Unique Atomic Structure of FCC-HCP Dual Phase High Entropy Alloy after Low Temperature Annealing

Strength and ductility are the most important physical properties in evaluating structural materials. Unfortunately, common strengthening strategies in most conventional alloys cause sacrifice of ductility inevitably and vice versa. Recently, however, high entropy alloy (HEA) is regarded as a good candidate that can overcome the strength-ductility trade off. It was shown that mechanical property of HEA could be enhanced by TRIP or TWIP behavior. Especially when dual phase high entropy alloy (DP-HEA), which is constituted by FCC and HCP phase, has multiple deformation mechanisms, strength and ductility showed good balance. Designing method for alloy which has various deformation mechanisms is controlling the stacking fault energy (SFE). SFE is well known as a key parameter which decide the deformation mechanism. By reducing the SFE, TRIP of TWIP mechanism is activated and even dual phase microstructure is stabilized at room temperature.<br/>In the present study, we designed a DP-HEA by reducing the SFE which could be implemented by CALPHAD. Furthermore, we successfully designed a unique DP-HEA microstructure with nano-sized austenite film in hard martensite matrix. This lamellar microstructure of austenite and martensite could enhance the overall toughness of the HEA. Initial microstructure was constituted by FCC and HCP phase after homogenization and quenching process. Cold rolling and heat treatment were done to make a better microstructure for the subsequent low temperature annealing, which is crucial for the partial austenitization. After low temperature annealing, the DP-HEA exhibited a novel ‘composite-like’ microstructure with metastable austenite film in martensite HEA matrix. XRD was implemented to confirm that partial austenitization occurs after low temperature annealing. Phase fraction and morphology of austenite was analyzed by SEM and EBSD. It was possible to make fine alternating lamellar structure of FCC and HCP phase even at 573K. Furthermore, we could characterize atomic ordered structures in HCP martensite after low temperature annealing by the STEM analysis. This heterogeneous atomic structures in martensite could have an effect of strengthening, acting as an obstacle for dislocation moves. This result can provide a guideline on how to overcome strength-ductility trade off by precise tailoring microstructure in DP-HEA.