Tensile Properties of Forged Mg-Al-Zn-Ca Alloy

Continuously-casted Mg-9Al-1Zn-1Ca (in mass%) alloy (Mg-Ca alloy) and Mg-9Al-1Zn alloys (Ca-free Mg alloy) were forged at 573 K and their mechanical properties were investigated by tension tests at ambient temperature and 573 K. The forged Mg-Ca alloy showed higher 0.2% proof stress than the forged Ca-free Mg alloy. The high strength for the Mg-Ca alloy was attributed not only to grain refinement by hot forging, but also to the strengthening mechanisms arising from the difference in thermal expansion and geometrical incompatibility between Mg matrix and second phase. The Ca addition decreased the elongation to failure; however, the decrease was reduced for the forged specimens, compared to the unforged specimen. This results from segmentation of the second phases by the hot forging. Also, the forged Mg-Ca alloy showed a large elongation of 284% at 573 K

Effects of Homogenization Annealing on Dynamic Recrystallization

Compression tests were conducted at the temperature of 573 K with the true strain rates of 10 À3 -1 s À1 on as-cast and homogenized Mg6Al-2Ca-2RE (RE = rare earth) (in mass%) alloy specimens, and their dynamic recrystallization (DRX) behaviors were investigated. Strain hardening occurred after yielding, followed by strain softening. The flow stress of the as-cast specimen was higher than that of the homogenized specimen. The DRX grain size depended minimally on the Z-parameter in both of the as-cast and homogenized specimens. This is likely to be due to the particle-stimulated nucleation mechanism involving the second-phase particles. When the specimens were deformed to the true compressive strain of 1.6, non-recrystallized regions were not observed in the homogenized specimen; however, they were partially observed in the as-cast specimen. The grain size in the recrystallized region in the as-cast specimen was smaller than that in the homogenized specimen. Elemental analyses revealed Al segregation around the second-phase particles in the as-cast specimen. Therefore, it is suggested that DRX in the present Mg-Al-Ca-RE alloy is affected by not only the second-phase particles, but also the Al segregation

Effect of Reduction in Thickness and Rolling Conditions on Mechanical Properties and Microstructure of Rolled Mg-8Al-1Zn-1Ca Alloy

A cast Mg-8Al-1Zn-1Ca magnesium alloy was multipass hot rolled at different sample and roll temperatures. The effect of the rolling conditions and reduction in thickness on the microstructure and mechanical properties was investigated. The optimal combination of the ultimate tensile strength, 351 MPa, yield strength, 304 MPa, and ductility, 12.2%, was obtained with the 3 mm thick Mg-8Al-1Zn-1Ca rolled sheet, which was produced with a roll temperature of 80°C and sample temperature of 430°C. This rolling process resulted in the formation of a bimodal structure in the α-Mg matrix, which consequently led to good ductility and high strength, exclusively by the hot rolling process. The 3 mm thick rolled sheet exhibited fine (mean grain size of 2.7 μm) and coarse grain regions (mean grain size of 13.6 μm) with area fractions of 29% and 71%, respectively. In summary, the balance between the strength and ductility was enhanced by the grain refinement of the α-Mg matrix and by controlling the frequency and orientation of the grains

Effect of Texture of AZ31 Magnesium Alloy Sheet on Mechanical Properties and Formability at High Strain Rate

The mechanical properties and formability of AZ31 magnesium alloy strips having different textures were investigated at a high strain rate based on that occuring in mass production by press forming. Forming at a high strain rate on the order of 10 0 s À1 requires a high temperature of over 473 K. To obtain accurate stress-strain curves, a high-speed testing machine that can maintain a constant true strain rate was used, and the change in gauge length on a test piece in a furnace was measured during the testing time of about 0.5 s. For the specimens, rolled strips consisting of fine grains (about 10 mm) and an extruded strip consisting of coarse grains (about 40 mm) were used. The {0001} textures of the extruded strip and one of the rolled strips were strongly oriented parallel to the rolled surface, but the texture of another rolling strip had two peaks that were inclined at 5 $15 deg in front of and behind the rolling direction. At the high strain rate of 10 0 s À1 , elongation decreased for every specimen. Nevertheless, a limiting drawing ratio (LDR) of 2:1$ 2:2 was obtained under uniform heating above 503 K in all the specimens except for the extruded strip. The high LDR of the rolled strip having a two-peak texture was maintained in forming at temperatures down to 473 K, in contrast to the LDR of the strongly oriented rolled strip, which reduced rapidly when formed at temperatures less than 503 K

Ultra-High-Resolution Computed Tomography of the Lung: Image Quality of a Prototype Scanner

Purpose: The image noise and image quality of a prototype ultra-high-resolution computed tomography (U-HRCT) scanner was evaluated and compared with those of conventional high-resolution CT (C-HRCT) scanners. Materials and Methods: This study was approved by the institutional review board. A U-HRCT scanner prototype with 0.25 mm × 4 rows and operating at 120 mAs was used. The C-HRCT images were obtained using a 0.5 mm × 16 or 0.5 mm × 64 detector-row CT scanner operating at 150 mAs. Images from both scanners were reconstructed at 0.1-mm intervals; the slice thickness was 0.25 mm for the U-HRCT scanner and 0.5 mm for the C-HRCT scanners. For both scanners, the display field of view was 80 mm. The image noise of each scanner was evaluated using a phantom. U-HRCT and C-HRCT images of 53 images selected from 37 lung nodules were then observed and graded using a 5-point score by 10 board-certified thoracic radiologists. The images were presented to the observers randomly and in a blinded manner. Results: The image noise for U-HRCT (100.87 ± 0.51 Hounsfield units [HU]) was greater than that for C-HRCT (40.41 ± 0.52 HU; P <.0001). The image quality of U-HRCT was graded as superior to that of C-HRCT (P <.0001) for all of the following parameters that were examined: margins of subsolid and solid nodules, edges of solid components and pulmonary ves sels in subsolid nodules, air bronchograms, pleural indentations, margins of pulmonary vessels, edges of bronchi, and interlobar fissures. Conclusion: Despite a larger image noise, the prototype U-HRCT scanner had a significantly better image quality than the C-HRCT scanners