Emmanuelle Girardin

Effects of thermal treatments on microstructure and mechanical properties of a Co-Cr-Mo-W biomedical alloy produced by laser sintering.

Direct Metal Laser Sintering (DMLS) technology based on a layer by layer production process was used to produce a Co-Cr-Mo-W alloy specifically developed for biomedical applications. The alloy mechanical response and microstructure were investigated in the as-sintered state and after post-production thermal treatments. Roughness and hardness measurements, and tensile and flexural tests were performed to study the mechanical response of the alloy while X-ray diffraction (XRD), electron microscopy (SEM, TEM, STEM) techniques and microanalysis (EDX) were used to investigate the microstructure in different conditions. Results showed an intricate network of ε-Co (hcp) lamellae in the γ-Co (fcc) matrix responsible of the high UTS and hardness values in the as-sintered state. Thermal treatments increase volume fraction of the ε-Co (hcp) martensite but slightly modify the average size of the lamellar structure. Nevertheless, thermal treatments are capable of producing a sensible increase in UTS and hardness and a strong reduction in ductility. These latter effects were mainly attributed to the massive precipitation of an hcp Co3(Mo,W)2Si phase and the contemporary formation of Si-rich inclusions.

Neutron diffraction measurements for the determination of heat treatment effectiveness in generating compressive residual stress in an automotive crown gear

Abstract Thermal austenitizing and tempering treatments are being developed in automotive industry to prevent crack initiation and propagation, especially in components where stress intensity factors influence the stress field and ultimately the fatigue life of the component. This is the case of crown gears, where the tooth root typically undergoes impulsive and very high loads which frequently cause cracking if tensile residual stresses are present at the surface. The sign reversal of these stresses is the aim of austenitizing and tempering treatments. In this work neutron diffraction measurements of residual stress (RS) on a UNI55Cr3 steel crown gear, carried out at HMI-BENSC 1 , are presented. The sample was submitted to a new multi-frequency induction technique whose effectiveness was checked. Comparisons with X-ray measurements are shown, and RS measured by X-rays on a similar shot-peened sample are also mentioned.

Defect Analysis on Optical Waveguide Arrays by Synchrotron Radiation Microtomography

In recent years, great attention has been devoted to the study and realization of polymeric optical waveguides embedded in printed circuit boards due to the increasing need of transferring large amounts of data at high speed within computer and telecommunication devices. Nonuniform microstructural defects that can be induced during the manufacturing process can dramatically influence the waveguide performance. The synchrotron radiation computed microtomography technique was used to obtain 3-D microstructural information, specifically to observe small defects, such as porosities, in a nondestructive way. Porosity level and pore size range were evaluated.

Neutron diffraction measurements for the determination of residual stresses in MMC tensile and fatigue specimens

Abstract The experiments here described have been carried out in the framework of a more general research, aiming to develop a set of complementary models to predict the in-service performances of particle reinforced MMC automotive and aeronautical components. As MMCs are highly heterogeneous materials, residual stresses are present in both the matrix and the particles microstructure, prior to any macroscopic loading. They vary with the temperature and with the type and level of loading imposed to the material, having a strong influence on the mechanical behaviour of MMCs. Neutron diffraction measurements have been performed at RISO (Roskilde, DK) and HMI-BENSC (Berlin, D), for the determination of residual stress in AA2124+17% SiCp and AA359+20% SiCp specimens, submitted to tensile and fatigue tests. For each of the investigated samples, the macrostress has been separated from the elastic and thermal mismatch microstresses. The results show that, in general, the main contribution to the stress state of both matrix and reinforcement is given by the thermal microstresses, already existing due to heat treatment prior to mechanical tests.

