Effects of Fe on L12 phase precipitation and mechanical properties in Al0.3CrFeCoNi high-entropy alloy

Abstract

The newly developed L12-strengthened high-entropy alloy shows great potential to possess superior combinations of physical and mechanical properties. It has been extensively studied how to control the L12 phase by adjusting L12-phase forming elements such as Al, Ti, Ta and Nb. However, the effect of transition-metal elements on L12 precipitation is still unclear. This work investigated the effect of Fe on the stability and distribution of the L12 phase by reducing the level of Fe in a well-studied high-entropy alloy Al0.3CrFeCoNi. A close examination of the precipitation behaviour in the Al0.3CrFeCoNi and Al0.3CrFe0.6CoNi1.4 alloys was conducted using transmission electron microscopes. When the aging temperature reaches 700 °C in the Al0.3CrFeCoNi alloy, nanosized L12 particles would transition to the B2 phase. By reducing the amount of Fe, the stability and density of the L12 phase were both enhanced. After 700 °C annealing, L12 nanoparticles are still stable in the Al0.3CrFe0.6CoNi1.4 alloy and there is no evidence of B2 phase precipitation. Less Fe can lower the concentration of Al in both the fcc and L12 phases during aging at a lower temperature of 620 °C when the L12 phase in both alloys is stable. A lower Fe concentration can also lead to more Cr and Fe atoms occupying Al sites in the L12 phase, which can result in the formation of more L12 particles. Since more L12 particles were precipitated when the amount of Fe was reduced, the Al0.3CrFe0.6CoNi1.4 alloy exhibits better strength and higher hardness than the Al0.3CrFeCoNi alloy. These findings give us insight into how transition metals affect the behaviour of L12 precipitation and suggest ways to develop and improve L12-strengthened HEAs.