en Research Paper Research Paper The effect of cooling rate after annealing at 900 °C on the microstructure and hardness of high entropy alloys was investigated using two typical samples with the chemical composition of Co₁₆Cr_14.5Fe₂₉Mn_11.5Ni₂₉and Co_11.5Cr₇Fe₂₇Mn₂₇Ni₂₇(Nb_0.08C_0.5) (at%). The microstructural characterisation and hardness measurements were carried out by optical microscopy, scanning electron microscopy, wavelength-dispersive X-ray spectroscopy, electron back scattered diffraction, X-ray diffraction technique and Vickers hardness testing. A face centred cubic crystal structure matrix was observed in both alloys before and after annealing and regardless of cooling conditions. SEM analyses revealed an extensive precipitation in Co_11.5Cr₇Fe₂₇Mn₂₇Ni₂₇(Nb_0.08C_0.5) alloy after annealing. It was also found that air/furnace cooling can enhance grain growth-coarsening just in Co₁₆Cr_14.5Fe₂₉Mn_11.5Ni₂₉. However, the hardness results generally showed insignificant hardness variations in both alloys after water-quenching, air-cooling and furnace-cooling. The results suggested that the hardness is mainly controlled by solid solution strengthening. High entropy alloys,Cooling rate,Annealing,Microstructure,Hardness 130 141 http://ijmse.iust.ac.ir/browse.php?a_code=A-10-1801-1&slc_lang=en&sid=1 E. Abbasi engabasi@gmail.com 1800319475328460011426 1800319475328460011426 Yes Amirkabir University of Technology K. Dehghani Dehghani@aut.ac.ir 1800319475328460011427 1800319475328460011427 No Amirkabir University of Technology T. Niendorf niendorf@uni-kassel.de 1800319475328460011428 1800319475328460011428 No Universität Kassel S. V. Sajadifar sajjadifar@uni-kassel.de 1800319475328460011429 1800319475328460011429 No Universität Kassel