Effect of severe quenching on hardness of commercially pure aluminium in correlation to structural changes

Typical non‐conventional routes of severe quenching treatment, both cyclic quenching from room temperature up to 4 cycles and single time quenching from 600 °C after short‐duration (5 min) holding using liquid nitrogen as the quenchant, has been adopted on an initially annealed commercially pure aluminium. The grain refinement effect is observed for single time quenching from 600 °C; but not for cyclic quenching from room temperature. Lattice strain is found to be accumulated up to 2 cycles followed by a decline to a consistent lower value for 3 and 4 cycles in the specimens subjected to cyclic quenching treatment. The specimen, single time quenched from higher temperature (600 °C), exhibits much higher lattice strain as compared to all cyclic quenched specimens. The accumulation of lattice strain is found to be consistent with the evolution of regions of extremely low misorientation angle in the aluminium matrix. As a result of significantly high lattice strain generation and grain refinement effects, the hardness of commercially pure aluminium is substantially enhanced to 88 HV 10 on execution of single time liquid nitrogen quenching from 600 °C after short‐duration (5 min) holding; while compared with an initial hardness of 36 HV 10 in annealed condition.