Computational Design of a Novel High-entropy Alloy with Multi-strengthening Mechanisms

Abstract

Efforts have been made to improve the strength of the Co-Cr-Fe-Mn-Ni high-entropy alloy using various strengthening mechanisms, including solid solution strengthening, transformation-induced plasticity, and precipitation hardening. A combination of these mechanisms can overcome the strength-ductility trade-off dilemma, but it is difficult to intentionally activate multiple mechanisms in a single alloy and individual strengthening effects have not been sufficiently exhibited in previous alloys with multi-strengthening mechanisms. When activating multiple mechanisms in a single alloy, a computational approach based on thermodynamic calculation is effective. However, a robust thermodynamic database is required for an accurate prediction of phase equilibria. Although commercial thermodynamic databases cover the Co-Cr-Fe-Mn-Ni system and extended systems, the prediction of phase equilibria even for the well-known Co-Cr-Fe-Mn-Ni HEA system is still challenging due to the incompleteness of the thermodynamic descriptions for the constituent ternary systems. Therefore, at first, we develop a self-consistent thermodynamic description for the quinary system by completing thermodynamic descriptions for all the constituent ternary sub-systems, newly carrying out thermodynamic assessments for the Co-Cr-Mn and Cr-Mn-Ni ternary systems. In addition, we also extend the developed thermodynamic description to higher-order systems with various additional elements (V, Mo, Cu, etc.). Then, we design a novel high-entropy alloy with multi-strengthening mechanisms through a stepwise design approach utilizing CALPHAD type thermodynamic calculation based on the developed thermodynamic description. The target strengthening mechanisms are introduced step by step, from solid solution strengthening, the addition of precipitation hardening and transformation-induced plasticity, based on the calculation. The finally designed Co21Cr11Fe49Mn4Ni4V2C1Mo3Si5 alloy simultaneously benefits from solid solution strengthening due to Mo and V addition, precipitation hardening from nanoscale precipitates, grain boundary strengthening by grain refinement, and transformation-induced plasticity by BCC deformation-induced martensite transformation. Individual strengthening effects is sufficiently exhibited in the designed alloy, which leads to an excellent combination of yield strength, ultimate tensile strength, and ductility.