Selective Electron Beam Additive Manufacturing of a Nanoparticle-Strengthened Medium-Entropy Alloy for Cryogenic Applications

A strong, ductile medium-entropy alloy, MEA, (CoCrNi)94Al3Ti3 with few defects was fabricated using the selective electron beam additive manufacturing technique. Due to preheating the substrate at 1173 K, the as-printed MEA showed coherent L12 nanoparticles (dia.∼112 nm, volume fractioñ8.9 %) in 68 μm grains with a dual {100}<011> + {100}<001> texture. The as-built alloy exhibited a yield strength (YS) of 612 MPa, an ultimate tensile strength (UTS) of 1020 MPa, and an elongation to failure (ε) of 37 % at 298K. These values increased when testing was at 77 K, i.e. YS of 811 MPa, UTS of 1339 MPa, and an ε of 39 %. After tensile failure, the LAGBs and GND densities sharply increased at both temperatures to accommodate strain gradient along with <111> fiber texture. The lack of annealing twin boundaries led to a relatively low strain hardening rate (SHR) of 2500 MPa at 298 K and 3890 MPa at 77 K (at 5 % strain) as compared to values of thermo-mechanically processed (CoCrNi)94Al3Ti3. The SHR curves exhibited three distinct stages, where an abrupt upturn at an intermediate strain level can be attributed to the occurrence of DTs and SFs at 77 K, and the appearance of SFs at 298 K.