| Description |
Al 5xxx alloys are widely used in marine and offshore structures for their excellent balance of weight, strength, ductility, weldability, and corrosion resistance. However, they can become sensitized when exposed to elevated temperature for a long time, which is caused by the precipitation of intergranular β phase. β phase is anodic to Al matrix and can be selectively dissolved by corrosive solutions, such as sea water, and cause intergranular corrosion and stress corrosion cracking. In the present study, Al 5xxx alloys (Al 5083, Al 5456, Al 5050, Al 5052, and Al 5154) were aged at constant temperatures (40, 50, 60, 70℃) and cyclic temperatures (40-45, 30-70, 50-70℃) for as long as 57.5 months. The microstructure was investigated using electron backscatter diffraction (EBSD), scanning transmission electron microscopy (STEM), energy-dispersive X-ray spectroscopy (EDS), atom probe tomography (APT), and small angle neutron scattering (SANS). Experimental results reveal that a phase transformation process from GP zones to β’/β phases occurs for precipitates formed in both Al 5083 H131 and H116 aged at 70℃. The size of intergranular and intragranular β phase increase with aging time. In addition, a model based on local equilibrium of chemical potential and multiclass precipitates number evolution was adopted to predict the multiphase precipitation process in the Al-Mg binary system. The overall trend of precipitate radius and number density predicted by the model match well with experimental results. Moreover, the particle size distribution (PSD) of different Mg-rich precipitates demonstrates their different stages of precipitation. A classical nucleation-growth-coarsening theory for the description of intergranular precipitation is formulated, which adopts a collector plate mechanism, an equivalent average Mg concentration at the grain boundary, and a new coarsening expression. Three coarsening mechanisms, the modified LSW (Lifschitz-Slyozov-Wagner), the Kirchner mechanism, and a combination of these two mechanisms, are compared. Modeling results reveal that the Kirchner mechanism will break down when continuity ( ) is close to 1. According to the new model, the coarsening mechanism still accounts for a small fraction (only 10%) in the final growth rate after aging at 70℃ for 40 months, which is confirmed by the precipitate size distribution data. Thickness and continuity results predicted by the new model agree well with experimental results obtained from scanning transmission electron microscopy (STEM) images of Al 5083 H131 alloys aged at 70℃ for different times. In addition, the new model is also applied to a high-temperature 180℃ situation, where precipitate coarsening is observed. ASTM G-67 Nitric Acid Mass Loss Test (NAMLT) results of Al 5050 H32, 5052 H32, 5154 H32, 5083 H116, 5083 H131, and 5456 H116 alloys were obtained to evaluate the Degree of Sensitization (DoS). A linear relationship between continuity and mass loss was adopted to predict the mass loss of Al 5083 H116 and H131 aged at constant and cyclical temperature, and the modeling results agree well with experimental mass loss data. |
