Comparing CALPHAD Predictions with High Energy Synchrotron Radiation X-ray Diffraction Measurements During in situ Annealing of Al0.3CoCrFeNi High Entropy Alloy

Abstract High-entropy alloys (HEAs) offer the unique ability to tailor microstructures through thermal processing techniques. One family of alloys of interest is AlxCoCrFeNi, where x varies from 0.0 to 2.0 molar fraction which allows for a transition from face-centered cubic (FCC) to body-centered cubic (BCC) crystal structure, as well as the formation of various secondary phases, such as B2, sigma, and L12. In this study, we focus on the Al0.3CoCrFeNi HEA. Based on CALPHAD simulations, the Al0.3CoCrFeNi HEA consists of the following phases from highest to lowest temperature: a) FCC at high temperature, followed by b) B2 precipitation in an FCC matrix, c) then B2 and σ phase co-precipitation in an FCC matrix, d) then B2, σ phase, and L12 phase in an FCC matrix, and finally e) B2, σ phase, and L12 phase in a BCC matrix at low temperature. The purpose of our study is to compare phases observed using high-energy synchrotron radiation X-ray diffraction measurements during in situ annealing with the equilibrium phases predicted using CALPHAD. Phase evolution and dissolution are discussed and compared with the CALPHAD simulation predictions. Our experimental results matched well with the CALPHAD predictions of the phase evolution for the B2 phase, however, the phase dissolution of L12 and σ phases was incomplete, which is likely only observable using high resolution synchrotron radiation X-ray diffraction. Better CALPHAD models are needed as well as longer annealing times due to the complexity of achieving phase equilibrium in multi-phase HEAs.