Precipitation behavior during low-temperature aging in Al–Mg–Si alloy using STEM-EDS intensity correlograms

Abstract

Al–Mg–Si alloys are extensively used in automotive panels because of their high formability and age-hardening capabilities during paint baking. Natural aging that occurs after the solution treatment inhibits formation of β″ precipitates during subsequent artificial aging, reducing the hardness. The precipitation behavior of Al–0.65Mg–0.81Si mass% alloy during isothermal aging at 100 °C over various durations is investigated in this study using scanning transmission electron microscopy (STEM) and STEM energy-dispersive X-ray spectroscopy (EDS). The analysis reveals the formation of Guinier–Preston (GP) zones and β″ precipitates, which influence the mechanical properties of the alloy. After 1 h of aging, no GP zones are observed; however, clusters with diameters of 1.0–1.5 nm formed, increasing and decreasing in density at 10 and 400 h, respectively. Further, β″-related structures with a Mg/Si ratio of ∼1 started forming at 10 h and grew in number by 400 h, with most particles being spherical and some elongated along the [100] Al direction. These microstructural changes correlate with yield strength variations, highlighting the role of pre-aging in promoting β″ formation during subsequent artificial aging. In addition, a novel STEM-EDS intensity correlogram method is introduced to effectively distinguish aging products from the matrix and deepen the understanding of microstructural evolution in Al–Mg–Si alloys. • Microstructural evolution of Al-Mg-Si alloys was investigated during isothermal aging. • Clusters and β″ precipitates were identified during aging using STEM-LAADF and STEM-EDS. • Novel STEM-EDS correlogram method effectively distinguishes aging products. • Clusters with widely distributed compositions formed after 1 h of aging. • β″-related structures with Mg/Si ratio of ∼1 started forming between 1 and 10 h of aging.

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