Gaosong Yi

Sensitization Prediction and Validation for Al 5xxx Alloys Exposed to Long-Term Cyclical and Constant Heating at Low Temperatures

A number of Al 5xxx alloys were aged at constant temperatures (40, 50, 60, 70°C) and cyclic temperatures (40 to 45, 30 to 70, 50 to 70°C) for as long as 57.5 months. ASTM G67 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. An intergranular β-phase growth model that considers nucleation, growth, and coarsening processes was developed to predict the formation and continuity of the β phase that leads to sensitization. Results obtained from scanning transmission electron microscopy were used to verify the model. 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.

Characterization of Al-Mg Alloy Aged at Low Temperatures

Long-term aged [343 K (70 °C) for 30 months and natural exposure for over 10 years] Al 5456 H116 samples were characterized using electron backscatter diffraction (EBSD), scanning transmission electron microscopy (STEM), state-of-the-art energy-dispersive X-ray spectroscopy (EDS) systems, and small-angle neutron scattering (SANS). ASTM G-67 mass loss tests of the sensitized Al 5456 alloy samples were conducted. Intragranular Mg-rich precipitates, such as Guinier–Preston (GP) zones, were confirmed in Al 5456 H116 aged at 343 K (70 °C) for 30 months, and the volume of these precipitates is 1.39 pct. β′ phase is identified at the grain boundary of a navy ship sample, while high-resolution STEM results reveal no intragranular precipitates. Intergranular corrosion (IGC) of Al 5456 was found to be related to the continuity of intergranular precipitates.

Precipitates in Long Term Aging Al 5083 Alloy

Al 5083 alloy aged at 343 K (70 °C) for 3–30 months was evaluated by high resolution transmission electron microscopy (HRTEM) results show that both thickness and continuity of β phase formed at grain boundaries increase with time. GP zones are observed in Al 5083 aged at 343 K for 3 months for the first time. TEM results reveal that GP zones can form homogeneously with the help of vacancies as well as heterogeneously at dislocations near grain boundaries, and the mechanism are explained in detail. Mg2Si precipitate was found to form at the grain boundary and energy-dispersive X-ray spectroscopy (EDS) results reveal that a Mg, Si (atomic ratio 1:1) rich phase nucleates at the interface of Mg2Si-matrix. β’ phase(Al3Mg2, hexagonal) is observed to form at grain boundaries of Al 5083 alloy aged at 343 K for 30 months. β phase was found at the grain boundary of Al 5083 aged at 343 K for 12 months using EDS.