Microstructure and mechanical properties of ultrafine grained CoCrFeNi and CoCrFeNiAl0.3 high entropy alloys reinforced with Cr2O3/Al2O3 nanoparticles

Abstract CoCrFeNi and CoCrFeNiAl0.3 high entropy alloys reinforced with in-situ oxide nanoparticles have been successfully fabricated by mechanical milling in combination with hot pressing and hot extrusion. The as-fabricated CoCrFeNi samples exhibited an ultrafine grained microstructure with in-situ Cr2O3 oxide nanoparticles formed during hot pressing and extrusion process and distributed both on grain boundaries and in grain interiors. Increasing extrusion temperature from 1000 to 1100 °C caused a significantly increase of grain size from 700 nm to 2.5 μm owing to recrystallization and growth of the recrystallized grains, as well as the coarsening of Cr2O3 nanoparticles with their mean size increasing from 57 to 140 nm. When adding Al elements, finer Al2O3 nanoparticles (mean size: 30 nm) formed in the CoCrFeNiAl0.3 high entropy alloy, leading to refinement of the grains (mean size: 504 nm) due to their pinning effect on grain boundary immigration. After T6 heat treatment, the CoCrFeNiAl0.3 high entropy alloy demonstrated improved mechanical properties with yield strength of 1335 MPa, ultimate tensile strength of 1346 MPa and an elongation to fracture of 7.9%. With quantitative analysis based on the current models, it was concluded that grain boundary strengthening and Orowan strengthening acted as the dominant strengthening mechanisms.