Microstructure observation of T-phase in Al-Zn-Mg alloy with low Zn/Mg ratio

For the first time, this study clarifies the interface of the T'', T', and T-phase in Al-Zn-Mg alloy with a low Zn/Mg ratio and evaluates the shape of the precipitate morphology according to different zone directions of the Al matrix. The orientation relationship, shape, misfit, and interfacial conditions between the T'', T', and T-phase and the Al matrix were investigated using high-resolution transmission electron microscopy (HR-TEM). We differentiated the T'' (a =1.387 nm), T' (a =1.422 nm), and T (a =1.456 nm) phase by calculating the lattice parameters in HR-TEM images. We also identified the T'', T', and T-phase by diffraction patterns along the <100>, <110>, <111>, and <112> zone directions of the Al matrix. Additionally, we introduce the different orientation relationships of the T-phase with the Al matrix, The [$\overline{11}$$\overline{11}$0] zone direction of the Al is parallel to the [$\bar{3}$$\bar{3}$20] direction of the T-phase, and the plane of ($\overline{11}$$\overline{11}$1) of the Al is parallel to the plane of ($11\bar{2}$$11\bar{2}$) of the T-phase. Among Al alloys, the 7xxx series is known for its superior strength due to the co-precipitation of the η and T-phase. While previous research has extensively examined the precipitation sequence, composition, atomic structure, and phase transformation mechanisms of the η-phase, the T-phase has received less attention. Our observations provide insight into the behavior of T-phase crystals, which is crucial for understanding the microstructural evolution of Al-Zn-Mg alloys and for designing materials with desired properties.