Study On The Microstructure Evolution And Cracking Mechanism Of AZ31 Magnesium Alloy Under Cyclic Loading

As the lightest engineering metal structure materials,magnesium alloys have the advantages of low density,outstanding electrical and thermal conductivity and good damping properties,as a result of have a broad application prospect in the fields of transportation,aerospace and electronics industry.Magnesium alloys are regarded as the green material for sustainable development of resources and environment in the 21 st century.However,compared with steels,aluminum alloys and other materials,magnesium alloys exhibit serious tension-compression asymmetry at room temperature due to the hexagonal close-packed structure,which seriously affects the service life of magnesium alloy under complex alternating loads.In essence,the phenomenon of tension-compression asymmetry in magnesium alloy materials is due to the different plastic deformation mechanisms(sliping or twinning)that are activated when the grains are subjected to stresses in different directions.Therefore,a comprehensive and profound understanding of the evolution of the microstructure and the potential microscopic mechanism of the formation and propagation of microcracks for the magnesium alloys under cyclic loading in different directions,is of great theoretical and engineering significance for the design,development and improvement of a new generation of magnesium alloy materials with excellent properties.In this dissertation,the microstructural evolution,the selection of twin variant,and the formation characteristics of interfacial microcracks of AZ31 magnesium alloy are analyzed and discussed when cyclic loading is applied in different directions by using the characterization methods including optical microscopy,scanning electron microscopy and electron backscatter diffraction as well as the Schmid factor(SF)and the geometrical compatibility parameter.The main conclusions are shown as following:(1)The {10 (?) 2} twinning-detwinning phenomenon occurs when a cyclic loading is applied along the Rolling direction(RD)of the rolled AZ31 magnesium alloy sheet.During the deformation process,most of the grains activate a single {10(?)2} twin variant or a {10(?)2}twin variant pair inside the grain.A large number of basal dislocation slip traces generate inside the {10(?) 2} twins,leading to a significant extrusion characteriation.Fatigue microcracks nucleate and propagate along the {10(?)2} twin interface.The local stress concentration induced by the interaction between the dislocations inside the twin and the twin interface is considered to be the main reason for the formation and propagation of microcracks at the twin interface.(2)When the cyclic loading is applied along the Normal direction(ND)of the rolled AZ31 magnesium alloy sheet,multiple {10(?)2} twin variants are activated within the same grain.Compared with the case of cyclic loading applying along RD,the interaction between different twin variants prevents the reverse migration of {10(?)2} twin interfaces,which leads to more difficulty in the occurrence of the detwinning process when cyclic stress is applied along ND.In addition,under this loading condition,a type of {10(?)2}-{10(?)2} secondary twin that does not satisfy Smit’s law generated in the grains.The results of the analysis of the Schmid factor and the geometrical compatibility parameter indicate that the formation of this{10 (?) 2}-{10 (?) 2} secondary twin is related to the local high stress state caused by the dislocation-twin interface interaction.(3)When the strain-controlled tensile-tensile fatigue loading is applied to the AZ31 magnesium alloy sheet along the RD direction,the plastic deformation is mainly carried by dislocation slip and the hysteresis curve is symmetrical in character.In this case,the fatigue cracks are mostly exhibited long or short cracks which perpendicular to the loading direction.The cracks mainly generate at the interface between the hard and soft oriented grains,and then propagate along the grains or the slip traces inside the grains.When the strain-controlled tensile-tensile fatigue loading is applied along the ND direction,the deformation twin is the main plastic deformation mode,and the hysteresis curve also has obvious symmetry.In this case,most of the fatigue cracks are short flat cracks which are confined inside the grains,mainly forming and propagating along the twinning boundary.