Hardness enhancement of commercially pure aluminium by a typical cyclic heat treatment process involving repeated ice‐brine quenching

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A typical cyclic heat treatment process has been adopted up to four cycles on initially annealed commercially pure aluminium, primarily containing iron impurity, that involves short‐duration (5 minutes) holding at 600 °C followed by ice‐brine quenching in each cycle. As a consequence, a significant grain refinement effect and a gradual accumulation of lattice strain during the progress of cyclic heat treatment are observed in relation to an evolution of the regions of extremely low misorientation angle in the aluminium matrix. Apart from quench‐in strain, the presence of grain boundary iron aluminide phase, possessing a different coefficient of thermal expansion than that of aluminium matrix, appears to be an additional reason for gradual lattice strain development. In addition, a preferred growth of (202) plane along [101] direction towards the final stage of cyclic heat treatment (four cycles) is also revealed. As a result of gradual lattice strain accumulation and grain refinement effects, the hardness of commercially pure aluminium is found to be considerably enhanced (98 HV 10) after 4 cycles of heat treatment as compared to an initial hardness (36 HV 10) in annealed condition.