(702e) Corrosion-Stress Relaxation Effects On Surface and Tensile Properties of an AZ31 Magnesium Alloy

Magnesium alloys exhibit the attractive combination of low densities and high strength per weight ratios, along with good damping capacity, castability, weldability, and machinability [1-4].  Because of these characteristics, Mg alloys are being increasingly used in electronics, automobile, and aerospace industries [2-5].  Of the various commercial magnesium alloys, those developed from the Al-Zn ternary system (i.e. the as-named AZ alloys) have found the largest number of industrial applications [3].   Although Mg alloys are some of the best candidates of lightweight structural materials with a relatively high strength-to-weight ratio, they have not been used more widely in vehicles because of their low resistance to corrosion [4, 6-9] and creep [10]. While the creep/stress relaxation properties can be improved considerably through alloy chemistry design, cost-effective methods for mitigation of corrosion are still lacking. When combining both corrosion and creep/stress relaxation mechanisms, AZXX Mg alloys are highly susceptible to stress corrosion cracking [11]. This is primarily due to the Mg₁₇Al₁₂secondary phase located at the grain boundaries (intergranular) causing microgalvanic cells and the microstructure interaction (transgranular) with a water environment causing hydrogen embrittlement [11].  This synergetic interaction between mechanical and chemical processes then becomes most important.  Localized corrosion taking place on slip planes due to mechanically enhanced anodic dissolution causes crack initiation and propagation then the plasticization of the metal assisted by the chemical reaction causes additional dislocation flux and enhanced plasticity [10].

     Researchers at the Center for Advanced Vehicular Systems (CAVS) at Mississippi State University have begun to investigate this phenomenon as it relates to tensile and surface properties.  An extruded AZ31 alloy was exposed to two environments, an immersion bath and a cyclic salt spray chamber, while under a constant strain (ε_a≈ 0.0036 mm/mm) over 60 hours.  Tensile and surface properties were examined at 0, 1, 4, 12, 36, and 60 hours to examine the effect that the coupled chemical and mechanical mechanisms had on the specimens and to determine if or when stress corrosion cracking developed.

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