High Strain Rate Superplastic Behavior Research Articles

Al-Mg-Mn-Zn-Zr alloy with refined grain structure to develop Al-B fiber-reinforced metal matrix composites compacted in superplastic conditions

Homogeneous nanostructured and ultrafine-grained states in an Al-Mg-Mn-Zn-Zr alloy were produced by high pressure torsion at room temperature and by continuous equal channel angular pressing at 200 °C. The alloys with refined grain structure in both states exhibited low temperature and high strain rate superplastic behavior. The nanostructured alloy with enhanced mechanical behavior was used for MMC fabrication under superplastic conditions guided with the help of finite element simulations. Computer-aided “foil-fiber-foil” process using a woven-alike boron fiber network and nanostructured Al alloy provided smooth defect-free compaction of an Al-B fiber-reinforced composite under superplastic conditions at a temperature of 300 °C.

Effect of strain rate on the microstructure evolution and superplastic deformation behavior of cold-rolled medium Mn lightweight steel

This work investigates the high-temperature tensile behavior of a Medium Mn lightweight steel (MMLS) with a chemical composition of Fe-0.3C-7.0Mn-3.5Al. The MMLS was prepared by hot-rolling followed by cold-rolling to produce a complete martensite microstructure with a high density of dislocations. The tensile tests were conducted at 800 °C within the strain rate range of 5 × 10−4 s−1-2.5 × 10−2 s−1 to explore the effect of microstructural evolution and deformation mechanism by strain rates during the tensile deformation. The results showed that the deformation behavior and mechanism of the MMLS depend not only on the initial grain size and morphology but also on the microstructure evolution during deformation. The MMLS exhibits high strain rate superplasticity (HSRS) behavior, and the deformation mechanisms are dependent on the strain rate. At lower strain rate (5 × 10−4 s−1), dislocation slip/creep is more pronounced in the γ-phase, and grain boundary sliding (GBS) is inhibited during the early deformation period, contributing to the significant texture enhancement. At moderate rate (5 × 10−3 s−1), GBS dominates deformation, while at high strain rate (2.5 × 10−2 s−1), dislocation activity and dynamic recrystallization (DRX) play a significant role. The HSRS behavior of MMLS is related to the dynamic reverse phase transition from cold-rolled ferrite to austenitic/ferrite fine-grained dual-phase structure. This work provides a theoretical basis for the development of superplastic MMLS with improved HSRS behavior and controlled the microstructure/mechanical property of component parts.

High Strain Rate Superplasticity and Microstructure Evolution in a Coarse-Grained Mg-Gd-Y-Zr Rolled Sheet

Superplasticity at high deformation rates is desirable in order to make superplastic forming more practical. High strain rate superplastic behavior and microstructure of the rolled Mg-Gd-Y-Zr alloy sheet were investigated. For the purposes, tensile tests at the strain rate of 0.01 s-1were conducted, which revealed that the sheet exhibited elongations of 180%~266%. Post-deforming microstructures were characterized by optical microscopy, scanning electron microscopy and transmission electron microscopy, while crystallographic orientation information was obtained from macro-texture analysis. The results show that the high strain rate superplasticity was attributed to class-I creep accommodated by dynamic recrystallization. It is suggested from microstructural analysis results that the interaction between second phases and dislocation facilitated dynamic recrystallization. The macro-texture at the strain of 0.8 still exhibited some characteristics of the crystal rotation arising from dislocation slip despite the occurrence of DRX.

Partial melting effect on tensile elongation behavior and activation energy for plastic flow in high-strain-rate superplastic 2124 Al alloy and 2124 Al composites

It is now well recognized that high strain-rate superplasticity (HSRS) can be obtained from powderprocessed aluminum and aluminum matrix composites [1–6]. Though the origin of HSRS is still controversial, it has been proposed that the presence of a liquid phase is important for achievement of HSRS as it serves to help the accommodation process for grain boundary sliding by relaxing the high stress concentration around the matrix/particulate interfaces [7]. Mabuchi and Higashi [8] showed that the optimum superplastic temperature for the MMCs is often close to the onset temperature for partial melting. Mabuchi et al. [8–10] and Koike et al. [11] conducted differential scanning calorimeter (DSC) tests on the 20% Si3N4 2124 Al composites reinforced by particulates or whiskers and discovered that for each material two downward peaks appeared in the DSC trace that were small and sharp, and large and continuous, interpreted to represent the local melting at the reinforcement/matrix interfaces and the melting of the aluminum alloy matrix, respectively. The temperatures for partial melting were 816 K, 784 K and 849 K for the 20% Si3N4 (dp= 0.2μm, where dp is the size of particulate reinforcement), 20% Si3N4 (dp= 1μm) and 20% Si3N4w (whisker) 2124 Al composites. Recently, Zahid et al. [12] reported HSRS in a PM 18 vol% SiCp (dp= 3μm)/2124 Al composite where tensile elongations over 300%, with the maximum value of 450%, were obtained in the temperature range between 743 K and 783 K at 7× 10−2 s−1. Their DSC test, however, did not show the sharp peak for partial melting. The onset temperature for a continuous endothermic curve was 861 K, which is significantly higher than the optimum temperatures where superplasticity was obtained. Kim et al. [13] recently investigated the HSRS in a PM 20 vol% SiCp (dp= 3μm)/2124 Al composite at the temperatures between 773 K and 838 K. Tensile elongations of about 300% could be obtained at temperatures between 793 K and 823 K near 10−1 s−1, with the largest tensile elongation of 400% at 808 K. Kim and Hong [14] also examined the HSRS behavior of a PM 2124 Al alloy where tensile elongations over 300% were achieved at temperatures between 773 K and 823 K at the optimum strain rate of 10−2 s−1, with the tensile elongation as large as 700% at 823 K. The aim of the present study is to look for evidence for partial melting in the HSR superplastic 2124 Al alloy and 20 vol% SiCp/2124 Al composite using a DSC technique, and to compare the DSC results with those reported by other investigators. The tensile ductility and activation energy for superplastic flow are also discussed based on the DSC test results. The chemical composition of the matrix of the composite is 3.5%Cu-1.24%Mg-0.53%Mn-0.14%Fe0.01%Zn-bal Al by weight percent. The 2124 Al alloy and 20% SiC/2124 Al composite were prepared by hot pressing at 843 K and hot extrusion at 723 K. DSC tests were conducted on the as-extruded and the superplastically deformed samples in a ∼50 mg specimen during heating and cooling at a rate of 10 K min−1. Two different DSC schedules were arranged for each material: 1) the sample was heated to 993 K; 2) the sample was heated to 878 K, held there for 30 min, and cooled down to 673 K. Then it was heated again to 993 K. Fig. 1 shows the DSC results for the superplastically deformed 2124 Al alloy (803 K, 10−1 s−1, 280%) and 20% SiCp/2124 Al composite (808 K, 9× 10−2 s−1,

