Mina Golshan

Energy calibration and full-pattern refinement for strain analysis using energy-dispersive and monochromatic X-ray diffraction

Precise channel-to-energy conversion is very important in full-pattern refinement in energy-dispersive X-ray diffraction. Careful examination shows that the channel-to-energy conversion is not entirely linear, which presents an obstacle to obtaining accurate quantitative data for lattice strains by pattern refinement. In order to establish an accurate quadratic channel-to-energy conversion function, a Matlab program was written to find the best quadratic coefficient and hence the whole energy conversion function. Then this energy conversion function was used to perform a whole-pattern fitting of the energy-dispersive X-ray diffraction pattern of a Ti64 sample. The strain across the Ti64 bar calculated from the fitting results has been compared with values obtained by single-wavelength X-ray diffraction utilizing a Laue monochromator.

The role of residual stress in the fracture properties of a natural ceramic

The role of residual stress in enhancing the fracture properties of the shell Ensis siliqua (razor shell) is reported. Using energy-variable synchrotron X-ray diffraction it is shown that the calcium carbonate crystals are preferentially orientated as a function of depth. Using scanning electron microscopy the structure is found to be mainly divided into two regions, namely crossed lamellar and prismatic. The latter are shown to be at a higher residual compressive stress than the former which gives rise to delaminations of the structure upon mechanical deformation. The role of residual stress in the structure is also shown to be critical to the fracture properties of the material and this has implications for the fabrication of man-made laminate ceramics.

Synchrotron Energy-Dispersive X-Ray Diffraction Analysis of Residual Strains around Friction Welds between Dissimilar Aluminium and Nickel Alloys

Friction welding processes, such as friction stir welding (FSW) and inertia friction welding (IFW) are popular candidate procedures for joining engineering materials (including dissimilar pairs) for advanced applications. The advantages of friction welding include lack of large scale material melting, ability to join dissimilar materials, and relatively low propensity to introduce defects into the weld joint. For these reasons FSW and IFW have become the subjects of a number of studies aimed at optimising the joining operations to obtain improved joint strength and reduce distortion and residual stress. In the present study we used the diffraction of high energy polychromatic synchrotron X-rays to measure interplanar lattice spacings and deduce nominal elastic strains in friction stir welds between dissimilar aluminium alloys AA5083 and AA6082, and in coupons from inertia friction welds between dissimilar nickel-base superalloys IN718 and RR1000. Energy-dispersive diffraction profiles were collected by two detectors mounted in the horizontal and vertical diffraction planes, providing information about lattice strains in two nearly perpendicular directions lying almost in the plane of the plate samples mounted perpendicularly to the incident beam. Two-dimensional maps of residual stresses in friction-welded joints were constructed. Apart from the 2D mapping technique, the sin2ψ method (transmission) was also used in the case of inertia friction-welded joint between nickel alloys. Comparison between the two results allowed the variation of the lattice parameter with the distance from the bond line to be deduced. It was found that friction welding of two dissimilar materials with significant strength mismatch may lead to the creation of a region of compressive stress in the vicinity of the bond line, in contrast with the behaviour observed for joints between similar materials.

Analysis of plastic deformation and residual elastic strain in a titanium alloy using synchrotron x-ray diffraction

This paper presents the results of a study of elastoplastic deformation of titanium alloy Ti-6Al-4V subjected to four point bending prior to residual elastic strain measurements by high energy synchrotron diffraction. Both white-beam and monochromatic x-rays were used at the SRS Station 16.3 in order to record diffraction patterns as a function of beam position across the sample. This allowed the comparison between the two techniques to be readily made. Residual (elastic) strain was calculated as a function of position across the sample, based on different reflections of hcp titanium. Inelastic bending analysis was used to extract the plastic strains. The results demonstrate that (i) the level of plastic deformation can be deduced from the x-ray diffraction profile, (ii) the asymmetry of the material response to plastic deformation in tension and compression can be identified and (iii) differences in the behaviour of different grain groups can be seen.

