quenching of high entropy alloys after annealing

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 co_16cr_14.5fe_29mn_11.5ni_29 and co_11.5cr_7fe_27mn_27ni_27(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_11.5cr_7fe_27mn_27ni_27 (nb_0.08c_0.5) alloy after annealing. it was also found that air/furnace cooling can enhance grain growth-coarsening just in co_16cr_14.5fe_29mn_11.5ni_29. 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.