Papers by Author: Xavier Sauvage

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Authors: Xavier Sauvage
Abstract: Concentration gradients resulting from long range diffusion during Severe Plastic Deformation (SPD) have been investigated with the 3D Atom Probe technique (3D-AP). First, in a pearlitic steel where alloying elements (Mn, Si and Cr) are partitioned between the ferrite and carbides in the non-deformed state. After processing by High Pressure Torsion (HPT), they are homogeneously distributed in the nanostructure, indicating that long range diffusion occurred along with the dissolution of carbides. 3D-AP data of a Cu-Fe composite processed by HPT show as well a significant interdiffusion of Cu and Fe, probably promoted by additional vacancies. On the basis of these experimental data, and using the theory described for irradiated materials, vacancy fluxes and vacancy production rates were estimated assuming that new vacancies are continuously produced and eliminated on grain boundaries.
Authors: Hirotaka Matsunaga, Z. Horita, Kazutaka Imamura, Takanobu Kiss, Xavier Sauvage
Abstract: An age-hardenable Cu-2.9%Ni-0.6%Si alloy was subjected to high-pressure torsion. Aging behavior was investigated in terms of hardness, electrical conductivity and microstructural features. Transmission electron microscopy showed that the grain size is refined to ~150 nm and the Vickers microhardness was significantly increased through the HPT process. Aging treatment of the HPT-processed alloy led to a further increase in the hardness. Electrical conductivity is also improved with the aging treatment. It was confirmed that the simultaneous strengthening by grain refinement and fine precipitation is achieved while maintaining high electrical conductivity. Three dimensional atom probe analysis revealed that fine precipitates with sizes of ~20 nm or smaller were formed in the Cu matrix and some particles consist of Ni and Si with no appreciable amount of Cu.
Authors: Xavier Sauvage, Gerhard Wilde, Ruslan Valiev
Abstract: Some nanocrystaline Ni was prepared by repeated cold rolling with intermediate folding (F&R). The material was then processed by High Pressure Torsion (HPT) to study the grain evolution under additional Severe Plastic Deformation (SPD). Microstructures were characterized by Transmission Electron Microscopy (TEM) and the impurity distribution was analyzed by Atom Probe Tomography (APT). In this paper, we discuss about the influence of impurities on the grain growth during HPT and on the grain size reduction mechanism during SPD.
Authors: Juraj Balak, Xavier Sauvage, Duk Lak Lee, Choong Yeol Lee, Philippe Pareige
Abstract: Microstructures of cold drawn pearlitic steel wires were investigated by three-dimensional atom probe (3D-AP) to understand the influence of alloying elements on the decomposition of cementite. Before cold drawing, Si is mostly located in the ferrite phase, while Cr is located in the Fe3C phase and the amount of Mn is similar in Fe3C and in ferrite. Higher Si amount leads to higher dissolution rate of cementite and Cr has a little effect on cementite decomposition during drawing.
Authors: Guo Fan Zhang, Xavier Sauvage, Jing Tao Wang, Nong Gao, Terence G. Langdon
Abstract: The phase decomposition was investigated in Cu-Al alloys processed to a nanostructure condition by High Pressure Torsion (HPT). The microstructures are characterized by optical microscopy (OM), X-ray diffraction (XRD) and Atom Probe Tomography (APT). The results show that the’ → (1 + decomposition reaction begins in the early stage of annealing and it is much faster than in the coarse-grained state although there are similar phases after annealing.
Authors: Abdelahad Chbihi, Xavier Sauvage, Cécile Genevois, Didier Blavette, Dmitriy Gunderov, Alexander G. Popov
Abstract: A Fe50Pd50 alloy was severely deformed by High Pressure Torsion (HPT). For a processing temperature ranging from 20°C to 300°C, the Severe Plastic Deformation (SPD) induces a significant grain size reduction (in a range of 50 to 150 nm) but also a strong disordering of the long range ordered L10 phase. However, Transmission Electron Microscopy (TEM) data clearly show that few ordered nanocrystals remain in the deformed state. The deformed material was annealed to achieve a nanoscaled long range ordered structure. The transformation proceeds via the nucleation and growth of ordered domains along grain boundaries. Aging at lower temperature (400°C) gives rise to a smallest domain size and thus the highest coercivity.
Authors: Shamil Kh. Mukhtarov, Xavier Sauvage
Abstract: This paper presents an overview and some original results about the mechanical properties and phase analysis of a nanostructured (NS) nickel-iron based alloy INCONEL 718. This structure was obtained by severe plastic deformation (SPD) via high pressure torsion (HPT) and multiple isothermal forging (MIF) of the alloy with an initial coarse-grained (CG) structure. Materials before and after SPD were analyzed by scanning, transmission electron microscopes and atom probe tomography (APT). Experimental data indicate that after HPT at room temperature - phase was partly dissolved and that precipitation of the -phase occurs during post deformation aging. A hardness up to 8 GPa was recorded for the NS alloy after SPD and annealing at 600°C.
Authors: Xavier Sauvage, Jean Jacques Malandain, Anton Hohenwarter
Abstract: Here we report about the microstructure of a metal-polymer composite that was processed by severe plastic deformation. The composite was prepared by compaction of a sandwich made of Al foils and polyethylene films. This aluminum-polyethylene composite was processed by high pressure torsion and the microstructure was characterized by optical microscopy and scanning electron microscopy. Our experimental data clearly show that in the early stage, the deformation is not homogeneous within the sample, indicating that significant softening occurred. However, at larger number of revolution the deformation progressively reaches the sample centre and the final material exhibits an ultrafine grained composite structure.
Authors: N.I. Vlasova, V.S. Gaviko, A.G. Popov, N.N. Shchegoleva, L.A. Stashkova, Dmitriy Gunderov, Xavier Sauvage
Abstract: Equiatomic FePd alloy in the ordered state has been processed by means of high-pressure torsion deformation (HPTD) and then annealed. X-ray diffraction (XRD), transmission electron microscopy (TEM), and magnetic measurements have been carried out. HPTD results in an order-disorder transformation of the initial ordered L10-phase (s.g. P4/mmm) into a disordered fcc phase (s.g. Fm-3m) through the body-centered tetragonal (bct) phase (s.g. I4/mmm). Subsequent annealing restores the L10-phase.
Authors: Julia Ivanisenko, Ian MacLaren, Xavier Sauvage, Ruslan Valiev, Hans Jorg Fecht
Abstract: The paper presents an overview of a number of unusual phase transformations which take place in pearlitic steels in conditions of the severe deformation, i.e. combination of high pressure and strong shear strain. Strain-induced cementite dissolution is a well-documented phenomenon, which occurs during cold plastic deformation of pearlitic steels. Recently new results which can shed additional light on the mechanisms of this process were obtained thanks to 3DAP and HRTEM investigations of pearlitic steel deformed by high pressure torsion (HPT). It was shown that the process of cementite decomposition starts by carbon depletion from the carbides, which indicates that the deviation of cementite’s chemical composition from the stoichiometric is the main reason for thermodynamic destabilisation of cementite during plastic deformation. Important results were obtained regarding the distribution of released carbon atoms in ferrite. It was experimentally confirmed that carbon segregates to the dislocations and grain boundaries of nanocrystalline ferrite. Another unusual phase transformation taking place in nanocrystalline pearlitic steel during room temperature HPT is a stress induced α→γ transformation, which never occurs during conventional deformation of coarse grained iron and carbon steels. It was concluded that this occurred due to a reverse martensitic transformation. The atomistic mechanism and the thermodynamics of the transformation, as well as issues related to the stability of the reverted austenite will be discussed.
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