Abstract: High entropy alloys are the focus of current research. An accurate description of their phase-transformation path, however, is a challenge when their phase constituent and transformation process are complex. In this study, a FeCoNiSn_x eutectic high entropy alloy (EHEA) system was investigated and a novel FeCoNiSn EHEA composed of BCC + HCP phases was reported. The transition from the hypoeutectic to the fully eutectic and then to the hypereutectic microstructure with the Sn addition was characterized by the electron backscatter diffraction (EBSD) technology, and the phase-transformation path was clarified by crystallographic orientation relationships. The studies reveal that the primary phase of FeCoNiSn_x (x = 0.2, 0.4) is FCC structure, and a further Sn addition induces an obvious phase-transformation from FCC to BCC in both the primary phase and eutectic lamellar, which satisfies the Kurdjumov-Sachs (K-S) or Nishiyama-Wasserman (N-W) variant orientation relationship. The mechanical results confirm that the phase structure and microstructure transition caused by Sn addition do significantly improve the strength and hardness of FeCoNiSn_x EHEAs, but have serious adverse effects on plasticity. This study would be of significance to understanding the phase-transformation process in HEAs and preparing the HEAs with aimed mechanical properties.

Key words: Eutectic high-entropy alloy, Orientation relationship, Phase-transformation, Microstructure, FeCoNiSn