Materials Science Forum Vols. 524-525

Abstract: Strain/stress experiments for characterization of micro- and macro-strain distribution and texture experiments were carried out on a varistor ceramic using neutron-TOF-diffraction. By a radial strain scan, residual strain has been determined on the (1010 ), (0002), (1012), and (1120 ) Bragg reflections of the hexagonal ZnO as well as on the (400) reflections of the cubic spinel Zn7Sb2O12. Residual strain values in the range of -1.5x10-4 (contraction) and 4x10-4 (dilatation) were observed for ZnO, with indications of macro-strains across the cylindrical sample and substantial micro-strains between the different phases. The texture on both phases (ZnO, Zn7Sb2O12) is characterized by a weak, non-ideal fibre texture around the cylinder axis. The results indicate that the texture pattern might coincide with the strain. To our knowledge, these are the first texture and residual strain investigations in metal oxide varistor elements and they will be discussed in detail.

Abstract: Manufacturing diamond-cobalt composites by sintering results in residual stresses due to the mismatch of thermal expansion coefficients mainly. To understand the influence of manufacturing process parameters on residual stresses of sintered diamond-cobalt composites samples are produced by different process parameters. The investigated diamond composites are pressureless sintered as well as pressureless sintered combined with hot isostatically pressing. Here the influence of powder characteristics and process parameters like compaction pressure and sintering temperature on the residual stresses and microstructure was analysed by X-ray diffraction, microscopy and tomography. The aim of this study is to correlate residual stresses with manufacturing parameters and to give hints for optimising the residual stress state and for improving the lifetime of diamond-cobalt composites.

Abstract: Residual stress in polycrystalline coatings can be determined by X-ray diffraction. The data collection requirements are summarized and evaluated in this paper. First, general requirements for stress measurements are described. Then, requirements related to the diffraction geometry and the specimen manipulation are considered. Finally, requirements with respect to specimen characteristics, including various coating-substrate combinations are presented. Polycrystalline coatings can be nanocrystalline, randomly orientated or highly textured. The substrates can be of any nature: amorphous, polycrystalline or single crystal. The complete set of requirements leads to a measurement advice for a particular coatingsubstrate specimen, which includes the choice of diffraction geometry and the data collection strategy. Based on two complementary test cases it is demonstrated that the set of rules is complete and that they can be applied to any type of polycrystalline specimen.

Abstract: Thin film diffusion couples (Pd-Cu, individual layer thicknesses: 50 nm) have been prepared by DC-magnetron sputtering on silicon substrates coated with amorphous inter-layers (Si3N4 on top of SiO2). The microstructural development, phase formation and the stress evolution during diffusion annealing have been investigated employing Auger-electron spectroscopy in combination with sputter depth profiling, ex-situ and, in particular, in-situ X-ray diffraction measurements. Upon annealing at relatively low temperatures (175°C to 250°C) for durations up to 100 hours, considerable diffusional intermixing occurs. Interdiffusion is accompanied by the sequential formation of a new phase (Cu3Pd). The detected stress changes are discussed in terms of possible mechanisms of stress generation.

Abstract: Two kinds of electrodeposited copper foils (thickness is 8 and 20 μm) were loaded statically, and the deformation behavior was observed. In-situ X-ray stress measurement was carried out under tensile loading. Fatigue tests were also conducted to observe the effect of the thickness on the fatigue strength. Change in the line broadening with stress cycles was observed to evaluate the fatigue damage. The tensile strength of 8 μm foil was higher than that of 20 μm foil. When the foils were loaded within elastic region, the stress measured by the X-ray method agreed with applied stress. When the plastic deformation occurred, difference between the measured stress and the applied stress became large. The difference of 20 μm foil was larger than that of 8 μm foil. Fatigue strength of 8 μm foil was also higher than that of 20 μm foil. The value of the full width at half maximum, FWHM, increased rapidly at the first cycle, and then the value became nearly constant. Just before fracture, the value increased again. The change in FWHM corresponded to the change in the accumulated ratchet strain.

