Unprecedented combination of strength and ductility in laser welded NiCoCr medium entropy alloy joints

Abstract CrMnFeCoNi high entropy alloys holding several principle elements have displayed exceptional strength, ductility and damage tolerance, and these properties are even better at cryogenic temperature. Among CrMnFeCoNi and its derivatives, NiCoCr, called medium entropy alloy (MEA), has shown the best mechanical properties. Yet, research on the welding of NiCoCr is still in its infancy, which, to certain extent, renders the application of such advanced alloys in engineering structures. Here, for the first time, laser welding is employed to join NiCoCr MEA to achieve butt-type joints; microstructure and mechanical properties at 300 K and 77 K are then examined by experiments and molecular dynamic (MD) simulation. It is found that the ultimate tensile strength (UTS) and ductility of the weld at 300 K are 525 ± 36 MPa and 85 ± 15%, respectively. At 77 K, the UTS is enhanced to 828 ± 99 MPa but the ductility is reduced by 10%. The combination of UTS and ductility at both tested temperatures is the best among the joints of CrMnFeCoNi and its derivatives. The reason for this unprecedented combination is explored by EBSD and atomistic simulation. Slip bands and low-density deformation twins are found in the deformed microstructure of the weld fractured at 300 K as evidenced from EBSD observation. High-density twins besides the slip bands are seen in the weld deformed at 77 K. Real-time atomistic deformation is revealed from MD simulation, including dislocation nucleation and its motion, the formation of twin boundaries, and the interaction between the dislocation and twin boundaries. The unprecedented combination of strength and ductility in laser welded NiCoCr joints provides significant promise of NiCoCr MEA for application in engineering structures, especially at cryogenic circumstances.