Microstructural influences on macro properties of hpdc magnesium alloy AZ91

The thesis aims to investigate the influence of microstructural features on the mechanical properties of cold chamber high-pressure die cast AZ91 magnesium alloy. The studies were carried out on various thick and heat-treated pressure die castings due to their wide spread industrial importance. The investigations revealed that the microstructure of AZ91 Mg alloy has complex multi-length scale features such as pore bands, inclusions, pores (shrinkage and gas) and phases (a and b). These different length scale features govern the deformation and fracture processes, particularly multiple fracture micro-mechanisms contribute to the overall fracture. The results demonstrated that the fracture path and mechanical properties were greatly influenced by the spatial correlations and microstructural gradients which in turn depended on spatial arrangement, size distribution and shape of every microstructural feature. To throw light on them and to study the microstructures influence on mechanical properties an automatic image processing algorithm has been developed. The effect of casting thickness and heat treatment of the material in micro and macro levels are explained by this quantification results. The results also demonstrated that the ultimate tensile strength, yield strength and fracture strain are mainly influenced by the total amount, size distribution and spatial arrangement of pores and phases. Fracture surface analysis and in-situ tensile tests confirmed among the shrinkage pore and b-phase, the former is more predominant in affecting the fracture process and mechanical properties. In addition it is concluded from these analyses that the fracture mode is intergranular failure. In order to explain the behavior of the material, a continuum mechanical model was used with the combination of linear elastic fracture mechanics. Finite element (FE) simulations of real microstructures were exploited to explain the influence of pores on the macro scale properties. Further, the local deformation behavior of the material was explained by FE simulations of real and artificial microstructures.