Microstructure evolution, mechanical behavior, and fracture analysis of ultrasonic-assisted stir-squeeze cast high strength AA7068/ZrO2p/Grp composite under thermal aging

Abstract

Herein, high strength Al-Zn(-Mg) alloy designated as AA7068 based composites containing twin unlike dispersoids of ZrO2p and Grp were synthesized through an enhanced stir-squeeze caster. The density of the casted materials accompanying porosity was investigated through the liquid displacement technique following the Archimedes principle. Mechanical behavior, such as hardness, impact toughness, tensile strength, and ductility was examined under the as-casted, artificially aged (T6), and retrogressed and reaged (RRA) conditions. Microstructural analysis and determination of intermetallic compounds were carried out using FESEM-EDX and XRD techniques. The fractured surface under tensile loading was also examined through FESEM to recognize the governing mode of failure. Hardness, 0.2% proof stress, and UTS of the composite containing 4 wt.% of ZrO2p were improved over monolithic alloy up to 29.8, 68.2, and 37.5% at the as-casted condition and 43.5, 86.3, and 85% at artificially aged condition, respectively. However, the addition of Grp decreased the properties mentioned above. On the contrary, the addition of both the particulates of ZrO2 and Gr (added separately or concurrently) decreased the ductility and impact strength irrespective of the material condition. FESE micrographs reveal the uniform distribution of dispersoids in the matrix phase. Furthermore, fractography depicts that the monolithic AA7068 matrix fractured in a ductile manner, while interfacial decohesion and particle cracking were the prime causes of failure in the composites.