高強度Ａｌ‐Ｚｎ‐Ｍｇ‐Ｃｕ系Ｐ／Ｍ合金押出材のミクロ組織微細化と高速超塑性特性

The effects of thermo-mechanical treatment (TMT) on grain refining and high temperature deformation properties have been investigated for the alloy with chemical composition of Al–9.6Zn–3.0Mg–1.4Cu–0.04Ag–0.4Zr in mass% fabricated by hot-extrusion from rapidly solidified powders The present TMT consisted of aging at 498 K for precipitation of T phase (Mg32(Zn, Al)49) and subsequent warm rolling at the same temperature. Due to aging up to 7.2 ks at 498 K, extremely fine T phase precipitated homogeneously in the matrix. At the next warm rolling step, those fine precipitates contributed to form the uniform dislocation cell structure with high number density. The microstructure analysis by means of XRD and TEM made clear that their dislocation cell structure recrystallized continuously and transformed to very fine (sub-) grain structure with average size of 0.45 μm at the tensile testing temperature of 718 K. The fine grain structure was stabilized by the distribution of fine Al3Zr particles. Thus the high strain rate superplastic behavior realized for the warm rolled specimens after aging at 498 K for about 7.2 ks. On the other hand, the warm rolled specimens after aging at 498 K for the longer time than 18 ks formed a very coarse grain structure and resulted in a poor superplasticity. The reason has been discussed on the basis of discontinuous recrystallization theorem.

The Effect of Fe-Addition to Al-10Ti Alloy on Superplasticity at High-Strain Rates

AbstractUltra-fine microstructure consisting of equiaxed Al-grains and aluminide particulate was produced by powder metallurgy process using gas-atomized powders of Al-10wt%Ti-2wt%Fe alloy. High strain rate superplasticity (HSRS) has been investigated at 873-923K and strain rates higher than 10−3s−1in tension, and total elongation up to 500% was observed at the strain-rate of 10−1s−1. The strain rate vs. flow stress behavior exhibits the typical aspect of HSRS such as the increase of strain-rate sensitivity exponent with increase in strain-rate and an apparent activation energy higher than that for lattice diffusion in aluminum. The concept of threshold stress has been incorporated to illustrate the HSRS behavior, where the stress exponent of 3 describes the experimental data. The determined threshold stress showed strong temperature dependency as in the case of a similarly processed Al-10wt%Ti alloy, which exhibited the stress exponent of 2 in the same testing conditions. Solute drag mechanism has been postulated for the Al-Ti-Fe alloy.

Effect of volume fraction of ain particle on superplasticity of AlN/6061 aluminum alloy composite

Ceramic whisker or particulate reinforced aluminum alloy composites which exhibit higher elastic modulus and tensile strength, excellent heat resistance and wear resistance, high thermal conductivity and can produce high strain rate superplasticity (HSRS), are expected to have application in engine and wear resistant components and even in semi-conductor packaging for the automobile and aerospace industries. The purpose of this study is to examine the effect of volume fraction of AlN on HSRS behavior of an AlN particulate reinforced 6061 aluminum alloy composite and the maximum volume fraction by which the HSRS could produce in order to understand the deformation mechanism of the HSRS.

High strain rate superplasticity of a powder metallurgy SiC particulate reinforced 6061 Al composite (6061/SiC/17.5p)

One of the major drawbacks of conventional superplastic forming is that the phenomenon is only found at relatively low strain rates, typically about 10[sup [minus]4] to 10[sup [minus]3] s[sup [minus]1]. Recent studies have demonstrated that superlasticity can exist at considerably higher strain rates than 10[sup [minus]3] s[sup [minus]1]. This high-strain-rate superplasticity (HSRS) phenomenon has now been observed principally in metal matrix composites and mechanically alloyed materials, but it has also been observed in more conventionally produced metallic alloys. Metal matrix composites can be fabricated by various techniques, e.g., squeeze casting, vortex methods, compocasting, powder metallurgical (PM) methods, and spray deposition. However, metal matrix composites that show HSRS are fabricated primarily by PM methods. Recently, Hikosaka et al. demonstrated that an ingot metallurgy (IM) 20 vol% SiC[sub p]-reinforced 6061 Al can also exhibit the HSRS phenomenon. The purpose of this study is to investigate the HSRS behavior of a SiC particulate-reinforced 6061 Al prepared by a powder metallurgy method and to compared its behavior with that of the composites produced by IM methods. The effect of thermomechanical processing and testing temperature will be discussed.