Deformation Characteristics Of Mg-Al-Zn Magnesium Alloy Sheets In Moderate Temperatures

Due to the low density, excellent specific strength, specific rigidity and recycling ability, magnesium alloy is considerd the research focal point in the 21st century. The poor plasticity at room temperature and the difficulty for cold working greatly restrain the development and the application of magnesium. Therefore, to confirm the deformation craft and extend the application domain in high speed and low temperature domain, even in room temperature, it has the most important significance of understanding the deformation behavior of magnesium alloy at moderate temperature.Twinning is one of the most important mechanisms at moderate temperature. However, the research on the micro-nature of twinning is absence. Otherwise, the strong basal texture which is obtained by normal processing for sheets seriously limits the deformation performance. By this reason, to explore the relationship between twinning and grain orientation in moderate temperature is the key to understand the deformation mechanism and improve the deformation ability of the sheet. The objective of this dissertation is to explore the deformation behavior of Mg-Al-Zn alloy sheets in moderate temperatures. Based on obtaining the sheets with different grain orientation, normal rolling, equal channel angular rolling (ECAR), extrusion and cryogenic treatment were utilized. The deformation behavior and anisotropy in moderate temperatures of these sheets was researched. The main conclusions can be drawn as the following.(1) For the AZ31 rolled sheet owning strong basal texture, {10(1|-)1} and {10(1|-)3} contraction twinning which has high nucleation rate and growth velocity including some basal slip has been proven to be the dominant model in the deformation process at low temperature (room temperature - 423K). The initiation of stacking fault and stacking fault bands would be discovered in the low-temperature deformation process. Stacking fault bands which have certain orientation relationship with basal body were thought to be the embryo of deformation twinning.(2) Contraction twinning in the deformation process twinning has both softening and hardening effects. In the low temperature, twinning induced hardening overrides both the twinning-induced softening. The initiation of twinning was greatly effected by stress, temperature, strain velocity, cavity, grain size and pre-treatment etc. In the process of bending, the amount of twinning increases with the distance between pressure head and the location of subjecting stress. Moreover, to be an immediate twinning which is sensitive to deformation conditions, contraction twinning in magnesium alloys also increases with strain rate and decreases with temperature. Most of cavities which hinder the growth of twinning distribute on the grain boundaries and twinning boundaries. Cryogenic treatment may lead to the increase of second phase particle and generate frame-type twins and nano-twins.(3) In the 423K and strain rate 10^-2s^-1, twinning including some a+c dislocation slip are the dominant mechanisms. In the 473K and strain rate 10^-2s^-1, obvious dynamic recovery occurs and a great mount of serrated grains emerge. With the Z parameter further decrease, the mount of twinning remarkable reduced and dynamic recrystallization gradually acts significant function. The dominant recrystallization mechanisms in moderate temperature include continuous recrystallization and twinning recrystallization. For AZ31 extruded magnesium alloy sheets which compress with ED direction in 473K-573K, the stress exponent n is 8.57 and active energy is 174.18KJ/mol. This means that the dynamic recrystallization and recovery are controlled by cross slip.(4) During two-stage deformation, due to the limitation of temperature, the recrystallization in 423K can not occur and there is no great difference between directly deformation and two-stage deformation. When the temperature elevate to 523K, the recrystallized grains occur in the first stage lead to softening in the second stage.(5) For extruded AZ31 magnesium alloy, in the process of compression in the ED direction, the main twinning mode is {10(1|-)2} tension twinning. The {10(1|-)2} twins lead to the rotation of grains and make most of grains turn to hard orientation. In the process of compression with the TD direction, twinning also reduces most of grains turn to hard orientation. Because of the grain rotation induced by twinning, the anisotropy of extruded magnesium alloy is obvious and the compression plasticity in TD direction is higher than ED direction.(6) After equal channel rolling, the intensity of sheet is reduced from 6.8 to 3.7. The reduction of basal texture leads to the increment of the grains with soft orientation. Therefore, the plasticity of ECAR sheets is higher than normal rolling sheets.