Influence of aging on discontinuous dissolution in Al-22at.%Zn alloy

The influence of prior aging and volume diffusion on the kinetic parameters of discontinuous dissolution were studied in Al-21.55at.%Zn alloy. The following kinetic parameters were considered: the critical temperature of discontinuous dissolution Tcr, the rate of discontinuous dissolution Vd1, the diffusivity KDb.mλ at the boundary of receding cells, and the activation energy of discontinuous dissolution. A new method of calculation of KDb.mλ is proposed. It is concluded that Tcr, Vd1, and KDb.mλ are strongly affected by the conditions of prior aging. The estimated diffusivities for discontinuous dissolution are more than two orders of magnitude smaller than those for diffusion at stationary grain boundaries. This result is caused by volume diffusion ahead of the precipitation front and within a-phase lamellae. However, the discontinuous dissolution is still controlled by diffusion at the boundary of receding cells. The influence of aging temperature and volume diffusion on KDb.mλ values was found to be connected with the change in pre-exponential factor in the Arrhenius equation.