Characterization of Ultra-fine Grained and Nanocrystalline Materials Using Transmission Kikuchi Diffraction

One of the challenges in microstructure analysis nowadays resides in the reliable and accurate characterization of ultra-fine grained (UFG) and nanocrystalline materials. The traditional techniques associated with scanning electron microscopy (SEM), such as electron backscatter diffraction (EBSD), do not possess the required spatial resolution due to the large interaction volume between the electrons from the beam and the atoms of the material. Transmission electron microscopy (TEM) has the required spatial resolution. However, due to a lack of automation in the analysis system, the rate of data acquisition is slow which limits the area of the specimen that can be characterized. This paper presents a new characterization technique, Transmission Kikuchi Diffraction (TKD), which enables the analysis of the microstructure of UFG and nanocrystalline materials using an SEM equipped with a standard EBSD system. The spatial resolution of this technique can reach 2 nm. This technique can be applied to a large range of materials that would be difficult to analyze using traditional EBSD. After presenting the experimental set up and describing the different steps necessary to realize a TKD analysis, examples of its use on metal alloys and minerals are shown to illustrate the resolution of the technique and its flexibility in term of material to be characterized

Comportement mécanique d'une structure multicouches obtenue par co-laminage de tôles nanostructurées par smat

Le but de cette étude consiste au développement de structures multicouches à hautes caractéristiques mécaniques obtenues par co-laminage de tôles en acier inoxydable 316L préalablement nanostructurées à travers le procédé SMAT (Surface Mechanical Attrition Treatment), puis à la modélisation par éléments finis de ces structures composites. Afin de quantifier l'influence de certains paramètres expérimentaux et d'évaluer la réponse mécanique des structures nanostructurées et co-laminées, un certain nombre d'essais ont été menés. Des mesures de dureté Vickers et de rugosité nous ont permis de caractériser la surface des matériaux traités. De plus, pour pouvoir suivre l'évolution de la microdureté des échantillons et pour pouvoir évaluer la taille de grains, des essais de nanoindentation ont été réalisés sur une section transverse des échantillons. Des essais de traction combinés aux mesures de dureté et microdureté ont permis de remonter aux caractéristiques mécaniques des différentes sous-couches et ainsi de simuler le comportement de la structure multicouche obtenue

Modélisation du grenaillage de précontrainte via l'analyse dimensionnelle : du profil de contrainte vers la pièce déformé

Le Grenaillage est un procédé de traitement mécaniquede surface, qui consiste à bombarder la surface du matériau avec des billes projetées à différentes vitesses.L’impact provoque une déformation plastique et donc des contraintes résiduelles de compression en surface. L’objectif de cette étude est l’amélioration de la maîtrise du procède de grenaillage et de prévoir la distribution de contraintes résiduelles après grenaillageà l’aide d’un modèle conçu grâce à l’analyse dimensionnelle. Les contraintes résiduelles dépendent des propriétés physiques des billes et du matériau traité, des conditions du procédé etc, et ces paramètres sont prisen compte dans le modèle. Une comparaison entre les profils de contrainte modélisés et des profils expérimentaux est proposée afin de tester leur fiabilité. Pour simuler le grenaillage sur des pièces complexes nous proposons de calculer le profil de contrainte avec le modèle et de l’introduire ensuite dans un modèle éléments finis de la pièce complexe qui fournira, après calcul, l’état de contraintes et déformations sur toute la pièce. La possibilité d’utiliser un modèle dynamique des billes qui permet de simuler le grenaillage ultrason (GUS) et de fournir la distribution des vitesses d’impact permet l’utilisation de ce modèle pour le GUS. La prise en compte du profil de contrainte résiduelle dans le dimensionnement d’une pièce est extrêmement importante. L’avantage du modèle proposé est sa rapidité d’exécution, et la simplicité du passage entre les profils de contrainte modélisés à la pièce déformé à l’aide d’un code FEM et avec un calcul de rééquilibrage statique. Les futures développements concernentles méthodes d’introduction des contraintes afin d’effectuer des calculs sur des pièces à géométrie de plus en plus complexes

A model for the influence of work hardening and microstructure on the evolution of residual stresses under thermal loading – Application to Inconel 718

This study proposes a model for the influence of work hardening and microstructure on the thermal relaxation of residual stresses. To construct such a model, an experimental campaign is first conducted on shot peened samples of Inconel 718 to generate different levels of residual stress and work hardening. The effect of the grain size and the size of the strengthening precipitates is investigated by producing two modified microstructures. Two shot peening conditions are used to introduce several profiles of residual stress and work hardening. These profiles are evaluated using X-ray diffraction. A thermal loading is then applied at 550°C with varying holding times, leading to a rapid but not complete relaxation of the residual stresses and work hardening. The experimental results exhibit the fact that the work hardening levels have a significant influence on this relaxation while the grain size and the size of the strengthening precipitates have a very moderate influence. Based on these experimental results, a model is proposed that considers the influence of work hardening on the thermal relaxation of residual stresses with some predictive applications. It is therefore possible to estimate the relaxation of residual stresses at any point on a shot peened part

