Materials Science Forum Vols. 571-572

Abstract: There have been many theoretical studies to predict the stress fields around the tip of a growing fatigue crack. However, until recently the highly-localized, small scale nature of the stresses has meant that direct measurement has not been possible. With the current generation of synchrotron X-ray sources, sub-millimetre sampling dimensions are now possible, and it has become possible to evaluate directly the stresses at the tip of a fatigue crack and to see how the stresses evolve as the result of an overload, for example. In this paper we present results of synchrotron X-ray diffraction analysis of the stress fields around a fatigue crack in aluminium alloy 5091 (Al-Mg-Li-C-O); this is a dispersion-strengthened alloy with a fine grain size, which makes it ideal for such experiments. Compact tension (CT) specimens were prepared with constant amplitude fatigue loading. The energy dispersive X-ray diffraction (EDXRD) technique was used for measuring strains around the crack tip along the mid thickness of the specimen under in-situ loading. The measurement was carried out at the ESRF (European Synchrotron Radiation Facility), Grenoble, France on the ID15A beam line. The experimental crack tip stresses have been compared with the analytical fracture mechanics solution.

Abstract: Current analytical techniques for simulating the generation of residual stresses within welded structures make approximations and simplifications relating to the mechanical properties of the weld metal. This is due to the paucity of knowledge on the anisotropic nature of weld metal. Consequently, the study of elastic and plastic anisotropy, within metals has been a subject of considerable interest to both experimentalists and modellers alike. This study investigates the elastic and plastic response of crystal lattice planes within single-pass and multi-pass austenitic steel weld metal using time-of -flight (TOF) neutron diffraction. As this material is often used for the fabrication of high temperature, pressure vessel components, it is important to understand the evolution of lattice strains within each set of planes during elastic and plastic loading. Neutron diffraction measurements were carried out for both single and multi-pass weld metal subjected to in-situ uni-axial tensile loading. The elastic response of individual reflections was recorded by diffraction measurements. All measurements were carried out at the ENGIN-X facility at ISIS, UK. The objective of the measurements was to determine the Young's modulus and characterise the elastic and plastic anisotropy of the weld metal. Measurements indicate a significant variation between the Young's modulus for the two materials, with values close to 210 GPa for the singlepass weld metal and 90-130 GPa for the multi-pass welds. These results are explained in terms of the texture of the welds.

Abstract: An ex-situ in-plane biaxial low cycle fatigued sample of cruciform geometry made from austenitic stainless steel AISI 321 H was studied with the help of neutron diffraction strain scanning and also in-situ uniaxial loading using a stress rig on the FSD stress-diffractometer at the IBR-2 pulsed nuclear reactor (Dubna). The objectives of the experiment were to measure the macroscopic and microscopic residual stresses, crystallographic phase composition and the mechanical characteristics of the alloy under fatigue conditions. To the best of our knowledge, no neutron diffraction investigations of structural alloys subjected to biaxial loading have previously been reported. Experimental data interpretation and analysis are presented and discussed.

Abstract: This paper describes the thermo-elasto-plastic analysis of bath and spray quenched AISI 316L cylinders. A suitably detailed continuum deformation analysis approach presented here is implemented within the framework of commercial Finite Element (FE) package ABAQUS. The results of the numerical analysis are compared with the residual elastic strains measured experimentally using neutron diffraction. The good agreement between measured and modelled residual elastic strains provides a basis for careful analysis of the residual elastic strain development resulting from two different quench methods. The conclusions drawn from the analysis provides a better understanding of quench processsing, so that the effects of different heat removal efficiencies of such processing technique can be taken advantage of to generate favourable residual elastic stress and deformation and microstructural distributions with quench processed components.

Abstract: Stored energy is generally considered as a main driving force of recrystallization process. After plastic deformation a high dislocation density and residual stress field remain in a material. Both quantities are at the origin of the stored energy and we call them as the “plastic” and “elastic” parts of this energy. Their orientation distributions can be determined using diffraction and deformation models. Both components of the stored energy are studied in the present work. Their distributions and characteristics are studied for f.c.c. and b.c.c. materials.

