Ageing behaviour of SiCp-reinforced AA 7075 composites

Abstract

The precipitation behaviour in 7075 aluminium alloy matrix composites reinforced with 0–40 vol% particulate SiCp (12.5 μm) was studied using macrohardness (HV) measurements and differential scanning calorimetry (DSC). In the low volume percentage (5,10) SiCp composites, the hardness-ageing curves and DSC scans are similar to those of the unreinforced alloy. However, the age-hardening quantities and DSC Gurnier-Preston (GP) zone peak size are smaller than those of the unreinforced alloy. Additionally, the high-temperature peaks (ageing curves at 200 °C or DSC scanning curves) are broader. In the high volume percentage (20, 30, 40) SiCp composites, the hardness-ageing curves and DSC scans are very different from those of the unreinforced alloys. Only a high-temperature broad peak was observed during the DSC scanning. On the hardness-ageing curves no hardening phenomena were detected, but rather a softening phenomenon occurred in the 30% or 40% SiCp composites, suggesting that during ageing an exothermic dislocation recovery softening process coexists with precipitation hardening. A model was introduced by dividing the matrix of the composite into Region I (normal precipitation) and Region II (particular precipitation). The precipitation of GP zones is completely suppressed and the precipitation of η′ phase is accelerated in Region II. The matrix of the low volume fraction SiCp composite comprises Regions I and II, whereas that of the high volume fraction SiCp composite comprises only Region II. The ageing behaviour and DSC scans of the composites can be fully explained by this model.