Materials Science Forum Vols. 783-786

Abstract: Magnesium alloys are extensively used in electronics, automotive and aerospace industries due to their low densities and high specific strengths. However, limited deformability of magnesium alloys at room temperature restricts the applications. Grain refinement and texture weakening as a result of recrystallization can be used to enhance the deformability of these alloys. In this study, recrystallization behavior of cold rolled and swaged AZ31 alloy is investigated at different temperatures in the range of 200-300°C. Effects of unidirectional and complex deformation modes on recrystallization behavior and microstructure development are studied. Microstructural analyses consisted of the examination of the grain structure and twinned regions by using optical and scanning electron microscopes. The volume fraction of recrystallized grains is determined by image analysis and supported by the hardness measurements.

Abstract: In this study, Mg-6.8Gd-4.5Y-0.5Zr alloy was fabricated by the spray atomization and deposition technique. The microstructure and mechanical properties of the alloy were studied using optical microscopy, scanning/transmission electron microscopy, X-ray diffraction, and tensile tests. The secondary phases in the microstructure of the spray-deposited alloy were examined. The tensile test results indicate that the spray-deposited Mg-6.8Gd-4.5Y-0.5Zr alloy displays superior tensile strength due to grain refinement and the presence of precipitating strengthening phases.

Abstract: In the present work, the microstructure and properties of a novel Al-0.92Mg-0.78Si alloy with adding Zn element were investigated by electron back scattering diffraction (EBSD), transmission electron microscopy (TEM), high transmission electron microscopy (HREM), mechanical properties tests and Erichsen test. The alloy sheet was cold-rolled to about 1mm, then solution treated at 545 °C for 30mins, and immediately water quenched. After solution treatment, the alloy was pre-aged at 140 °C for 10mins. The average grain size of the alloy was about 20μm. The yield stress and elongation of pre-aged alloy was 137MPa and 30% respectively. The I_E value of the alloy was 8.6mm. After paint baking at 170 °C for 30mins, the increment of yield stress was about 100MPa. The GP(II) zones were formed in the alloy during paint baking process through adding Zn element, which should play a very important role for improving paint hardening response significantly. GP(II) zones and η ́ phases were formed after artificial ageing at 170 °C for 10h.

Abstract: Experimental studies proved that structures and properties of misfit dislocations and their intersections (nodes) in semi-coherent interfaces strongly affect thermal and mechanical stability of interface. Employing atomistic simulations, we reveal that misfit dislocation lines can exhibit a spiral pattern (SP) or remain straight in association with dislocation character at nodes. By analyzing nodes formation processes in terms of kinetics and energetics, we found that the variation is ascribed to the competition between core energy of misfit dislocation and interface stacking fault energy with respect to coherent interface.

Abstract: AZCa912, 8.8mass%Al-0.68mass%Zn-0.25mass%Mn-1.9mass%Ca-Bal.Mg, continuous casting bars were formed and their hot compression properties were measured. AZCa912 alloy is a noncombustible magnesium alloy. AZCa912 bars 50 mm in diameter were formed by a direct chill continuous casting method. The surfaces of the bars had excellent metallic luster. Grain sizes in the bars were 148, and the dendrite arm spacing was 16.9 μm. Cracks on the side surfaces of the samples were formed by compression at 250-300°C, 0.01-1/s and 350°C, 1/s. However, inner cracks were not observed and-5 μm fine-grains were formed. Some samples had an unchanged casting structure after hot pressing, and the percent of the structure remaining unchanged decreased with low-temperature compression.

Abstract: Twin Roll Casting (TRC) has been successfully employed for the past sixty years to produce aluminum, steel and, in the past ten years, magnesium sheet. Although the TRC process is relatively simple, its application for commercial-scale magnesium strip production has proven difficult. This is primarily due to inherent characteristics of magnesium alloys, such as their high reactivity to oxygen, low specific heat and latent heat of fusion, and large freezing ranges, which can induce formation of casting defects if various TRC processing parameters, such as metal delivery design, heat transfer in the roll gap, and casting speed, aren’t tightly controlled. Research is underway worldwide to concurrently gain a better understanding of TRC processing variables in order to provide optimum casting conditions which will reduce defects, and develop new magnesium alloys with properties tailored to the TRC process. The opportunities and challenges associated with magnesium TRC will be outlined and include: 1) defect formation during TRC of magnesium alloy AZ31, 2) the feasibility of producing clad magnesium strip via TRC and 3) the effect of scale-up (moving from a laboratory unit to commercial production) will have on the TRC process for magnesium.

