Development and characterization of highly formable magnesium sheet alloys

Abstract

Lightweight magnesium sheet has attracted considerable interest for applications in automotive, light-rail, high-speed trains and consumer electronics. Therefore, the development of magnesium sheet with sufficient formability and mechanical properties has been one of the major themes of contemporary magnesium research. One of the technical issues that has restricted the wider application of magnesium sheet, is a lack of sheet alloys that have a good combination of formability, ductility and strength. In the commercially available magnesium sheet alloy Mg-3Al-1Zn (alloy compositions in this thesis are in weight percent), designated AZ31, the majority of magnesium grains have a preferential orientation, with their basal planes parallel to the surface of the sheet, i.e. a strong basal texture. The strong basal texture, which is developed during the rolling process, results in an intrinsic difficulty for the forming of the sheet alloy at near room temperature. Over the past 15 years, considerable efforts have been made towards texture weakening and formability improvement of Mg sheet alloys. It is now well established that the dilute addition of Ca or rare-earth (RE) elements, such as Nd, Gd, Ce, to Mg sheet alloys can significantly weaken the basal texture, and therefore improve the ductility and formability. Moreover, the combined additions of RE and Zn or Ca and Zn can result in an even weaker texture and better ductility than the single addition of RE or Ca. However, the Mg-Zn-RE and Mg-Zn-Ca sheet alloys still have lower ductility and formability than aluminum sheet alloys, such as Al-1.2Si-0.4Mg (designated 6016). This project involves the development of magnesium sheet alloys that are superior to Mg-Zn-RE and Mg-Zn-Ca alloys. It is found that combined additions of Gd and Ca elements result in significant improvement in ductility and formability, with respect to individual addition of Gd or Ca. With the individual addition of 0.4 wt.% Gd or 0.2 wt.% Ca to Mg-1Zn alloy, the resultant Mg-1Zn-0.4Gd and Mg-1Zn-0.2Ca alloys have similar ductility, about 28% and 27% respectively in total elongation, and similar formability to each other, about 1.86 in limit drawing ratio (LDR). In contrast, with the combined addition of 0.4 wt.% Gd and 0.2 wt.% Ca, the Mg-1Zn-0.4Gd-0.2Ca-0.5Zr alloy has much enhanced ductility and formability, its total elongation and LDR are about 38% and 1.96, respectively. Furthermore, micro-alloying elements, including Sr, La and Zr, are also added to the Mg-Zn-Ca sheet alloys. With the addition of 0.5 wt.% Zr to Mg-1Zn-0.5Ca, the Mg-1Zn-0.5Ca-0.5Zr alloy has the best ductility and formability among the Mg-Zn-Ca based alloys, about 36% in total elongation and 1.90 in LDR. In comparison, the total elongation and LDR are about 29% and 1.92, respectively for 6016 alloy, and about 23% and 1.62, respectively for AZ31 alloy. This project also involves the examination of the influence of Zn and Ca additions on sheet ductility. Previously, it was reported that the enhanced ductility of the Mg-Zn-Ca alloy sheet is caused predominantly by the weakened basal texture, and hence considerable efforts were made to investigate the effect of texture weakening on the ductility improvement. However, the effects of deformation modes and grain boundary cohesion on sheet ductility did not attract sufficient attention. In this study, the microstructures and textures of Mg-0.80Zn-0.16Ca, Mg-1.06Zn, Mg-0.16Ca, Mg-0.64Ca and pure Mg sheets before and after tensile testing are examined using scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD). It is found that the Mg-Zn-Ca sheet has a much weaker texture and better ductility than the pure Mg and the binary alloy sheets. The traces of both basal and non-basal slips are observed in the ternary alloy sheet after the tensile testing. In contrast, in the pure Mg and the binary alloy sheets, only basal slip traces are observed. Moreover, the formation and propagation of cracks along grain boundaries in the ternary alloy sheet are much more difficult than those in the pure Mg and the binary alloy sheets, indicating an enhanced grain boundary cohesion in the ternary alloy sheet. The experimental observations suggest that the improved ductility of the Mg-Zn-Ca sheet is caused by the weakened texture, enhanced