An assessment of the high-entropy alloy system VCrMnFeAlx

A major consideration when choosing materials for nuclear reactors is the future radioactive waste produced by the irradiation of their component parts. Only a handful of structural metals can be considered ‘low activation’ in a fusion environment. In recent work, we showed that the low-activation multicomponent equiatomic alloy VCrMnFe comprises a single BCC (A2) phase at 1200°C. Here, we examine its stability on ageing at lower temperatures, and the effect of Al additions (to create VCrMnFeAlx alloys) to destabilise the sigma phase and form strengthening superlattice structures. It is found that substantial volume fractions of sigma phase form after ageing VrCrMnFe at 600°C and 800°C for 1000 h. The addition of Al was found to destabilise the sigma phase, as predicted using thermodynamic modelling, with it being eliminated at all temperatures with additions of 6.6 at% Al. Increasing Al additions also led to the formation of superlattice structures: B2 and L21 (Heusler). Higher Al content had a slight increasing effect on the alloys’ hardness, but also embrittled the alloys (at room temperature). Significant hardening was produced by nano-segregation induced in the higher Al x = 0.25, 0.5 and 1.0 alloys after aging at 600 °C. This alloy system presents an attractive opportunity to fine-tune the composition to obtain a balance of ductility and high-temperature strength and stability. Of particular interest was the formation a two-phase basket weave cube-on-cube orientated, coherent, microstructure in VCrMnFeAl1.0 after aging at 800 °C.