Study On The Stress Corrosion Cracking Properties Of Cast AZ91 Magnesium Alloy

As a light-weight engineering structural material,magnesium alloys have been widely used in automotive,3C and other industrial fields owing to their high specific strength and specific stiffness,excellent damping property and electromagnetic shielding characteristics,good machinability and easy to be recycled.However,magnesium alloys have poor corrosion resistance,especially the magnesium alloy structure components are employed in the service process combined the external load and service environment,which could induce stress corrosion cracking?SCC?and reduce the load-bearing capacity of magnesium alloy components.This not only causes security risks in use but also limits the scope of application of magnesium alloys.Thus,it is of great significance to expand the application scope of magnesium alloys to reveal the stress corrosion cracking mechanism and improve the stress corrosion cracking resistance of magnesium alloys.In the paper,the cast AZ91 magnesium alloy was used as the research object.By studying the pre-exposure embrittlement behavior of AZ91 magnesium alloy,the mechanism of hydrogen embrittlement was verified.The microstructures of AZ91 magnesium alloys were optimized by heat treatments?T4,T6?and rare earth?Er,Ce?alloying.The behavior of the stress corrosion cracking of AZ91 was systematic studied in humid atmosphere.The microstructures and phase compositions,the characteristics of static corrosion,the stress corrosion cracking behavior and fracture morphologies of SCC were investigated by scanning electron microscope?SEM?,X-ray diffraction?XRD?,X-ray photoelectron spectroscope?XPS?,weight loss and potentiodynamic polarization analysis,slow strain rate tensile?SSRT?test and so on.The effects of T4,T6 heat treatments and rare earth Er and Ce on the SCC behavior of AZ91 magnesium alloy and its mechanism were discussed in detail.The results show that the hydrogen atoms formed in hydrogen evolution would dissolve into the Mg matrix and lead to hydrogen embrittlement during the slow strain rate tensile test.At the same time,the formation of stress concentration near the pitting on the surface of the alloy also reduced the strength and plasticity.After a hydrogen charging step,the strength and ductility values of the pre-exposure sample did not reach that of the unexposed sample,which was ascribed to the residual hydrogen atoms and irreversible anodic damage.Therefore,the pre-exposure embrittlement behavior of AZ91 magnesium alloy should be attributed to the combined effects of hydrogen embrittlement and stress concentration.AZ91 magnesium alloy occurred stress corrosion cracking in humid atmosphere under the coupling effects of anodic dissolution and hydrogen induced cracking.However,the stress corrosion cracking resistance of AZ91 magnesium alloy was significantly improved after T4and T6 treatments.The coarse?-Mg₁₇Al₁₂ phase dissolved after T4 treatment,and then the decomposed Mg and Al atoms diffused to the?-Mg matrix to form a single phase supersaturate solid solution.The corrosion resistance of Mg matrix was improved by increasing the content of Al in the?-Mg matrix and reducing the number of the cathode phase.While the?-Mg₁₇Al₁₂phase precipitated in the form of flake and granular along the grain boundary and in grain after T6 treatment,respectively,which could reduce the corrosion rate of AZ91 magnesium alloy through forming a continuous corrosion barrier to impede the expansion of corrosion.In addition,T4 and T6 treatments could block the cathodic hydrogen evolution process to some extent,and decrease the number of hydrogen atoms.Also the dissolution or even precipitation of?-Mg₁₇Al₁₂ phase after heat treatment could reduce the segregation of hydrogen.The stress corrosion cracking susceptibility of AZ91 was reduced obviously by T4 and T6 treatments through reducing the effect of the corrosion of Mg matrix and weakening the hydrogen embrittlement.The results indicated that the coarse and bone like?-Mg₁₇Al₁₂ phase was transformed from discontinuous network into small island distribution and dispersed homogeneously after the addition of Er and Ce,and the formation of granular Al₃Er and needle like Al₁₁Ce₃ rare earth intermetallic compounds reduced the volume fraction of?-Mg₁₇Al₁₂ phase.Rare earth Er and Ce could improve the corrosion resistance of AZ91 magnesium alloy due to the decrease of the volume fraction of?-Mg₁₇Al₁₂ phase and the reduction of the surface area ratio of the cathode to anode,which could inhibit the acceleration of the galvanic corrosion of?-Mg₁₇Al₁₂phase.Moreover,the presence of rare earth oxide enhanced the densification and stability of the corrosion product film,which could isolate the corrosive medium effectively.The addition of Er and Ce greatly improved the SCC resistance of AZ91 magnesium alloy in humid atmosphere,because the local corrosion resistance was improved,so the processes of initiation and expansion of cracks were suppressed and the influence of hydrogen embrittlement on SCC resistance was weakened.The fractography of SCC was changed from brittle cleavage fracture into quasi-cleavage fracture and the local corrosion at the edge of fracture surfaces was greatly alleviated.