Influence of Isochronal and Isothermal Annealing on Microstructure and Hardness of Nanocrystalline CoCrFeNi-B4C High Entropy Alloy Composites

Abstract

Adding nano-sized reinforcements to nanocrystalline alloys is an effective strategy for achieving stable microstructures with enhanced mechanical properties. This study investigates the effect of nano-sized B4C particles on the microstructural stability and mechanical properties of nanocrystalline CoCrFeNi high-entropy alloys (HEAs) synthesized by high-energy mechanical alloying. Comprehensive characterizations, including x-ray diffraction (XRD), focused ion beam (FIB) microscopy, and transmission electron microscopy (TEM) revealed that B4C addition improved microstructural stability resulting in an average grain size of 225 nm after annealing at 900 degrees C for 1 h. The in-situ formation of secondary phases, such as Cr7C3 and Cr2O3, during annealing process further contributed to the retardation of grain growth in CoCrFeNi HEA, improving the overall thermal stability. The mechanical properties, particularly hardness, were enhanced by the addition of B4C following mechanical alloying and annealing at 900 degrees C for up to 6 h. Beyond this duration, the hardness values plateaued, exhibiting minimal variations with B4C additions. Specifically, the as-milled hardness increased from 518 HV to 573 HV with incorporating 4 wt.% B4C. After annealing for longer than 6 h at 900 degrees C, the B4C containing HEA showed a hardness of 309 HV, compared to 278 HV for the HEA without B4C. Despite some reduction after prolonged annealing at elevated temperatures, this enhancement shows the alloy's potential to maintain mechanical performance under such conditions, while further optimization is needed to ensure durability over extended temperature exposures.