Comprehensive study of phase transitions in equiatomic AlCoCrFeNi high-entropy alloy

Highlights: • Precipitation of FCC at the B2/BCC grain boundaries leads to σ formation at 644.8 ℃. • Activation energy of FCC+ σ precipitation found to be −155.14 ± 13.47 KJ/mol using Kissinger's method. • At 960.1 ℃, σ decomposed to B2 particles with "dot-like" precipitates of BCC. • Above 1100 °C, a complex phase transition was identified where BCC → B2 at 1154 ℃, and FCC → B2, between 1203 and 1248 ℃. • This work leads to new insights regarding the phase transitions in AlCoCrFeNi and will used to refine its phase diagram. -- Abstract: In recent years, high-entropy alloys (HEAs) have attracted research attention because of their promising properties of high strength at elevated temperatures and high resistance to oxidation and corrosion. One of the most investigated families is the AlCoCrFeNi-based HEA. This series tends to form simple structures such as face-centered cubic (FCC), ordered and disordered body-centered cubic (B2 and BCC, respectively) and sigma phase (σ). Many studies aimed at calculating and predicting the thermodynamic and phase transitions in this system have been based on existing binary or ternary phase diagrams, in the absence of relevant thermodynamic experimental data. This has led to inaccurate phase predictions. In the present work, the phase transitions, thermal expansion and activation energies in the AlCoCrFeNi system have been measured and analyzed. It was found that at about 590 ℃, a FCC phase starts to precipitate from the B2/BCC matrix: the high-energy interphase at the FCC/BCC boundary leads to σ-phase precipitation. The activation energy for the growth of these phases was found to be 155.14 ± 13.47 kJ/mol. At 960 ℃, the σ-phase decomposed to a B2 matrix with "dot-like" BCC particles. The last phase transition was found to be a complex event, which contained two transitions: BCC → B2 at 1154 ℃ and FCC → B2 between 1203 and 1248 ℃: this transition was associated with a large volumetric shrinkage. Finally, at temperatures near melting there is a single B2 phase region.