operation of non-basal slip and improved grain boundary cohesion. This project discovers for the first time an interesting phenomenon: an annealing treatment of Mg-0.80Zn-0.16Ca sheet at 80–200°C, after some plastic strain (0.01–0.06) in tension, leads to a remarkable strengthening, rather than softening, effect. The strength increment due to annealing can be as large as 30% with respect to the yield strength of the Mg-0.80Zn-0.16Ca sheet without straining. Microstructural characterizations using EBSD and high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) suggest that basal slip is the dominant deformation mode, and that the annealing strengthening is caused by the pinning of gliding basal dislocations by GP zones and possibly solute atoms segregated to the dislocations. In this project, a quasi-in-situ EBSD method is used for the first time to monitor the texture evolutions during the cold rolling process of Mg-Zn-Ca and Mg-Zn alloys. In previous studies, while it has been established that dilute additions of Ca or RE elements can lead to a weakened basal texture after recrystallization, it is still unclear whether such alloying additions can also result in a weakened basal texture in the as-cold-rolled state. Moreover, it is also unclear how the Ca alloying addition affects texture evolution during the cold rolling process, primarily due to the lack of direct observations of the texture evolution. The quasi-in-situ observations made in this project show that, during the cold rolling process, the deformation texture in the Mg-0.80Zn-0.16Ca alloy is significantly weaker than that of Mg-1.06Zn alloy under similar cold rolling conditions. However, with an increase in the thickness reduction to 40%, a strong basal texture is eventually developed in the Mg-0.80Zn-0.16Ca alloy, which is very similar to that in the Mg-1.06Zn alloy. The delayed development of a strong basal texture is caused by the Ca addition—the addition of only 0.16 wt.% Ca to the Mg-0.80Zn alloy significantly retards the formation and growth of deformation twins, and hence effectively preserves the parent grains and their random texture. During the annealing process, the texture weakening in RE- or Ca-containing alloys was reported in previous studies to be caused by particle stimulated nucleation, shear band induced nucleation or deformation twin induced nucleation, but which one is dominant for texture weakening is controversial. In addition, it is unclear whether the development of the weakened basal texture is also affected by grain growth behaviours. By monitoring the microstructural and textural evolutions, using the quasi-in-situ EBSD method, of cold-rolled Mg-0.80Zn-0.16Ca, Mg-1.06Zn, Mg-0.16Ca and Mg-0.64Ca alloys during annealing, an alternative interpretation is provided in this project. It is found that, in the ternary alloy, recrystallized grains with randomised orientations form in high-angle grain boundaries of deformed grains, and these recrystallized grains grow with similar rates. It is the comparatively equal-rate growth of recrystallized grains with randomised orientations that gives rise to the weakened basal texture in the ternary alloy. In contrast, in the binary alloys, the preferential growth of recrystallized grains whose c-axes are nearly parallel to those of parent grains is dominant. Hence, the strong basal texture of deformed parent grains are largely preserved after the annealing process. Moreover, such preferential grain growth leads to a replacement of // RD texture in the as-cold-rolled state by // RD after full recrystallization. On the basis of these quasi-in-situ observations, the grains that preferentially grow are identified to be those with a nearly 30°[0001] misorientation with respect to their deformed parent grains. Characterization of the grain boundaries using HAADF-STEM and energy dispersive X-ray spectroscopy (EDXS) shows that both Zn and Ca atoms are segregated to grain boundaries in the ternary alloy, while Zn or Ca atoms are individually segregated to grain boundaries in the binary alloys. Hence, it is speculated that the co-segregated Zn and Ca atoms leads to a much stronger pinning effect of grain boundaries, and that such stronger pinning of grain boundaries significantly suppresses the preferential growth of recrystallized grains with a nearly 30°[0001] misorientation to their parent grains.