Papers by Author: Pascal J. Jacques

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.

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].

Abstract: The orientation relationships that apply to the fcc (γ) – bcc (α) phase transformation in high-performance hot-rolled TRIP-aided steels were characterised by EBSD techniques. A statistical treatment of the experimental data allows the mean orientation relationship to be determined. This mean orientation relationship was compared to the models commonly proposed in the literature and confronted qualitatively to the predictions of the phenomenological theory of martensite crystallography (PTMC). The variant selection phenomenon was also characterized quantitatively at the level of individual austenite grains. The reconstruction of the EBSD maps evidences that bainite grows by packets in which the bainite laths share a common {111}γ plane in the austenite. This growth mechanism is not influenced by the prior hot deformation of the austenite. The hot deformation has a critical influence on the number of packets that forms. The analysis of the crystallographic features of the bainite packets reveals that all possible variants are formed in a packet, though in different proportions.

Abstract: Fe and Fe-C based alloys present the exceptional feature that the processing route can be adapted to lead to various phases that present antagonist mechanical properties ranging from soft ferrite to high strength martensite. Among the different deformation mechanisms that can be exhibited by these phases, the TRIP effect brings about large enhancements of the work-hardening rate. The current TRIP-assisted multiphase steels present a ferrite-based matrix with a distribution of islands of bainite and retained austenite obtained at the end of specific thermal or thermomechanical treatments. The present study aims at characterising the interactions occurring between ferrite recrystallisation and austenite formation during the intercritical annealing of cold rolled Nb-added TRIP-aided steels. It is shown that the addition of niobium retards the ferrite recrystallisation during heating. As a consequence, ferrite may not be completely recrystallised before the nucleation and growth of the austenite grains. Strong interactions between these phenomena can then be observed, i.e. a strong hindering of the ferrite recrystallisation due to the austenite formation. Furthermore, the heating rate from room temperature to the intercritical temperature range influences the thermodynamic conditions prevailing at the ferrite / austenite interface and dictates the phase proportions.

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.

Abstract: TRIP-assisted multiphase steels exhibit an excellent balance of strength and ductility, which makes them very attractive for the automotive industry. These remarkable mechanical properties can be attributed mainly to their composite-like microstructures and to the transformation of retained austenite into martensite during straining (Transformation-Induced Plasticity). The aim of this study is to highlight the interactions between the hot rolling conditions, the transformation of austenite and formation of the microstructure, and the resulting mechanical properties. Various rolling simulation techniques were employed to determine how the composite microstructure is formed during the various steps of multi-stage thermomechanical processing.

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.

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.