Phase evolution and thermal stability of AlCoCrFe high entropy alloy with unsolicited carbon from the milling media were investigated. A single BCC phase was evolved during mechanical alloying, and BCC/B2 phases with Cr-rich M23C6 carbide was observed after spark plasma sintering. Unsolicited carbon during milling has led to the formation of M23C6 secondary phase after sintering. The alloy exhibits a very high hardness of 1050 +/- 20 HV1 and excellent phase stability. The hardness reduction after annealing at 900 degrees C for 600 h is less than 10%, indicating excellent resistance to anneal softening. (C) 2017 Elsevier B.V. All rights reserved.