Study Of Dynamic Mechanical Behavior Of AZ31B Magnesium Alloy

Magnesium alloys are the most attractive as the’green project materials’developed in future of the21st century due to their many special advantages, such as low density, high specific strength, high specific rigidity, good casting capability, electromagnetic shielding performance and easy recycling. And their structural parts are extensively used in automobile, airplane, computer and communication equipments, etc. As a wrought magnesium alloy, AZ31B alloy shows characteristics of high ductility and strength, it has been widely used in a variety of applications. However, it is inevitable for the magnesium alloy to suffer from dynamic loading in its application. So the researches on the daynamic mechanical behaviors of the wrought magnesium alloy and the relationship between the behaviors and the micro structures are significant for the safe design and the reasonable use of its components.In this paper, dynamic tensile, dynamic impact and low cycle fatigue behaviors of AZ31B magnesium alloy were investigated. The influence of texture on the deformation mechanisms and the dynamic mechanical behabiors was studied. The deformation mechanisms under various loading conditions were discussed. The results showed that the samples (Axial sample) having{0001} parallel to extruding direction (ED) present a typical true stress-true strain curve with concave-down shape under tension at low strain rate, while the samples (Radial sample) having texture with ED distributed along<0001>-<11-20> and <0001>-<10-10> present a linear shaped true stress-true strain curve. However, under dynamic tension, the generation of a large number of {10-12} tension twins results in the horizontal step appeared in true stress-true strain curves, and the width of the step increases with the increment of the number of the tension twins. The step appears before yielding for Axial samples, but after yielding for Radial samples. Because{10-12} tension twinning,{10-11} compressive twinning, basal<a> slip, prismatic<a> slip and pyramidal<c+a> slip have different critical shear stress (CRSS), their contribution to the degree of deformation are very differential. In addition, Schmid factor plays an important role in the activity of various deformation modes, and it is the key factor for the samples with different texture exhibit various mechanical behabiors under dynamic tensile loading. In ultra-rapid tensile loading, the yielding strengh, fracture strength and total strain are increased with the increasing of strain rate. The sample fracture behaves from ductile to brittle as strain rate increases. Under the dynamic impact tests, the yielding strengh, fracture strength, total strain all increase with strain rate increasing for both Axial samples and Radial samples, corresponding to number increment of {10-12} tension twins in deformed microstructure.During cyclic deformation at large strain amplitude under the tensile-tensile loading, the maximum tensile stress decresces with the cyclic number increasing, while the maximum compressive stress increases. Under compressive-compressive loading, the maximum tensile stress and the maximum compressive stress are both increased with the increment of cyclic number. There are three turning points during unloading and reversal tension process on the true stress-true strain curves under the tensile-tensileloading, while there is only one turning point during unloading and reversal compression process on the true stress-true strain curve under the compressive-compressive loading. For both cyclic loading,{10-12} tension twinning was activated in compressive process and then its detwinning happens in reversal tension process during which Bauschinger effect has a greater impact on the detwinning.The strain controlled fatigue experiments showed that under compressive-compressive asymmetric loading, the Axial samples exhibit a continuous cyclic hardening characteristics, and the hardening rate increases with strain amplitude increasing below1.0%strain amplitude; while at1.0%strain amplitude, stress amplitude essentially unchanges with the cyclic number increasing. Cyclic strain hardening in tensile process is main factor leading to the asymmetry of the hysteresis loops, and the asymmetry is enhanced with strain amplitude increasing. The difference between plastic deformation mechanisms in tensile and compressive processes is the reason for the asymmetry of the hysteresis loops. At lower strain amplitude, dislocation slip is the dominant plastic deformation mechanism responsible for cyclic hardening. Twinning-detwinning is the key deformation mechanism which is the main reason for the stress unchanged with cyclic number increasing at1.0%strain amplitude. Under tensile-tensile asymmetric loading, except for0.3%strain amplitude, as-received and annealed Axial sample exhibits a weak cyclic hardening characteristics. Further-extruded sample (RR=6.25, having a weak texture with{0001} parallel to ED) shows a cyclic softening in the early fatigue process at0.3%and0.5%strain amplitude, and the stress amplitude remain unchanged as the cyclic number increasing. As the strain amplitude increasing, the stress amplitude remains unchanged at the whole fatigue process. At the same strain amplitude, the stress amplitude of further-extruded sample is larger than that of as-received and then annealed Axial sample. At0.5%strain amplitude, the hysteresis loops exhibit asymmetry for as-received and then annealed Axial sample, while at0.7%strain amplitude, the hysteresis loops exhibit asymmetric for furher-extruded sample. For both kinds of samples, the asymmetry was enhanced by the increased strain amplitude. Du to the intensive texture, the asymmetry of as-received and then annealed Axial sample is more evident than that of further-extruded sample at the same strain amplitude.Under tensile-compressive symmetric loading, from0.35%to0.75%amplitude, the as-received and then annealed Axial sample shows obvious cyclic hardening; while at0.25%strain amplitude, the stress amplitude increases at the initial stage, then keeps constant.During the cyclic fatigue, the elastic strain of the tensile process is larger than that of compressive process. At the same strain amplitude, the fatigue life is longest under incompressive-compressive asymmetric loading, followed by the tensile-compressive symmetric loading and then the tensile-tensile asymmetric loading.