Papers by Keyword: Superplasticity

Abstract: Effect of one-step and two-step homogenization treatment on precipitation of Al₃(Sc,Zr) dispersoids, grain structure after annealing of cold rolled sheets and superplastic behaviour of a novel Al-Mg based alloy were studied. Heterogeneous nucleation of Al₃(Sc,Zr) phase on dislocations and subboundaries dominated at one-step annealing and both homogeneous and heterogeneous nucleation of Al₃(Sc,Zr) were observed at two-step annealing modes. It was shown that two-step treatment mode provides high density of Al₃(Sc,Zr) precipitates and 650 % of elongation at the constant strain rate of 10^-2 s^-1 in the studied alloy.

Abstract: Surface observations are used to elucidate the deformation mechanisms responsible forsuperplasticity in the Ti-6Al-4V and Ti-4Al-2.5V-1.5Fe titanium alloys. First, stress relaxation testsare used to quantify the difference in superplastic behaviour of each alloy. Then, high-temperature insitutensile tests are performed in the scanning electron microscope at 700, 800, and 900°C to relatedifferences in formability to micro-mechanisms. These are found highly dependent on: (i) the grainsize; (ii) the alpha-to-beta volume fraction; (iii) the crystallographic texture; and (iv) the nature and angle ofthe dominant grain boundaries.

Abstract: Modelling and predicting the flow behaviour of metallic materials subjected to superplastic deformation is mandatory for providing useful information about the metal forming process. This information helps the designers to reduce the manufacturing time and costs by choosing appropriate deformation conditions based on the models results. The study developed a constitutive model to predict the flow behaviour of various Ti-based alloys (Ti-2.5Al-1.8Mn, Ti-6Al-4V and Ti-4Al-1V-3Mo) at elevated temperatures. The constant strain rate tests within the superplastic temperature and the strain rate ranges for each alloy were performed. The experimental tensile tests results were used to develop the hyperbolic sine Arrhenius-type constitutive models for each alloy. The performance of the developed model for each alloy was evaluated regarding the correlation coefficient (R), the mean absolute relative error (AARE) and the root mean square error (RMSE). The results revealed that the predicted flow stresses have a good agreement with the experimental flow stresses for the studied alloys.

Abstract: The mechanical behavior of an AZ31 magnesium alloy processed by high-pressure torsion (HPT) was evaluated by tensile testing from room temperature up to 473 K at strain rates between 10^-5 – 10^-2 s^-1. Samples tested at room temperature and at high strain rates at 373 K failed without any plastic deformation. However, significant ductility, with elongations larger than 200%, was observed at 423 K and 473 K and at low strain rates at 373 K. The high elongations are attributed to a pronounced strain hardening and a high strain rate sensitivity. The results agree with reports for a similar alloy processed by severe plastic deformation. However, the level of flow stress is lower and the strain rate sensitivity and the elongations are larger than observed in this alloy processed by conventional thermo-mechanical processing.

Abstract: The 7075 (Al-Zn-Mg-Cu) aluminium alloy is the reference alloy for aerospace applications due to its specific mechanical properties at room temperature, showing excellent tensile strength and sufficient ductility. Formability at high temperature can be improved by obtaining superplasticity as a result of fine, equiaxed and highly misoriented grains prone to deform by grain boundary sliding (GBS). Different severe plastic deformation (SPD) processing routes such as ECAP, ARB, HPT and FSP have been considered and their effect on mechanical properties, especially at intermediate to high temperatures, are studied. Refined grains as fine as 100 nm and average misorientations as high as 39o allow attainment of high strain rate superplasticity (HSRSP) at lower than usual temperatures (250-300oC). It is shown that increasing misorientations are obtained with increasing applied strain, and increasing grain refinement is obtained with increasing processing stress. Thus, increasing superplastic strains at higher strain rates, lower stresses and lower temperatures are obtained with increasing processing strain and, specially, processing stress.

