Effects of mechanical milling on microstructure and mechanical properties of CrMnFeCoNi high-entropy alloy fabricated via spark plasma sintering

Improving the strength of CrMnFeCoNi high-entropy alloys is one of the key strategies to expand their applications. In this study, mechanical milling was applied to alloy powder to improve the strength of CrMnFeCoNi alloy. The effects of the mechanical milling on the microstructure and mechanical properties of spark plasma-sintered alloys were systematically investigated using electron backscatter diffraction (EBSD), transmission electron microscopy (TEM), small-angle neutron scattering (SANS), and compression testing. The mechanical milling of gas-atomized powders transforms spherical powders into faceted powders, with the powder size initially decreasing and then gradually increasing. EBSD analysis revealed that mechanical milling formed a fine-grained structure in the sintered alloy by introducing high-density low-angle grain boundaries (LAGBs). The addition of stearic acid as a process-control agent in mechanical milling introduces C and O into the milled powder, promoting the formation of Cr₇C₃ and Cr₂₃C₆ carbides and (Cr,Mn)₃O₄ and Mn₃O₄ oxides during sintering. SANS analysis indicated that mechanical milling increased the volume fraction and reduced the size of the oxide particles. The compressive yield strength increased significantly from 307 MPa to 886 MPa by mechanical milling. Yield strength modeling revealed that this enhancement resulted from grain boundary strengthening (470 MPa) and oxide strengthening (311 MPa). After compression, the milled alloy exhibited an ultrafine grain structure with a relaxed compression texture.