Low Carbon Fabrication of High-Entropy Alloy via High Energy Milling and Hydrogen Reduction—A Study of Reaction Kinetics and Mechanical Behavior

High-entropy alloys (HEAs) are increasingly being recognized as excellent materials in various sectors such as the marine and defense fields due to their remarkable chemical and mechanical properties in extreme environments. This study attempts to fabricate and analyze CoCrFeNi as a notable high entropy alloy candidate through milling and hydrogen reduction because of its extraordinary mechanical characteristics as well excellent corrosion and oxidation resistance. In this study, an equimolar CoCrFeNi high entropy alloy was fabricated by the powder metallurgical process of high energy milling at 2400 r.p.m and hydrogen reduction at 1000 °C. Reaction kinetics of the resulting HEA nanopowder was investigated as a preliminary examination of the feasibility of hydrogen reduction of chromium oxide in chromium-based HEAs; as a function of milling time. Thermogravimetric analysis (TGA) was used to investigate the hydrogen reduction process with the utilization of the Kissinger-Akahira-Sunose (KAS) method being applied to the TGA results to determine the apparent activation energy and thermal stability. Scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDS) mapping and X-Ray Diffraction (XRD) confirmed the nano sizing, homogeneity and phase transformation of the high entropy alloy from increasing milling time to after hydrogen reduction. The mechanical properties were observed via hardness and tensile tests. The nano sizing of the HEA powder was confirmed to be from 500 nm to 50 nm reduction of particle size as milling time increased from 5 h to 30 h through SEM images; homogeneity of the elements was observed in EDS mapping results. The milled and hydrogen reduced HEA powder exhibited FCC phase peaks at the lattice parameter of 3.59 Å confirming the formation of solid solution and the reduction of particle size as milling time increased. The TGA results demonstrated a weight loss in the HEA powder during hydrogen reduction from 100 % to 71.94 %; with the reduction of activation energy occurring at two reactions, firstly from 387 to 245 kJ/mol and 474 to 22 kJ/mol. The hardness was observed to be around 160 HV and the tensile strength was observed to be around 712 MPa. These results provide the primary data for comprehension of the microstructural development of the CoCrFeNi high entropy alloy with increasing milling energy and time as well as the hydrogen reduction behaviour of the alloy so as to fabricate the most optimal structure for application in extreme environments with minimal carbon emissions. Therefore, the microstructural development of spherical CoCrFeNi nanopowder agglomerates fabricated by the hydrogen reduction process, was demonstrated to produce high entropy alloys as a potential alternative process for the conventional gas atomization process.