Microchemical and Microstructural Characterization of the Early Stages of the Discontinuous Precipitation Reaction in Al-22 at.% Zn Alloy

Abstract.

High-spatial energy dispersive X-ray (EDX) microanalysis has been applied in order to reveal changes of chemical composition accompanying the formation of the lamellar structure due to discontinuous precipitation reaction in Al-22 at.% Zn alloy. The line-scans performed in the reaction zone have shown that the region left behind the grain boundary (GB) after its displacement from the original position is solute depleted, similar to the de-alloying case of diffusion induced grain boundary migration. This enables the allotriomorphs to be transformed into solute-rich β lamellae and development of lamellar α+β structure, especially if the GB moves in the direction perpendicular to its original location. If the direction of movement is strongly inclined to the original location of the GB, the growth usually stops after a short period of time, mainly due to formation of the precipitation free zone (PFZ) ahead of the reaction front. The detailed EDX examination revealed that there is a sharp change in solute content at the α lamella/PFZ and PFZ/matrix alloy interfaces. The solute content in the PFZ is lower than in the neighbouring α lamella and it corresponds to the equilibrium state. Therefore, one can suppose that the PFZ is formed due to backward movement of the reaction front in order to consume the excess of solute atoms stored in the α lamellae in comparison with the equilibrium state. As soon as the equilibrium state is attained within the PFZ, the growth of the colony is completed.