Cyclic Deformation Behavior Of Wrought AZ31 Magnesium Alloy

Magnesium alloys are among the lightest metallic materials for structural applications. Due to high specific strength and rigidness, as well as good machinability and recyclability, magnesium alloy has been known as the 21st century 'green' engineering material. In recent years, with the rapid progress of automotive and electronic industries, a number of magnesium alloy components have been manufactured to replace those made from plastics, aluminium alloy and steel ones. It can be expected that magnesium alloys will become the most important structural materials in commercial metal materials. Wrought magnesium alloys can exhibit the higher strength and better plasticity than cast magnesium alloys, and show the more significant potential in further applications of magnesium based materials. As structural materials in service, magnesium alloys are usually subjected to repeated reverse loading, and therefore the cyclic deformation behavior of these materials needs to be studied in detail for safety reasons. However, many investigations were based on the macroscopic fatigue properties, neglecting the deformation characteristics of magnesium alloys. Due to hcp crystal lattice, twinning and slip are important to deformation of magnesium alloys. And in some cases, twinning may be the main deformation mechanism. As such, it is important to understand the role of twinning in fatigue process.So in this dissertation, the widely used AZ31 wrought magnesium alloy are chosen as model material to understand the role of twinning. In fact, some factos influence the behavior of twinning, such as texture, strain rate, grain size and initial twins. According to the initial texture of the extruded AZ31 plate investigated by X-ray diffraction, samples were cut along different directions to discriminate the role of twinning and slip in fatigue process. Under different frequencies, the degree of twinning is different and its effect on fatigue properties was analyzed. Magnisum alloy can be fined by equal channel angular pressing (ECAP), which suppress the activation of twinning. And the fatigue properties of ultrained AZ31 can be understood. Initial twins can alter the deformation mechanism under fatigue process, and the influence of initial twins on fatigue properties can be acquired by pre-deformation. From this work, the following conclusions are drawn:(1) The low-cycle tension-tension fatigue properties of extruded Mg-3%Al-1%Zn alloy plate have significantly different features in twinning-dominated samples and dislocation-dominated samples. The twinning-dominated samples show more pronounced cyclic hardening and longer fatigue life than those of the slip-dominated samples. The elongated lifetime of the twinning-dominated samples may be due to the roughness-induced crack closure, according to the calculated reverse plastic zone size.(2) A number of uniaxial stress-controlled cyclic loading experiments were conducted on extruded AZ31 magnesium alloy, in order to investigate the influence of tension-compression asymmetry on fatigue properties. The results show that the systeresis loops exhibit asymmetry during initial fatigue cycles, but this asymmetry vanishes after 200 cycles. The peak compressive strain gradually decreases, and at about 200 cycles, it reverses to tensile strain. Fatigue crack initiates at the twin bands in the surface, and the crack propagates along with specific twin boundaries. Due to texture and deformation mechanism, twining and detwinning behaviors are often observed in the fatigue process, which leads to the hysteresis loop asymmetry.(3) Fully reversed strain-controlled tension-compression fatigue tests were carried out at frequencies of 1Hz and 10Hz in ambient air to investigate the frequency effect. When the strain amplitude was lower than 0.2%, the fatigue life exhibited a positive correlation with loading frequency, and the activity of twinning was increased at 10Hz. When the strain amplitude was higher than 0.2%, significant twinning was observed both at both frequencies, and the fatigue life was found to be independent of frequency. The possible reasons for this frequency-related fatigue lifetime may be due to the dependence of twinning upon loading frequency and strain amplitude.(4) The low-cycle tension-compression fatigue tests were performed at ambient temperature on ultrafine grained AZ31 magnesium alloy processed by equal channel angular pressing. All samples exhibited cyclic softening, and the softening effect increased with increasing total strain amplitude, which may be due to the instability of microstructure. Observations by optical microscope revealed that pronounced recrystallization occurred, and the direction of larger axis of recrystallized grains was nearly 45°with respect to the loading axis. A model is proposed to account for the recrystallization, based on the characteristic distribution of defects introduced by equal channel angular pressing. Compared with the conventional extruded AZ31 alloy, the ECAP processed AZ31 alloy has lower hysteresis strain energy and leads to enhanced fatigue lives. And the dependences of the strain fatigue life on plastic strain amplitude and elastic strain amplitude can be described by the Coffin-Manson and Basquin equations, respectively.(5) The fatigue properties of a AZ31 magnesium alloy was investigated in both thermomechanically treated extruded and pre-compression conditions. For pre-compression materials, twinning was the dominant deformation mechanism during the tensile loading, and slips were the dominant during the compressive loading, which induced different mean stresses in extruded and pre-compressin materials. Expermental results show that in high strain amplitude, the fatigue lifetime was controlled by plastic strain amplitude which could lead to accumulated cyclic damage. On the other hand, in low strain amplitude, it is invalid for most of dislocation slips are reversal, and the fatigue lifetime was controlled by mean stress.