Papers by Author: Pascal J. Jacques

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Authors: You Liang He, Stéphane Godet, Pascal J. Jacques, John J. Jonas
Abstract: The crystallographic relationship between the g and a phases in samples of the Gibeon meteorite and a TRIP steel was investigated by means of EBSD techniques. The orientations of the two phases were measured and are represented in pole figures. The results are compared to predictions made on the basis of the Bain, Kurdjumov-Sachs (K-S), Nishiyama-Wassermann (NW), Greninger-Troiano (G-T) and Pitsch orientation relationships. The local misorientation between individual fcc and bcc crystals along their common interface was measured to demonstrate the way in which the exact orientation relationship varies along the boundary. The local orientations within lamellae and laths of kamacite and bainite are compared to that in recrystallised ferrite polygons. The occurrence of variant selection during the transformation of deformed austenite is analyzed using a recent dislocation-based model.
Authors: Stéphane Godet, You Liang He, John J. Jonas, Pascal J. Jacques
Abstract: The orientation relationships that apply to phase transformations in high-performance TRIP and TWIP steels were characterised by orientation imaging and EBSD techniques. The results are presented in the fundamental zones of Rodrigues-Frank space that correspond to the specific phase transformation under consideration (cubic to cubic or cubic to hexagonal). The use of Rodrigues-Frank space enables straightforward comparison to be made with orientation relationships proposed in the literature. The observations indicate that the active slip systems in the parent phase play important roles in variant selection.
Authors: Astrid Lenain, Nicolas Clément, Muriel Véron, Pascal J. Jacques
Authors: Stéphane Godet, Ph. Harlet, Francis Delannay, Pascal J. Jacques
Authors: You Liang He, Stéphane Godet, Pascal J. Jacques, John J. Jonas
Abstract: The mechanisms governing the formation of transformation textures during the austenite-to-ferrite transformation are the subject of major debate. In this study, two extreme cases were examined: those of undeformed and deformed austenite. The first involves the transformation of austenite into Widmanstätten ferrite under "equilibrium" conditions in the Gibeon iron-nickel meteorite. This meteorite passed through the transformation at the rate of a few degrees per million years. Such cooling rates cannot of course be reached under laboratory conditions. The second concerns the transformation of hot rolled austenite after a quench into the bainite temperature range. These two behaviors were investigated by means of optical microscopy and electron backscatter diffraction (EBSD) techniques. The orientations of both the parent and product phases were measured and the orientation relationships are represented in Rodrigues-Frank (R-F) space. From the orientation of a particular FCC crystal, the crystallographic orientations of the product BCC crystals can be predicted according to the Bain, Kurdjumov-Sachs (K-S) and Nishiyama- Wassermann (N-W) correspondence relationships. Comparison of the predicted and measured orientations reveals that the Bain rotation is never observed; the K-S and N-W relationships are both observed and there is a continuous distribution of orientations between the exact K-S and N-W positions. The formation of preferred orientations under non-equilibrium conditions is scrutinized. These results are compared to recent models accounting for variant selection.
Authors: Fan Sun, Jing Yong Zhang, Matthieu Marteleur, Thierry Gloriant, Philippe Vermaut, Philippe Castany, Caroline Curfs, Pascal J. Jacques, Frédéric Prima
Abstract: Titanium alloys typically exhibit a limited ductility (typically 20%) and little strain-hardening. An alloy design with new concept was conducted aiming at improving both ductility and strain hardening while keeping the mechanical resistance at an excellent level. An experimental validation was illustrated with the Ti-12(wt.%)Mo alloy, exhibiting true stress - true strain values at necking, of about 1000MPa and 0.38, respectively, with a large strain hardening rate close to the theoretical limit. In order to clarify the origin of this outstanding combination of mechanical properties, detailed microstructural investigation and phase evolution analysis were conducted by means of in-situ synchrotron XRD, in-situ light microscopy, EBSD mapping and TEM microstructural analysis. In the deformed material, combined Twinning Induced Plasticity (TWIP) and Transformation Induced Plasticity (TRIP) effects are observed. Primary strain/stress induced phase transformations (β->ω and β->α’’) and primary mechanical twinning ({332}<113> and {112}<111>) are simultaneously activated in the β matrix. Secondary martensitic phase transformation and secondary mechanical twinning are then triggered in the twinned β zones. The {332}<113> twinning and the subsequent secondary mechanisms are shown to be dominant at the early stage deformation process. The evolution of the deformation microstructure results in a high strain hardening rate (~2GPa) bringing both a high tensile strength and a large uniform elongation.
