Abstract: A Kossel microdiffraction experimental set up is under development inside a Scanning Electron Microscope (SEM) in order to determine the crystallographic orientation as well as the inter- and intragranular strains and stresses. An area of about one cubic micrometer can be analysed using the microscope probe, which enables to study different kinds of elements such as a grain boundary, a crack, a microelectronic component, etc. The diffraction pattern is recorded by a high resolution Charge-Coupled Device (CCD) camera. The crystallographic orientation, the lattice parameters and the elastic strain tensor of the probed area are deduced from the pattern indexation using a homemade software. The purpose of this paper is to report some results achieved up to now to estimate the reliability of the Kossel microdiffraction technique.
Abstract: This article gives an overview of different methods for data treatment in x-ray stress measurement, and how these methods should be replaced with the matrix method, which in general is more versatile, more accurate and, in most cases, also easier to handle. It also shows how much the accuracy could be improved by replacing the traditional methods with the matrix method.
Abstract: Patterns calculated by applying the Debye function to faulted spherical nanoparticles are used to test the accuracy of modern Line Profile Analysis theory of faulting for small crystallites. The relative deviation of the determined fault density is found to be dependent on the fault position, and on the particle size. The study of the average pattern from systems of 100 particles (D = 9.8nm) shows an overestimated deviation of the determined fault density by as much as 30%.
Abstract: Two-dimensional x-ray diffraction is an ideal method for examining the residual stress and texture. The most dramatic development in two-dimensional x-ray diffractometry involves three critical devices, including x-ray sources, x-ray optics and detectors. The recent development in brilliant x-rays sources and high efficiency x-ray optics provided high intensity x-ray beam with the desired size and divergence. Correspondingly, the detector used in such a high performance system requires the capability to collect large two-dimensional images with high counting rate and high resolution. This paper introduces the diffraction vector approach in two-dimensional x-ray diffraction for stress and texture analysis, and an innovative large area detector based on the MikroGap™ technology.
Abstract: Solid Oxide Fuel Cell (SOFC) is a high-performance electrochemical device for energy conversion. Usually, the electrolyte is made of dense YSZ (Yttria Stabilized Zirconia) and the anode is a porous YSZ-Ni composite. The electrolyte is submitted to high stresses mainly due to the thermal expansion coefficient mismatch between components and the volume change associated with the redox cycling of Ni. Because the mechanical integrity of the cell is a major issue during its life time, it is proposed in this study to determine both micro and macro stresses in the electrolyte. Macro stresses in the 10 µm-thick electrolyte were measured using the sin²(Ψ) method after different treatments of the cell : (i) manufacturing, (ii) thermal cycle, (iii) reduction and (iv) re-oxidation of the anode layer. After manufacturing, the electrolyte is under strong biaxial compressive stresses (-690 MPa). These stresses decrease after reduction of the anode. They finally reach tensile stresses and induce the cracking of the electrolyte for full re-oxidation. Micro stresses determinations were performed using the micro-diffraction setup of the BM32 beam line at ESRF (European Synchrotron Radiation Facility). Complete strain-stress tensor and crystallographic orientation determinations have been achieved within 5 µm grains. The accuracy of the method has been improved and is now 2.4 10-4 for strain values. On an average, local measurements are found to be consistent with global ones. Both strain and stress heterogeneities between grains with various orientations have been evidenced.
Abstract: The load transfer among ferrite orientations and between ferrite and martensite was analysed in DP 600 steel by means of neutron diffraction during in situ tensile tests on the multiple pulse overlap time-of-flight strain scanner POLDI. The material had 0.07 wt% C and a martensite volume fraction of 15%. In situ tests were done in “Young” as well as in “Poisson setup”. The martensite phase could not be probed due to its low tetragonality. The curves of the lattice plane strains as a function of the externally applied macroscopic stress reveal (1) plastic relaxations of transformation and intergranular stresses in the compliant <100> oriented grains, and (2) a second inflection point in the fully plastic part indicating the onset of plastic deformation of the hard phase.
Abstract: Low Transformation Temperature (LTT) alloys were developed in order to control the residual stress development by the martensitic phase transformation already during cooling of the weld metal. The positive effect of such LTT alloys on the mitigation of detrimental tensile residual stresses during welding has already been confirmed on the basis of individual laboratory tests. Within the current project it was experimentally investigated whether the phase transformation mechanisms are effective under increased restraint due to multi-pass welding of thicker specimens. The local residual stress depth distribution was analyzed non-destructively for V-type welds processed by arc welding using energy dispersive synchrotron X-ray diffraction (EDXRD). The use of high energy (20 keV to 150 keV) EDXRD allowed for the evaluation of diffraction spectra containing information of all contributing phases. As the investigated LTT alloy contains retained austenite after welding, this phase was also considered for stress analysis. The results show in particular how the constraining effect of increased thickness of the welded plates and additional deposited weld metal influences the level of the residual stresses in near weld surface areas. While the longitudinal residual stresses were reduced in general, in the transition zone from the weld to the heat-affected zone (HAZ) compressive residual stresses were found.
Abstract: Two autogenously edge welded beams made from SA508 ferritic steel were investigated with the purpose of validating residual stress modelling tools which are relevant for integrity assessment of structural power plant components. The two specimens were welded with two different torch travel speeds. The residual strain and phase distributions were non-destructively determined by high-energy synchrotron X-ray diffraction. Good agreement between numerical and experimental data was found for the specimen welded at fast speed. Furthermore, the texture changes in the specimen welded at slow speed were analysed by the same experimental technique.
Abstract: This paper examines the extent to which mechanical shot peening (MSP), ultrasonic impact treatment (UIT) and laser shock peening (LSP) can affect the tensile residual stresses in the fusion zone caused by welding for a 10mm multi-pass 'V' groove weld within a 20 mm thick 304L stainless steel plate. Stresses are measured by deep hole drilling, neutron diffraction and incremental center hole drilling. For the UIT and LSP treated samples, the tensile stresses present in the as-welded plate are reversed to compressive stresses to a depth in excess of 2-4mm. For MSP the affected depth is much less (~0.5mm). The depth of these compressive stresses is similar to those measured in 20 mm thick parent plate test coupons.