Effect of twinning on the orientation dependence of mechanical behaviour and fracture in single crystals of the equiatomic CoCrFeMnNi high-entropy alloy at 77K

Abstract

The plastic flow behaviour of single crystals of the CoCrFeMnNi high-entropy alloy is shown to be greatly influenced by the twinning multiplicity on the stress-strain (σ(ε)) curves, the strain hardening coefficient Θ = dσ/dε, plasticity and fracture. It is shown that a maximum effect of strain hardening is obtained during the onset of twinning in two systems in the [1¯11]- and [1¯44]-oriented crystals, while a transition to the development of twinning predominantly in one system simultaneously with slip, is accompanied by a decrease in strain hardening. At 77K in the [1¯11]-, [1¯23]- and [011]-oriented crystals, when twinning develops in one system with slip, the extension of the linear hardening stage and plasticity increase as a result of the suppression cross-slip and shifting the neck formation according to the Considère condition to higher stress levels. The development of deformation by twinning only in several systems and the suppression of the relaxation of local stresses during twin-twin intersection by slip or twinning, leads to the failure of quasi-cleavage in the [1¯44]-oriented crystals at 77K, after 50% strain. With the development of twinning predominantly in one system and a high density of dislocations, the [1¯11]-, [1¯23]- and [011]-oriented crystals show ductile failure, since the stresses for neck formation according to the Considère condition are reached before the stresses for brittle fracture.