Study and develop magnesium alloys is one aspect of the outline of China's the Eleventh Five-year Plan. Magnesium (Mg) alloys are very attractive materials in order to achieve high performance and energy saving of machines and structures, because of their advantages such as light weight, high strength-to-weight ratio and high specific stiffness. Therefore, they have recently been increasing interest as tructural materials in many applications. For applications to load-bearing components, it is necessary to evaluate fatigue properties. However, there have been limited studies on the influence of texture and twin on fatigue behavior of magnesium alloy. Besides, the basic fatigue properties of some new developed magnesium alloys are unknown at present.
In the present study, axial plastic strain controlled fatigue tests have been performed using smooth specimens of a conventional extruded AZ31 Mg alloy in laboratory air at ambient temperature with an aim to study the influence of texture and twin on fatigue behavior of magnesium alloy; Total strain controlled fatigue tests performed using extruded Mg-12Gd-3Y-0.5Zr (GW123K) in order to know the basic fatigue behavior of the new developed magnesium alloy. In this dissertation, low cycle fatigue behaviors of single pass equal channel angular pressed (ECAP) magnesium alloy AZ31 and Mg-12Gd-3Y-0.5Zr (GW123K) also investigated. The main results are as follows:
1. Conventional extruded AZ31 with strong fiber texture exhibits large tension-compression yield asymmetry. The ratio of yield stress between compression and tension less than 0.5 which result from different micro-deformation mechanism. It is believed that tension twinning dominates deformation in compression and the stress needed to activate tension twinning less than the stress needed to activate prism slip which dominates deformation in
tension when loading along extruded direction.
2. EBSD in-situ observation shows clearly that twinning-detwinning operate alternately during fatigue.
3. The conventional extruded AZ31 exhibits cyclic hardening asymmetry that the compressive stress amplitude increased slightly while tensile stress amplitude increased significantly with the number of cycles increasing. Besides, pesudo-elastic and pesudo-plastic behavior as well as yield asymmetry were found on the hysteresis loop. These behaviors are result from twinning-detwining during fatigue.
4. The conventional extruded GW123K (Mg-12%Gd-3%Y-0.5%Zr) alloy maintained cyclic stability during most of the cycling times. Fracture morphology observation indicated that cracks initiated at large Gd-rich regions as well as inclusions at surface or subsurface.
5. The yield asymmetry of AZ31 processed by ECAP for single pass decreased significantly which result from the weakened fiber texture and grain size refining.
The fatigue life of ECAPed AZ31 alloy increased slightly. For GW123K alloy, after one passes of ECAP, the ductility of the alloy increased significantly. The ECAPed GW123K alloy maintains cyclic stability during fatigue and the fatigue life of the material keep the same level as the initial state.
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