Differential scanning calorimetry and energy dispersive spectrometry studies of phase transformations in supersaturated Al–39·5 at.-%Zn alloy

Abstract

The transformation processes in a supersaturated Al–39·5 at.-%Zn solid solution have been studied and characterised by using an SEM equipped with an energy dispersive spectroscopy (EDS) elemental composition microanalyser and a differential scanning calorimetry analyser. With the aid of the SEM examinations of the alloy after various specific aging temperatures, the EDS analysis showed that a transformation, solid solution α_ss phase → spherical Zn precipitates → ZnAl precipitates with an average Zn concentration of 70·5 at.-%, had taken place as a result of increasing the aging temperature. On the basis of the Johnson–Mehl–Avrami equation and assuming diffusion controlled continuous transformation processes, it was concluded that the above transformations were controlled by the three-dimensional mechanism. The average overall activation energy associated with the first precipitation process is determined as 28·13 kJ mol^-1. The other transformation processes are not dominated by their reaction kinetics but are dominated by their temperature dependent thermodynamic equilibrium.