| Magnesium due to its ultra-light structure and high specific strength received great attention in various industries such as aerospace, automobile and telecommunication. However, its high temperature time dependent deformation (known as creep) needs to be improved to be able to use in a practical manner. To improve the creep resistance of Mg, researchers tried to add some alloying elements such as Al, Zn, Mn, Bi, Si, Ca etc. They result in some intermetallic phases (precipitates) in the matrix (e.g. Mg17Al12, Mg2Si, Mg3Sb2, and Mg3Bi 2) that can act as obstacles to grain boundary migration and dislocation glide, and improve the creep resistance of the alloy. The basis of this work is to study the creep behavior of the AZ series of Mg alloys, AZ31 and AZ91. The AZ31 includes 3wt.% Al and 1wt.% Zn and the AZ91 consists of 9wt.% Al and 1wt.%Zn. The experimental part of this thesis focuses on the AZ31 and a comprehensive study on the creep behavior of AZ91 is provided in the literature review.;Investigation of the microstructure development of high temperature deformed AZ31, the fracture analysis and determination of the dominant creep mechanism(s) are objectives of this study. The creep tests were conducted in a wide range of stresses (1 ∼ 70 MPa) and temperatures (220 ∼ 350Z ) to characterize the underlying deformation mechanism(s), dislocation density evolution, fracture analysis, possible dynamic recrystallization (DRX) and dynamic recovery (DRV). The results show that the DRV, grain growth, decrease of the dislocation density, and a combination of ductile and brittle fractures are characteristics of the low stress creep regime. Further analysis revealed that grain boundary sliding (GBS) is the dominant creep mechanism in this region. At intermediate stresses, DRX along with slightly increased dislocation density were observed. Dislocation glide creep (DGC) is noted to be the underlying creep mechanism in this region. At elevated stresses, where dislocation climb creep (DCC) is applicable, the microstructure contains a very large amount of dynamically recrystallized grains along with increases in the dislocation density.;In this study, two different types of creep tests were conducted: i) steady state (St-St) creep tests, in which the test continued to reach to the St-St region and ii) rupture tests that samples passed the St-St stage and reached to the fracture point. The former one is applicable to find the possibility of DRX, dislocation density variation and dominant creep mechanism whereas the latter is important to find the total life time of the sample and the fracture mechanisms. Because Ti with hexagonal close pack (HCP) crystal structure has similar (to Mg) limitations for deformation at low temperatures, in addition to the theoretical and experimental study of the creep mechanisms of the AZ series Mg alloy, the thesis presents a comprehensive study on the severe plastic deformation (SPD) method of equal channel angular process (ECAP) of the commercially pure Titanium (CP-Ti). It is presented in the appendix A and focuses on finding the best experimental parameters for CP-Ti to attain the most homogeneous microstructure and mechanical properties. |
