Strengthening and deformation behavior of as-cast CoCrCu1.5MnNi high entropy alloy with micro-/nanoscale precipitation

Abstract

The microstructure of as-cast CoCrCu1.5MnNi high entropy alloy (HEA) exhibited the dual fcc phase structure with Co–Cr rich dendritic and Cu–Mn rich interdendritic regions. Long submicron-scale needle-shaped Co–Cr precipitates were observed in Cu–Mn rich interdendritic region whereas much smaller nanoscale Cu–Mn rich needle-shaped precipitates were found in Co–Cr rich dendrites. The compositionally inverse relationship between the precipitate and the matrix in dendritic and interdendritic regions was induced by the decreasing solubility of the elements in two phases due to the reduced contribution of entropy during cooling. Excellent as-cast yield strength/ductility combinations at both room (431.5 MPa/55%) and cryogenic (600.2 MPa/67%) temperatures are attributed to the precipitation strengthening and quasi-linear strain hardening in the as-cast CoCrCu1.5MnNi. The quasi-linear strain hardening in the as-cast CoCrCu1.5MnNi is associated with the accumulation of deformation-induced defects such as planar array of dislocations, extended stacking faults and deformation twins in the matrix with closely spaced precipitates with the inter-precipitate spacing smaller than the critical cross-slip annihilation distances. The predicted yield strengths of CoCrCu1.5MnNi with nanoscale and submicron-scale precipitates strengthening in dual fcc phase structure at RT and 77 K were in good agreement with the experimental strengths.