“Designing Precipitation Strengthened FCC-based High Entropy Alloys/Complex Concentrated Alloys”
While high entropy alloys (HEAs), based on single phase concentrated solid solutions, have attracted a lot of worldwide attention, their potential application as real engineering alloys is rather restricted, especially for high temperature applications. Furthermore, often the experimentally observed single phase HEA is the result of second phase precipitation constrained by thermodynamic and kinetic factors. Therefore, more recently there has been an emphasis on expanding the scope of single-phase alloy exploration in HEAs to include the engineered design of multi-phase HEAs, also referred to as complex concentrated alloys (CCAs). This presentation will focus on designing HEAs strengthened using second (or more) phases, exploiting the impact of thermo-mechanical processing on the phase transformation pathways. These pathways lead to different combinations of phases, at multiple length scales, within these alloys. Example systems to be considered include FCC-based 3d transition series alloys with small additions of Al and Ti. The influence of Al additions on such FCC-based 3d transition series alloys has been investigated using a combinatorial approach based on compositionally-graded alloys, processed via additive manufacturing technologies. Based on the trends revealed by the combinatorial assessment, a model precipitation strengthenable alloy, Al0.3CoCrFeNi, was identified, and subsequently using solution thermodynamic based CALPHAD approaches, two other alloys have been developed; Al0.5Co1.5CrFeNi1.5, and Al0.2Ti0.3Co1.5CrFeNi1.5. All these alloys inherently exhibit a competition between the precipitation of different ordered intermetallic phases, such as the L12, B2, and L21, within the FCC solid solution matrix. The thermodynamic rationale underlying such competition is the complex interplay between the driving force and the nucleation barrier associated with each of these phases. This interplay will be investigated in detail as a function of different thermo-mechanical treatments, resulting in differences in the homogeneous versus heterogeneous nucleation of the intermetallic phases. The resultant microstructural diversity within the same HEA can lead to dramatically different mechanical properties, an aspect which can be used for tuning their properties for various applications.