Peter A. Lynch

X-ray diffraction line broadening from thermally deposited gold films

Diffraction profiles were analysed from thermally deposited 111-oriented gold films, ranging in thickness from 300 to 1900 A. The data were collected using the high-resolution powder diffractometer on beamline BM16 at the European Synchrotron Research Facility (ESRF) set at a wavelength of 0.3507 A. The profiles were measured under conventional symmetric θ–2θ reflection conditions and by asymmetric transmission diffraction to ensure that only crystallites oriented normal to the substrate contribute to the diffraction. An analysis of the instrument profile shape of the diffractometer was undertaken using the SRM 660 LaB6 line profile standard. A parallel study of the films using atomic force microscopy and transmission electron microscopy was also undertaken to provide information on the dimensions of the crystallite columns in the films and the presence of dislocations. All the films displayed diffraction broadening arising from both crystallite-size effects and dislocation-induced strain effects. Analysis of the magnitude and anisotropy of the dislocation-induced broadening with hkl indicates that the dislocations have a mixed screw/edge character and tend to form primarily on (111) slip planes parallel to the substrate at densities of ∼1015 to 1016 m−2.

Determining the Dislocation Contrast Factor for X-ray Line Profile Analysis

An important step in quantifying dislocation broadening of a line profile is the evaluation of the dislocation contrast factors. In this way the elastic properties of the material, displacement field of the dislocations and geometric orientation of the diffraction vector relative to the dislocation slip-system are incorporated into the analysis.

Investigation of the interfacial structure of ultra-thin platinum films using X-ray reflectivity and X-ray photoelectron spectroscopy

Ultra-thin films of platinum deposited on highly polished 100 silicon have been investigated using X-ray reflectivity (XRR) and X-ray photoelectron spectroscopy (XPS) as part of a study on the interface structure of multilayers used for X-ray mirrors. In this paper results are presented for films deposited by electron beam evaporation and by DC-magnetron sputtering. The reflectivity was fitted by assuming an intermediate platinum silicide layer exists between the platinum and the silicon. XPS data clearly confirmed the existence of such a platinum silicide layer. According to the XRR data this layer was estimated to be approximately 10 A thick for e-beam samples and approximately 30 A thick for magnetron samples. For e-beam films the fitted density for the platinum silicide layer was found to decrease from 16.4 g·cm−3 for a nominal 80 A film down to 2.65 g·cm−3 for a nominal 20 A film. For magnetron sputtered films the fitted density was always within the range of 6.1–6.8 g·cm−3. The fitted density of the platinum layer from the e-beam results was always within 5% of the density of bulk platinum whereas for the magnetron sputtered films the density decreased uniformly with decreasing film thickness for films with a nominal thickness less than 30 A. The XPS data show that the magnetron deposited platinum penetrates through the native oxide layer and into the silicon substrate to a far greater degree than the e-beam deposited platinum.

Development of a laboratory-based transmission diffraction technique for in situ deformation studies of Mg alloys

A laboratory-based transmission X-ray diffraction technique was developed to measure elastic lattice strains parallel to the loading direction during in situ tensile deformation. High-quality transmission X-ray diffraction data were acquired in a time frame suitable for in situ loading experiments by application of a polycapillary X-ray optic with a conventional laboratory Cu X-ray source. Based on the measurement of two standard reference materials [lanthanum hexaboride (NIST SRM 660b) and silicon (NIST SRM 640c)], precise instrumental alignment and calibration of the transmission diffraction geometry were realized. These results were also confirmed by the equivalent data acquired using the standard Bragg–Brentano measurement geometry. An empirical Caglioti function was employed to describe the instrumental broadening, while an axis of rotation correction was used to measure and correct the specimen displacement from the centre of the goniometer axis. For precise Bragg peak position and hkil intensity information, a line profile fitting methodology was implemented, with Pawley refinement used to measure the sample reference lattice spacings (do(hkil)). It is shown that the relatively large X-ray probe size available (7 × 7 mm) provides a relatively straightforward approach for improving the grain statistics for the study of metal alloys, where grain sizes in excess of 1 µm can become problematic for synchrotron-based measurements. This new laboratory-based capability was applied to study the lattice strain evolution during the elastic–plastic transition in extruded and rolled magnesium alloys. A strain resolution of 2 × 10−4 at relatively low 2θ angles (20–65° 2θ) was achieved for the in situ tensile deformation studies. In situ measurement of the elastic lattice strain accommodation with applied stress in the magnesium alloys indicated the activation of dislocation slip and twin deformation mechanisms. Furthermore, measurement of the relative change in the intensity of 0002 and 10\overline 13 was used to quantify {10\overline 12}〈\overline 1011〉 tensile twin onset and growth with applied load.

