Simultaneous Tomography and Diffraction Analysis of Creep Damage

Time-resolved x-ray stroboscopic phase tomography using Talbot interferometer for dynamic deformation measurements

Time-resolved x-ray phase tomography using a Talbot interferometer and white synchrotron radiation can provide a three-dimensional movie for visualizing the structural change of materials consisting of light elements. In this study, time-resolved x-ray stroboscopic phase tomography using a Talbot interferometer is demonstrated for a vibrating object under 24 Hz compression-stretch fatigue loading. Moiré patterns are recorded by synchronizing drivers for a shutter, grating displacement, and sample rotation with an x-ray camera with a 200 µs exposure, and phase tomograms are reconstructed at specific motion phases of the vibration. The measurement lasts for a few minutes and the δ value changes before breaking, which is considered due to plastic deformation of soft materials under external vibration are depicted three-dimensionally.

Evidence of damage evolution during creep of Al–Mg alloy using synchrotron X-ray refraction

In order to provide further evidence of damage mechanisms predicted by the recent solid-state transformation creep (SSTC) model, direct observation of damage accumulation during creep of Al–3.85Mg was made using synchrotron X-ray refraction. X-ray refraction techniques detect the internal specific surface (i.e.surface per unit volume) on a length scale comparable to the specimen size, but with microscopic sensitivity. A significant rise in the internal specific surface with increasing creep time was observed, providing evidence for the creation of a fine grain substructure, as predicted by the SSTC model. This substructure was also observed by scanning electron microscopy.

In-situ Observation of Cross-Sectional Microstructural Changes and Stress Distributions in Fracturing TiN Thin Film during Nanoindentation

Load-displacement curves measured during indentation experiments on thin films depend on non-homogeneous intrinsic film microstructure and residual stress gradients as well as on their changes during indenter penetration into the material. To date, microstructural changes and local stress concentrations resulting in plastic deformation and fracture were quantified exclusively using numerical models which suffer from poor knowledge of size dependent material properties and the unknown intrinsic gradients. Here, we report the first in-situ characterization of microstructural changes and multi-axial stress distributions in a wedge-indented 9 μm thick nanocrystalline TiN film volume performed using synchrotron cross-sectional X-ray nanodiffraction. During the indentation, needle-like TiN crystallites are tilted up to 15 degrees away from the indenter axis in the imprint area and strongly anisotropic diffraction peak broadening indicates strain variation within the X-ray nanoprobe caused by gradients of giant compressive stresses. The morphology of the multiaxial stress distributions with local concentrations up to -16.5 GPa correlate well with the observed fracture modes. The crack growth is influenced decisively by the film microstructure, especially by the micro- and nano-scopic interfaces. This novel experimental approach offers the capability to interpret indentation response and indenter imprint morphology of small graded nanostructured features.

Diffraction/scattering computed tomography for three-dimensional characterization of multi-phase crystalline and amorphous materials

The three-dimensional characterization method described herein is based on diffraction and scattering techniques combined with tomography and uses the variation of these signals to reconstruct a two-dimensional/three-dimensional structural image. To emphasize the capability of the method in discriminating between different poorly ordered phases, it is named diffraction/scattering computed tomography (DSCT). This combination not only allows structural imaging but also yields an enhancement of the weak signals coming from minor phases, thereby increasing the sensitivity of structural probes. This article reports the suitability of the method for discrimination of polycrystalline and amorphous phases and for extraction of their selective local patterns with a contrast sensitivity of about 0.1% in weight of minor phases relative to the matrix. The required background in tomography is given and then the selectivity of scattering signal, the efficiency of the method, reconstruction artefacts and limitations are addressed. The approach is illustrated through different examples covering a large range of applications based on recent literature, showing the potential of DSCT in crystallography and materials science, particularly when functional and/or precious samples with sub-micrometre features have to be investigated in a nondestructive way.

X-RAY COMPUTED MICROTOMOGRAPHY OF RUBBER

In rubber science, X-ray computed microtomography (micro CT) is becoming an increasingly used technique to characterize 3D microstructures. As a first step, experimental methods, limitations, and data analysis are described. A review of published micro CT studies for rubber is reported. Examples of our recent works are presented, including investigations on samples or complex structures, for compact or foam rubbers. Micro CT is used to describe the evolution of microstructures relative to different processing steps, to environmental interaction, and to adaptation to a mechanical deformation. New insights and better understanding of damage mechanisms due to quasistatic, creep, and fatigue solicitations are presented from in situ micro CT experiments. Perspective studies are outlined.

