In-situ formed oxide enables extraordinary high-cycle fatigue resistance in additively manufactured CoCrFeMnNi high-entropy alloy

Abstract

The high-cycle fatigue (HCF) properties and deformation behavior of additively manufactured equiatomic CoCrFeMnNi high-entropy alloys (HEAs), strengthened by in-situ formed oxides, were investigated. A CoCrFeMnNi HEA manufactured by selective laser melting (SLM) had a heterogeneous grain structure and dislocation network-induced sub-structures. Furthermore, nanosized oxides dispersed at the sub-structure and grain boundaries of SLM-built HEA. The results of tensile tests indicated that this SLM-built HEA had superior yield strength of 774.8 MPa with an elongation of 30.8%. The S-N curves revealed that the resistance of the SLM-built HEA to HCF was extraordinarily high compared with that of a homogenized (conventional casting + hot rolling + heat treatment) HEA. The corresponding fatigue limits of the SLM-built and homogenized HEAs were 570 MPa and 280 MPa, respectively. The excellent fatigue resistance of the SLM-built HEA is attributed to its unique microstructural characteristics (i.e., heterogeneous grain structures, dislocation networks, and in-situ formed oxides), and the deformation twins generated during cyclic load. The un-melted powder generated during the additive manufacturing (AM) process also contributed to the HCF resistance of the SLM-built HEA. Based on these findings, the correlations among the unique microstructure, internal defects, HCF properties, and fatigue fracture mechanism of the SLM-built HEA are also discussed.