techniques need to be chosen when making statements about phase stability and structural evolution in compositionally complex alloys.

Highlights

Near-equiatomic, multi-principal component alloys – commonly termed compositionally complex alloys (CCAs) or high-entropy alloys (HEAs) – were conceptually introduced about a decade ago [1,2,3]
Wet chemical analysis of the overspray powder of the AlCoCr0.75Cu0.5FeNi alloy confirmed a close proximity to the nominal composition
The diffractogram intensity was plotted against the scattering vector for a better visualization of the agreement between both measurements, as the reference pattern was collected with a different type of X-ray diffraction source and step size, i.e. Cu-Kα type radiation and 0.5°, respectively

Summary

Introduction

Near-equiatomic, multi-principal component alloys – commonly termed compositionally complex alloys (CCAs) or high-entropy alloys (HEAs) – were conceptually introduced about a decade ago [1,2,3]. Combinations of 3d transition metals seem to provide a suitable playground for exploration of such single-phase HEAs (among other HEA classes such as refractory metal-based alloys) This opened up new avenues, breaking the traditional concepts, and a change in paradigm of alloy design. A multitude of alloy compositions, satisfying the Hume-Rothery rules and predicted to show the high-entropy effect, have shown phase constitutions ranging from (seemingly) single-phase solid solutions [5] to up to four or five different coexisting phases [6], termed CCAs. The phase constitution of an alloy sometimes changes by a slight concentration variation of one of the alloying elements, e.g. by the Al concentration in the AlxCoCrCuFeNi system [7]. This renders the establishment of comprehensive microstructureproperty relationships a challenging task and requires first of all a thorough microstructural characterization, at small scales

Results

Discussion

Conclusion