Sensitization Behavior And Environmentally Assisted Cracking Of AA5083-H128 Al-Mg Navy Alloy

Al-Mg alloys are widely used in ship building.However,the use of Al-Mg alloys is limited by intergranular corrosion(IGC)and environmentally assisted cracking(EAC)due to the precipitation of a variety of Mg-rich phases along the grain boundaries(i.e.sensitization).In this study,a newly developed Al-Mg alloy,AA5083-H128,was utilized to investigate the effects of sensitization,test environments,sample location and sample orientation on Mg-rich precipitates,EAC and corrosion fatigue crack growth behaviors.Proper parameters of reversion heat treatment were explored to improve the Mg-rich phases precipitation and IGC.For AA5083-H128 thick plate,the deformation was not homogenously distributed throughout the plate thickness and this inhomogeneity could lead to the differences in grain size,dislocation density,pre-existing second phases and corrosion resistance from the surface to the mid-section.The top specimens exhibited greater amounts of Mg-rich precipitates,a greater IGC/EAC susceptibility and faster fagitue crack growth(da/dN)than the middle specimens.The peak Mg concentration,thickness and the coverage of Mg-rich phases along the grain boundaries increased with increasing exposure time and elevated temperature.IGC/EAC susceptibility increased with increasing exposure time and elevated temperature.The relationships between mechanical test results and the grain boundary coverage of Mg-rich phases were established.The mechanisms of EAC involved the anodic dissolution of Mg-rich precipitates and hydrogen embrittlement of second phase/matrix interfaces.Large flat planar regions outlined by a sharp ridge-step of ductile shear were observed on the fracture surfaces.Intergranular delamination was present on the fracture surfaces of L-T specimens and increased with an increasing severity of sensitization.The delamination occurred at the high angle grain boundaries between ‘hard' grain and ‘soft' grain,and was caused by the hydrogen embrittlement of Mg-rich precipitates/matrix interfaces.As the test frequency decreased,corrosion fatigue crack growth(da/dN)first increased and then decreased.Waveform also had a great effect on da/dN and da/dN ranks in this order: square > sinusoidal > posive saw > negative saw.Reversion heat treatment could decrease Mg-rich preciptate and improve the IGC susceptibity.Mg-rich preciptates and the mass losses decreased with increasing reversion temperature.No Mg-rich precipitate was present and the mass loss nearly equaled to zero in the specimen after 260?/1h treatment.