Stress corrosion cracking of aluminum alloys

The stress corrosion behaviour of precipitation hardened Al-9Mg, Al-22Zn and Al-3Mg-6Zn alloys has been studied in aqueous environments and ethanol. The stress corrosion susceptibility defined as the reciprocal of failure time has been investigated as a function of alloy-environment system, isothermal aging treatment, microstructure, applied tensile stress, and temperature using smooth and notched specimens. Constant load tests, load-relaxation tests and tensile tests in different environments have been used to evaluate the stress corrosion characteristics of aluminum alloys. A limited study of Mg-9Al has also been carried out in aqueous environments. The process of stress corrosion generally consisted of three parts: 1) A slow initiation stage 2) a rapid propagation stage 3) mechanical fracture due to tensile overload. With a few exceptions, the initiation time was greater than the propagation time. The crack initiation and propagation rates were stress and thermally activated and could be expressed by a general equation of the form Rate = [formula omitted] where α is the applied tensile stress, Q is the apparent activation energy of the rate controlling process and A(0) and n are constants for a given alloy-environment system. The apparent activation energy of the rate controlling process was different in the two environments. It also changed between initiation and propagation stages. The aluminum alloys when ranked in order of increasing susceptibility were: 1) Al-3Mg-6Zn, 2) Al-9Mg, 3) Al-22Zn. The alloys which were given heat treatments correlating to the presence of coherent or partially coherent phases, were found to be most prone to stress corrosion cracking. The environments placed in an order of increasing aggressiveness were dessicant-dried air, double distilled water, ethanol, ambient air, deionized water and NaCl/K₂CrO₄solution. The ductility of susceptible aluminum alloys was found to be significantly decreased by NaCl/K₂CrO₄and deionized water at low strain rates and enhanced by dessicant-dried air. Fractography showed the cracking to be intergranular in aluminum alloys and transgranular in the Mg-Al alloy. The stress corrosion surface was characterised by a rough or corroded appearance while the mechanically fractured surface exhibited slip steps and dimples caused by void formation. The hydrogen mechanism of cracking was examined in light of hydrogen charging experiments and other evidence and was found to be unsatisfactory. Models involving either dissolution or deformation alone were also inadequate in explaining the present results. Therefore a new model was postulated which involves the generation of a continuous path of chemical heterogeneity by shearing and link up of coherent precipitates followed by their dissolution. The rate controlling step in the deformation process is believed to change during the transition from initiation to propagation. The postulated model is consistent with the present results but its further development must await better knowledge of the kinetics of dissolution of precipitates and that of deformation processes at the crack tip.Applied Science, Faculty ofMaterials Engineering, Department ofGraduat