Microstructure and texture characterisation of friction stir welded CoCrNi and CoCrFeMnNi multi-principle element alloys

This work investigates the microstructure formed in friction stir welds of FCC alloys, focused on two multi-principal alloys: a CoCrFeMnNi high-entropy alloy (HEA) and a CoCrNi medium-entropy alloy (MEA). A commercial stainless steel AISI 304 is used for comparison. The largest nugget was formed in the MEA, while the smallest was formed in the HEA. Grain refinement occurs in the stirred zone in all welds. Discontinuous dynamic recrystallisation is the predominant restoration mechanism during friction stir welding of the three investigated alloys. A sharp decrement in the Σ3 boundary fraction occurs in the stirred zone of the AISI 304 and HEA welds, while comparable values with the base metal are found for the MEA weld. The peak in the maximum index of crystallographic texture is observed on the advancing side of the stirred zone of the AISI 304 weld. A strong < 001 > θ-fibre texture is formed in the advancing side of the nugget in the AISI 304 from a well-established {123}< 634 > S-type texture in the base metal. Multiple crystallographic texture components without specific fibres are identified in most regions of the welds, indicating the complex shear path history during friction stir welding.