Ageing Processes in Al-Mg-Si Alloys during Continuous Heating

Abstract

The precipitation behaviours during continuous heating in the Al-Mg 2 Si alloys with excess Mg and Si contents have been investigated by means of Vickers hardness measurements, differential scanning calorimetry and transmission electron microscopy. The first exothermic reaction occurring at the lowest temperatures is due to the annihilation of quenched-in vacancies by the collapse of vacancy clusters and the migration of vacancies to various sinks such as grain boundaries. The prismatic dislocation loops formed by this collapse are frequently observed. The second reaction detected as broad hardening increases with increasing excess Si content and can be interpreted as the formation of solute atom clusters. The sharp and large exothermic reaction inducing the largest hardening corresponds to the precipitation of β needles. The following exothermic reaction arises from the precipitation of β-Mg 2 Si particles. The formation of β and Type-B rods can be recognised in the quasi-binary and the excess Si at slightly higher temperatures.

Effects of Excess Mg and Si on the Isothermal Ageing Behaviours in the Al-Mg<SUB>2</SUB>Si Alloys

“…In the previous continuous heating experiment, 18) the intermediate phases observed were limited, and the formation of Type-A and C rods could not be recognised. The phases precipitated during isothermal ageing processes are summarised in Table 2.…”

“…Since the precipitation of cuboid β-Mg 2 Si particles has been discussed in detail in the previous studies, 8,18) the formation of other intermediate phases is examined here. In the excess Si alloys, Type A rods starts to precipitate after β needle and fine Si particle formation at 210…”

Effect of Mn or Fe Addition on Age-Hardening Behaviour of Al&ndash;Mg₂Si Alloys

“…Dutta and Allen 24) reported 33.1 kJ/mol for 6061 alloy, however Doan et al 25) determined 79 kJ/mol for the same process of the same alloy. In addition, Ohmori et al 26) calculated activation energy of 52.364.5 kJ/mol for this process of AlMgSi alloy. Therefore, our value for the activation energy of the clustering process is within the most acceptable values found in the literature.…”

Atomic-scale pathway of early-stage precipitation in Al–Mg–Si alloys

“…The second group of clusters, speculated to be descendants of the first Si-rich clusters, is shown to exhibit Mg-enrichment while growing 9,24,25,30 . It has been reported that a larger Si content in the alloy composition promotes cluster nucleation and leads to a finer distribution of β precipitates at later stages of aging 9,31,32 . The morphology of early-stage precipitates is generally reported from atom probe investigations to be spherical 9,24,28,29 .…”