In this work, the effect of Al contents and the associated phase transformations on the impact toughness of AlxCoCrFeNi high-entropy alloys (HEM) (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. (C) 2017 Elsevier B.V. All rights reserved.