Microstructural Stability of As-Cast and Directionally Solidified AlCoCrFeNi2.1 Eutectic High-Entropy Alloys at Elevated Temperatures

Abstract

In this article, we investigate the microstructural stability of as-cast and directionally solidified (DS) AlCoCrFeNi2.1 eutectic high-entropy alloys at elevated temperatures (900 °C to 1100 °C). The microstructure of the as-cast alloy is composed of a lamellar and mesh-like structure consisting of NiAl-rich (B2) and CoCrFeNi-rich (L12) phases. Annealing at 900 °C to 1100 °C results in the coarsening of the lamellar or mesh-like structures and the migration of termination. For the as-DS alloy, the solid–liquid interface undergoes a transition from planar to cellular morphology along with the decrease of interlamellar spacing with increasing the withdrawal rates. After the same heat treatment, the stability of DS microstructure is relatively excellent at 6 μm/s where only a little coarsening and migration of the lamellar structure occur. With increasing withdrawal rates, the microstructural degradation, like the coarsening of lamellar or mesh-like structures and the migration of termination, becomes severe because of the decrease of interlamellar spacing. Moreover, an interesting phenomenon is that the coarsening of the lamellar structure occurs along with the thinning of the adjacent lamellae at 1100 °C for the as-cast alloy or the DS alloy at 120 μm/s due to the fine interlamellar spacing. The relevant coarsening and migration mechanisms are discussed. In addition, the precipitate forms at 900 °C in all the experimental alloys and is identified as Ni-rich phase (FCC).