Materials Science Forum Vols. 551-552

Abstract: Ultrahigh carbon steel containing 1.6 wt pct C was processed to create microduplex structure consisting of fine-spheroidized carbides and fine ferrite grains. Elongation-to-failure tests were conducted at strain rates from 10-4s-1 to 15×10-4s-1, and at temperatures from 600 °C to 850 °C. The steel exhibited superplasticity at and above 700 °C when testing at a strain rate of 10-4s-1, and at 800 °C when testing at strain rates of 7×10-4s-1 and slower. The grains retained the equiaxed shape and initial size during deformation; dynamic grain growth was not observed after superplastic deformation, whereas carbide coarsening was observed. It is concluded that the fine ferrite grains or austensite grains are stabilized by the grain boundary carbides, and grain-boundary sliding controlled by grain boundary diffusion is the principal superplastic deformation mechanism at temperatures in the range of 700-850 °C.

Abstract: Superplastic deformation (SPD) behaviors of two fine-grained materials produced by ECAE and hot rolling methods have been contrastively studied in this paper. It is found that the optimum superplastic condition in as-ECAEed material was at 350°C and 1.7×10-3s-1 with elongation to failure about 800%; while in as-rolled material, the largest elongation to failure about 1000% was obtained at 480°C and 5.02×10-4s-1. Microstructure observation showed that grain evolution and cavitation behavior were different in these two materials during superplastic deformation. The controlled mechanisms for superplasticity, i.e. grain boundary sliding (GBS), dislocation creep and diffusional creep, at different deformation conditions were discussed in terms of strain rate sensitivity coefficient, stress exponent and activity energy.

Abstract: Superplastic forming (SPF) of magnesium alloys has received increasing attention in the recent past. The aim of this presentation is to review recent works dealing with SPF of Mg alloys with a three-fold objective: i. How to produce fine or ultra fine grained (UFG) microstructures ii. Are there specifities in superplastic deformation mechanisms iii. How SPF Mg alloys resist to cavitation Deformation mechanisms as well as damage variations in the superplastic regime will be preferentially discussed in relation with grain size, content in intermetallic particles and diffusion kinetics. For the sake of illustration, some results concerning the superplastic behaviour of UFG magnesium alloys produced by severe plastic deformation will be presented since such microstructures exhibit particularly attractive superplastic properties at quite low temperatures.

Abstract: Accurate constitutive modeling of superplastic deformation is critical for successful simulation and optimization of superplastic forming. The selection of the forming pressure profiles in gas blow forming of superplastic materials is still based on trial and error due to the limited predictive capabilities of current models describing superplastic deformation. In general, these models are based on uniaxial loading condition, assuming isotropic behavior. In this work, we examine the biaxial bulge forming of AZ31 magnesium alloy using pressure profiles derived from different available analytical models. The results clearly show the need for more accurate description of superplastic deformation to improve predictive capabilities. In addition, a pressure profile that is generated using FE in conjunction with a microstructure-based constitutive model is used and the results indicate better ability to predict the height of the bulged sheet.

Abstract: Magnesium alloys show promise in meeting the demand for materials of lighter weight and higher rigidity. Mg alloys are hard to process and normally require grain refining for improved formability and mechanical properties. To process these fine-grained Mg alloys effectively, it is important to relate their load stress and mechanical properties to changes in their microstructures. Using a biaxial tensile machine and cruciform specimens, to evaluate the mechanical properties, microstructure, and plasticity, in a high temperature biaxial stress state, used of AZ31 Mg alloy sheet. With biaxial deformation, grain boundary slide occurred more frequently than with uniaxial deformation, causing grain boundary separation and formation of micro-voids between the grains. In the vicinity of the cracks and at the locations of grain boundary separation, although deformation temperature at higher than the recrystallization temperature, fine grains (about 2 )m) showing in duplex grain structures were formed locally. The formation of duplex grain structures as a result of local formation of fine grains during the deformation process is a major issue to be solved from the viewpoint of plasticity processing.

