Influence Of Initial Microstructure On Static Recrystallization Of AZ31Magnesium Alloy

Mg alloys with hexagonal close packed crystal structure can only initiate limitedslip systems deformed at room temperature leading to a poor working capability, whichgreatly limits their applications. Annealing usually serves as an important process tocontrol and improve the material properties. Therefore, it is important to investigate thestatic recrystallization behavior with the aim to improve plastic forming properties ofMg alloys.In this thesis, two types of samples with c-axis of grains nearly perpendicular andparallel to the thickness reduction direction (ND samples and TD samples) respectivelywere rolled at150℃and then annealed at different temperatures. The influence ofreduction and annealing parameters on static recrystallization were studied bymicrohardness, optical microscopy, X-ray diffraction (XRD) and electron backscatterdiffraction (EBSD). The effects of initial microstructure on the nucleation, grain growthand texture evolution during the static recrystallization were also discussed.The results show that:①For ND samples, the basal texture is strengthened andshear bands increases as the strain increases. While, for TD samples, the prismatictexture is weakened and the basal texture is enhanced with increased rolling reduction.A large number of extension twins exist after9%rolling reduction, while shear bandsappears after17%reduction.②The recrystallization in both the two samples areaccelerated obviously with enhanced annealing temperature and increased the rollingreduction. After9%rolling reduction, the grain size nearly keeps stable in ND samples,while the grain grow up obviously in TD samples as the annealing temperatureincreased. With the rolling reduction increasing, the recrystallized grains of bothmaterials are refined and the difference between two samples is reduced as well.③Asthe deformation strain increases, the static recrystallization activation energy decreasessignificantly. After9%deformation, the activation energy of TD samples is muchhigher than that of ND samples. However, after17%deformation, the activationenergies of two samples are nearly equal.④Shear bands are the main nucleation sitesfor ND samples during recrystallization, and the recrystallized nuclei are orientatedrandomly to some extent and the grain are refined significantly after recrystallization.For TD samples, after9%reduction, no obvious nucleation can be detetected and the{101ˉ2} tension twinning expands consuming the deformation matrix during the grow stage, thus cannot refine gain size. However,17%reduction induced much shear bandswhich provide favorable sites for nucleation. The orientations of recrystallized nucleiare also very scattered.⑤Texture of recrystallized ND samples is similar as the rolledone except for a weakened basal texture, while, for TD samples, texture evolutionduring recrystallization process is different due to the rolling reduction. The basaltexture of TD samples with9%reduction is enhanced at the expense of the prismatictexture. However, the strong basal texture is retained and weakened for the TD sampleswith17%rolled reduction similar as that in ND samples.