Rietveld refinement of energy-dispersive synchrotron measurements

In the past two decades the energy-dispersive diffraction (EDD) method has become a powerful tool in many fields of materials research such as residual stress, texture, and crystal structure analysis, because of its favorable ratio of a comparatively low experimental effort in form of a simple and fixed instrumental setup to a high information content included in the measured diffraction patterns. However, mainly due to the rather poor instrumental resolution only little work has been done so far to apply the well-established methods of diffraction line profile analysis to the EDD data. In the paper, a Rietveld program is introduced that allows for particle size and strain broadening analysis by refining the whole EDD äpattern. With the examples of synchrotron measurements performed on the materials science beamline EDDI at BESSY II on instrumental standard as well as samples exhibiting size and/or strain broadened diffraction lines, it is demonstrated that the generalized Thompson, Cox & Hastings approach (TCH) using pseudo-Voigt functions for describing the diffraction line profiles yields sound and reliable results on the materials microstructure. For a first proof of the theoretical assumptions this Rietveld program is based on, the Pawley approach was used to extract the peak intensities obtained from powder samples affected by microstructural broadening. An excellent agreement with the results of the size-strain round robin was obtained. Future enhancements of the program code which aim at its application to full residual stress and microstructure analysis in the near surface zone and also in the material voläume of polycrystalline materials are discussed.

Determining grain resolved stresses in polycrystalline materials using three-dimensional X-ray diffraction

An algorithm is presented for characterization of the grain resolved (type II) stress states in a polycrystalline sample based on monochromatic X-ray diffraction data. The algorithm is a robust 12-parameter-per-grain fit of the centre-of-mass grain positions, orientations and stress tensors including error estimation and outlier rejection. The algorithm is validated by simulations and by two experiments on interstitial free steel. In the first experiment, using only a far-field detector and a rotation range of 2 × 110°, 96 grains in one layer were monitored during elastic loading and unloading. Very consistent results were obtained, with mean resolutions for each grain of approximately 10 µm in position, 0.05° in orientation, and 8, 20 and 13 × 10−5in the axial, normal and shear components of the strain, respectively. The corresponding mean deviations in stress are 30, 50 and 15 MPa in the axial, normal and shear components, respectively, though some grains may have larger errors. In the second experiment, where a near-field detector was added, ∼2000 grains were characterized with a positional accuracy of 3 µm.

High-speed digital-image correlation method

Provided that the software and hardware equipment in a digital-image correlation method is unchanged, the only major factor affecting integer-pixel correlation computation precision and the integer-pixel processing time is the size of the computing window. The bigger the size of the computing window is, the more reliable the computation result is. However, a bigger computing window means longer processing time. For reduction of the integer-pixel processing time without affecting the reliability of the result, a two-step correlation computation method (which uses a small window and a big window in the correlation computation) is proposed. The experiment results confirm the validity of the new method, and the total processing time of this method can be approximately reduced to a twentieth of that of the traditional method.

Investigation of misalignment in analyzer crystal based-CT and its effect

Analyzer crystal-based computed tomography (ACB-CT) is a novel x-ray phase contrast computed tomography technique. In this paper, in order to extract the refraction angle, we analyzed the effect of misalignment on tomography-sliced images of ACB-CT. Two different methods were considered: the first, proposed by Chapman and Dilmanian, was based on two sets of projective image data taken at both sides of the rocking curve and with a half circle sample rotation and the second, recently proposed by our team, was based on only one set of projective image data taken at one side of the rocking curve but with a full circle sample rotation. Theoretical analysis and experimental results will show that the second method improves the quality of reconstructed CT images, also simplifying the ACB-CT data acquisition procedure.

Aspects of Residual Stress Determination Using Energy-Dispersive Synchrotron X-Ray Diffraction

In this paper we discuss certain aspects of residual stress measurements using energy-dispersive synchrotron X-ray diffraction using very high X-ray energies in the range up to 200keV. In particular, we focus on the strain resolution and its relation to the geometric contribution to the instrumental resolution. This energy range together with the brilliance of insertion devices allows measurements in bulk materials with penetration approaching those of neutrons, and the technique is demonstrated to have a high potential for residual stress determination. However, the use of high X-ray energies implies a relatively small diffraction angle and in turn a relatively elongated gauge volume, which favours the application of the technique to essentially 2D problems.

The Materials Science Beamline EDDI for Energy-Dispersive Analysis of Subsurface Residual Stress Gradients

In April 2005 the materials science beamline EDDI (Energy Dispersive DIffraction), which the HMI operates at the Berlin synchrotron storage ring BESSY, started user service. The high energy white synchrotron beam up to about 150 keV used for the diffraction experiments is provided by a superconducting 7 Tesla multipole wiggler. Starting with some basic information on the technical parameters of the beamline, its set-up and measuring facilities, the paper focuses on the application of white beam diffraction to the analysis of residual stress fields in the near surface zone of polycrystalline materials. The concept of a program system is introduced, which we offer to our users for preparing and evaluating their measurements performed at the EDDI beamline.