Laurent Barrallier

Evolution of residual stress in the diffusion zone of a model Fe-Cr-C alloy during nitriding*

Abstract Development and evolution of compressive residual stress during nitriding treatment are studied. A model carbon iron-based alloy Fe-3wt.%Cr-0.35wt.%C was nitrided in gas at 550 °C for different times. Microstructural investigation indicated an influence of the transformation and the associated carbon diffusion on the in-situ relaxation of residual stress induced by nitride precipitation. The transformation of initially present carbides into nitrides and the associated carbon accumulation ahead of the nitriding front is particularly taken into account. The distribution of residual stress in the nitrided case was predicted with a self-consistent mechanical model, using the volume changes associated with the phase transformations. To this end the nitrogen and carbon concentration distributions were converted into the equilibrium phase fractions of carbides and nitrides. An excellent correlation was obtained between experimental (X-ray diffraction analysis) and, thus, calculated independent residual stress distributions in the ferrite matrix.

Neutron and Synchrotron Evaluation of Residual Stresses in Coatings

Abstract The present study is dedicated to the evaluation of residual stresses at surfaces, in the bulk of materials and at interfaces, by classical X-ray and neutron diffraction and high-energy synchrotron measurements. It is mainly focused on the improvement of these experimental techniques. The new developed methods have been applied to a coating that consists of leucite moulded on a Palladium alloy substrate. These materials are employed in dental applications.

Identification of shear bands in wrought magnesium alloy friction stir welds and laser beam welds

Abstract The microstructural study of hot rolled magnesium friction stir and laser beam welds revealed the presence of shear band features starting from the weld edge and propagating in the base metal. These shear bands present a highly twinned microstructure and strain localisation occurs preferentially in this region. This influences the resulting mechanical properties of wrought magnesium alloy friction stir and laser beam welds.

Evaluation by synchrotron radiation of shape factor effects on residual stress in nitrided layers

Nitriding is a thermo-chemical treatment to improve fatigue life of steel parts what are exposed to high cyclic loading on and close to the surface like gears for example. During the nitrogen diffusion, the precipitation of nitrides and carbides generates residual stresses and increases the superficial hardness. These residual stresses are function of the conditions of the nitriding process but also critically depend on the geometry of the steel components. Indeed, the result of the diffusion process is different for a plane geometry or a curvilinear one. In this present work, the sample is a part of a gear, composed by two teeth. Between two teeth of gear, the determination of in-depth stress gradients by classical X-rays diffraction cannot be done with a great accuracy: the spatial resolution is not sufficient because the irradiated area has the same dimension than the surface curvature of the component. Furthermore, it is very difficult to take into account the removal of matter that is required to determine in-depth stress profile because of the particular geometry. The synchrotron diffraction technique is a well-adapted method to determine such stress gradients in strongly absorbing materials due to the capability of penetration power of high energy X-rays. The removal of matter is not required and it is possible to determine an in-depth map of the stress tensor. After measurements on the ESRF ID15 beam line, stress profile has been calculated without the σ33 equal to zero hypothesis. The results are as following: compressive residual stresses were found close to the surface, and the calculated σ33 component of stress tensor is really not equal to zero. This important result seems to show the geometrical effect on stress state near non-plane surface.

Neutron Determination of Residual Stress in a Nitrided Notched Part

Nitriding is an hardening thermomechanical treatment generally used to improve fatigue life of steel parts, like gear for example. Another effect of this treatment is generating superficial stress, influenced by nitriding conditions, composition of steel and geometry of the part. This work deals with the effect of shape on the residual stress profile obtain after nitriding on a gear tooth. The residual stress profile was determined using neutrons diffraction technique.

Mechanical Properties, Microstructure and Crystallographic Texture of Magnesium AZ91-D Alloy Welded by Friction Stir Welding (FSW)

The objective of the study was to characterize the properties of a magnesium alloy welded by friction stir welding. The results led to a better understanding of the relationship between this process and the microstructure and anisotropic properties of alloy materials. Welding principally leads to a large reduction in grain size in welded zones due to the phenomenon of dynamic recrystallization. The most remarkable observation was that crystallographic textures appeared from a base metal without texture in two zones: the thermo-mechanically affected and stir-welded zones. The latter zone has the peculiarity of possessing a marked texture with two components on the basal plane and the pyramidal plane. These characteristics disappeared in the thermo-mechanically affected zone (TMAZ), which had only one component following the basal plane. These modifications have been explained by the nature of the plastic deformation in these zones, which occurs at a moderate temperature in the TMAZ and high temperature in the SWZ.

Variation of Residual Stresses in Drawn Copper Tubes

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.

A Perspective of Pulsed Laser Deposition (PLD) in Surface Engineering: Alumina Coatings and Substrates

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.

Synchrotron Evaluation of Residual Stress in Palladium Alloy Substrate

The aim of this work is to improve some experimental techniques dedicated to the evaluation of residual stress at the surface and in the bulk of materials, by means of high-energy synchrotron measurements. This method has been applied to a metallic substrate used in dental applications.

Percolation Properties of Internal Wetted Polycrystals: Effect of Stresses and Material Structure

It is well established from studies of bicrystals that the wetting behavior of grain boundary (GB) depends on the GB crystallography and stress. However, only in several researches of polycrystalline materials properties this boundary-boundary variability is taken into account. In polycrystalline materials the whole network of GB is never completely wetted. The number of wetted boundaries and the topology of their network vary as a function of many factors such as texture, intensity and sign of residual or applied stresses, impurities, content, and specimen size. The role of these factors is not clearly understood yet. This article is devoted to the effect of material structure and stresses on the relative number of wetted GBs and on the connectivity of intergranular liquid in polycrystalline materials. Percolation approach is employed to describe the liquid phase connectivity in internal wetted polycrystals. Some examples are regarded. The relative number of wetted GBs in Zn/Ga system is established on the basis of the distribution of GB orientations and GB wetting statistics. The influence of stresses on the network topology and percolation threshold (the critical concentration of wetted boundaries at which an infinite network appears) value is discussed. The computer modeling is fulfilled to evaluate the threshold as the function of grain shape. The topology of liquid phase inclusions is discussed in terms of critical indexes.