Microstructure and mechanical properties of ultrafine grained CoCrFeNi and CoCrFeNiAl_(0.3) high entropy alloys reinforced with Cr_2O_3/Al_2O_3 nanoparticles

摘要

CoCrFeNi and CoCrFeNiAl_(0.3) 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 Cr_2O_3 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 Cr_2O_3 nanoparticles with their mean size increasing from 57 to 140 nm. When adding Al elements, finer Al_2O_3 nanoparticles (mean size: 30 nm) formed in the CoCrFeNiAl_(0.3) 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 CoCrFeNiAl_(0.3) 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.