The effect of temperature on the stress-strain responses under cyclic plastic deformation on a rolled AZ31B sheet was investigated by performing in-plane cyclic tension-compression experiments at various temperatures ranging from room temperature to 200℃. The change in the crystallographic textures during the cyclic deformation was examined by XRD analysis. From room temperature to 137℃, three deformation modes, i.e., slip, twinning and detwinning, were active. In particular at room temperature, the three deformation modes sequentially occurred in compression- tension experiment, and as a result, an unusual cyclic stress-strain curve with tension-compression asymmetry was observed. At 200℃, the cyclic-stress-strain curve was symmetric and its stress level was much lower than that at R.T., since, at this elevated temperature, CRSSs of nonbasal slip systems were low because slip deformation occurs rather than twinning.
In a previous investigation concerning the ductility of Mg-La-Zr alloys, it was revealed that the tensile-tested Mg La-Zr alloys developed ultrafine grains within the deformed areas of the specimens. On the basis of those observations, in the present study, cast Mg-La-1.5Zr alloys with lanthanum content varying between 0 and 5.4 wt.% were hot-rolled and the microstructures examined by optical microscopy, X-ray diffraction, EBSD and KAM analyses to establish the characteristics of the hot-rolled and annealed specimens. It was revealed that the microstructures within the primary and eutectic αMg of hot-rolled Mg-5.4La-1.5Zr materials consisted of fine grains of 3.6 and 0.3 μm in average size, respectively. The grain growth of these fine grains within the primary αMg did not expand into the eutectic regions during annealing, while the fine grains within the eutectic αMg remained as fine as those observed in the hot-rolled materials even after annealing at 400 ℃. This is most likely due to the proximity of a eutectic Mg12La phase that may act as an obstacle to grain growth, suggesting the possibility of developing materials with bimodal microstructures. It was also found that the level of the basal texture in the hot-rolled and annealed materials of Mg-La-1.5Zr alloys was significantly lower than that of Mg-Zr alloy, demonstrating the beneficial effects of lanthanum.
The deformation behaviour during the trimming of ultra-high-strength steel sheets was observed to reduce the flying speed of cut scraps. In the trimming of ultra-high-strength steel sheets, the flying speed is high owing to the high strength, and thus the trimming operation becomes noisy and scraps jump out from the disposal box. As the sheet strength and trimming speed increased, the flying speed became high. The flying speed for an 1180MPa sheet was accelerated to 2.7 times the free-fall speed, whereas the speed for the mild steel sheet was 1.2 times. As the strength of the sheet increased, the noise level in trimming increased. To reduce the flying speed, punches having single and double bevels were developed. Although the flying speed became high for the punch having a double bevel, the speed decreased when the punch had a single bevel. It was found that the trimming punch having a single bevel is effective in reducing the flying speed of scrap and the noise level in the trimming of ultra-high-strength steel sheets.