Constitutive Modelling, Processing Maps and Dynamic Restoration Behavior of High Entropy Alloys

Abstract

The novel concept of high-entropy alloys (HEAs) has emerged as a new research field that has created an enormous drive for new alloy design strategies involving various combinations of multi-principal elements in equal or close-to-equal proportions. The alloy compositions in the central region of multi-component phase diagrams contemplate high configurational mixing entropies that distinguish these alloys from traditional solid solution alloys. This multi-dimensional approach can create practically infinite alloy compositions, and ironically, only a tiny area of alloy compositions has been researched so far. Although the initial design approach was to stabilize a single-phase solid solution, newer compositions were designed by adding certain elements in non-equiatomic proportions, facilitating the formation of intermetallic phases and/or multiphase structures in order to improve the balance of strength and damage tolerance.

Further, the face centered cubic (FCC) based HEAs possess exceptional mechanical properties exceeding those of conventional alloys intended for structural applications. In order to produce bulk components for potential structural applications, techniques involving high temperature mechanical processing or hot working of bulk metallic materials, such as hot forging, rolling, or extrusion, are conducted to impose large strains in a single operation step without inducing the onset of failures, such as cracks or fractures. However, mechanical processing at elevated temperatures is difficult, and trial-and-error methods are often used in selecting hot working parameters. Therefore, careful optimization of processing parameters is of vital importance since they may have a significant influence on the deformation mechanisms, microstructural evolution, and final mechanical properties.

In this work, a study was made of the influence of hot deformation parameters, consequent structural evolution, and associated deformation mechanisms of two selected FCC-based HEAs (CoCrFeMnNi and Al<SUB>0.3</SUB>CoCrFeNi) intended for advanced engineering and structural applications. Initially an empirical model was generated, establishing a quantitative relationship between constitutive flow stress behavior and imposed deformation conditions, using the hyperbolic-sinusoidal Arrhenius-type modeling approach. Further, the processing maps of the two selected HEAs were constructed using dynamic materials modeling, in order to identify their ‘safe’ hot working domains for realizing defect-free microstructures. Finally, the dynamic recrystallization mechanism that occurred in the ‘safe’ hot working domain identified using the processing maps was characterized and modeled. This was done to predict the progress of microstructural reconstitution by establishing a kinetic model based on the imposed processing parameters.