Author: Jing Su

Publisher:

Published: 2013

Total Pages:

ISBN-13:

"Magnesium alloys have attracted a great deal of attention as structural materials for automotive applications, due to their low density, high specific strength and high specific stiffness. However, poor formability, due to a strong crystallographic texture in Mg alloy sheet, is a major issue. In this thesis, alloys have been designed to form precipitates during hot compression at the industrial hot rolled temperatures, since this might influence dynamic recrystallization and texture. Based on thermodynamic calculations, two micro-alloyed Mg alloys, Mg-0.3Al-0.2Ca (AX0302) and Mg-0.1Al-0.5Ca (AX0105), were designed such that the equilibrium precipitation (Al2Ca and Mg2Ca, respectively) temperatures are in the hot rolling range (300 °C to 400 °C).Both alloys were cast in a copper mould, and the ingots were solution treated at 500 °C for 8 h to dissolve the eutectic precipitates and homogenize the distribution of the alloying elements. In order to investigate precipitation characteristics, isothermal heat treatment (aging) were performed at 250 °C to 400 °C for a time range from 10 min to 32 h. To understand the effect of deformation parameters on hot deformation characteristics, including precipitation, hot compression tests were conducted on homogenized specimens at temperatures from 300 °C to 450 °C with three strain rates of 0.1s-1, 0.01s-1, 0.001s-1 to strains of 10%, 30%, 60% and 90%. As well as precipitation, the deformation substructures and dynamic recrystallization was characterized using the flow curves, SEM with BSE and EDS detectors and optical microscopy was used to characterize microstructure mainly on DRX and twins, X-ray diffraction was used to measure texture and micro-hardness was conducted to study the room temperature properties of aged and hot compressed samples.Strain induced dynamic precipitation occurred in both alloys during hot compression at under all conditions except at temperatures where precipitation was not predicted (by FactSage) to occur. The effect of temperature and strain rate on the size and volume fraction of DRX grains was studied.The results indicated that the size and amount of DRX grains increased as Z parameter decreased. However, compared with two alloys, the effect of Z on DRX is more sensitive to AX0302 than AX0105, which could be related to larger amount of precipitates in AX0105. Texture was not clearly affected by changes in precipitate amounts, but higher temperature slightly decreased the intensity of the basal texture, which may due to the activation of non-basal slip." --