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Columnar to equiaxed transition and grain refinement of cast CrCoNi medium-entropy alloy by microalloying with titanium and carbon
Thermomechanical processing has been used to control the grain size/shape of the equiatomic CrCoNi medium-entropy alloy (MEA) and obtain excellent strength and ductility. However, in the cast state, the alloy has coarse columnar grains with average widths and lengths of approximately 120 and 1000 μm, respectively, resulting in inferior mechanical properties. To overcome this deficiency, here we microalloyed with Ti and C and successfully changed the grain shape (from columnar to equiaxed) and refined the grain size. The degree to which the microstructure changes depends on the amount of Ti and C added, with the best results obtained at 0.4 at.% each. In the optimal alloy [(CrCoNi)99.2Ti0.4C0.4], the as-cast grains were nearly equiaxed with a uniform size of ∼75 μm. Associated with this change in grain shape/size was a significant improvement of yield strength, ultimate tensile strength and elongation to fracture at both 293 and 77 K. The columnar to equiaxed transition is attributed to the strong mutual affinity of C and Ti, which leads to their build-up ahead of the solid-liquid interface and, in turn, to enhanced constitutional undercooling.