Materials Science Forum Vols. 838-839

Abstract: Although superplasticity has a long history, dating back to the first laboratory-scale observations in 1934, the major new developments in superplasticity have occurred almost exclusively over the last four decades. Furthermore, this corresponds to the period associated with the ICSAM conferences which started with a first conference in San Diego, California, in June 1982 and has continued to ICSAM-2015 in Tokyo, Japan. Major developments over this time include the growth of a vibrant and effective superplastic forming industry and an extension of the concept of metallic superplasticity to include both ceramics and geological materials. This paper examines the significance of these developments and discusses future prospects and new opportunities within the field of superplastic research.

Abstract: Superplasticity of metallic alloys has been widely investigated for many years but despite numerous studies, some important questions regarding superplastic behavior of metals can be considered as still open. Among others, one can mention the role of grain boundary sliding as contributing mechanism of deformation, the microstructural requirements for getting superplasticity in metallic alloys or the link between mechanisms of deformation and mechanisms of damage. Interesting information were recently obtained by the use of new experimental techniques. One can mention for instance the development of fine grids allowing more detailed studies of the respective contributions of intra-and inter-granular strains in superplastic alloys or fast X-ray micro tomography allowing in situ 3D damage studies. The aim of this paper is to give information about current findings related to superplastic deformation of metallic alloys and to suggest some perspectives for future.

Abstract: Recent studies have revealed that ultrafine-grained (UFG) metals and alloys produced by severe plastic deformation (SPD) can demonstrate extraordinary superplasticity at low temperatures and/or high strain rates. This work presents new results on superplasticity in several UFG Al and Ti alloys focusing on microstructural evolution and strain hardening, as well as the challenges of their application. Grain refinement in these alloys was accomplished using severe plastic deformation techniques, including new modifications of equal channel angular pressing (ECAP). Unusual behavior of UFG alloys originates both from the formation of ultrafine grain by SPD processing as well as the state of grain boundaries in these materials. It is established that superplastic deformation allowed not only to attain their efficient forming, but also to improve the ultrafine-grained structure and to obtain enhanced mechanical properties in the articles produced. The results demonstrate the possibilities of new applications of superplastic forming using bulk nanostructured materials.

Abstract: Superplasticity in fine-grained oxide ceramics has been generally elucidated on the basis of their experimental strain rate-flow stress relationship and phenomenological analysis of cavity nucleation and growth. It has been widely accepted that the high temperature superplastic flow and failure in ceramics is significantly influenced by the atomic structure and chemistry of grain boundaries. Such phenomenon cannot be explained based on the classical phenomenological analysis. Our research group has therefore proposed to establish a new research field, grain boundary plasticity, to describe the superplastic deformation related to the grain boundary atomic structure. This paper aims to point out the importance of the atomistic analysis of grain boundary to develop new superplastic ceramics.

Abstract: High-temperature tensile deformation was performed using an oxide-dispersionstrengthened (ODS) ferritic steel,, which has grain structure largely elongated and aligned in one direction, in the perpendicular direction. In the superplastic region II, two-dimensional grain boundary sliding (GBS) was achieved, in which the material did not shrink in the grain-axis direction and grain-boundary steps appeared only in the surface perpendicular to the grain axis. In this condition, a classical grain switching event was observed. Using kernel average misorientation maps drawn with SEM/EBSD, dominant deformation mechanisms and accommodation processes for GBS were examined in the different regions. Cooperative grain boundary sliding, in which only some of grain boundaries slide, was also observed.

Abstract: The synthesis of ultrafine-grained (UFG) materials is very attractive because small grains lead to excellent creep properties including superplastic ductility at elevated temperatures. Severe plastic deformation (SPD) is an attractive processing technique for refining microstructures of metallic materials to have ultrafine grain sizes within the submicrometer to even the nanometer level. Among the SPD techniques, most effective processing is conducted through equal-channel angular pressing (ECAP) and high-pressure torsion (HPT) and there are numerous reports demonstrating the improved tensile properties at elevated temperature. This report demonstrates recent results on superplasticity in metals after ECAP and HPT. Moreover, superplastic flow of the UFG materials is evaluated by using flow mechanisms developed earlier for coarse-grained materials and depicted by plotting deformation mechanism maps which provide excellent visual representations of flow properties over a wide range of testing conditions.

Abstract: Texture change during superplastic deformation was examined and compared in two magnesium alloys with different chemical composition. These alloys were extruded to refine the microstructure. The pre-existing basal texture of both alloys became slightly more random within the bulk probably owing to grain boundary sliding and the accompanying grain rotation. However, the texture changes differed between tensile and compressive deformation along the extrusion (longitudinal) direction. This fact suggests that dislocation slip is important in superplastic deformation. It was concluded that dislocation slip acts primarily as an accommodation mechanism for grain boundary sliding.

Abstract: The evolution of surface, grains and dislocation structures during superplastic deformation was studied in Al–6.8%Mg–0.6%Mn–0.25%Cr alloy by SEM, EBSD, TEM techniques. The effective activation energy of superplastic deformation was calculated. Contribution of grain boundary sliding was defined during superplastic deformation. Low value of grain boundary sliding, significant dynamic grain growth in stress direction, high dislocations activity and permanent continuous formation of sub-grain boundaries during superplastic deformation were found.

Abstract: Changes in crystallographic orientation distribution during superplastic deformation in a fine-grained Al-Zn-Mg-Cu alloy and an Al-Mg-Mn alloy consisting of the coarse-grained surface and the fine-grained center layers have been reviewed in order to reveal contribution of dislocation slips to deformation. The strain rate and grain size dependences of the deformation behavior were examined by SEM/EBSD (scanning electron microscopy/ electron back scatter diffraction) analysis. Intragranular misorientation increases after deformation at high strain rates, presumably due to dislocation activity, while it was low in the specimen deformed at a low strain rate in the early stage of 35% strain. Progressive randomization of the initial texture was also found during deformation at the low strain rate. Further, grain structure and grain boundary character are analyzed in detail to discuss the deformation mechanism.