Materials Science Forum Vols. 524-525

Abstract: The rail/ wheel contact comprises interactions of thermal and mechanical loadings which lead to microstructural changes in the wheel. These were investigated on a wheel from a regional train using metallographic examinations, X-ray diffraction for phase and residual stress analyses. The results show that the microstructural alterations, the carbon content and the residual stress distribution depend on the loading profile of the wheel. The formation of two different martensites (tetragonal and cubic martensite) on a wheel surface could be detected at different positions. The martensites are characterized by high hardness values, increased carbon contents in the lattice and an increased level of compressive residual stresses. Detailed structural analyses of the martensites which were formed under locally different loading and time conditions gave evidence for different structural evolutions.

Abstract: Microstresses due to intergranular and inter-phase interactions in an austenitic-ferritic super duplex steel (SAF 2507) under uniaxial compressive deformation have been studied by in-situ neutron diffraction experiments. Lattice strains of several hkl planes of austenite respective ferrite were mapped as a function of sample direction at a number of load levels during loading into the plastic regime and unloading. The analysis of the experimental results has shown that during loading both grain-orientation-dependent and inter-phase stresses were generated under plastic deformation that was inhomogeneous at the microstructural level. Residual stresses depending on the grain-orientation and phase have been found after unloading. The results also indicate stronger intergranular interactions among the studied hkl planes of austenite than those of ferrite.

Abstract: X-ray diffraction line profiles of a ferritic-pearlitic steel at different stages of monotonic and cyclic incremental strain paths were recorded and peak broadening was regarded. Evaluation of integral peak widths shows, that the distortion of structure is strongly dependent on the deformation path. The results for peak broadening at different deformation states correlate with flow stress behaviour and can be explained with certain dislocation arrangements.

Abstract: The deformation behaviour of the highly textured Mg-base wrought alloy AZ31 subjected to tensile and compressive elasto-plastic loading was characterized by means of synchrotron radiation. In former publications it was shown that there exists an asymmetry in the deformation behaviour of the investigated alloy, which can be related to the deformation mechanism of the hexagonal structure due to the crystallographic texture relative to the loading direction. A local X-ray stress analysis was carried out on highly textured metal sheets for different {hkil}- planes of the hexagonal crystal structure. The load transfer was investigated within in-situ loading experiments in order to account for suitable XEC´s, thus ensuring accurate stress evaluations. An X-ray imaging method was applied in order to provide bending stress distributions with a high local resolution using synchrotron radiation (beamline G3, Hasylab (DESY)). Stress analyses were carried out on the side face of a bending bar being subjected to elasto-plastic bending up to total strains in the outer layers of approximately 2%. It is distinguished between loading stress distributions measured in in-situ loading experiments as well as residual stress distributions monitored after unloading of elasto-plastically bended bars. Furthermore the loading direction was alternated, in order to investigate the effect of the twin formation as well as the reversibility of the twinning on the results of X-ray stress analysis.

Abstract: The brass alloys CuZn10 and CuZn37 were cold deformed by uniaxial compression in steps of 5 % res. 10 % to 20 %. Using X-ray synchrotron radiation d versus sin2ψ-distributions were measured. All diagrams showed strong non-linearities which were furthermore different for different hkl. The amplitudes of the oscillations increase with increasing deformation but come to a saturation, for CuZn37 at approximately 10 % compression, for instance. The development of the hkl-dependent residual stresses and the oscillations could be related to different microstructures induced by cold deformation. The Voigt-model suits well to describe the oscillations in d-sin2ψ curves associated with the hot extrusion process which was used for the fabrication of the samples. Moreover, it is shown that additional information obtained from TEM and X-ray line profile analysis is necessary to reproduce the experimental oscillations of d versus sin2ψ-distributions measured in cold deformed samples.

Abstract: The low volume fraction of carbide phase in carbon steel determines that it is difficult to estimate the stress state in it by diffraction method. In the present study, different from the studies before, we improve the technique of surface treatment on specimen and have successfully finished the stress estimations of carbide phase in carbon steels by X-ray diffraction method under normal conditions. Moreover, we investigate the affection of spherical cementite particle size on the residual stress distribution in both phases during the plastic deformed steels. We observed that the steels with small-sized cementite particles showed higher stress states than the steels with relatively large-sized cementite particles.

Abstract: Raman spectroscopy has been used by various workers to provide a measure of the stresses within the oxides grown on metal substrates at high temperatures. In this paper, we consider thermally grown oxides produced on a Type 316 austenitic stainless steel and an iron 3% silicon ferritic steel. The oxides were grown in air at temperatures of 950oC and 650oC respectively over a range of times. These oxides have been characterised by producing cross-sections using focused ion beam milling. The variation of the Raman spectra wave number (He, Ne laser; λ = 633nm) for the oxides produced on the polycrystalline austenitic stainless steel and the ferritic steel were measured as a function of oxide thickness. This shift in wave number was a function of stress. For a fixed oxide thickness on the stainless steel substrate the specimen has been subject to a bending force. A back face strain gauge fixed to the metal substrate provided a measure of the applied strain. The peak wave number varied with applied strain. The results are discussed with respect to the potential to characterise the stresses produced in thermally grown oxides and as a tool to monitor applied stress.

Abstract: Mass gain during oxidation, texture and residual stresses in oxide layers on polycrystalline Armco iron substrates with different surface conditions are investigated using thermogravimetry microscopy and synchrotron X-rays. The mass gain during oxidation in all samples follows a parabolic law. The parabolic oxidation constant increases with increasing roughness of a mechanically ground respectively polished oxide layer. Electrolytic polishing (grain surface etching) reduces while grain boundary etching increases the parabolic oxidation constant compared to the mechanically polished sample. All oxide layers show columnar growth of the magnetite and a moderate fiber texture. The magnetite contains compressive residual stresses. Under the conditions chosen for the oxidation treatment the magnitude of these compressive residual stresses does not depend on the substrate surface condition.

Abstract: True Baroque organ music can only come back to life in the 21st century by developing Cu-based alloys and implementing them in the organ reed pipes. Reed pipes contain a vibrating part, the brass tongue that crucially influences its sound. Energy dispersive synchrotron X-ray diffraction has been performed in order to investigate residual stresses in the tongues. The in depth analysis gives us an important indication on the processes the tongues were submitted to during their manufacturing: hammering, annealing, filing to the neat thickness, curving of the tongues. A biaxial stress state in the organ tongues was considered. The residual stress values and behaviour were correlated to the manufacturing processes.