Materials Science Forum Vols. 768-769

Abstract: The paper deals with methods for X-ray stress analysis (XSA), which allow for the evaluation of near surface in-plane residual stress gradients σ_||(τ) and σ_||(z) in the LAPLACE- and the real space, respectively. Since the ‘robustness’ of residual stress gradient analysis strongly depends on both, the quality of the measured strain data and the number of experimental data points, the discussion aims at those approaches which are based on processing various diffraction lines or even complete diffraction patterns. It is shown that these techniques, which were originally developed for angle-dispersive (AD) diffraction, can be adapted and enhanced for energy-dispersive (ED) diffraction employing high-energy synchrotron radiation. With the example of a shot-peened ferritic steel it is demonstrated, that sin²ψ-data measured in the Ψ-mode of XSA employing the ED diffraction technique can be analyzed on different levels of approximation.

Abstract: Elastic-plastic deformation properties of austenitic stainless steel foils were evaluated by using polychromatic laboratory X-rays. The transmitted optical system was used for stress measurement. The diffraction elastic constants for several diffraction planes were measured under monotonic loading by the cos2χ method. The diffraction energy decreased almost linearly with increasing cos2χ, and the slope of the cos2χ diagram decreased with increasing applied stress. Measured diffraction elastic constants were compared with the theoretical values calculated by the Kröner model. The experimental value agreed well with the theoretical value. The lattice strain measured during plastic deformation depended on the diffraction plane. The full width at half maximum increased with applied plastic strain. From the the diffraction-plane dependence of the lattice strain, the full width at half maximum and the diffraction intensity can be evaluated using polychromatic laboratory X-rays.

Abstract: Multireflection grazing incidence X-ray diffraction (MGIXD) was applied to measure residual stresses in thin surface layers and the problem of X-ray elastic constants (XEC) used for the interpretation of results was studied. To show the influence of the X-ray elastic constants on the interpretation of MGIDX results, polycrystalline materials having low (Ti alloy) and high elastic anisotropy of crystallites (Ni alloy) were investigated.

Abstract: The engineering neutron diffractometer E3 at Helmholtz Centre Berlin is constantly being upgraded. Since the installation of a new monochromator in 2007, the instrument has become much faster and more attractive for the user community. Here, the continuation of the upgrade activities, including a radial oscillating collimator, a new slit system and further advantageous tools to improve the performance on E3, is presented.

Abstract: Possibilities for depth resolving texture analysis applying energy-dispersive X-ray synchrotron diffraction are presented. Exploiting the advantage of having the complete diffraction spectra observed in a fixed but arbitrary measuring direction, two different approaches for high spatial resolution analyses are discussed. The first allows fast access of intensity distribution from plan families {hkl} parallel to the sample surface. The latter allows successful pole figure assessment despite the complex and time consuming slit alignment and data processing. The size of the sampling volume can be tailored to the sample problem ranging from 10 to 100 µm in height or more if necessary.

Abstract: Recently, the ‘stress scanning method’ has been introduced in the field of depth resolved residual stress analysis. The principle of this method is based on depth scans that are performed in several inclination angles with a gauge volume characterized by a height dimension in the range of 10 µm. This method has been used in the energy-dispersive mode of diffraction for rather long-range depth gradients. In this case the variation of the residual stresses is negligible on the scale of the gauge volume height dimension. In this contribution it is shown that the stress scanning method can be extended to the analysis of steep residual stress depth gradients that vary significantly even within the height dimension of the gauge volume, but a careful evaluation of the measured data is necessary and must be adapted to the special case.

Abstract: Precise determination of diffraction peak positions is of particular importance for the evaluation of residual strains. Neutrons are commonly used to probe residual strains from material volumes in depths of several millimetres under the sample surface. However, neutron strain analyses are critical for the near surface region. When scanning close to a sample surface, aberration peak shifts arise, which can be of the same order as the peak shifts related to residual strains [1]. Series of Monte Carlo (M.C.) simulations using the software package RESTRAX/SIMRES [2] were carried out to simulate the peak shift as a function of gauge volume depth, monochromator curvature and other instrumental parameters, which can be used to quickly optimise the experimental setup for direct measuring residual strains near the sample surface at an arbitrary surface orientation. The M.C. simulations were compared and agree very well with the experimental data, not only for a stress free steel sample but as well for a deep rolled steel sample, measured at the STRESS-SPEC diffractometer at the research reactor FRM II, Garching (Germany).

Abstract: Complex strain paths are often applied to materials during production processes. This paper shows the first successful in-situ biaxial mechanical tests during neutron diffraction performed on a cruciform steel sample and reports on the differences compared to uniaxial deformation. Digital image correlation is demonstrated to be an appropriate tool to monitor spatially resolved the macroscopic straining. The new, modular biaxial machine that will be installed at the neutron diffractometer POLDI is presented.

Abstract: Aluminum alloy (Al-7075-T6) samples were analyzed to determine the in-depth residual stress profile induced by a shot-peening treatment. The influence of coverage degree and Almen intensity on the surface residual stress and on the sub-surface residual stress gradient was investigated. Residual stress profiles were obtained using three different techniques: (i) standard laboratory X-ray diffraction (XRD) residual stress analysis with progressive chemical layer-removal; (ii) XRD residual stress analysis with synchrotron radiation using different X-ray energies, thus changing the penetration depths, and (iii) Blind Hole Drilling (BHD). A comprehensive comparison of the results given by the used techniques is shown.