Grenaillage ultrason : expériences et simulations

On étudie le procédé de grenaillage ultrason sous deux aspects : a) un modèle numérique 3D de la dynamique des billes dans l'enceinte de traitement basé sur la théorie des gaz granulaires, b) un dispositif expérimental développé pour mesurer la distribution des vitesses des billes dans l'enceinte et au niveau de la surface traitée. L'intérêt d'une telle étude est de proposer une méthode expérimentale qui donne accès aux trajectoires de billes et de valider le modèle numérique pour optimiser les paramètres de ce procédé, utilisé par exemple en aéronautique

A calibration procedure for the assessment of work hardening part I: Effects of the microstructure and load type

This paper presents a methodology to define and quantify the level of work hardening locally in a material. The methodology is proposed after a thorough experimental study based on three complementary experimental techniques for microstructural characterizations: microhardness, X-ray diffraction (XRD) and Electron Backscatter Diffraction (EBSD) applied on Inconel 718 samples. In our analysis, several loading histories including single tension, single compression, high strain rates and low cycle fatigue have been investigated. The effects of the microstructure have been further investigated by modifying the size of the grains and the size of the strengthening precipitates. Experimental tests have also been simulated to choose a model variable able to represent work hardening. A reciprocal link between work hardening and experimental characterizations has then been established. Correlation curves have been proposed that enable to quantify the level of work hardening from the knowledge of the experimental data. Accuracy and complementarity of the three experimental approaches are discussed as well as the impact of the microstructure of the material on the measured quantities

A calibration procedure for the assessment of work hardening Part II: Application to shot peened IN718 parts

The objective of this paper is to discuss the application of the calibration methodology exposed in the previous part to shot-peened Inconel 718 specimens. Shot peening is commonly used to increase the fatigue life of critical parts such as Inconel 718 turbine discs. This surface treatment induces residual stresses, work hardening and possibly, gradients of microstructures that, in turn, affect fatigue life. Work hardening is a quantity that represents a set of physical and mechanical phenomena related to the level of disorder reached in the microstructure of the material. Work hardening is seldom taken into account in fatigue life assessment mainly because it is not possible to characterize this quantity directly. We propose to use the calibration methodology (see part I of this paper [1]) on samples shot peened with several conditions. The three complementary experimental techniques (microhardness, XRD and EBSD) are then used to determine through correlation curves the work hardening gradients. The meth-odology for characterizing the work hardening within shot peened specimens is first presented. A dis-cussion of the applicability of the method in this context is then provided. The results obtained for the different characterization methods and microstructural effects are analyzed in two different sections. Finally, the influence of shot peening conditions on residual stresses and on work hardening is dis-cussed, showing the interest of the proposed procedure to obtain a real picture of the mechanical state after shot peening

Numerical modelling of grain refinement around highly reactive interfaces in processing of nanocrystallised multilayered metallic materials by duplex technique

Microstructure evolution around highly reactive interfaces in processing of nanocrystallised multilayered metallic materials have been investigated and discussed in the present work. Conditions leading to grain refinement during co-rolling stage of the duplex processing technique are analysed using the multi-level finite element based numerical model combined with three-dimensional frontal cellular automata. The model was capable to simulate development of grain boundaries and changes of the boundary disorientation angle within the metal structure taking into account crystal plasticity formulation. Appearance of a large number of structural elements, identified as dislocation cells, sub-grains and new grains, has been identified within the metal structure as a result of metal flow disturbance and consequently inhomogeneous deformation around oxide islets at the interfaces during the co-rolling stage. These areas corresponded to the locations of shear bands observed experimentally using SEM-EBSD analysis. The obtained results illustrate a significant potential of the proposed modelling approach for quantitative analysis and optimisation of the highly refined non-homogeneous microstructures formed around the oxidised interfaces during processing of such laminated materials

Severe plastic deformation for producing superfunctional ultrafine-grained and heterostructured materials: An interdisciplinary review

Ultrafine-grained and heterostructured materials are currently of high interest due to their superior mechanical and functional properties. Severe plastic deformation (SPD) is one of the most effective methods to produce such materials with unique microstructure-property relationships. In this review paper, after summarizing the recent progress in developing various SPD methods for processing bulk, surface and powder of materials, the main structural and microstructural features of SPD-processed materials are explained including lattice defects, grain boundaries and phase transformations. The properties and potential applications of SPD-processed materials are then reviewed in detail including tensile properties, creep, superplasticity, hydrogen embrittlement resistance, electrical conductivity, magnetic properties, optical properties, solar energy harvesting, photocatalysis, electrocatalysis, hydrolysis, hydrogen storage, hydrogen production, CO2 conversion, corrosion resistance and biocompatibility. It is shown that achieving such properties is not limited to pure metals and conventional metallic alloys, and a wide range of materials are currently processed by SPD, including high-entropy alloys, glasses, semiconductors, ceramics and polymers. It is particularly emphasized that SPD has moved from a simple metal processing tool to a powerful means for the discovery and synthesis of new superfunctional metallic and nonmetallic materials. The article ends by declaring that the borders of SPD have been extended from materials science and it has become an interdisciplinary tool to address scientific questions such as the mechanisms of geological and astronomical phenomena and the origin of life