Understanding the microstructure and fatigue behavior of magnesium alloys

This thesis is mainly focused on the study of magnesium alloy’s fatigue behavior. The experimental material is obtained from racing wheels. Racing wheels manufactured from metal are cast, forged, machined, or manufactured from a combination of these methods. For many years, aluminum was chosen as the material of racing wheels because it is relatively convenient to manufacture and more economical. However aluminum wheels are relatively heavy because of the density to strength ratio of the element. Therefore it is necessary to choose a lighter and stronger material for solving this problem. Introducing magnesium alloy would be a recommended way as magnesium alloy wheels can be used on racing vehicles. However, magnesium alloy wheels have some drawbacks. First of all, magnesium is relatively expensive. In addition, magnesium is a quite brittle metal due to the fact that it does not have five different slip systems available at room temperature and it does not satisfy Von Mises criterion for full plasticity. There would be considerable risks involved with failure if it is not being designed and manufactured to provide adequate strength. This study will cover the microstructure of magnesium alloys and the fatigue properties of magnesium. The specimens are mainly provided by Dymag who manufacture magnesium racing wheels. The microstructure has been examined by optical microscopy. Electron imaging methods such as Scanning Electron Microscope (SEM) and Electron Backscattered Diffraction (EBSD) have been used to characterize the microstructure. Fatigue testing has been performed by a rotating-bending test machine (or Gill machine). The fatigue fracture was also been examined by electron microscopy to determine the fatigue mechanism. The results show that fatigue behavior differs through the whole wheel cross section. After microstructures of different locations of the specimen have been examined by microscopy, it is found that these differences were caused by manufacture method. Therefore, future works will mainly focus on techniques to improve the fatigue behavior. In order to further understand the mechanical performance of the magnesium alloys studied here, the properties of the different phases in the microstructure have been determined by nanoindentation testing. This has allowed the Young’s modulus and hardness of the α-Mg and β-Mg17Al12 phases to be determined. The results are discussed in terms of the implications for the fatigue performance of the material.