Study of precipitation in Al–Mg–Si alloys by Atom Probe Tomography I. Microstructural changes as a function of ageing temperature

Abstract Atom Probe Tomography (APT) analysis and hardness measurements were used to characterise the early stages of precipitation in an Al–0.51 at.%Mg–0.94 at.%Si alloy. The changes in microstructure were investigated after single-stage or multi-stage heat treatments including natural ageing at 298 K (NA), pre-ageing at 353 K (PA), and automobile paint-bake ageing conditions at 453 K (PB). A wide range of particle sizes and compositions was typically present after a given heat treatment. Changes in particle sizes and compositions were found to be continuous, without abrupt changes of chemistry or morphology during the evolution of the initial clusters into GP zones and fine-scale precipitates. No phases were observed which were intermediate between GP zones and β″. NA had a deleterious effect on the PB hardening response, which was caused by a decrease in the rate of formation of stable, elongated precipitates during the subsequent PB. This decrease appeared to be due mainly to the formation of clusters of low average Mg/Si ratio at 298 K that did not act as effective precursors to the development of elongated precipitates upon PB. The presence of these clusters lowered the matrix solute concentrations, and suppressed nucleation of more efficient (higher Mg/Si ratio) precursors of the elongated phases.