Formation of hypoeutectic structure and strengthening mechanism of Co4Cr1.5Fe1.5Ni5 high entropy alloy alloyed by Al

Abstract

To reveal the phase formation mechanism and the strengthening mechanism, the AlxCo4Cr1.5Fe1.5Ni5 alloys (x = 1.0, 1.2, 1.4, 1.6, 1.8, 2.0) were prepared by arc melting. Phase formation and prediction, tensile properties, strengthening mechanism and deformation mechanism were researched in detail. Results show that the microstructure of AlxCo4Cr1.5Fe1.5Ni5 alloys changes from single FCC phases (x ≤ 1.4)) to FCC and BCC phases (x＞1.4), which was in good agreement with the calculated phase diagram (CALPHAD) prediction. The lower mixing enthalpy of Ni-Al leads to the segregation of Al element at the inter-dendritic region, which promotes the formation of BCC and FCC lamellar structure in hypoeutectic Al2Co4Cr1.5Fe1.5Ni5 alloy. The ultimate tensile strength of Al2Co4Cr1.5Fe1.5Ni5 alloy is 835 MPa, meanwhile the fracture strain is 31.8 %. The tensile strength of Al2Co4Cr1.5Fe1.5Ni5 alloy is attributed to the interface strengthening and the solid solution strengthening. The massive cross slip of dislocations in primary FCC phases and the dense dislocations blocked by the FCC-BCC interface increase the tensile strength. The interface barrier strength of the BCC-FCC interface in Al2Co4Cr1.5Fe1.5Ni5 alloy was estimated to be 3.9 GPa, which shows the higher barrier strength than conventional BCC-FCC interfaces. Coupled effects of the primary FCC and the lamellar structure of FCC and BCC phases will maintain good mechanical properties.