Exploring microstructural evolution in various high entropy alloys: A journey from TRIP/TWIP HEAs to refractory metal based CCAs

The development of non-equiatomic complex concentrated alloys enabled researchers worldwide to explore new compositional space in the field of high entropy alloys. The present study provides an account of the development of several transformative high entropy alloys with flexible microstructures, obtained through a systematic combination of Thermocalc based alloy design and friction stir processing, involving the synergistic application of strain, strain rate and temperature. Starting with a Fe-Mn-Co-Cr-Si based composition, various alloys were designed to obtain different combinations of single-phase or fcc-hcp dual-phase microstructures in the as-cast state. Upon subsequent friction stir processing and thermomechanical processing by conventional methods such as rolling and annealing, it was possible to activate a variety of underlying deformation mechanisms such as epsilon martensitic transformation, twinning induced plasticity and precipitation strengthening. As a result, significant simultaneous enhancement of work-hardenability, strength and ductility was achieved, as reflected in their tensile and fatigue behavior. Additionally, some of these alloys also exhibited excellent corrosion resistance. The key insight deals with how all the properties are rooted in microstructural engineering, which may be utilized to develop a class of advanced materials for structural applications. The second part of this investigation of various complex concentrated alloys deals with another important type of alloy constituents, namely the refractory metals. The alloy design strategy endeavors to combine lighter refractory metals with some non-refractory elements to produce a new compositional space of Fe-Cr-Mo-Nb-Ti based alloys. These alloys were fabricated through vacuum arc melting route and heat treatment was used to tailor a microstructure consisting of bcc solid solution and Laves phases that enabled a unique combination of low density, high strength and corrosion resistance. Further research is ongoing to develop more novel compositions with a wider range of property attributes that might be suitable for diverse applications such as biomedical applications.