Stress Corrosion Cracking Behavior And Improvement Of Cast AZ91 Magnesium Alloy

Magnesium alloy has the advantages of good damping performance,low density,high recovery,high specific strength and good ductility,which has broad application prospects in the automotive industry,3C and other fields.It enjoys the reputation of“green metal structural materials in the 21st century”.However,due to its low potential and relatively active chemical properties,magnesium alloys exhibit strong stress corrosion susceptibility in corrosive environments,resulting in stress corrosion cracking under relatively low stress.This"bottleneck"severely limits the application of magnesium alloys in various fields.What is even more terrifying is that under the dual effects of the environment and low external stress,the components are suddenly brittle,which is likely to cause serious safety hazards and huge losses in the national economy.Therefore,revealing the stress corrosion cracking mechanism of magnesium alloys and exploring ways to improve the stress corrosion resistance of magnesium alloys are of great significance for the popularization of magnesium alloys.In this paper,the AZ91Gd series magnesium alloy modified with different content of rare earth Gd was prepared by gravity casting method.The stress corrosion cracking performance of rare earth Gd element on AZ91 magnesium alloy in humid atmosphere was systematically studied.By optical microscopy?OM?,scanning electron microscopy?SEM?,total immersion corrosion weight loss method,X-ray diffraction?XRD?,electrochemical polarization curve,electrochemical impedance,slow strain rate tensile test?SSRT?,fracture morphology characterization and fracture cross-section crack propagation methods reveal the mechanism and law of the influence of rare earth Gd on the stress corrosion cracking of as-cast AZ91 magnesium alloy in humid atmosphere.Meanwhile,two kinds of magnesium alloys T4,and T6 heat treatment methods,by electrolysis of an aqueous solution of hydrogen charging method further analyzed the effect of heat treatment on the cracking behavior of cast AZ91 magnesium alloy hydrogen induced in detail,revealed the effect and mechanism of heat treatment on the hydrogen induced cracking behavior of AZ91 magnesium alloy.The results show that the cast AZ91 magnesium alloy exhibits high stress corrosion cracking sensitivity in a humid atmosphere,the fracture morphology is cleavage morphology,the brittle river pattern is obvious,and the crack propagation is intergranular and intergranular.The stress corrosion sensitivity in the humid atmosphere containing Cl^-1is much higher than that in the non-Cl^-1humid atmosphere,and the fracture edge is strongly corroded.After the addition of the rare earth Gd element,the original coarse skeletal?-Mg₁₇Al₁₂2 phase in the matrix changes to an island shape dispersed at the grain boundary,and the volume fraction is significantly reduced.At the same time,white agglomerated Al₂Gd and Al-Mn-Gd intermetallic compounds were formed in the matrix.By inhibiting the action of the cathode phase?-Mg₁₇Al₁₂,the large anode and small anode phenomenon are changed,and the rare earth oxide is embedded in the surface film,which improves the compactness and stability of the surface film.The combination of the two acts to improve the corrosion resistance of the AZ91 magnesium alloy.The low stress corrosion cracking sensitivity of the modified AZ91 magnesium alloy will be attributed to the increase of corrosion resistance,which will greatly reduce the corrosion degree of the magnesium alloy surface and reduce the number of crack sources.On the other hand,it is attributed to the fact that the rare earth Gd element can suppress the cathodic hydrogen evolution process to a certain extent,so that the hydrogen induced cracking effect is weakened.The fracture morphology of the modified AZ91 magnesium alloy also changed from brittle cleavage fracture before modification to quasi-cleavage fracture.When the content of rare earth Gd element is 1.0%,the modified AZ91 magnesium alloy exhibits better resistance to stress corrosion cracking.The stress corrosion cracking resistance of magnesium alloy does not increase with the increase of rare earth content,which will be attributed to the fact that too much rare earth will not only form a large amount of rare earth intermetallic compounds,but also the content of Al dissolved in magnesium matrix.On the other hand,the size of the cathode phase?-Mg₁₇Al₁₂2 in the matrix is not further inhibited,but is increased,and the two together act to increase the corrosion rate of the magnesium alloy matrix.T4 and T6 treatment can improve the hydrogen embrittlement resistance of magnesium alloy.T4 treatment makes?-Mg₁₇Al₁₂2 solid solution into the magnesium matrix,resulting in a decrease in the number of hydrogen traps,while T6 treatment of?-Mg₁₇Al₁₂2 phase with grain boundary layer and crystal internal granular precipitation.Therefore,the improvement of the hydrogen embrittlement resistance is attributed to the dissolution or uniform precipitation of the?phase after heat treatment,which reduces the segregation of hydrogen,and the solid solution strengthening and precipitation strengthening.The hydrogen-filled slow-stretch fractures show a tendency to change from quasi-cleavage to cleavage,and the crack propagation mode is changed from intergranular to intergranular and transgranular.