Microstructure and Properties of AlCoCrFeNiBx(x=0, 0.1, 0.25, 0.5, 0.75, 1.0) High Entropy Alloys

The influences of B element on the microstructure and properties of AlCoCrFeNiBx (x=0, 0.1, 0.25, 0.5, 0.75, 1.0, mol ratio) high entropy alloys were investigated. The AlCoCrFeNi high entropy alloy exhibits equiaxed grain morphology, and then turns to dendritic structures when B content x=0.1. The spinodal decomposition microstructure can be clearly observed in equiaxed grains. When x>0.1, both of the dendrite and the spinodal decomposition microstructure gradually disappear, but much borides form instead. The transformation is attributed to the high negative mixing enthalpy of Cr-B and Co-B. The microstructures of AlCoCrFeNiBx high entropy alloys change from B2+bcc structures to B2+bcc+fcc structures, and finally formed B2+bcc+fcc and borides mixing structures with the increased B elements. The hardness HV declines from 4860 to 4607 MPa, then rises to 6157 MPa with the addition of B element. The lowest hardness value is obtained when x=0.1. The compressive fracture strength shows a distinct decrease with B addition. The maximum compression strength is 2227 MPa when x=0.25. But when x reaches 0.75, the samples fracture during the elastic deformation due to the formation of hard and brittle borides. The coercive forces and the specific saturation magnetizations of the alloys decrease as the contents of B element increase. The decreasing coercive forces show a better soft magnetic behavior.