Abstract
The microstructure and phase constituents in stir zone of friction stir weld for 2024-T4 aluminium alloys were observed and analysed by transmission electron microscopy (TEM), high-resolution transmission electron microscopy (HRTEM) and X-ray diffraction analysis. Homogeneous amorphous structure in some area of the weld nugget zone was detected. The amorphous structure with halo pattern has a structure characteristic of the long-range disorder. X-ray diffraction analysis indicated that a diffuse scattering peak in the shape of steamed bun with amorphous characteristic exists in the degree scope of 15°–29°. Some influencing factors, such as higher temperature gradient, hydrostatic pressure and shear stress and strain, may induce the formation of amorphous structure in weld nugget zone of friction stir weld.

Key words
friction stir welding, amorphous structure, transmission electron microscopy, X-ray diffraction, microstructure

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