Effect of transition-metals (Fe, Co and Ni) on the L12-phase strengthening in the Al0.3CrFeCoNi high-entropy alloy

Abstract

L12-strengthened high-entropy alloys have become research of interest for their great potential to achieve a superb balance between ductility and strength. While much attention has been paid to strong L12-phase forming elements, little discussion has focused on the role of transition-metals. The concentration of Fe, Co, and Cr in the Al0.3CrFeCoNi alloy was decreased to investigate how transition-metals affect the stability and distribution of the L12 precipitate. The transmission electron microscope was used to characterize the precipitation behavior in the four alloys. The results show that reducing the level of transition-metals promotes the stability of the L12 phase. In the case of the base Al0.3CrFeCoNi alloy, B2 phase replaces nanosized L12 particles as the main precipitates when aging at 700 °C. In contrast, uniformly dispersed L12 nanoparticles remain stable after 700 °C aging, with almost no B2 phase observed. Adjusting the content of these transition-metals can also alter the solid solubility of Al in the fcc matrix and change the site occupancy of Al sites in the L12 phase, enabling a shift in the volume percentage of the L12 phase. Of the three elements tested, Fe reduction effectively increases the fraction of the L12 phase, while Cr reduction even have a little negative influence on the density of the L12 particles. As a result, the microhardness and strength decrease in the following sequence: Al0.3CrFe0·6CoNi1.4, Al0.3CrFeCo0·6Ni1.4, Al0.3CrFeCoNi and Al0.3Cr0·6FeCoNi1.4. This study provides significant insights into the role of transition-metals and facilitates the design of L12-strengthened high-entropy alloys with excellent mechanical properties.