Influence Of Different Deformation Modes On The Microstructure Characteristic And Mechanical Properties Of AZ31 Magnesium Alloy

Due to the extensive application of magnesium alloy, it has widely used in different environment. And the magnesium alloy needs to bear the different types of load. Such as the static load, the dynamic load and the cyclic load. In this paper, by observing and studying the microstructure, mechanical properties and fracture morphology after dynamic plastic deformation(DPD), fatigue deformation and static compression deformation of magnesium alloy, we looking for the influence law of different deformation modes on material. The main conclusions are as follows:(1) The dynamic plastic deformation makes the AZ31 magnesium alloy produce a large amount of {10-12} twin lamellas, with the increase of deformation, the number of twin boundaries increases firstly and then decreases, and twins grow up to merge, when the deformation reaches 5%, the twin is saturated basically. Samples after DPD predeformation occur the detwinning when tensile along the RD direction(0°), so the yield strength decreases significantly. With the increase of deformation, the yield strength has a certain improvement due to the texture hardening, but the tensile strength and elongation have little change. When the tension is along the TD direction(90°), with the increase of deformation, the yield strength and tensile strength have a certain improvement due to the twin boundaries hinder dislocations slip. For the sample of 5% DPD deformation, the strength and plasticity of material are both improved due to a large number of twin lamella organizations with smaller spacing. The fracture of 0° sample after DPD has more and deep dimples, and the plasticity is good. With the increase of deformation, the dimples reduce and the cleavage steps appear, so the plasticity decreases. The fracture of 90° sample mainly presents the cleavage steps and less plasticity. With the increase of deformation, the cleavage steps widen. When the deformation reaches 5%, the fracture have more dimples and high plasticity, when the deformation continues to increase, the dimples reduce and the plasticity decreases.(2) The static compression deformation makes the AZ31 magnesium alloy produce a large amount of {10-12} twins and a small number of {10-11} twins. With the increase of deformation, the number of {10-12} twin boundaries decreases, and twins grow up to merge until they are saturated. And very little {10-11}-{10-12} second twins are produced. The stress- strain curve of compression deformation is concave, with the increase of deformation, the work hardening rate decreases firstly, then increases and decreases finally. This is caused by different deformation mechanisms. The shear fracture of the material occurred along 45° direction with the compressed axis after the static compression. The fracture has a large amount of cleavage steps, river patterns and a small amount of secondary cracks. It also can be observed the small flat facet is directional at high magnification, so the fracture type is brittle fracture.(3) After the high cycle fatigue deformation, due to the incomplete detwinning of the different orientations samples, the {10-12} tensile twins won't disappear completely, and will produce residual twin boundaries. With the increase of fatigue cycle number, the detwinning is suppressed, resulting in the increase of residual twins. The increase of residual twins will hinder dislocations slip, causing the cyclic hardening. Therefore the sample with more cycles has the high strength and low plasticity. The fatigue softening which is caused by material recovery after fatigue deformation, results in the decrease of strength and the increase of plasticity. The fatigue source area of TD sample is relatively rough, and it has tearing ridges. It can be seen that consisted of grounds and lags which are formed of twins at high magnification. The fatigue crack growth area of RD sample is small, and it has many dimples at high magnification. A typical high cycle fatigue fracture morphology includes crack nucleation zone, crack growth zone and rapid crack zone three parts. The surface of rapid crack zone has dimples, and the surface of crack nucleation zone has secondary cracks. The cracks nucleate in the boundaries of twin layers.