Phase Engineering of High-entropy Alloys through Transformations

The objective of this study is to improve the fundamental understanding of phase transformation behaviors and their influence on the properties of high-entropy alloys (HEAs). As a relatively new concept that has been developed in the recent two decades, HEAs possess many significant properties compared with traditional alloys. This work focuses on utilizing phase engineering of HEAs to improve their mechanical properties through transformations, including both strain-controlled and temperature-controlled phase transformations. Firstly, TiZrHfNbx refractory HEAs (RHEAs) are studied to investigate their phase transformation behaviors under uniaxial loading, which can be categorized as strain-controlled phase transformations. With different Nb contents, these metastable RHEAs are expected to show various transformation behaviors from body-centered cubic (BCC) to hexagonal-close packed (HCP) during uniaxial plastic deformations since Nb serves as a BCC stabilizer in this system. By adjusting the Nb content to proper values, the ductility/plasticity could be improved due to the transformation-induced plasticity (TRIP) effect. Using one of the RHEA compositions in the first part featuring both BCC and HCP phases, the transformation behaviors under hydrostatic pressures are investigated in the second part. The influence of the hydrostaticity of the environment is also studied by altering the pressure-transmitting medium. In the third part, novel HEAs are designed and developed in the Al-Co-Cr-Fe-Mn-Ni-Ti system to have enhanced strengths. The strengthening strategy is to nucleate and grow precipitates with the ordered L1₂ structure in the face-centered cubic (FCC) matrix. Moreover, the evolution of the precipitates is investigated to understand the temperature-controlled phase transformation behavior in HEAs. Mechanical tests combined with in-situ neutron/synchrotron X-ray diffractions as well as various microstructural characterization methods are utilized to understand the deformation mechanisms and their correlations with phase transformation behaviors in this study. Theoretical simulations/calculations are also performed and coupled with the experimental work to give a better understanding of the underlying physics.