Papers by Author: Laurent Barrallier

Abstract: The evolution of residual stresses during short time gas nitriding of 33CrMoV12-9 steel grade is studied. It aims understanding the influence of nitriding parameters (temperature and nitriding potential) on the generation and evolution of residual stresses in the very first stage of nitriding. The samples are gas nitrided using a thermobalance during 2h30 and 5 hours for various temperatures and nitriding potentials. Residual stress analyses are carried out by laboratory X-ray diffraction.

Abstract: UMo-Al specimens are analyzed using X-ray diffraction techniques. One specimen was partially irradiated using a heavy ion beam ¹²⁷I. Another specimen was thermally annealed 2h at 400°C. Those treatments result in the formation of an interaction layer between UMo particles and Al matrix. UMo, Al and UAl₃ phases are identified in the treated specimen using X-ray diffraction. Only aluminium phase exhibits a crystallographic fiber texture, the other phases having an isotropic crystallographic texture. X-ray stress analyses are performed. After irradiation, stress analyses show that UMo phase is in a compressive stress state whereas the stress level in the formed UAl₃ in the interaction layer is not that high.

Abstract: Nitriding is a well-established thermochemical surface treatment of carbon micro-alloyed steels aiming enhancing surface properties such as fatigue, wear and corrosion resistances. The idea is taking benefits from the high hardening level due to a fine nitride precipitation and also the compressive residual stress state. Due to some complex interactions of phenomena during nitriding, the last has not been completely explained yet. When interest is focused on stress depth gradient and time evolution, difficulties find origins in pronounced heterogeneities whether it is the chemical gradient due to nitrogen diffusion, the resulting gradient of microstructure or the gradient of volumetric misfits. Relaxation of residual stresses is so usually described using a thermally controlled creep phenomenon due to the couple of diffusion and stress, but depend on phenomenological descriptions. A key point is also disregard that is the diffusion of carbon and its redistribution across the nitrided surface during the treatment. Based on experimental characterizations of model carbon iron-based alloys, the role of phase transformations, especially carbides, is explored in order to give better understandings of the residual stress development during nitriding.

Abstract: Residual stresses are to be determined to predict mechanical behaviour of vital parts in aircraft. This part constituted by Ti-10V-2Fe-3Al is analysed using x-ray diffraction (XRD). However, the broadening and sometime the disappearance of peaks involve a major difficulty to quantify the strains and the stresses which are close to the surface. In the current study, some investigations are conducted in order to understand the difficulties to evaluate stresses by XRD. The microstructure of samples is studied by taking into account the history of surface generation: after the forging and after the shot-peening process. Shot-peening induces a distortion of the primary phase α which is close to the surface. Different deformation rates introduced by forging process have generated a different organisation in the second phase constituted by α secondary and β. To complete the investigation, crystallographic textures are performed on α phase. The main difficulty in analysing when using XRD is due to the peak broadening. The conclusion of this study identifies three separate zones beneath the surface: where the stresses can be determined, it is possible to analyze it using different method of analysis with different levels of accuracy.

Abstract: This experimental study focuses on the influence of shot peening on parts initially treated by carburizing or carbonitriding. Experimental investigations have been carried out: optical observations of the microstructures, in-depth hardness measurement, X-ray diffraction analysis of residual stresses. A comparison is made between the carburizing anad the carbonitriding treatments.

Abstract: This work deals with the development of residual stresses during nitriding of steels. The main features of a chemico-thermo-echanical model of nitriding are presented. A micro-macroapproach is applied based on volume change computation in agreements with thermochemical modifications. Results are correlated with the characterization of a ternary Fe-C-3w.%Cr alloy nitrided at 550°C for various time. Residual stress-depth analyses are carried out by X-rays diffraction. Residual stress generation is deeply dependant on chemical and thermodynamical evolutions during the treatment, taking advantage on microstructural effects.

Abstract: In this article, two original studies of the alumina as porous substrate and PLD (pulsed laser deposition) thin films in view of its biomedical and tribological applications are presented. The first biomedical study aimed to evaluate the role of Al2O3 on thin deposited nanostructures. For this purpose, cerium stabilized zirconia doped hydroxyapatite thin films were deposited by PLD onto high purity, high density alumina substrates with different low porosities. For deposition, an UV KrF* (λ=248 nm, τ ~ 25 ns) excimer laser was used for the multi-pulse irradiation of the targets. The nanostructured surface morphologies of the thin films with micro droplets were evidenced by atomic force microscopy and scanning electron microscopy and the compositions with a Ca/P ratio of 1.7 by energy dispersive spectroscopy. The films were seeded with mesenchymal stem cells for in vitro tests. The cells showed good attachment and spread and covered uniformly the surface of the samples. Different functions of substrate porosities are observed in the efficiency of developing long filopodia and of obtaining the optimal intracellular organization. The second study aimed to understand the influence of micro-structural and mechanical characteristics on the tribological behaviour of stainless steel samples with PLD alumina coatings produced using an UV KrF* (λ=248 nm, τ ~ 20 ns) excimer laser and a sintered alumina target. Various microscopic observation techniques were used in order to connect the tribological response to the amorphous microstructure of the coatings. The results correspond to the determination of the mechanical characteristics by nanoindentation tests, scratch tests, and a tribological behaviour analysis of the treated steel against 100Cr6. The films were stoichiometric, partially crystallized with an amorphous matrix and their surfaces had few particulates deposited on. The obtained values of hardness and elastic modulus of the films were in good agreements with literature data.

Abstract: Seamless tubes are used for many applications, e.g. in heating, transport gases and fluids, evaporators as well as medical use and as intermediate products for hydroforming and various mechanical applications, where the final dimensions normally are given by some cold drawing steps. The first process step – piercing of the billet, for example by extrusion or 3-roll-milling - typically results in ovality and eccentricity in the tube causing non-symmetric material flow during the cold drawing process, i.e. inhomogeneous deformation. Because of this non-axisymmetric deformation and of deviations over tube length caused by moving tools, this process step generates residual stresses. To understand the interconnections between the geometrical changes in the tubes and the residual stresses, the residual strains in a copper tube had been measured by neutron diffraction.

Abstract: A simple model including the recovery and nucleation kinetics is proposed to simulate recrystallization using a Monte Carlo technique. Nucleation is simulated on the basis of equations describing stored energy evolution and subgrain growth kinetics. The parameters describing the stored energy evolution during recovery are shown to influence not only the relative number, orientation and local distribution of nuclei, but also the recrystallization kinetics and the final texture. The estimated kinetics of nucleation, the mean stored energy evolution as well as the preferential texture of first nuclei are in agreement with experimental observations for annealing 40% cold-rolled IF-Ti steel and cold drawn copper wires.

Abstract: Orientational patterns of wetted grain boundaries (GB's) in stressed NaCl polycrystals have been used for describing the GB penetration process on the basis of combined interfacial and mechanical energy balance considerations. The response of the internally wetted polycrystals to static loading has been shown to obey “pressure solution” constitutive laws involving dissolution at stressed surfaces, diffusive transport of dissolved matter and precipitation at less stressed surfaces. Direct experimental evidence for pressure solution mechanism is presented.