On the origin of the heterogeneous precipitation and dissolution in Al-Mg alloys

The autocatalytical continuous reaction in Al-8 wt. % Mg alloy, due to heterogeneous nucleation and growth into grains, is a reversible phenomenon because the dissolution of precipitated particles is also a heterogeneous reaction. The study of the defect called rosette which leads to these heterogeneous precipitation and heterogeneous dissolution reactions has shown that it is difficult to confirm precisely its origin. It is not the concentration of a given element because microanalysis (line scan, point counting and X-ray image) has not shown any enrichment in this element, and microhardness measurements give the same value in different regions of the matrix (with and without rosettes). However, the observation of a complex network of dislocations confirm the idea that rosettes are probably a steady structure defect of this type.