Abstract: Currently magnesium alloys are used for different applications in the transportation industry where cast magnesium alloys dominate the market. Although cast alloys predominate over wrought products such as extrusions, forgings, sheet and plate, the latter are also being used in a variety of different applications. Recently, a growing interest in the automotive industry in looking at potential applications for magnesium turned back towards wrought alloys. Typically, applications of magnesium sheets are sought in automotive or aeronautics industry. However, the spectrum of potential applications can be significantly expanded. For example, body protection systems for civil services like police, custom officers and prison personnel currently include anatomically shaped aluminium alloy sheets. Replacement of aluminium alloys by magnesium to result in substantial weight savings up to 30%.

Abstract: To enhance the heating efficiency and the formability of AZ31 magnesium alloy, the assistance heating method is adopted during the hot bulging process. The free bulging test of coarse grained and fine grained AZ31 magnesium alloy sheet was carried out. During the forming process, the effects of pulse current on the formed sheet combine both thermoelectricity and electro-plasticity. Directional and asymmetrical deformation of the coarse-grained AZ31 magnesium alloy sheet by the effect of pulse current is observed and analyzed. But in the same processing condition, the deformation of fine grained AZ31 magnesium alloy sheet is symmetrical. To investigate the influence of electron wind force on pulse current auxiliary bulging process, the simulation of AZ31 magnesium alloy pulse current auxiliary bulging based on the electron wind force value calculated by mathematical model was performed using the Marc software. The bulging simulation result showed a special phenomenon that the shape of the bulging specimen is unsymmetrical and the dome deviated from the symmetry axis to the side of positive pole. The simulation results were basically consistent with the experimental results. The deformation properties, microstructure characteristics, dislocation movement of the AZ31 magnesium alloy during gas bulging processing by the pulse current are investigated.

Abstract: ATI 425^® Alloy, nominal composition Ti-4.0Al-2.5V-1.5Fe-0.25O, is a new alpha/beta Ti alloy of significant commercial interest as a viable replacement for Ti-6Al-4V, CP-Ti, and other titanium alloys in a variety of aerospace applications. ATI 425^® Alloy offers properties comparable to Ti-6Al-4V alloy with significant improvements in formability, both at room and elevated temperatures. The reasons for the improved formability, particularly at low temperatures, are not well understood. The development of a thorough understanding is complicated by the wide array of phases, microstructures, and deformation paths available via thermomechanical processing in alpha/beta titanium alloys. In this paper, theories of strengthening and dislocation mobility in titanium and HCP metals will be reviewed and applied to better understand why ATI 425^® Alloy offers a unique combination of strength and formability not obtainable by conventional alpha/beta titanium alloys. Subsequently, the application of the improved formability to a range of product forms including sheet, tubing, and forgings will be discussed.

Abstract: In this investigation, microstructure evolution of Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy during thermo-mechanical processing at temperatures in beta single-phase and alpha+beta two-phase fields was studied. Microstructure analyses indicate that: (1) in the beta single-phase field, dynamic recovery accompanied by geometric dynamic recrystallization at large strains takes place dominantly within elongated large prior beta grains with serrate grain boundaries during deformation at higher temperatures and lower strain rates; and discontinuous dynamic recrystallization occurs along elongated small prior beta grain boundaries during deformation at lower temperatures and higher strain rates. During discontinuous dynamic recrystallization, recrystallized grain size is a function of Zener–Hollomon parameter, and a modified Avrami recrystallized kinetic model was established. (2) In the alpha+beta two-phase field, the globularization process is a thermally activated process controlled by parameters of temperature and strain rate. A modified Avrami globularized kinetic model was established. The primary alpha grain size is a function of Z on a ln-ln scale.