Grain boundary versus particle stimulated nucleation in hot deformed Al-4.5Zn-1Mg alloy

In the present research, an analytical model of recrystallisation combining grain boundary (GB) and particle stimulated nucleation (PSN) in hot deformed Al-4.5Zn-1Mg alloy was developed. The effects of evolving subgrain sizes and associated stored energy in addition to the Zener drag pressure on recrystallisation were incorporated into the model. A number of constants, fitting parameters and activation energies for recrystallisation nucleation were determined for the Al-4.5Zn-1Mg alloy by investigating recrystallisation after axisymmetric hot compression testing. It was found that with increasing temperature and decreasing strain rate, the number density of the nuclei decreased. Both GB and PSN played roles in the recrystallisation of this alloy. The temperature of subsequent annealing was found to be not strongly influential on the mode of recrystallisation, i.e. recrystallisation originated by GB nucleation or PSN. However, deformation conditions (strain, strain rate and temperature) clearly affected the mode of recrystallisation. Nucleation by the GB mechanism was especially marked. However, PSN played a dominant role in recrystallisation after deformation to large strains. In addition, when strain was <1, nucleation by the GB mechanism was only dominant if the strain rate was smaller than 1 s(-1) and the deformation temperature was higher than 450 degrees C.