Abstract: The commercial Zr-modified 5083 aluminum alloy was homogenized to precipitate nanoscale Al₆Mn particles and then undergone to equal-channel angular pressing (ECAP) at 300 °C to a true strain of ~12 via B_C route. The obtained ultrafine-grained material was subjected to friction-stir welding (FSW). The welding variables were selected to provide reasonable homogeneous microstructure distribution across the weld zone and thus to ensure a highly uniform elongation during subsequent superplastic tests of the joints. Superplastic behavior of the obtained welds is discussed.

Abstract: The bulk ultrafine-grained (UFG) materials usually show superior mechanical properties. Since the occurrence of superplastic flow generally requires a grain size smaller than ~10 μm, it is anticipated that materials processed by severe plastic deformation (SPD) will exhibit superplastic ductilities when pulled in tension at elevated temperatures. Recent advances in the processing of UFG metals have provided an opportunity to extend the understanding of superplastic flow behavior to include UFG materials with submicrometer grain sizes. Recent studies showed the UFG materials demonstrated the development of plasticity in micro-mechanical response at room temperature by the significant changes in microstructure attributed to high-pressure torsion (HPT). Accordingly, this study summarizes recent results on excellent ductility and plasticity in a UFG Zn-22% Al alloy. Specifically, the alloy demonstrated the occurrence of exceptional superplastic flow at high temperature after equal-channel angular pressing and HPT and excellent room temperature plasticity of the alloy after HPT where the plasticity was evaluated by the nanoindentation technique. The significance of purity of the alloy is also considered for enhancing the ductility at room temperature.

Abstract: Ti55 is a type of new near-α titanium alloy featuring good heat resistance and thermal stability at a temperature below 550°C. This paper explores the superplastic behaviors of Ti55 titanium alloy sheets at high temperature. The results showed that at a strain rate between 8.3×10^-4 and 1.32×10^-2s^-1 and a deformation temperature between 885 and 935°C, this material exhibited favorable superplasticity. A 872% tensile elongation was achieved even at a high temperature of 925°C and a high strain rate of 1.32×10^-2s^-1. For a deformation temperature of 925°C and a strain rate range of 8.3×10^-4～1.32×10^-2s^-1, the strain rate sensitivity index (m) was equal to or higher than 0.38, which depends on the specific strain rate and reached its peak at 6.64×10^-3s^-1. The study showed that tiny cavities tended to appear around the rare earth phase particles. As the deformation temperature and the deformation strain rate increased, both size and volume fraction of the cavities decreased significantly. When the plastic deformation further increased, the cavities experienced not only longitudinal aggregation along the tensile strain direction, but also transverse aggregation normal to the tensile strain direction. The typical high-temperature ductile fracture morphology is related to both aggregation and growth of the large number of microscopic cavities and dimples.

Abstract: The results of the course of martensite transformation in steel under the action of the magnetic field are presented. These results indicate that the formation of stress-assisted martensite in the temperature interval M_d-M_s (where superplasticity was observed) is possible. It was found that during hardening of steel products in magnetic field, martensite can be formed not only below M_s, but higher than this point (in the temperature range). The significant structural transformations and steel properties improvement under thermal treatment in a magnetic field depend on the catalytic effect of a field on the transformation development. This effect occurs when the initial phase is paramagnetic, and the product of the transformation is ferromagnetic. Stress-assisted martensite and cooling-induced martensite are formed in the superplastic temperature range of the austenite. This makes it possible for the undeformation hardening of steel products in a magnetic field. The undeformation hardening is caused by a magnetic field holding the product and the formation of martensitic structure oriented predominantly towards the magnetic field vector.

Abstract: Numerical simulation of superplastic forming limit of AZ31B magnesium alloy sheet was investigated. The damage evolution equation based on the law of the micro-damage evolution and statistical mechanics was derived, and damage characteristic parameters as well as the critical value of damage variable were identified to provide a theoretical ground on which the plastic forming technology of magnesium alloy sheet can be optimized. The theoretical prediction was made with the numerical simulation program, and the results were verified by experiments. The forming limit curve of the theoretical prediction drawn by numerical simulation was established by the basic adaptation of the forming limit curve based on the experimental data.