High-strength ultrafine-grained CoCrFeNiNb high-entropy alloy prepared by mechanical alloying: Properties and strengthening mechanism

An equiatomic CoCrFeNiNb alloy was prepared by conventional induction melting and by 8 h of mechanical alloying and compaction via spark plasma sintering. The alloy prepared via mechanical alloying showed a uniform ultrafine-grained microstructure composed of an FCC solid solution strengthened by HCP Laves phases. A detailed TEM inspection revealed the presence of nanocrystalline Cr2O3 particles at the triple junctions of the present grains as well as stacking faults and nanotwins found exclusively in the interior of the FCC solid solution grains. The as-cast alloy had a high initial hardness of 648 ± 18 HV 30 and ultimate compressive strength of 1374 MPa. On the other hand, the mechanically alloyed alloy compacted at 1000 °C showed even higher hardness of 798 ± 9 HV 30 as well as an ultra-high strengths that reached 2412 MPa. Based on the TEM quantitative analyses considering the contributions of different structural constituents and lattice defects, the aforementioned strength value was found to be in good agreement with results from microstructure strengthening modelling, which indicated a calculated mean value of 2300 ± 300 MPa. Moreover, the mechanically alloyed alloy also showed exceptional thermal stability even after long-term annealing/testing at 600 °C because it maintained a hardness of 777 ± 5 HV 30, and strength of 2284 MPa.