Abstract
In this study we report the deformation microstructures and strength of medium entropy alloys (MEAs) and high entropy alloys (HEAs), which are defined as alloys composed of four or less, and five or more principal elements, respectively, with (near-) equi-atomic concentrations. The friction stress (fundamental resistance to dislocation motion in the crystal lattice) and Hall-Petch relationship of various MEAs (CoCrFeNi, CoCrNi, etc.), taken as subsystems of the equi-atomic CoCrFeMnNi HEA, were precisely measured at room temperature. Experimental values of the friction stresses were found to fit with a theoretical model proposed by Toda-Caraballo et al. very well, which indicates that the strength of the alloys is closely related to a heterogeneously distorted crystal lattice. At the same time, values of the average lattice distortion in the alloys were found to be comparable to those in some dilute alloys, contradicting the belief that “severe” lattice distortion is a reason for the higher strength than in dilute systems. Finally, a strengthening mechanism due to element-element interactions was proposed as an additional mechanism in FCC HEAs and MEAs.
Highlights
Metallic materials have been designed based on one kind of solvent element, such as Fe, Al, etc., where a small amount of alloying elements, such as C, Cr, Cu, etc., are added to achieve superior properties
Experimental values of the friction stresses were found to fit with a theoretical model proposed by Toda-Caraballo et al very well, which indicates that the strength of the alloys is closely related to a heterogeneously distorted crystal lattice
The importance of element-element interactions was demonstrated by Zhang et al [19] very recently, based on their theoretical modeling of short-range ordering (SRO) in high entropy alloys (HEAs). This effect of weak elemental inhomogeneity has been omitted in conventional dislocation theory, but the present results suggest that the strengthening effect of element-element interactions cannot be neglected in high-alloy systems such as HEAs and medium entropy alloys (MEAs) because of their high concentration of alloying elements
Summary
Metallic materials have been designed based on one kind of solvent element, such as Fe, Al, etc., where a small amount of alloying elements, such as C, Cr, Cu, etc., are added to achieve superior properties. As derivatives of HEAs, a concept of medium entropy alloys (MEAs), composed of four or fewer alloying elements with (near-) equimolar concentration was proposed. Some of these MEAs have been found to possess similar, or even superior, physical properties compared with HEAs. For example, Gludovatz et al investigated mechanical properties and fracture toughness of equiatomic CoCrFeMnNi HEA [4] and CoCrNi MEA [5] with FCC single-phase and found that these materials showed excellent resistance to fracture
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