Phase stability and coarsening mechanism in AlCrFeNi3 eutectic medium-entropy alloy during prolonged isothermal annealing

Abstract

This study systematically investigates the effects of prolonged isothermal annealing (up to 400 h) at medium (700 °C) and high (1000 °C) temperatures on the microstructural evolution and mechanical properties of an as-cast AlCrFeNi₃ eutectic medium-entropy alloy (EMEA). The results revealed that the lamellar structure becomes unstable at 1000 °C, leading to the fragmentation and spheroidization of the B2 phase, accompanied by the formation of fine spherical B2 precipitates due to Al enrichment. In contrast, the lamellar structure remains largely stable at 700 °C, with only discontinuous coarsening observed at some eutectic cell boundaries after extended holding. The nanoscale L1₂ precipitates within the FCC phase coarsen into elongated shapes at 700 °C, while they refine at 1000 °C. Concurrently, the Cr-rich BCC precipitates within the B2 phase dissolve at 1000 °C but continue to grow at 700 °C. Notably, significant Cr segregation at the B2/FCC phase interfaces at 700 °C leads to the formation of irregular, coarse Cr-rich BCC precipitates, which are considerably larger than those in the interior of the B2 phase. The microstructural changes directly govern the mechanical properties. Annealing at 1000 °C enhances ductility at the expense of a slight reduction in strength. Conversely, treatment at 700 °C significantly increases yield strength but sacrifices ductility. A two-step annealing process (1000 °C/100 h followed by 700 °C/100 h) effectively enhances the strength with only a marginal decrease in ductility.