Electron beam welding of L12-nanoparticle-strengthened strong and ductile medium-entropy alloys for cryogenic applications

The weldability, microstructures, and mechanical properties of two L1(2)-nanoparticle-strengthened medium-entropy alloys (MEAs) Ni43.4Co25.3Cr25.3Al3Ti3 and Ni42.4Co24.3Cr24.3Al3Ti3V3 (at.%) are explored after electron beam welding (EBW). Strong yet ductile defect-free joints were produced with coarse columnar grains (118-245 mu m) in the fusion zones, which were larger than the equiaxed grains in the heat-affected zones (15.6-22.3 mu m) and in the base materials (4.6-5.6 mu m). Both EBWed MEAs showed high yield strengths (838-858 MPa), high ultimate tensile strengths (1416-1420 MPa), and good fracture strains of 20-21 % at 77 K, which are 66 similar to 83 %, 84-89 %, and 57-81 %, respectively, of those of the respective thermo-mechanically treated (TMT) MEAs. The V-doping improved the cryogenic mechanical properties of the TMT MEA while not influencing those of the EBWed MEA. High back-stress hardening contributes to over 50 % of the cryogenic strength. Both EBWed MEAs exhibited abundant dislocation networks, stacking faults, and nanoscale deformation twins after fracture, producing a high strain hardening rate and good ductility.