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Physical Methods for Characterizing Solids
Published in Elaine A. Moore, Lesley E. Smart, Solid State Chemistry, 2020
The difference between X-ray and neutron diffraction techniques lies in the scattering process: X-rays are scattered by the electron cloud around the nucleus, whereas neutrons are scattered by the nucleus. The scattering factor for X-rays thus increases linearly with the number of electrons in the atom, so that heavy atoms (large atomic number Z) are much more effective at scattering than light atoms (small Z), but because of the size of the atoms relative to the wavelength of the X-rays, the scattering from different parts of the cloud is not always in phase, so the scattering factor decreases with sin θ//λ due to the destructive interference (Figure 2.12a). Because the nucleus is very small, neutron scattering factors do not decrease with sin θ/λ, and because nuclei are similar in size, they are all fairly similar in value and that of hydrogen is anomalously large due to the nuclear spin. The fact that the neutron scattering factors are almost invariant with sin θ/λ, means that the intensity of the data does not drop at high angles of θ as is the case with X-ray patterns, and so a neutron powder pattern tends to yield considerably more data. Neutron scattering factors are also affected randomly by resonance scattering, when the neutron is absorbed by the nucleus and released later. This means that neutron scattering factors cannot be predicted but have to be determined experimentally and they vary for different atoms and indeed for different isotopes (Figure 2.12b).
Analytical Methods
Published in Colin R. Gagg, Forensic Engineering, 2020
This is another important residual stress measurement technique. The first article on record on measuring strains using neutron diffraction was published in 1985, so it is not as mature as XRD or hole drilling. One of the main advantages of neutron diffraction is that neutron beams can penetrate much deeper into materials than lab X-rays, enabling residual stress measurements deep inside polycrystalline materials. The International Organization for Standardization (ISO) has been publishing and revising a neutron diffraction standard since 2005 (ISO/TS 21432:2005) and, although the standard was last reviewed in 2010, it is again under review (May 2018).
Diffraction
Published in Peter E. J. Flewitt, Robert K. Wild, Physical Methods for Materials Characterisation, 2017
Peter E. J. Flewitt, Robert K. Wild
A benefit of neutron diffraction is that it can be used to probe, non-destructively, the state of strain within a polycrystalline material which for typical engineering components can extend to a depth of many centimetres. By limiting the irradiation volume and the field of view of the detector, it is possible to sample a small volume, ~1 mm3, within the body of a specimen. In general, one of the following two techniques is used for these measurements: conventional θ/2θ scanning as described for X-ray diffraction or a time-of-flight approach (Winholz and Kravitz 1996, Todd et al. 1997, Withers and Bhadeshia 2001). The former is well suited to neutron beams produced by reactor sources, whereas the latter is appropriate to the pulsed beam generated from a spallation source. In the time-of-flight method, the Bragg angle is held constant, usually at 2θ = π/2, and the incident wavelength is varied. In this case, each pulse of neutrons leaving the moderated spallation target has a range of neutron energies. The most energetic neutrons arrive at the target in advance of the least energetic so that the energy and, hence, the wavelength of each detected neutron can be obtained from the time that has elapsed since the neutron pulse was produced. In this case, the strain ε is given by () ε=Δtt where t is the time of flight. As strain resolution depends upon the accuracy of the measurement of t, high-resolution instruments invoke large flight paths, ~100 m. An approach adopted to analyse these diffraction spectra is to use the Rietveld refinement (Young 1993, Webster 2000) to derive a single value of the lattice spacing by simultaneously fitting a curve to the intensity profile obtained from all the diffraction peaks obtained within the time-of-flight measurements (Daymond et al. 1997a). This value is weighted towards the most intense peaks. The approach has been experimentally and theoretically demonstrated to provide a good representation of the bulk elastic response as compared to the relative insensitivity to tensile and compressive shifts of the various diffraction peaks. A complication when interpreting neutron diffraction data is the contribution of apparent strain arising as a result of diffraction from internal and external surfaces because the diffracting volume is only partially filled. These contributions are accommodated by including the diffraction geometry and attenuation. An alternative is to use the so-called z-scan geometry where the surface is approached by bringing the specimen into the gauge volume vertically to eliminate lateral displacement of the centre of gravity of the scattering volume, thereby eliminating geometrical peak shift (Spooner and Wang 1997).
Hydrogen bonding and clusters in supercritical methanol–water mixture by neutron diffraction with H/D substitution combined with empirical potential structure refinement modelling
Published in Molecular Physics, 2019
Koji Yoshida, Shigeru Ishida, Toshio Yamaguchi
Figure 3 shows the coordination number distribution for the methanol-water mixture. The coordination number distribution of water–water and methanol-water under a supercritical condition shifts to a lower coordination number, compared with that under an ambient condition. These features are also observed in the structural results of pure water [29] where the coordination number distribution of water–water shifts to a lower coordination number with increasing temperature at constant pressure (30 MPa). In an ambient condition, there exist few water monomers. The coordination number of the highest population is 3 and 2 for OW-OW and O-OW, respectively, implying that the structure of the methanol-water mixture is a branched chain-like one. This model is consistent with the previous structure study by X-ray [9–12], neutron [13–20] diffraction and MD simulation [56]. In a supercritical condition, the coordination number of the highest population is 2 and 1 for OW-OW and O-OW, respectively. The results can propose the following structure model: the chain composed of methanol and water molecules is broken shortly and methanol molecules locate at both termini of the chain. The coordination number distribution of methanol-methanol does not change with temperature. Almost all of the methanol molecules exist as a monomer at both ambient and supercritical conditions because of a low concentration of methanol (xm = 0.3).
Relation between residual stresses and microstructure evolution in Al–Si alloys based on different casting parameters
Published in Philosophical Magazine, 2019
S. S. Mohamed, A. M. Samuel, H. W. Doty, S. Valtierra, F. H. Samuel
Residual stresses can be quantified by many techniques, including mechanical techniques such as sectioning, hole drilling, curvature measurements, and crack compliance methods. These techniques involve strain gauges placed on the surface of sample pieces that register the stress relief that results from the removal of material by cutting, machining or drilling. In the diffraction techniques, such as electron, X-ray, and neutron diffraction, the residual stresses are quantified by measuring the elastic strains in components. Finally, other techniques, including magnetic, ultrasonic, piezo spectroscopy are also used to measure the residual stresses developed within parts.
A novel modeling approach for multi-passes butt-welded plates
Published in Journal of Thermal Stresses, 2021
S. Trupiano, V. G. Belardi, P. Fanelli, L. Gaetani, F. Vivio
Residual stress measurements can be performed through experimental approaches and are classified as destructive and nondestructive. The most widely used nondestructive measurements are diffraction-based methods like synchrotron diffraction, neutron diffraction, X-ray diffraction [5–7]. Conversely, destructive measurements are founded on stress relaxation phenomena. The principal destructive methods are deep hole drilling, slotting, tube splitting, layer removal, and hole-drilling strain gauge method [8–10].