Abnormal temperature dependence of impact toughness in AlxCoCrFeNi system high entropy alloys

In this work, the effect of Al contents and the associated phase transformations on the impact toughness of AlxCoCrFeNi high-entropy alloys (HEAs) (x denotes Al molar ratio; x = 0, 0.1, 0.75, and 1.5) in the as-cast state were investigated at T = 77 K, 200 K, and 298 K. For the alloys with x = 0 and 0.1, the alloys have the FCC structure, and an inverse temperature dependence of impact toughness between 298 K and 77 K was observed. The enhanced impact toughness (397.87 J) at 77 K is attributed to the capability of extensive nano-twinnings as well as ductile dimple fracture. Increasing Al contents to x = 0.75 and 1.5 induces the formation of brittle BCC phase and ordered B2 compound and accordingly embrittlement, leading to tremendously reduced Charpy-impact energies. The present study demonstrates that FCC AlxCoCrFeNi system HEAs with low Al contents (x < 0.5) may be well suitable for cryogenic applications.