Ordering Effects on Deformation Substructures and Strain Hardening Behavior of a CrCoNi Based Medium Entropy Alloy

Abstract

A CrCoNi based medium entropy alloy with a small addition of Ti, Al and Nb (denoted as (CrCoNi)93Al4Ti2Nb) in the as-quenched condition, exhibits tensile properties comparable to those of the equiatomic CrCoNi alloy at room temperature. Atom probe tomography (APT) together with electron diffraction show that nanosized L12-type ordered γʹ domains exist in this alloy, which may be formed through spinodal decomposition during the quenching process. The evolution of deformation substructure with plastic deformation in this alloy was characterized using electron backscattered diffraction (EBSD) and scanning transmission electron microscopy (STEM) based techniques including the recently developed weak beam dark field STEM imaging. Plastic deformation occurs by the slip of a/2 dislocations, which are narrowly dissociated into Shockley partial dislocations on {111} slip planes. Their dissociation distances in the (CrCoNi)93Al4Ti2Nb alloy are much smaller than the widths of the corresponding partials in the equiatomic CrCoNi alloy due to one or more of the minor alloying elements (Al, Ti, Nb). Dislocation slip in this alloy has a pronounced planar character. The leading dislocations in slip bands glide as pairs due to the existence of ordered γʹ domains. Multipoles were formed through the slip of dislocations with opposite signs on adjacent {111} slip planes. Those multipoles serve as important building blocks for the formation of subgrain structures consisting of fine slip bands. The distances between slip bands were continuously refined during plastic deformation and dynamic refinement of slip bands plays a crucial role in strain hardening. The effects of ordered γʹ domains on planar dislocation slip, the deactivation of deformation twinning and strain hardening of this alloy are discussed.