Neutron and Synchrotron Non-Destructive Methods for Residual Stress Determination in Materials for Industrial Applications

Neutron and synchrotron radiation techniques are very powerful non-destructive methods for the characterisation of a wide variety of materials. In particular, neutron and synchrotron radiation diffraction is nowadays widely used for the evaluation of residual stresses induced by thermal and mechanical treatments in materials and components for industrial applications. A review is presented of the techniques mentioned above, and some applications to materials for technological applications will be presented.

X-ray and neutron diffraction determination of residual stresses in a pressed and welded component

Abstract X-ray and neutron diffraction experiments have been carried out, in order to determine the residual stress (RS) field in a pressed and welded mock-up of an engine support for motor-bike technology. Such investigation is suggested by the need to know the stress state of the component after forming and welding. This allows to assess the quality of the first machining and further to theoretically simulate its performances under operation. Results are presented below, showing a good agreement between X-ray and neutron diffraction experiments.

Residual stress determination in several MMC samples submitted to different operating conditions

Abstract The experiments here described have been carried out in the framework of a more general research, aiming to develop a set of complementary models to predict the in-service performances of particular reinforced MMC automotive and aeronautical components. As MMCs are highly heterogeneous materials, residual stresses are present both in the matrix and the particles microstructure, prior to any macroscopic loading. They vary with the temperature and with the type and level of loading imposed to the material, and have a strong influence on the mechanical behaviour of MMCs. Neutron and X-ray diffraction measurements have been performed at HMI-BENSC (Berlin, Germany), and LLB (Saclay, France) for the determination of residual stresses in a prototype component an automotive brake drum (AA359+20% SIC). As expected, the results show that in each gauge point the main contribution is generally given by thermal microstresses.

Advanced Synchrotron Radiation and Neutron Scattering Techniques for Microstructural Characterization in Industrial Research

The rapid development of new materials and their application in an extremely wide variety of research and technological fields has lead to the request of increasingly sophisticated characterization methods. In particular residual stress measurements by neutron diffraction, small angle scattering of X-rays and neutrons, as well as 3D imaging techniques with spatial resolution at the micron or even sub-micron scale, like micro-and nano-computerized tomography, have gained a great relevance in recent years.Residual stresses are autobalancing stresses existing in a free body not submitted to any external surface force. Several manufacturing processes, as well as thermal and mechanical treatments, leave residual stresses within the components. Bragg diffraction of X-rays and neutrons can be used to determine residual elastic strains (and then residual stresses by knowing the material elastic constants) in a non-destructive way. Small Angle Scattering of neutrons or X-rays, complementary to Transmission Electron Microscopy, allows the determination of structural features such as volume fraction, specific surface and size distribution of inhomogeneities embedded in a matrix, in a huge variety of materials of industrial interest. X-ray microtomography is similar to conventional Computed Tomography employed in Medicine, allowing 3D imaging of the investigated samples, but with a much higher spatial resolution, down to the sub-micron scale. Some examples of applications of the experimental techniques mentioned above are described and discussed.

Advanced Microstructural Characterizations of Some Biomaterials and Scaffolds for Regenerative Orthopaedics

In the last decades, very significant advances have been made for what concerns bone and joint substitution and in the repair and regeneration of bone defects. Though some strong requirements are still to be met, biomaterials for these purposes have known an impressive evolution, for what concerns their mechanical behaviour, their bioresorbability and finally their capability to generate new bone tissue in a stable way in long periods. The validation of such materials necessarily depends on a suitable characterization of their properties. In this article a brief review of some works in this field, carried out by the authors’ research group, is presented. It was shown in particular how advanced experimental methods, such as synchrotron radiation µCT and synchrotron radiation diffraction can offer very important information, can be not only complementary methods to more standard techniques (electron microscopy, X-ray diffraction), but can also offer the possibility to measure parameters that cannot be obtained otherwise.

Residual Stress Analysis by Neutron and X-ray Diffraction

Residual stresses are self-equilibrating stresses occurring in a specimen after the original cause of the stress (external forces, heat gradient, mismatch of thermal coefficients during cooling or heating of multiphase materials), has been removed.