Grain refinement of a NiCoFe medium entropy alloy: composition design from solute interaction perspective

The medium entropy alloys (MEAs) with mixing entropy between 1 R and 1.5 R e.g., NiCoFe alloy, exhibit unique properties, such as excellent specific strength and fracture toughness at 77 K. However, their coarse columnar microstructures lead to poor mechanical properties. Here, the solute effects on as-solidified structures were studied with the aim of grain refinement. First, sole addition of C leads to columnar to equiaxed transition (CET) and achieves an average grain size of 210 mu m for (NiCoFe)97C3 MEA. Further, based on the consideration of solute interaction, Cr was added into the C-doping MEA. The interaction of C and Cr ahead of the solid-liquid interface during solidification results in further grain size reduction to 164 mu m for the (NiCoFeCr0.25)97C3 MEA. The CET and grain refinement are attributed to the additional constitutional supercooling ahead of solidification interface. The grain refinement leads to significant improvement of yield strength, ultimate tensile strength, work hardening rate and slight sacrifice of elongation.(c) 2023 Published by Elsevier B.V.

Automated summary

Medium entropy alloys (MEAs) with a mixing entropy between 1R and 1.5R, such as NiCoFe alloy, show excellent specific strength and fracture toughness at 77 K. However, their coarse columnar microstructures result in poor mechanical properties. By adding C alone, the columnar to equiaxed transition (CET) can be achieved, with an average grain size of 210 μm for (NiCoFe)97C3 MEA. Further addition of Cr into the C-doping MEA leads to the interaction of C and Cr during solidification, resulting in a further reduction in grain size to 164 μm for the (NiCoFeCr0.25)97C3 MEA.