Abstract: The superplasticity of a hot-rolled AZ31 Mg alloy was investigated by uniaxial tensile tests at temperature range 250-450oC and strain rate range 0.7×10-3-1.4×10-1s-1. Superplastic formability of the alloy was evaluated by gas bulging test at elevated temperatures. The threshold stress for grain boundary sliding (GBS) was calculated and the topography during superplastic deformation was observed by SEM. It is found that, at 400 oC and 0.7×10-3 s-1, the maximum elongation reaches 362.5%. GBS is the predominant deformation mechanism and characterized by a pronounced improvement in homogeneity with increasing temperatures, indicating a transformation of GBS mode from cooperative GBS (CGBS) to individual GBS (IGBS). The improved homogeneity of GBS can be interpreted in terms of decreased threshold stress with increasing temperatures. Gas bulging test demonstrates that the temperature for the best superplastic formability is 400 oC and a hemispherical part with a specific limiting dome height of 0.51 was obtained, suggesting good application prospect for this alloy.

Abstract: The superplasticity of magnesium alloy is important in industrial application. However the superplastic deformation of casting magnesium alloy is hard to be realized. In this paper, the stress–strain behaviors of casting AZ31 magnesium alloy with various strain rates at different deformation temperatures were investigated. The alloy was tested in the tensile condition with initial grain size of 25μm. It was found that the elongation of the alloy at 400°C with ε& = 4.25×10-4 s-1 is almost 200%. According to the results of uniaxial tensile experiment, the alloy exhibited superplastic deformation behavior with the slow stain rate in a temperature range of 350 to 450°C. The microstructures deformed and undeformed samples were observed with aid of optical microscope.

Abstract: A new SPF/DB technology using gasification agent as pressurization mediator was proposed in this paper. The forming principle of SPF/DB using the agent was represented. Dies used for the forming process was designed and manufactured, and new gasification agent N11 which is white solid powder at normal temperature and it is easy to encapsulate and keep was developed for magnesium alloy. ZK60 magnesium alloy with a thickness of 1mm and grain size of 7.9μm was selected to conduct the experiment of SPF/DB. Corrugated ZK60 magnesium alloy parts were well formed under the temperature between 653K and 673K and forming time of 50 minutes. SEM was used to observe the microstructures of the diffusion bonding joints. The result shows that the joints were well bonded.

Abstract: The superplasticity of LY12 alloy was reviewed in this paper. Complex component was extruded by taking advantage of the superplasticity of supplied LY12 alloy. The research demonstrates that in isothermal compression process dynamic recrystallization may occur in supplied LY12 alloy. In superplastic forming test grain refinement was combined with initial extrusion step utilizing dynamic recrystallization to complete grain refinement, which ensured the superplasticity in supplied LY12. In the early stage of extrusion, using high deformation speed and large amount of deformation can result in grain refinement, which primarily satisfied the demand of the superplasticity. In the final stage, the forming speed decreased sharply so that the optimum strain rate was satisfied and the complicated component can be extruded successfully. The resulted product has reasonable flowing traces, which improves its service performance.

Abstract: In this paper, an ultra-fine grained AZ31 magnesium alloy sheet with grain size less than 3μm was generated by three-run accumulative roll bonding of as cast alloy at a deformation temperature of 350°C and a reduction of 80% for each pass. The microstructures on the different ARB stages were observed and superplasticity examination in the ultrafine grained AZ31 alloy were carried out at a fixed temperature of 300°C and varied strain rate ranging from 10-4 to 10-1 s-1. It is indicated that significant grain refinement was mainly achieved in the first run and gradual uniformity of grain size in the next by continuous dynamic recrystalization. Besides, a superplastic deformation with a moderate elongation-to-fracture of 316% was obtained at a strain rate of 10-2 s-1 indicating a low temperature and high strain rate superplasticity, while a maximum elongation-to-fracture of 562% at10-4 s-1. The strain rate sensitivity exponent as high as of 0.34-0.41 implies the dominant role of grain boundary sliding in superplastic deformation at strain rate ranging from 10-3 to 10-2 s-1. The results indicate a possible approach to produce magnesium alloy sheet with fine grain and excellent deep drawing workability.