Commencement of Ordering in Al–0.69Mg–0.31Si Alloy

Early stages of ordering in dilute Al–0.69(at.%)Mg–0.31(at.%)Si alloy, aged at 373 K for 120 h and 7200 h and at 453 K for 20 h, have been studied using scanning transmission electron microscopy techniques. Starting from the solute enrichment in (110)-type parallel planes, merely after 120 h of aging, isothermal growth at 373 K has been observed to lead to the formation of the small-ordered region after 7200 h of aging. Scanning transmission electron microscopy images from the aforesaid three varieties of samples and the corresponding Fourier-transformed images have been the basis to suggest the possible path of evolution of β″ ordering in this dilute alloy. Based on this, the present study reveals the mechanism of formation of silicon pillar, which is the early stage of β″ that transforms to a fully developed β″. This corroborates with the concept of getting the final β″-strengthened product from an under-aged alloy by a suitable heat treatment.