Development of a multiscale internal state variable inelasticity-corrosion damage model for magnesium alloys

This dissertation proposes a multiscale Internal State Variable (ISV) inelasticity-corrosion damage model that is motivated by experimental microstructure-property relations of magnesium alloys. The corrosion damage framework was laid out based on observation of different corrosion mechanisms occurred on an extruded AM30 magnesium alloys. The extruded AM30 magnesium alloy was studied under two corrosion environments (cyclical salt spray and immersion) in order to observe the corrosion rates under different exposure environments. The coupons were examined at various times to determine the history effects of three corrosion mechanisms: (1) general corrosion; (2) pitting corrosion in terms of the nucleation rate, growth rate, and coalescence rate; and (3) intergranular corrosion. The multiscale ISV corrosion model was developed by bridging the macroscale corrosion damage to the mesoscale electrochemical kinetics, microscale material features, and nanoscale material activation energies. The corrosion testing results of Mg alloys (pure Mg, Mg-2% Al, and Mg-6% Al) were used to develop, calibrate, and validate the model, and good agreement was found between the model results and the corrosion testing data. Finally, the simultaneous effects of corrosion and cyclic loading were tested but not modelled for the extruded AM30 magnesium alloy by conducting fatigue experiments in a 3.5 wt.% NaCl solution environment. The corrosion fatigue life of the AM30 alloy was significantly reduced due to corrosion pit formation on specimen surface, hydrogen diffused into the material, and the fracture surface dissolved into the solution. The corrosion damage that arose on the fatigue specimens reduced the crack nucleation process and enhanced the crack propagation rate.