Microstructure evolution and mechanical properties of friction stir welded Fe50Mn30Co10Cr10 high-entropy alloy

Abstract

Metastable Fe50Mn30Co10Cr10 HEA has a significant strength-ductility synergistic effect and a wide application prospect in nuclear power. The present work reports the microstructure evolution and tensile behavior of friction stir-welded joints for Fe50Mn30Co10Cr10 high entropy alloy (HEA) at various rotational speeds. Defect-free welds of Fe50Mn30Co10Cr10 alloy were obtained by a W-Re alloy welding tool. Significant grain refinement occurs in the nugget zone, where the most of HCP phases in the base metal transformed into FCC phase. Additionally, strain-induced Σ3 mechanical twins are generated in the FCC austenite grains. The microstructure of thermo-mechanically affected zone exhibited multiple deformation mechanisms, including stacking faults, martensite lath, martensite variants, and reverse transformation. The microhardness of the joint primarily depends on the grain size of the FCC phase and the fraction of twins and HCP phase. In the nugget zone of the joint welded at 300 rpm, a high microhardness zone ranging from 266.7 to 310 HV was detected. While the reverse transformation and fresh twinning in the joint at 600 rpm contribute to the enhancement of tensile properties, the increase in grain size and ε martensitic fraction have resulted in a noteworthy reduction in ultimate tensile strength (UTS) and elongation (EL). Conversely, the joint of 300 rpm exhibited optimum UTS and EL owing to the grain refining after FSW and the transformation induced plasticity (TRIP) effect. These insights confirm that FSW is an ideal approach for achieving superior joint performance in dual-phase HEAs.