Tuning the degree of chemical ordering in the solid solution of a complex concentrated alloy and its impact on mechanical properties

Using the binary enthalpies of mixing in a Co-Cr-Fe-Ni base alloy system, a high entropy alloy (HEA) or complex concentrated alloy (CCA), the equiatomic CoFeNi has been identified, which should form a random solid solution. Subsequent experimental validation established that this alloy is indeed a near-ideal, random face centered cubic (FCC) solid solution. The same thermodynamic basis has been employed to systematically engineer the degree of chemical ordering within the random CoFeNi alloy, from localized domains of short-range ordering (SRO), also referred to as clustered ordering, to well-defined long-range ordered (LRO) domains, by adding controlled amounts of Al and Ti, since these elements have a strong ordering tendency (negative enthalpy of mixing) with Co, Fe, and Ni. A series of seven alloys were designed in this study, based on enthalpies of mixing among 3d transition metals. This change in the degree of chemical ordering has a strong influence on the tensile yield strength of the alloy, for the same nominal grain size, ranging from similar to 181 MPa in case of CoFeNi to, similar to 793 MPa in case of the Al0.3Ti0.2Co0.7FeNi1.7 CCA. These experimentally measured yield strengths of the candidate CCAs are in close agreement with predicted values afforded by simple strengthening models. (C) 2021 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Automated summary

By utilizing the binary enthalpies of mixing in a Co-Cr-Fe-Ni base alloy system, a high entropy alloy (HEA) or complex concentrated alloy (CCA) has been identified as equiatomic CoFeNi solid solution, and the degree of chemical ordering within the alloy has been systematically engineered by adding controlled amounts of Al and Ti. This change in chemical ordering significantly influences the tensile yield strength of the alloy, ranging from 181 MPa for CoFeNi to 793 MPa for the Al0.3Ti0.2Co0.7FeNi1.7 CCA. The experimentally measured yield strengths of the candidate CCAs are in close agreement with predicted values afforded by simple strengthening models.