Microstructural evolution and delayed hydride cracking of FSW-AZ31 magnesium alloy during SSRT

Evolution of microstructure including texture and fractography in a friction-stir welded (FSW) AZ31 magnesium alloy was investigated. The texture was measured using a neutron diffractometer. The microstructure and fractography of stress corrosion cracking (SCC) samples were observed by optical and scanning electron microscopy, respectively. An X-ray diffraction study was carried out on the fractured surfaces of the SCC specimens. The results indicated that a strong basal fiber was formed on the base material, whereas the grains in the stir zone were reoriented with their most basal planes tilted 25° to the welding direction. Feather-like twins and hydride formed under slow strain rate tensile (SSRT) stress in air and aggressive solutions, respectively. Transgranular cracks propagated and finally failed on the retreating side in the solution. The hydride phase confirmed to sit on the fracture surface demonstrated the delayed hydride cracking (DHC) mechanism of the alloy.