Abstract
The CoCrFeMnNi alloy is one of the most notable first-generation high-entropy alloys and is also known as a Cantor alloy. This alloy was first proposed in 2004 and shows promising performance at cryogenic temperatures (CTs). Subsequent research has indicated that the equiatomic ternary CoCrNi medium-entropy alloy (MEA), as a subset of the Cantor alloy family, has better mechanical properties than the CoCrFeMnNi alloy. Interestingly, both the strength and ductility of the CoCrNi MEA are higher at CTs than at room temperature. CoCrNi-based alloys have attracted considerable attention in the metallic materials community and it is therefore important to generalize and summarize the latest progress in CoCrNi-based MEA research. The present review initially briefly introduces the discovery of the CoCrNi MEA. Subsequently, its tensile response and deformation mechanisms are summarized. In particular, the effects of parameters, such as critical resolved shear stress, stacking fault energy and short-range ordering, on the deformation behavior are discussed in detail. The methods for strengthening the CoCrNi MEA are then reviewed and divided into two categories, namely, modifying microstructures and adjusting chemical compositions. In addition, the mechanical performance of CoCrNi-based MEAs, including their dynamic shear properties, creep behavior and fracture toughness, is also deliberated. Finally, the development prospects of CoCrNi-based MEAs are proposed.
Highlights
The traditional alloy design strategy takes one element as the principal constituent and adds other minor elements to optimize the properties
The effect of HCP nano-lamellae on the tensile response in the CoCrNi alloy was investigated by employing molecular dynamics simulations[45]
Whether at room temperature (RT) or cryogenic temperatures (CTs), the deformation process of the alloy was mainly divided into three stages, namely, the dislocation, twinning and phase transformation stages
Summary
The traditional alloy design strategy takes one element as the principal constituent and adds other minor elements to optimize the properties. The high-angle ADF-STEM (HAADF-STEM) image in Figure 12A shows a complex nanotwin-HCP lamella in a specimen strained to 53% strain at RT There are both well-developed multilayer HCP structures and local HCP stacking due to the slip of the single partial dislocations at twin boundaries. The extensive cross-slip activities and the subsequent dislocation multiplication and interactions was found to be related to the high strength during plastic deformation of the CoCrNi alloy at CTs[89] Another difference is that the volume fraction of the nanotwin-HCP lamella increased more rapidly with strain at CTs than at RT. This phenomenon explains the high strain hardening rate at CTs. the effect of HCP nano-lamellae on the tensile response in the CoCrNi alloy was investigated by employing molecular dynamics simulations[45]. Because of the increase of lamellar width, the blocking ability of the dislocation slip was stronger and the density of the phase boundaries was higher when the interspacing was smaller, providing more barriers for the dislocation glid in the other slip systems and resulting in higher strength
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