Papers by Keyword: Continuous Dynamic Recrystallization

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Authors: Keita Sekiguchi, Hiroshi Masuda, Hirobumi Tobe, Eiichi Sato
Abstract: A new class of superplastic titanium alloy, Ti–4.5Al–2.5Cr–1.2Fe–0.1C–0.3Cu–0.3Ni, was deformed at 1073 K with strain rates of 1×10−4–1×10−1 s−1, and microstructures in the condition between superplastic regions II and III (= 1×102 s−1) were observed using scanning electron microscope and electron back-scattered diffraction. Continuous dynamic recrystallization was observed, resulting in grain refinement both in α and β phases. The grain size decreased significantly in α phase at the early stage of the deformation and in β phase at the later stage. In the recrystallized microstructure, the major sub-boundaries formed perpendicularly to slip directions <11−20> in α phase and parallel to slip planes {110} in β phase, which might be caused by the difference in the symmetry of the crystal structures.
Authors: Arthur Galiyev, Rustam Kaibyshev, Taku Sakai
Authors: Matteo Caruso, Hector Verboomen, Stéphane Godet
Abstract: The aim of this research is to investigate the effect of Cr and Al (strong ferrite formers) on the strain-induced γ-to-pearlite transformation in eutectoid steels. The microstructure evolution during the hot deformation of three eutectoid steel grades was investigated using hot torsion testing. More specifically, the steels were deformed to strain levels varying from ε = 0,5 to ε = 1,5 at their specific Ar1 temperature. Hot deformation of the undercooled austenite leads to strain-induced γ-to-pearlite transformation and to the almost instantaneous spheroidization of the formed carbides. The corresponding microstructures consist of submicronic cementite particles and ferritic grains that are 1-5 μm in size. It is shown that 1,5% Cr addition and 0,5% Al addition increase the equilibrium transformation temperature but slower significantly the kinetics of the strain-induced transformation and consequently reduce the kinetics of cementite spheroidization and of ferrite recrystallization.
Authors: Alla Kipelova, Ilya Nikulin, Sergey Malopheyev, Rustam Kaibyshev
Abstract: Microstructural changes during equal channel angular pressing (ECAP) at the temperatures of 250 and 300°C to the strains ~4, ~8 and ~12 were studied in a coarse-grained Al-5.4%Mg-0.5%Mn-0.1%Zr alloy. At a strain of ~4, the microstructural evolution is mainly characterized by the development of well-defined subgrains within interiors of initial grains and the formation of fine grains along original boundaries. Further straining leads to increase in the average misorientation angle, the fraction of high-angle grain boundaries and the fraction of new grains. However, only at 300°C, the plastic deformation to a strain of ~12 leads to the formation of almost uniform submicrocrystalline (SMC) grained structure with an average crystallites size of ~ 0.5 m. At 250°C, the microstructure remains non-uniform and consists of subgrains and new recrystallized grains. The mechanism of new SMC structure formation after ECAP is discussed.
Authors: Xu Yue Yang, Hiromi Miura, Taku Sakai
Authors: Anne Laure Etter, Sandrine Bozzi, Thierry Baudin
Abstract: Dynamic recrystallization mechanisms have been studied after 5182 aluminum Friction Stir Spot Welding (FSSW) and dissimilar friction stir spot welding of 6016 aluminum alloy to IF-steel using EBSD measurements. Moreover, welds have been ice quenched after welding to state on the post-dynamic microstructure evolution after the tool removal. For the Al/Al welds, fine recrystallized grains of the stir zone result from a continuous dynamically recrystallization mechanism followed by a post-dynamic recovery that reduces the low angle boundary fraction in the periphery of the pin. As far as the dissimilar Al/Fe welds are concerned, steel grains of the base metal were fragmented into sub-grains in the thermomechanically affected zone. Nevertheless, recrystallized grains of the stirred zone were about three times larger than these sub-grains. In this case, the continuously recrystallized grains undergo a post-dynamic grain-growth during friction stir welding cooling. In the upper aluminum sheet, the recrystallization mechanisms are the same as in the Al/Al welds.
Authors: Cédric Chauvy, Pierre Barbéris, Frank Montheillet
Abstract: Compression tests were used to simulate simple deformation paths within the upper a-range of Zircaloy-4 (i.e. 500°C-750°C). The mechanical behaviour reveals two different domains : at low temperatures and large strain rates, strain hardening takes place before flow softening, whereas this first stage disappears at lower flow stress levels. Strain rate sensitivity and activation energy were determined for both domains. Dynamic recrystallization was investigated using the Electron BackScattering Diffraction (EBSD) technique. It appears that the mechanism involved here is continuous dynamic recrystallization (CDRX), based on the increasing misorientation of subgrain boundaries and their progressive transformation into large angle boundaries. At low strains (e £ 0.3), CDRX kinetics are similar whatever the deformation conditions, while higher temperatures and lower strain rates promote recrystallization at large strains.
Authors: A. Morozova, Andrey Belyakov, Rustam Kaibyshev
Abstract: The microstructure evolution and strength properties of a Cu-0.096%Cr-0.057%Zr alloy subjected to equal channel angular pressing (ECAP) at a temperature of 673 K via route BC to total strains of 1 to 4 were examined. The planar low-angle boundaries with moderate misorientations form within initial grains during the first ECAP passes. Upon further processing the misorientations of these boundaries progressively increase and the formation of new ultrafine grains occurs as a result of continuous dynamic recrystallization. Partially recrystallized ultrafine grained structure evolves at strains above 4. After straining to 4 the (sub) grain size attains 0.65 μm. The large plastic straining provides significant strengthening. The ultimate tensile strength increases from 190 MPa in the initial state to 420 MPa after 4 ECAP passes. A modified Hall-Petch analysis is applied to investigate the contribution of grain refinement and dislocation density to the overall strengthening.
Authors: Xu Yue Yang, Hiromi Miura, Taku Sakai
Abstract: Grain refinement taking place in a magnesium alloy AZ31 was studied in a single- and multi-directional compression at a temperature of 573K. The structural changes observed by SEM/EBSD analysis can be characterized by the evolution of many mutually crossing kink bands at low strains, continuous increase in their number and misorientation angle in moderate strain and finally full formation of a fine-grained structure in high strain. The characteristics of new grain evolution process are sensitively affected by initial grain size (D0) and strain path. New grains are developed faster with decrease in D0. Multi-directional compression accelerates the evolution of fine grains and the improvement of plastic workability. The mechanism of new grain formation is discussed in detail.
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