A Focusing Laue Monochromator Optimised for Diamond Anvil Cell Diffraction Experiments

We have developed a sagittally bent Laue monochromator that is optimised for diffraction experiments on samples in diamond anvil cells. Test measurements have shown that the bandwidth of the monochromatic beam can be broadened with simultaneous focusing of the X‐rays. A gain in X‐ray flux of over 100 is achieved.

Measuring strain in polycrystalline CVD diamond films

Three-dimensional reciprocal-space mapping was performed to determine the true microstrain: that is, the distribution of strain within individual grains: and to calculate the precise lattice parameters and deviation from near-perfect samples. In order to accomplish this, Bragg reflections from individual grains were isolated (Fewster P F 1997 Crit. Rev. Solid State Mater. Sci. 22 69–110; Golshan M, Fewster P F, Andrew N L, Kidd P, Moore M and Butler J E 2001 J. Phys. D: Appl. Phys. 34 A44–6). This was achieved by increasing the 2θ resolution and by reducing the axial divergence of the beam, resulting in a smaller probe and partially eliminating the effect of projection on to the diffraction plane along the perpendicular direction. A suitable divergence was achieved by using two pairs of slits, placed after the monochromator and before the detector, respectively. The incident beam size was reduced to 50 μm2 to ensure that only a small part of the sample was illuminated. The experiments were carried out at Daresbury Laboratory, Stations 16.3 (Collins S P, Cernik R J, Fell B, Tang C C, Harris N W, Miller M C and Oszlanyi G 1998 J. Synchrotron Radiat. 5 1263–9) and at ESRF, ID10B. The results obtained from three different grades of CVD diamond films were compared. Nitrogen incorporated in one diamond during growth appeared to relieve the strain and give a sharper texture to the film.

High energy white beam x-ray diffraction studies of residual strains in engineering components

In order to predict the durability of engineering components and improve performance, it is mandatory to understand residual stresses. The last decade has witnessed a significant increase of residual stress evaluation using diffraction of penetrating radiation, such as neutrons or high energy X‐rays. They provide a powerful non‐destructive method for determining the level of residual stresses in engineering components through precise characterisation of interplanar crystal lattice spacing. The unique non‐destructive nature of these measurement techniques is particularly beneficial in the context of engineering design, since it allows the evaluation of a variety of structural and deformational parameters inside real components without material removal, or at worst with minimal interference. However, while most real engineering components have complex shape and are often large in size, leading to measurement and interpretation difficulties, since experimental facilities usually have limited space for mounting the sample, limited sample travel range, limited loading capacity of the sample positioning system, etc. Consequently, samples often have to be sectioned, requiring appropriate corrections on measured data; or facilities must be improved. Our research group has contributed to the development of engineering applications of high‐energy X‐ray diffraction methods for residual stress evaluation, both at synchrotron sources and in the lab setting, including multiple detector setup, large engineering component manipulation and measurement at the UK Synchrotron Radiation Source (SRS Daresbury), and in our lab at Oxford. A nickel base superalloy combustion casing and a large MIG welded Al alloy plate were successfully studied.In order to predict the durability of engineering components and improve performance, it is mandatory to understand residual stresses. The last decade has witnessed a significant increase of residual stress evaluation using diffraction of penetrating radiation, such as neutrons or high energy X‐rays. They provide a powerful non‐destructive method for determining the level of residual stresses in engineering components through precise characterisation of interplanar crystal lattice spacing. The unique non‐destructive nature of these measurement techniques is particularly beneficial in the context of engineering design, since it allows the evaluation of a variety of structural and deformational parameters inside real components without material removal, or at worst with minimal interference. However, while most real engineering components have complex shape and are often large in size, leading to measurement and interpretation difficulties, since experimental facilities usually have limited space for mounti...