Thermal stability and coarsening of coherent particles in a precipitation-hardened (NiCoFeCr)(94)Ti2Al4 high-entropy alloy

In this work, we studied the coarsening of L1(2) precipitates in a face-centered-cubic (fcc) (NiC-oFeCr)(94)Ti2Al4 high-entropy alloy (HEA) at temperatures between 750 and 825 degrees C. Temporal evolutions of the morphology, size, and volume fraction of these coherent precipitates were examined and evaluated using electron microscopes. These precipitates always appeared to be spheroidal and uniformly distributed in the alloy matrix. Treating the fcc-(NiCoFeCr)(94)Ti2Al4 alloy as a pseudo ternary Ni-Ti-Al alloy, we analyzed the coarsening kinetics of the precipitates and were able to obtain the activation energy for coarsening. The measured activation energy was compared with the diffusion activation energies of various elements in Ni-based alloys to shed lights on the rate limiting process for the coarsening of L1(2) precipitates. One of the major findings was that, owing to the sluggish diffusion in HEAs, coarsening of L1(2) precipitates in the current HEA was much slower than that in the conventional Ni-based alloys. This result demonstrated the remarkable thermal stability of the L1(2) precipitates. In this study, we also evaluated and modeled the strengthening effect in the (NiCoFeCr)(94)Ti2Al4 alloy, and correlated it with the coarsening behavior of the precipitates. (C) 2018 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.