Using concentration gradients to examine the effects of Al, Ga and Sn additions on the low-activation VCrMnFe system

A critical design criterion for future fusion reactor components is low activation. The equiatomic multi-principal element alloy VCrMnFe is comprised solely of low activation elements and forms a single-phase solid solution at temperatures over 1000 degrees C. However, at lower temperatures it forms detrimental sigma phase. In this work, compositional gradients of Ga, Sn or Al were induced in VCrMnFe using only a furnace to investigate their effect on intermetallic formation. By examining how the microstructure changed across a region with varying composition, phase stability limits could be assessed. For example, all three elements were found to prevent sigma phase from forming within the alloy when they were present at relatively low concentrations (2-5 at%). Al was found to be the most promising addition (in terms of not causing embrittlement), and the approach used enabled the characterisation of the VCrMnFe-Al pseudo binary phase diagram up to 50 at% Al after heat treatment of 800 degrees C/240 h followed by ageing at 600 degrees C/240 h, with numerous ordered phases found using electron diffraction. The level of Al addition required to suppress the sigma phase has been identified more precisely, which will be useful for future alloy development work.