| The evolution of texture and microstructure during the deformation of two Mg-based (+Al, Mn, Zn) alloys has been studied under various conditions of temperature and strain rate. The tested materials were taken from AZ31 and AM30 extruded tubes. The mechanism of recrystallization was investigated by using uniaxial compression, while twinning was studied under uniaxial tension testing. Uniaxial tension followed by annealing was also carried out in order to examine the potential of contraction and double twins for texture changes after recrystallization. Optical metallography, X-ray analysis and EBSD techniques were employed to study the texture and grain scale development.;The second part focused on the mechanisms associated with variant selection during contraction and double twinning. These twins are of practical interest since they have a high potential for texture randomization. For this purpose, magnesium alloy samples were deformed in tension at room temperature and a strain rate of 0.1s-1. The variants were characterized with respect to the orientation of the parent grain rather than of its host primary twin. This approach led to a regrouping of the 36 possible variants into 4 sets, namely the SA, SB, S C and SD, consisting of variants that are geometrically equivalent. A statistical analysis revealed that the observed secondary twins were almost entirely of the SA and SD types (misorientations of 37.5º and 69.9º, respectively). The results indicate that the formation of contraction twins and the SD type of double twins obeys Schmid's law. On the other hand, even though the SA variants are associated with significantly lower Schmid factors, they were observed to be the most common. This can be attributed to the ease of growth of this variant as well as to the very low associated accommodation strains. Similarly, the relative absence of the two other types of double twin variants (SB and SC) is explained by their geometrical configurations, which maximize the accommodation strain in the neighbouring matrix grains and minimize their potential for growth.;In the final part, the potential for decreasing the texture intensity generated during the bulk deformation of Mg alloys (AM30 and AZ31) was investigated using a combination of contraction twinning, double (secondary) twinning and static recrystallization. The results enabled the effects of initial texture and grain size on the texture and microstructural changes to be determined. A large number of twins was induced by tensile deformation at room temperature. Their volume fraction and the variants selected during straining were found to be largely responsible for the changes evident in the deformation texture. After recrystallization, most of the twin boundaries lose their character. The few that remain are contraction twins, indicating that the double twins recrystallize more rapidly. The new grains that form in the twinned regions were found to be of the order of few microns in diameter. However, these new grains do not expand into the matrix, so that the final texture is not changed significantly.;The first part of the investigation focused on the misorientations that apply to the different mechanisms of new grain formation during the hot compression of magnesium alloy AM30. This approach led to the identification of three types of microstructural features produced during deformation at high temperature (350°C at a strain rate of 0.001s-1). In the first, microbands were produced in grain interiors due to the accumulation of basal dislocations. Their orientation changes involve rotations of the basal planes to a more favorable orientation for glide, leading to geometric softening. In the second, two different types of DRX nuclei were observed, i.e. those formed i) by bulging and ii) by continuous dynamic recrystallization. Although visually distinct, the two mechanisms are formed as a result of dislocation-based processes that produce c-axis rotations toward the loading axis. The present observations indicate that new grain formation takes place in two separate stages. For low misorientation angles, the boundary character is consistent with the accumulation of basal dislocations. Once nuclei of the latter two types have formed, however, only those orientations belonging to the RD-TD fibre are able to grow. In this way, recrystallization leads to the retention of the main characteristics of the initial RD-TD texture. |
