Solid state processing of the cantor derived alloy CoCrFeMnNi by oxide reduction

Abstract

In recent years, high entropy alloys (HEAs), also known has Multi-Principal Element Alloys (MPEAs) have emerged as a new and exciting class of materials. One of the first (and most widely studied) HEA compositions is that of the so-called Cantor alloy, the equimolar CoCrFeNiMn composition. This paper reports on the synthesis of an HEA containing the same metallic elements via reduction of a mixture of the corresponding oxides. High purity precursor powders of Co(OH)2, Cr2O3, Fe2O3, MnO2, and NiO were milled and mixed using standard ceramic processing methods. The green pellets were subjected to a series of isothermal reduction anneals in flowing 3% H2–Ar (which is below the flammability limit), at temperatures ranging from 1000 to 1185 °C, with annealing times from 24 to 120 h. The microstructure and phase evolution of the samples were characterized using SEM, EDS analysis, and XRD. Reduction heat-treatment at 1185 °C resulted in samples developing a core-shell structure, with a dense outer metallic layer, and a core consisting of a mixture of metallic and ceramic phases. Using EPMA (electron microprobe analysis), the composition of the HEA shell was determined to be Co0·25Cr0·19Fe0·25Ni0·23Mn0.08. The hardness of the HEA was measured using Vickers micro-indentation. Examination of the individual indentation sites showed a strong positive correlation between the hardness value and the volume fraction of second phase particles (MnCr2O4, Cr2O3) within the indented area. A comparison between two sub-groups of indentation sites showed an 43% increase in average hardness (211.1 HV versus 148 HV) that was associated with a higher volume fraction of second phase (10.9% versus 2.9%). Consistent with thermodynamic considerations, experiments revealed that when a sample consisting solely of MnO2 was subjected to the same conditions, reduction to metallic Mn did not take place. A model is put forward whereby the free energy change related to the formation of the HEA solid solution provides an additional driving force for the reaction.