Abstract: The objective of this paper is to study the influence of residual stresses due to fabrication conditions on the thermomechanical behavior of carbon/epoxy laminate structures (cross ply). These studied laminates have undergone various cycles of thermal aging. The addition of a post-cure cycle after the end of the initial cycle makes it possible to reduce the residual stresses level. The incremental hole-drilling method is used to measure the residual strain in the laminates. These measured strains and the numerical calibration coefficients obtained by the finite element method allow to calculating the residual stress distribution in composite depth. The obtained results show that heat treatments of composite structures do not lead to an important reduction the initial residual stress due the fabrication conditions.

Abstract: The deformation behavior of metastable austenitic stainless steel AISI 301, suffering different initial cold rolling reduction, has been investigated during uniaxial tensile loading. In situ highenergy x-ray diffraction was employed to characterize the residual strain evolution and the strain induced martensitic transformation. Moreover, the 3DXRD technique was employed to characterize the deformation behavior of individual austenite grains during elastic and early plastic deformation. The cold rolling reduction was found to induce compressive residual strains in the austenite along rolling direction and balancing tensile residual strains in the ά-martensite. The opposite residual strain state was found in the transverse direction. The residual strain states of five individual austenite grains in the bulk of a sample suffering 2% cold rolling reduction was found to be divergent. The difference among the grains, considering both the residual strains and the evolution of these, could not be solely explained by elastic and plastic anisotropy. The strain states of the five austenite grains are also a consequence of the local neighborhood.

Abstract: The accurate prediction of the residual stresses present in welded structures can be of great importance to the fracture assessment of such components. Therefore, a large amount of benefit can be gained from improving techniques for measuring and numerically analysing these stresses. In recent years many advances have been made in the field of analysing residual stresses using finite element methods. That said, very little work has been conducted on the accurate modelling of welded ferritic components. This is largely due to the added complication of phase transformations that occur during the heating and cooling of such steels. The objective of the work presented in this paper was to improve understanding of the effect that phase transformations have on residual stresses present within welded ferritic structures. This was conducted by simulating such welding processes using the finite element package SYSWELD. An investigation was conducted to determine how phase transformations, and therefore residual stresses, are affected by the welding process used. Phase transformation and material property data available within SYSWELD were used for this analysis. An autogenously welded beam provided a simple basis for this qualitative investigation. In the future the manufacture and measurement of suitable test-pieces will enable these simulations to be validated.

Abstract: A new variety of duplex steels with superior mechanical properties has been studied. They exhibit a very interesting combination of strength (tensile strength of 680 MPa) and ductility values (more than 45% total elongation) due to the competition between different plasticity mechanisms. These steels contain two phases: austenite and ferrite and are characterized by low stacking fault energy at room temperature. In this work, four duplex steels with different chemical composition and phase volume fraction are studied. Residual and internal stresses in each phase were determined using the classical X-ray diffraction sin²ψ method. In the as-received state, both longitudinal and transverse residual stresses are in compression (until -350 MPa) for the ferrite and in tension (until +410 MPa) for the austenite. However, residual stresses in the austenitic phase decrease when its volume fraction increases. Moreover, internal stress distribution in one alloy was determined by X-ray diffraction during an in situ tensile test. The austenitic phase stress along the loading direction is higher than the macroscopic applied one, which is higher than the ferritic stress state, verifying a mixture rule and consistent with the initial residual stresses. For an applied macroscopic strain of about 1%, the austenite phase is subjected to a stress of about 600 MPa whereas the stress in the ferritic phase is about 300 MPa. It was also observed that as macroscopic strain increases, stress difference between the austenite and the ferrite decreases.

Abstract: A new method for determining the parameters characterising elastoplastic deformation of two-phase material is proposed. The method is based on the results of neutron diffraction, which are analysed using the self-consistent rate-independent model of elastoplastic deformation. The neutron diffraction method (time-of-flight technique) was applied and the self-consistent model was used to predict the second order stresses in austeno-ferritic duplex steel. Calculations based on the model were successfully compared with experimental results for both phases of the duplex steel.