Microstructure and Properties of T6I4 Aged 700-MPa-Grade Aluminum Alloy.

700-MPa-grade aluminum alloys (AAs) were processed by T6 and T6I4 aging treatments. The nanostructures and phases of obtained AAs were analyzed using a transmission electron microscope and x-ray diffraction, respectively. The effects of aging treatment on the tensile property and intergranular corrosion of AAs were investigated using a tensile machine and optical microscope, and the fracture morphologies and corrosion microstructure were observed using a scanning electron microscope and its equipped energy-dispersive spectrometer. The results show that the fine matrix precipitates, narrow precipitate free zones and discontinuous distribution of grain boundary precipitates (GBPs) are the characteristics of T6I4 aged AA, while those of T6 aged AA are the opposite. The matrix precipitates of T6I4 aged AA are transformed from Guinier–Preston zones to η′ phases, while those of T6 aged AA are transformed from η′ to η phases. The uniform distribution of fine matrix precipitates in the T6I4 aged AA is the main factor for its high tensile strength. The continuous GBPs in the T6 aged AA promote the formation of short-circuit cell to aggravate the intergranular corrosion, while those in the T6I4 aged AA are the opposite. [ABSTRACT FROM AUTHOR]