Effect of thermal treatments on the microstructure and local mechanical behaviour of a laser beam welded CoCuFeMnNi high-entropy alloy

An equiatomic CoCuFeMnNi high entropy alloy was subjected to laser beam welding and the effect of two post-weld thermal treatments (i.e. solution treatment and direct ageing) was studied. Multiple characterisation techniques were implemented to correlate the local mechanical behaviour (evaluated by direct image correlation) to the microstructural evolution, induced by welding and thermal treatments across the melted zone, the heat affected zone, and the base material. This in turn provided an understanding of the macroscopic mechanical behaviour of welded CoCuFeMnNi plates. Ageing treatment induces strengthening through a nanometric, maze-like structure resulting from spinodal decomposition, without affecting the grain structure. The dendritic microstructure is dissolved by solution treatment, which also activates the recrystallization of the melted zone, thus reducing the residual stresses caused by solidification. The local strengthening produced by spinodal decomposition, and the improved strain hardening capability resulting from grain refinement allow the melted zone to stand out in directly aged and solution-treated conditions, respectively.