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

The newly developed L1 2 -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 L1 2 phase by adjusting L1 2 -phase forming elements such as Al, Ti, Ta and Nb. However, the effect of transition-metal elements on L1 2 precipitation is still unclear. This work investigated the effect of Fe on the stability and distribution of the L1 2 phase by reducing the level of Fe in a well-studied high-entropy alloy Al 0.3 CrFeCoNi. A close examination of the precipitation behaviour in the Al 0.3 CrFeCoNi and Al 0.3 CrFe 0.6 CoNi 1.4 alloys was conducted using transmission electron microscopes. When the aging temperature reaches 700 °C in the Al 0.3 CrFeCoNi alloy, nanosized L1 2 particles would transition to the B2 phase. By reducing the amount of Fe, the stability and density of the L1 2 phase were both enhanced. After 700 °C annealing, L1 2 nanoparticles are still stable in the Al 0.3 CrFe 0.6 CoNi 1.4 alloy and there is no evidence of B2 phase precipitation. Less Fe can lower the concentration of Al in both the fcc and L1 2 phases during aging at a lower temperature of 620 °C when the L1 2 phase in both alloys is stable. A lower Fe concentration can also lead to more Cr and Fe atoms occupying Al sites in the L1 2 phase, which can result in the formation of more L1 2 particles. Since more L1 2 particles were precipitated when the amount of Fe was reduced, the Al 0.3 CrFe 0.6 CoNi 1.4 alloy exhibits better strength and higher hardness than the Al 0.3 CrFeCoNi alloy. These findings give us insight into how transition metals affect the behaviour of L1 2 precipitation and suggest ways to develop and improve L1 2 -strengthened HEAs. • The stability and density of the L1 2 phase were both improved by reducing Fe content. • Less Fe can lower the concentration of Al in both the fcc and L1 2 phases. • Less Fe can cause more Cr and Fe atoms to occupy Al sites in the L1 2 phase. • The Al 0.3 CrFe 0.6 CoNi 1.4 alloy exhibits better strength and hardness.