Study On The Dynamic Mechanical Behavior Of AZ31Magnesium Alloy Under The Multi-factor Coupling Effects

Stress corrosion and corrosion fatigue are two important indexes forevaluating the dynamic mechanical properties of materials, which are also twoimportant engineering failure forms for materials. Magnesium alloys areprospective green engineering structure materials of the21stcentury after steeland aluminum alloys. To truly realize the application in “3C, automotive,aerospace”, the study on their dynamic mechanical properties is very necessary.However, although magnesium alloys have series of advantages, such as lowdensity, high specific strength and stiffness, easiness of machining and recycling,and so on, they have poor corrosion resistance. In process of actual use, themagnesium alloy components not only need to bear various dynamic loads, butalso affected by many service environments. Simulating the dynamicmechanical performance of AZ31magnesium alloy in different media is of greatsignificance to expand the application scope of magnesium alloys.In the paper, the AZ31wrought magnesium alloy was used as the researchobject, four common media were used for the testing environments: air,3.5wt.%NaCl solution,3.5wt.%Na2SO4solution and gear oil. The stress corrosionand corrosion fatigue of the AZ31magnesium alloy were systematically studied.Moreover, the micro arc oxidation (MAO) and the organic coating treatmentswere adopted, and their dynamic protection mechanisms in differentenvironments were investigated. The corrosion features and fracturemorphologies of the AZ31magnesium alloy substrate, the MAO specimens andthe epoxy coated specimens after dynamic mechanical tests in different environments were observed by OM, SEM, EDS, TEM, XRD and polarizationmeasurements and so on. The influence of the MAO and epoxy coatings on thedynamic mechanical properties (including stress corrosion sensitivity, corrosionfatigue limit, crack initiation and propagation mechanisms) of the AZ31magnesium alloy in different environments were discussed in detail. Thecorrosion mechanisms between the AZ31magnesium alloy substrate, the MAOspecimens, the epoxy coated specimens and the surrounding environments werealso expounded. In this paper, the fatigue limit is named as the maximumapplied fatigue loading under which the fatigue lives of the specimen reached1.0E+7cycles.The results show that the stress corrosion and corrosion fatigue propertiesof the AZ31magnesium alloy are degraded in the order of air,(gear oil), Na2SO4and NaCl solutions. This is closely related with the ion selective corrosion ofmagnesium alloy. After the MAO treatment, the stress corrosion properties in thecorrosive environments are improved a little, while the stress corrosion crackingsensitivity in air is deteriorated, the corrosion fatigue properties in allenvironments are also reduced. However, the corrosion resistance of thespecimen after the MAO treatment is enhanced. It demonstrates that thedynamic mechanical properties of the AZ31magnesium alloy are not alwayslinear correlated with its electrochemical performance. By contrast, thecorrosion fatigue properties of the epoxy coated specimens are increaseddramatically, which are even higher than those of the substrate in air.From the perspective of the corrosion, the corrosion features have greatinfluence on the crack initiation, which is closely related to the specimen surfacecharacteritics, the surrounding environments, and the stress state on thespecimen.(1) The corrosion of the AZ31magnesium alloy in air and gear oil ishardly seen. In NaCl solution, the pit corrosion is observed, which has norelation with the stress state on the specimen. But in Na2SO4solution, thecorrosion features changed with the different loading stress on the specimen: in the slow tensile test, there are strip corrosion traces on the specimen surface,which is parallel to the tensile stress loading direction; while under the highfrequency fatigue cyclic loading, the corrosion feature is not obvious. Corrosioncan accelerate the crack initiation and propagation process. This is the mainreason for the reduction of the dynamic mechanical property of the material.(2)After the MAO process, in Na2SO4solution, some bend and small “cuts” areobserved on the fatigue specimen surface. The inherent defects, micro-arcoxidation holes, are easy to be torn apart under certain stress loading. However,the fatigue loading is alternating back and forth, not like the slow tensile stress,which is one-way and increases monotonically. The specimen cannot fracturedirectly by the MAO holes, which need to experience a certain damageaccumulation process.(3) After epoxy coating process, no obvious destructivecorrosion defects are observed, there are only some adsorbates on the coatedspecimen surface in Na2SO4solution and gear oil environments. But in NaClsolution, the corrosion features are changed as a large number of circle shapedfeatures distributing like crystalline grain. These circles are composed of layersof small pinholes around, which are very tiny and distributed uniformly, andthey extend in the depth direction of the coating and cannot aggregate. Thisimplies that it belongs to the uniform corrosion and is far less harmful than thepit corrosion. It is the main reason for the fatigue property improvement of theepoxy coated specimen.From the perspective of the fracture morphology,(1) the stress corrosionfracture surface of the AZ31magnesium alloy is uneven. There are several crackinitiation sources and a small amount of dimples in air. While for the fatiguespecimen, the fracture surface is neat, containing the fatigue crack initiation,propagation and transient breaking three typical regions. And there is only onefatigue crack initiation source, in which the fatigue striations phenomena areobserved. No dimples appeared. The TEM analysis shows that dislocation andslip are two main deformation forms for magnesium alloys under the effect of dynamic loads. The specimens after fatigue test have high dislocation densitiesthan that before test. The corrosive environments don’t change the deformationmode of magnesium alloys, which at the crack tip accelerates its propagationprocess till the fracture occure. Except the stress corrosion mix fracture in air,the stress corrosion fractures in NaCl and Na2SO4solutions and the corrosionfatigue fractures in all environments are cleavage fractures. This was due to thatthe corrosive environments and the high frequency fatigue cyclic loading stresscan accelerate the crack initiation, propagation and failure process of thespecimen, make the material embrittle, and the dimples have no time to appear.(2) The micro-arc oxidation holes are important channels for the penetration ofthe surrounding corrosive environments into the substrate, on which the stressconcentration was easy to product. Finally, the stress corrosion crack andcorrosion fatigue crack initiation happened.(3) Although the epoxy coatingcannot change the cleavage fracture mechanism, the fatigue crack is alwaysinitiated from the pores in the epoxy coating layer firstly. During the process ofcorrosion fatigue test, the strain matching ability between the epoxy coating andthe substrate is the key to determine the fatigue crack initiation position.