Papers by Author: Alain Lodini

Abstract: The TOF neutron diffraction measurements were done for Al/SiC_p metal–matrix composite (17% of SiC) subjected to T6 thermal treatment. Using three separated diffraction peaks of SiC phase and four peaks of Al phase, the lattice strains were measured for both phases independently during in situ tensile test. The experimental results were presented in comparison with elastoplastic model, which allows to find the values of parameters determining plastic deformation of Al matrix (critical resolved shear stress and hardening parameter). Additionally, the results of TOF method were compared with those which were obtained with monochromatic neutron radiation (LLB, Saclay). In the latter experiment Al/SiC_p composite containing 25% of SiC was measured. It was shown that after elastoplastic deformation the mismatch stresses determined for both phases relax during tensile deformation.

Abstract: The goal work of this work is to describe the qualitative and quantitative behaviour of titanium T40 during tensile test. Material characteristics were determined using EBSD and X-ray techniques. Textures, twin boundary fractions, residual stresses and coherent domain size were determined. It was found that deformation mechanisms and microstructure characteristics are different in the samples stretched along rolling and transverse directions. For example the average grain size, as determined from EBSD measurements, is higher in the sample stretched along rolling direction. Also smaller coherent domains form and residual stress is more easily relaxed in this sample. A strong appearance of tensile twins was observed in the samples deformed along transverse direction. In the present paper a complex study of material characteristics and deformation mechanisms is presented. A special emphasis is done on residual stress characteristics determined in the samples stretched in two perpendicular directions.

Abstract: The properties of the interface between biomaterials and the host tissue play an important role for the process of successful adaptation of implants. Extensive research has focused on shortening the time of osseointegration by modifying the surface in adding a coating such as hydroxyapatite (HAp). We have developed a new type of biocompatible nanohydroxyapatite (n-HAp) coatings, which are characterized before and after deposit on a Ti-6Al-4V substrate using neutron diffraction and scanning electron microscopy. Three months after the implantation in the sheep tibias, high-energy synchrotron radiation (ID15B, ESRF, Grenoble, France) diffraction studies of the cortical bone identify that the c-axes of HAp are preferentially oriented in the direction of the stresses that bone usually withstands. This non destructive analysis of the bone-implant interface proves that bone maturation is achieved successfully with this novel n-HAp coating and demonstrates that the mineralization is completed without spatial organization. None of these findings are obtained with uncoated titanium alloys. The presence of this n-HAp coating on Ti-6Al-4V substrate is decisive in obtaining this mature bone at the interface.

Abstract: The aim of this work is to understand the evolutions of the β₁ metastable austenite phase of a CuAlBe Shape Memory Alloy at macroscopic and microscopic scales under mechanical solicitation by neutron diffraction. The tensile specimen, taken in the raw material is subjected to superelastic cycles at room temperature on SMARTS diffractometer. Before loading, the mater ial is fully austenitic. During loading, after elastic deformation of austenite, phase transformation starts, martensite variants appear. The material follows a law of pseudo elastic behavior. At the end of the first mechanical cycle after unloading, the macroscopic curve does not fully return into its original point. A macroscopic deformation is observed. The evolution of first order microdeformations during mechanical cycles shows a large deformation of {400} plane family. This deformation is linked to the presence of <001> partial fibber characterizing the crystallographic texture of the material after elaboration. The FWHM of the (400) diffraction peak is also largely increased during loading. This increase is the signature of the generation of stacking faults during the transformation of β₁ metastable austenite into β₁ martensite.

Abstract: Asymmetric rolling can be used in order to modify material properties and to decrease forces and torques applied during deformation. This geometry of deformation is relatively easy to implement on existing industrial rolling mills and it can provide large volumes of a material. The study of microstructure, crystallographic texture and residual stress in asymmetrically rolled titanium (grade 2) is presented in this work. The above characteristics were examined using EBSD technique and X-ray diffraction. The rolling asymmetry was realized using two identical rolls, driven by independent motors, rotating with different angular velocities w₁ and w₂. It was found that asymmetric rolling leads to microstructure refinement, texture homogenization and lowering of residual stress.

Abstract: The multi-reflection grazing incidence X-ray diffrection was used to determine residual stress gradient in the mechanically polished Al-Mg alloy and CrN coating. Also, the root mean square values of the third order lattice strain was determined using Wiliamson-Hall method. The results obtained for Al-Mg alloy show that the stress field in the surface layer as well as the microstructure (density of dislocation) depend strongly on the sample preparation. A very high residual compressive stress, which does not change significantly with depth, was measured in the CrN coating. Moreover, a large value of the measured third order strains in the coating was found.

Abstract: The aim of this work was to study the implant-bone interface by synchrotron radiation in order to show the evolution of the crystallites of hydroxyapatite (Hap) reconstituted at the interface with the implant. The implant used an orthopaedic surgery is the Titanium alloy (Ti-6Al-4V), the implants are currently coated with (HAp), Ca₁₀ (PO₄)₆ (OH)₂, in order to obtain a stable and functional direct connection between the bone and the implant. In this work, two implants have been used, the first one coated with HAp and the second uncoated. At the implant-bone interface, the new bone reconstituted after two months of implantation must have the same properties like the natural bone in order to accept the implant. Therefore we studied the crytallinity index and texture of the new bone crystals reconstituted at the interface using synchrotron radiation on ID15 at ESRF in Grenoble, France.

Abstract: Asymmetric rolling is a promising forming technique offering numerous possibilities of material properties modification and the improvement of technological process parameters. This geometry of deformation is relatively easy to implement on existing industrial rolling mills. Moreover, it can provide large volume of a material with modified properties. The study of microstructure, crystallographic texture and residual stress in asymmetrically rolled titanium (grade 2) is presented in this work. The above characteristics were examined using EBSD technique and X-ray diffraction. The rolling asymmetry was realized using two identical rolls, driven by independent motors, rotating with different angular velocities ω₁ and ω₂. This ensured a wide range of rolling asymmetry: A=ω₁/ω₂. It was found that a strong shear stress induced in the asymmetrically rolled material allowed to obtain a microstructure refinement, texture homogenization and lowering of residual stress.

Abstract: The aim of the present study was to study the interface implant-bone by synchrotron radiation, the implant has two faces the first one coated with hydroxyapatite and the second uncoated. In orthopaedic surgery, Titanium (Ti-Al-4V) implants are currently coated with hydroxyapatite (HAp), Ca₁₀ (PO₄)₆ (OH)₂, in order to obtain a stable and functional direct connection between the bone and the implant. At the implant-bone interface, the new bone reconstituted after two months of implantation must have the same properties like the natural bone in order to accept the implant. Therefore we studied the texture of the reconstituted bone crystals at the interface applying non destructive x-ray diffraction. The required high spatial resolution was achieved utilizing high-energy synchrotron radiation on ID15 at ESRF in Grenoble, France.

Abstract: Grazing incidence geometry, called MGID-sin²y, was applied to measure surface stresses in very thin layers (depth of a few mm) of Al-Mg alloy samples subjected to different thermal and mechanical treatments. The Göbel mirror was used to parallelize the incident X-ray beam. Perfect collimation of the beam significantly increases accuracy of determined peak position and consequently allows to measure low stresses in surface layers.