Thermal stability and grain boundary strengthening in ultrafine-grained CoCrFeNi high entropy alloy composite

Thermal stability of CoCrFeNi high entropy alloy in as-milled and sintered conditions was investigated using Xray diffraction, differential scanning calorimetry, transmission electron microscopy, and atom probe tomography. Composite microstructure consists of FCC and carbide with a fine dispersion of oxide was observed in the sintered condition. Unsolicited contamination of carbon and oxygen in the as-milled powder due to the milling medium had led to the formation of composite microstructure. An exceptional thermal stability was observed upon exposure of sintered compact to higher temperatures (0.56 T-m to 0.68 T-m) for the prolonged duration of 600 h. Sintered compact exposed to 700 degrees C (0.56 T-m) for 600 h showed negligible change in hardness and grain size. Analysis based on the modified Hall-Petch model for two phase alloy indicates the phase boundaries act as a strong obstacle while the major contribution to strengthening comes from grain boundaries. (C) 2017 Elsevier Ltd. All rights reserved.