Compressive creep behavior of selective laser melted CoCrFeMnNi high-entropy alloy strengthened by in-situ formation of nano-oxides

초록

The present study has investigated the high temperature creep behavior of equiatomic CoCrFeMnNi high-entropy alloy (HEA) manufactured via selective laser melting (SLM). Additionally, the effect of in situ oxide, formed during SLM, on the creep property was examined. The SLM-built HEA exhibited a heterogeneous microstructure as well as cellular and columnar structures decorated by dislocations. Nanosized oxides were newly formed at the substructure boundaries due to high oxygen content in the pre-alloyed powders. SLM-built HEA with in-situ formed oxide revealed exceptional creep resistance at 600 degrees C, as compared to other HEAs, and stress exponents of n = 3.45 and n = 6.45 for low stress region (LSR) and high stress region (HSR), respectively. This indicated that the creep mechanisms of SLM-built HEA were viscous glide at the LSR and dislocation climb at the HSR. The activation energy for creep was determined to be 335 kJ/mol, which was similar to the activation energy of self-diffusion of the constituent elements. The prediction of the Cottrell-Jaswon dragging model on the steady-state creep rate for the SLM-built HEA was that the most sluggish species of the Ni element encouraged the solute drag phenomenon in the LSR. After the creep tests, the SLM-built HEA exhibited an accumulation of geometrically necessary dislocations at the substructure boundaries, unlike in other metallic materials, where the dislocations are concentrated at the grain boundaries. The nanosized oxides that formed during SLM suppressed the dislocation movement and acted as a site of dislocation accumulation. These results could suggest a way for improving the high-temperature creep resistance of HEAs using unique microstructures (i.e., substructures decorated by dislocations and in-situ-formed oxide via high oxygen content in pre-alloy powders) developed by additive manufacturing.

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