Application of white-beam X-ray microdiffraction for the study of mineralogical phase identification in ancient Egyptian pigments

High-brightness synchrotron X-rays together with precision achromatic focusing optics on beamline 7.3.3 at the Advanced Light Source have been applied for Laue microdiffraction analysis of mineralogical phases in Egyptian pigments. Although this task is usually performed using monochromatic X-ray diffraction, the Laue technique was both faster and more reliable for the present sample. In this approach, white-beam diffraction patterns are collected as the sample is raster scanned across the incident beam (0.8 µm × 0.8 µm). The complex Laue diffraction patterns arising from illumination of multiple grains are indexed using the white-beam crystallographic software package XMAS, enabling a mineralogical map as a function of sample position. This methodology has been applied to determine the mineralogy of colour pigments taken from the ancient Egyptian coffin of Tjeseb, a priestess of the Apis bull dating from the Third Intermediate to Late period, 25th Dynasty to early 26th Dynasty (747 to 600 BC). For all pigments, a ground layer of calcite and quartz was identified. For the blue pigment, cuprorivaite (CuCaSi4O10) was found to be the primary colouring agent with a grain size ranging from ∼10 to 50 µm. In the green and yellow samples, malachite [Cu2(OH)2CO3] and goethite [FeO(OH)] were identified, respectively. Grain sizes from these pigments were significantly smaller. It was possible to index some malachite grains up to ∼20 µm in size, while the majority of goethite grains displayed a nanocrystalline particle size. The inability to obtain a complete mineralogical map for goethite highlights the fact that the incident probe size is considerably larger than the grain size. This limit will continue to improve as the present trend is toward focusing optics approaching the diffraction limit (∼1000× smaller beam area).

A uniaxial tensile stage with tracking capabilities for micro X-ray diffraction applications

First results are presented for a uniaxial tensile stage designed to operate on a scanning micro X-ray diffraction synchrotron beamline. The new tensile stage allows experiments at typical loading cycles used in standard engineering stress–strain tests. Several key features have been implemented to support in situ loading experiments at the intragranular length scale. The physical size and weight of the load cell were minimized to maintain the correct working distance for the X-ray focusing optics and to avoid overloading the high-resolution raster scan translation stages. A high-magnification optical microscope and image correlation code were implemented to enable automated online tracking capabilities during macroscopic elongation of the sample. Preliminary in situ tensile loading experiments conducted on beamline 12.3.2 at the Advanced Light Source using a polycrystalline commercial-purity Ti test piece showed that the elastic–plastic response of individual grains could be measured with submicrometre spatial resolution. The experiments highlight the unique instrumentation capabilities of the tensile stage for direct measurement of deviatoric strain and observation of dislocation patterning on an intragranular length scale as a function of applied load.

Stress Evolution during Tensile Twinning in MgAZ31 Alloy Processed by Extrusion

An in-situ laboratory based X-ray diffraction technique has been developed to directly measure lattice strain and stress evolution associated with {10.2} < 10.1 > twin nucleation and growth in rolled and extruded Mg alloys during tensile loading. A transmission diffraction geometry was utilised to measure peak position and intensity for the (10.0), (00.2) and (10.1) lattice planes while the sample was loaded in uni-axial tension. Lattice re-orientation arising from deformation twinning is utilizedto estimate the twin volume fraction by measuring the increase in the (10.0) peak intensity along with a simultaneous decrease in the (00.2) peak intensity as a function of applied load. From observation of the lattice strain plotted against applied stress for different orientations it was found that the (10.1) orientation displayed the anticipated linear behaviour within the whole stress range. Yielding in the (10.2) and (10.3) orientations was identified at around 75 and 90 MPa respectively, indicating theonset of basal slip. Twin nucleation was observed at at a stress of approximately 110 MPa.

Characterizing Grain-Oriented Silicon Steel Sheet Using Automated High-Resolution Laue X-ray Diffraction

Controlling texture in grain-oriented (GO) silicon steel sheet is critical for optimization of its magnetization performance. A new automated laboratory system, based on X-ray Laue diffraction, is introduced as a rapid method for large scale grain orientation mapping and texture measurement in these materials. Wide area grain orientation maps are demonstrated for both macroetched and coated GO steel sheets. The large secondary grains contain uniform lattice rotations, the origins of which are discussed.

The Effect of Ageing on the Compressive Deformation of Mg-Sn-Zn-Na Alloy

The influence of ageing on deformation twinning in an extruded Mg-6Sn-3Zn-0.04Na alloy is investigated. In-situ compression tests have been carried out using high resolution synchrotron X-Ray Diffraction (XRD) to measure the influence of precipitates on twining activity. Synchrotron experiments revealed the increase in the critical resolved shear stress of twinning with ageing. The compressive yield strength (along the extrusion direction) of the aged sample increased by ~150% over the non-aged specimen. To obtain statistical insight into the twinning activity, the microstructure of the non-aged and aged samples (200°C, 24 hours) deformed up to ~1% plastic strain was studied using optical microscopy. A higher number of thinner twins were observed in the microstructure of the aged sample compared to the non- aged sample.