Abstract: This paper reports results of an in-situ compression experiment carried out on a hot rolled Zircaloy-4 plate at ENGIN-X, ISIS. The experiment was aimed at characterizing the plastic anisotropy of the alloy, which can give rise to high intergranular stresses in the polycrystal. As expected from the crystal anisotropy, the various lattice reflections had very different behaviours. In the compression directions, the basal <0002> reflections appeared to bear much more load than the other planes. The resulting intergranular elastic strains could therefore reach up to 5000 microstrain after 10% total deformation, and were responsible for high type II residual stresses after unloading. Considering the macroscopic behaviour, the normal direction had higher mechanical properties than the other two processing directions. The strong texture measured from EBSD measurements suggest that the crystal anisotropy has been brought to a macroscopic level. The experiment also evidenced a significant change in texture for compression along the rolling direction which indicates twinning activation.

Abstract: Neutron and synchrotron strain or stress evaluations are reliable when the probe volume is completely immersed in the studied material. However, acquisitions carried out close to interfaces are much more difficult to analyze. Under these conditions, it is indeed very difficult to characterize precisely the volume analyzed by the radiation and finally to define the measured depth. To solve this problem, a complete Monte Carlo simulation of neutron spectrometers and synchrotron experiments has been developed. This method allows defining precisely the size and shape of the probe used. It permits then predicting the evolution of the diffracted intensity versus the position of this volume in the matter. The calculations finally let to define the real analyzed depth, accounting for the local conditions of diffraction and absorption in the material. The method is illustrated by neutron and synchrotron experiments carried out to characterize stress fields existing close to interfaces. The simulations also permit predicting the shape of diffraction profiles that would be observed on perfect specimens. Such information can then be used to correct the instrumental broadening existing in real experiments. This allows a fine Fourier analysis of the diffraction peaks recorded for several orders of reflection and finally permits defining the mean size of the crystallites and the root mean squares of the strains of second and third kind. Such information is useful to characterize and analyze the mechanical behavior of materials.

Abstract: In recent years, nanostructured coatings by Plasma Thermal Spraying (PTS) attracted intense interest due to their enhanced mechanical properties as hardness, strength and ductility. The aim of this study was to evaluate the influence of coating the implant by nanohydroxyaptite, n-HAp, Ca10 (PO4)6(OH)2. The results obtained with n-HAp will also compared with the implant coated with HAp. Bone is a composite material in which are associated a mineral phase in the form of crystals of HAp and an organic matrix constituted by collagen. The c-axes of HAp and the collagen fibers are preferentially oriented in the direction of the stresses that the bones need to withstand. At the interface implant-bone, the new bone reconstituted after implantation must have the same proprieties of the original bone in order to have good fixation with the implant. Therefore, it is necessary to study the mechanical properties of this new bone crystals reconstituted at the interface with the implants coated with n-HAp and HAp by neutrons diffraction on D20 at ILL.

Abstract: For the investigation of retained strains during hot forming, which are related to the dislocation structure, single and double hit compression tests were carried out at different temperatures and strain rates for a stainless steel. Using microhardness measurements the retained strains after the first and second pass were investigated as a function of the amount of deformation, temperatures as well as strain rates and dwell durations. In general, the retained strain decreases with increasing dwell durations in both the deformed and recrystallized grains, respectively. At a given total amount of deformation in a double hit compression, the retained strains for the as deformed unrecrystallized grains are reduced for a lower deformation in the first hit. For the recrystallized grains the retained strain increases, when comparing double hit with single hit compression.

Abstract: In this work, the influence of temperature on the mechanical properties of duplex steel is studied by performing monotonic “in situ” tension and compression at 200oC. The lattice strains in both phases were measured using the time-of-flight neutron diffraction method (at the ISIS spallation neutron source, STFC Rutherford Appleton Laboratory, UK). A thermal-elastic selfconsistent model was used to predict the expansion of the interplanar spacings during heating to 200°C. Subsequently, the variation of phase stresses during tensile and compressive loading at room temperature (20°C) and at 200°C were theoretically calculated by the elastoplastic self-consistent model. Comparing the model data with experimental results the critical resolved shear stresses and work hardening parameters were determined individually in each phase of the DSS. Finally, the yield stresses in each phase of the studied steel have been estimated. It was found that both yield points (of austenite and ferrite) are lower at 200°C than those at room temperature.