Authors: C. Georges, Stéphane Godet, Pascal J. Jacques
Abstract: The present work describes the correlation between the microstructure and the mechanical properties of multiphase steels processed through a Continuous Annealing Process Line (CAPL). The mechanical and microstructural properties of one of the former experimental steel compositions was also processed via Batch Annealing (BA) and the results have been included for comparison reasons.
Authors: M. Sánchez-Araiza, Stéphane Godet, Pascal J. Jacques, John J. Jonas
Abstract: In warm rolled steels, the intensity of the <111>//ND annealing texture, which favours formability, has been related to the formation of shear bands during rolling. Coarse hot band grain sizes (HBGS’s) facilitate flow localization, the mechanism associated with the formation of shear bands.In this work, the effect of grain size after hot rolling was studied in a low carbon steel containing small additions of Cr and Mn. The formation of shear bands and their subsequent influence on the normal anisotropy rm and planar anisotropy Dr in the annealed steels were of particular interest. Two HBGS’s (18 and 30mm) were employed and the specimens were warm rolled to reductions of 65 and 80% at various temperatures between 640 and 700°C. The results show that the frequency of shear banding is slightly lower for the smaller grain size. The normal anisotropy was not affected by the HBGS; by contrast, much lower Dr values were associated with the finer grained steel.
Authors: Frédéric Prima, F. Sun, Philippe Vermaut, Thierry Gloriant, Diego Mantovani, Pascal J. Jacques
Abstract: During the last few decades, titanium alloys are more and more popular and developed as biomedical devices because of their excellent biocompatibility, very good combination of mechanical properties and prominent corrosion resistance [1-3]. Recently, a new generation of beta titanium alloys dedicated to biomedical applications has been developed. Based on biocompatible alloying elements such as Ta, Nb, Zr and Mo, these alloys were designed as low modulus alloys [4] or nickel-free superelastic materials [5, 6] mainly for orthopedic or dental applications as osseointegrated implants. Beta type titanium alloys take great advantages from their capacity to display several deformation mechanisms as a function of beta phase stability. Therefore, from low to high beta stability, stress assisted martensitic phase transformation (β-α’’), mechanical twinning or simple dislocation slip can alternatively be observed [7]. As a consequence, a very large range of mechanical properties can be reached, including low apparent modulus, large reversible elastic deformation or high yield stress. Although titanium alloys display now a long history of successful applications in orthopedic and dental devices, none of them have been commercially exploited in the area of coronary stents, despite their superior long term haemocopatibility compared to the 316L stainless steel. However, according to previous researches on the biocompatibility of various metals, the corrosion behavior of stainless steel is dominated by its nickel and chromium components, which may induce redox reaction, hydrolysis and complex metal ion–organic molecule binding reactions, whereas none are observed with titanium [8, 9].
Authors: Mathieu Iker, D. Gaude-Fugarolas, Pascal J. Jacques, Francis Delannay
Abstract: Twinning-Induced Plasticity steels (TWIP steels) are extensively studied due to their ultra-high strain-hardening rate, that brings about a remarkable combination of ductility and strength. Twinning can be observed in high manganese-carbon steels. This paper considers hardening by combination of mechanical twinning with carbide precipitation. The kinetics of precipitation and the morphological evolution of carbides with annealing time were studied for two different TWIP steels with high manganese and carbon contents. The steels are first cold-rolled and then annealed at 800°C for recrystallization and carbide precipitation. Depending on the steel composition, the kinetics of precipitation and the morphology of the carbides are quite different. The influence of the annealing cycle on the mechanical properties has also been assessed. The results are used to discuss the influence of composition, stacking fault energy (SFE) and carbide precipitation on twinning. We show that the usual criteria based on the SFE only are not sufficient to characterize the twinning ability of a steel.
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