Effect of Mg/Si Ratio on Precursor Evolution and Precipitation Hardening During Extended Pre-Aging of Al-Mg-Si Alloys

The pre-aging response of Al-Mg-Si alloys is controlled by both thermal history and solute balance. Three model alloys with similar total Mg + Si contents but Mg/Si ratios of approximately 0.5, 2, and 4 (MS0.5, MS2.0, and MS4.0) were pre-aged (PA) at 323 or 343 K for 0.6–6000 ks and subsequently aged at 473 K. Microhardness, differential scanning calorimetry (DSC), and bright-field and high-resolution transmission electron microscopy (BF-TEM, HRTEM) were used, including counts of precipitate areal number density (NA) and type. Without PA, peak hardness decreased from 71.9 HV0.1 for MS0.5 to 54.7 HV0.1 for MS4.0. After 343 K × 6000 ks pre-aging, peak hardness reached 89.3, 91.6, and 73.1 HV0.1, while NA increased to 4087, 4384, and 1383 μm−2, respectively. DSC showed that the precursor state formed during PA depends on composition. After 343 K PA, MS0.5 shifted toward β″ (74.5% of classified precipitates), whereas MS2.0 kept a mixed population and MS4.0 retained β′ and parallelogram-type precipitates. Raising the PA temperature from 323 to 343 K increased NA by 2.1–2.6 times, but increased peak hardness by less than 5 HV0.1. The Mg/Si ratio thus controls how a given PA treatment becomes the final precipitate population and hardness.