Materials Science Forum Vols. 571-572

Abstract: As an important industrial problem, the rolling contact fatigue damage is accumulated in rails during the repeated passage of trains over the rails, and rail failures may occur from the cracks grown in the rails. In order to prevent such rail failures, the estimation of the behavior of internal rail cracks is required based on the exact engineering analysis model as well as conducting rail test to search rail defects. The purposes of this paper are to apply the neutron stress measurement to rails, and to obtain residual stress state in the rails for the above purpose. The rail samples used were those that have been used in service line in Japan for about six years (222 million gross tons). The neutron measurement was conducted using the Residual Stress Analyzer (RESA) of the Japan Atomic Energy Agency (JAEA). The present measurement of stresses in rails by the neutron diffraction method was the first attempt in Japan.

Abstract: Bending of metal plates with high-energy laser beams presents a flexible materials forming technique where bending results from the establishment of a steep temperature gradient through the material thickness. This inevitably leads to non-uniform thermal expansion/contraction and subsequently residual stresses. Non destructive residual strain mapping with neutron diffraction through the 8mm thickness of a series WA 300 grade structural steel plate samples, focused on the region straddling the centerline of the heating bead location, shows the presence of large residual stress fields. Directly below the laser track the longitudinal strains are tensile and dominant, normal strains compressive and transverse strains slightly tensile. The magnitudes of the strains decrease outside the width of the laser beam footprint. The first laser pass induces throughthickness strains close to yield, whereafter their magnitudes decrease with increased number of laser beam passes. A comprehensive mapping of the longitudinal stresses as function of the number of laser passes is given.

Abstract: The evolution of residual stress and crystallographic texture during thermal treatment was studied using X-ray diffraction. Polycrystalline α-brass samples were examined after cold rolling and afterwards after annealing at different temperatures in the range of 50 0C - 450 0C. Additionally, the width of the diffraction peak was measured in order to estimate the variation of the dislocation density. The interpretation of experimental data was based on a fitting procedure for which the anisotropic diffraction elastic constants calculated by a self-consistent approach were used. As the result of analysis, the values of the first order and second order stresses were determined in each sample.

Abstract: Hexagonal close-packed and lower symmetry metals often exhibit anisotropic mechanical properties because the dominant slip system forbids slip in certain lattice directions. Rod-textured Zircaloy-2 is a model system which can act as a road map for understanding more complex cases. In this case prism slip is dominant and pyramidal slip is only initiated at higher applied stresses. Tensile twinning does not always play a role since its initiation depends on the starting texture. Along the rod axis, Zircaloy-2 exhibits a very strong weighting of poles lying within the basal plane of the structure such as <10 1 0>, <11 2 0> etc. However, perpendicular to the rod axis, all poles are present. The coefficients of thermal expansion are unequal along the a- and c-axes of the crystal structure, so there are always large intrinsic thermal strains. Likewise, the mechanical properties perpendicular to the rod axis are dominated by the interaction of grains with hard and soft plastic response. Over two decades, the residual strains and the in-situ strain response parallel and perpendicular to the rod axis have been measured by neutron diffraction for both tensile and compressive applied stress. The paper reviews our understanding of the strain development for tensile and compressive applied stress in Zircaloy-2 in terms of slip and tensile twinning, the crucial part played by the thermal strains and the simplifying role of the strong texture.

Abstract: Instrumented indentation testing has been carried out on steel samples subjected to uniaxial loading. It is distinguished between purely elastic as well as elasto-plastic loading up to plastic strains of 3%. The experimental approach allows the combination of local in-situ residual stress analyses by means of X-ray diffraction and the subsequent realisation of instrumented indentation testing for the same material volumes beforehand irradiated by the X-rays. The aim of the investigations was to explore a potential correlation between the imposed macroscopic stresses and the characteristic values of instrumented indentation testing evaluated from the measured force-indentation depth curves according to the standard ISO EN 14577. The studies illustrate that results of depth sensing indentation testing have to be handled with care for prestressed material states. The effect of mechanical stresses on the results of instrumented indentation testing is not stringently limited on the hardness measures alone. Other indentation parameters like e.g. the indentation modulus EIT or the elastic part of the indentation work η IT are likewise affected by the prestresses, whereas the impact is in the order of magnitude that it has to be taken into account during interpretation of indentation readings.

Abstract: By associating texture determinations and strains measurements by neutron diffraction, the elastoplastic behaviours of families of crystallites with the same crystallographic orientations were characterized in situ in a brass and a bronze alloys under uniaxial loading. The polycrystalline orientation analysis method proposed here allows an intermediate approach between a “local” (intragranular) and a “global” characterization, within the bulk of massive samples.

Abstract: The constant reduction of production costs and the development of materials during recent years are favoured the development of Austempered Ductile Iron (ADI) because of favourable combination of technological and structural properties. The process of forging on ADI makes it possible to obtain final parts with good dimensions. Moreover, Austempered Ductile Iron has a remarkable workability. ADI has many advantages, including the possibility of modifying and of improving the mechanical characteristics by thermo mechanical treatments while preserving a relatively low production cost, thus competing with many categories of steels. The study presented relates to the influence of the parameters of the thermo mechanical treatments on the proportion of residual austenite allowing modification of the mechanical characteristics of the material and on the evaluation of the residual stresses.

Abstract: A temperature behaviour of residual stresses in shot-peened steel coated with 3m CrN is characterized using in-situ energy dispersive synchrotron X-ray diffraction performed in the temperature range of 25-800°C. The samples are thermally cycled and the development of volumeaveraged residual stresses in the coating and residual stress depth gradients in the steel is characterized. The results reveal complex changes of stresses in CrN and in the substrate. The annealing results in the removal of stress gradients in the steel which starts at the temperature of about 600°C. After cooling down, there are no stresses detected in the steel. The temperature dependence of stresses in CrN is very complex and indicates the presence of phenomena like an annealing of intrinsic stresses about the deposition temperature of 350°C, a formation and a closing of micro-cracks in the tensile region and finally a stress relaxation of approximately 500 MPa after the cooling down. The presented approach allows a complex characterization of thermo-mechanical processes in coating-substrate composites and opens the possibility to understand phenomena related to the thermal fatigue of coated tools.

Abstract: In-situ neutron diffraction has been used to study the pseudoelastic-like behaviour of hydrostatically extruded AZ31 magnesium alloy during stress-strain cycles in compression and tension along the extrusion direction. It has been confirmed that the activation of reversal twinning processes during unloading is responsible for the macroscopically observed hysteresis effect. Moreover, neutron diffraction data reveals the existence of high tensile stresses in grains which have just experienced significant twinning activity prior to the start of the unload cycle. It is thus proposed that this tensile stresses provides the necessary driving force for the activation of untwinning in already twinned grains.

Abstract: This paper presents a study of the residual strain field within a high pressure die cast (HPDC) AZ91 Mg alloy bar subjected to four point bending. The technique employed for this purpose is high energy synchrotron X-ray diffraction. Strain scanning using polychromatic X-ray beam allows the collection of multiple peak diffraction patterns and monitoring of small peak shifts as a function of beam position. These shifts allow collective interpretation in terms of the equivalent macroscopic residual elastic strain. Residual elastic strain distributions were studied in the sections subjected to pure bending and also in sections of contact between the sample and the rollers. These experimental results are compared with the predictions from a finite element analysis of contact and deformation. Good agreement is found between the modelled and measured results. It is hoped that these results help improved understanding of complex deformation behaviour of thin-walled HPDC AZ91 components and provide useful background information for lifing assessment of such structures.