J.R. Santisteban

ENGIN-X: A third-generation neutron strain scanner

ENGIN-X, a new time-of-flight (TOF) neutron diffractometer optimized to measure elastic strains at precise locations in bulky specimens recently commissioned at the ISIS Facility in the Rutherford Laboratory, UK, is described. Fast counting times, together with a flexible and accurate definition of the instrumental gauge volume are the main requirements of neutron strain scanning and have been addressed on ENGIN-X through the design of a novel TOF diffractometer with a tuneable resolution and interchangeable radial collimators. Further, the routine operation of the instrument has been optimized by creating a virtual instrument, i.e. a three-dimensional computer representation of the diffractometer and samples, which assists in the planning and execution of experiments. On comparing ENGIN-X with its predecessor ENGIN, a 25× gain in performance is found, which has allowed the determination of stresses up to 60 mm deep in steel specimens. For comparison with constant-wavelength diffractometers, special attention has been paid to the absolute number of counts recorded during the experiments. A simple expression is presented for the estimation of counting times in TOF neutron strain scanning experiments.

Time-of-flight neutron transmission diffraction

The positions of Bragg edges in neutron transmission experiments can be defined with high accuracy using the time-of-flight (TOF) technique on pulsed neutron sources. A new dedicated transmission instrument has been developed at ISIS, the UK spallation source, which provides a precision of Δd/d ≃ 10−5 in the determination of interplanar distances. This is achieved by fitting a theoretical three-parameter expression to the normalized Bragg edges appearing in the TOF transmission spectra. The technique is demonstrated by experiments performed on iron, niobium and nickel powders. The applicability of using the instrument for the determination of lattice strains in materials has been investigated using a simple in situ loading experiment. Details of the calibration process are presented and the dependence of the resolution and the experimental times required by the transmission geometry on the instrumental variables are studied. Finally, the requirements for a Rietveld-type refinement of transmission data and the advantages and limitations over traditional neutron diffraction peak analysis are discussed.

High-resolution strain mapping in bulk samples using full-profile analysis of energy-dispersive synchrotron X-ray diffraction data

The feasibility of both high spatial and strain resolution is demonstrated using high-energy X-rays between 100 and 300 keV on beamline ID15A at the ESRF. The data analysis was performed using a multiple-peak Pawley-type refinement on the recorded spectra. An asymmetric peak profile was necessary in order to obtain a point-to-point uncertainty of 10−5. The measurements have been validated with complementary techniques or reference data.

Strain imaging by Bragg edge neutron transmission

The Bragg edges appearing in the transmitted time-of-flight spectra of polycrystalline materials have been recorded using a two-dimensional array of detectors. Subsequent analysis has enabled maps of the elastic strain to be produced.

Using pulsed neutron transmission for crystalline phase imaging and analysis

The total scattering cross section of polycrystalline materials in the thermal neutron region contains valuable information about the scattering processes that neutrons undergo as they pass through the sample. In particular, it displays characteristic discontinuities or Bragg edges of selected families of lattice planes. We have developed a pixelated time-of-flight transmission detector able to record these features and in this paper we examine the potential for quantitative phase analysis and crystalline phase imaging through the examination of a simple two-phase test object. Two strategies for evaluation of the absolute phase volumes (path lengths) are examined. The first approach is based on the evaluation of the Bragg edge amplitude using basic profile information. The second approach focuses on the information content of certain regions of the spectrum using a Rietveld-type fit after first identifying the phases via the characteristic edges. The phase distribution is determined and the coarse chemical species radiographic image reconstructed. The accuracy of this method is assessed.

In situ determination of stresses from time-of-flight neutron transmission spectra

The pulsed neutron transmission diffraction technique exploits the sharp steps in intensity (Bragg edges) appearing in the transmitted spectra of thermal neutrons through polycrystalline materials. In this paper the positions of these edges acquired by the time-of-flight (TOF) technique are used to measure accurately the interplanar lattice distances to a resolution of Δd/d ~10^-4 of specimens subjected to in situ uniaxial tensile loading. The sensitivity of the method is assessed for elastically isotropic (b.c.c. ferritic) and anisotropic (f.c.c. austenitic) polycrystalline specimens of negligible and moderately textured steels. For the more anisotropic austenitic steel, the elastic anisotropy is studied with regard to a Pawley refinement, and compared with previous results from conventional neutron diffraction experiments on the same material. It is shown that the method can be used to determine anisotropic strains, diffraction elastic constants, the residual and applied stress state as well as the unstrained lattice parameter by recording transmission spectra at different specimen inclinations, by complete analogy with the sin2ψ technique frequently used in X-ray diffraction. The technique is shown to deliver reliable measures of strain even in the case of moderate texture and elastic anisotropy.

Bragg Edge Determination for Accurate Lattice Parameter and Elastic Strain Measurement

The transmission spectrum of thermal neutrons through a polycrystalline sample displays sudden, well-defined increases in intensity as a function of neutron wavelength. These steps, known as Bragg edges, occur at the point at which the neutron wavelength exceeds the Bragg condition for coherent scattering from the respective lattice planes, and can be easily observed using the time-of-flight method. Accurate location of these edges and determination of their magnitude and shape can be used to extract information about the stress state, texture and phases present in the material. This paper describes a method for analysing these edges singly and collectively, using a Pawley-type refinement. Furthermore, experimental trials are presented which demonstrate the utility of the method for the accurate measurement of lattice spacings, and thus strain. These trials include measuring the lattice parameter in Cu/Zn alloys as a function of Zn content, and the determination of elastic strain of an iron rod under tensile/compressive straining. In the former case the results are compared with Bragg diffraction peak measurements made on HRPD and in the latter case with conventional strain gauge measurements.

Residual Strain Measurements in Thermal Spray Cermet Coatings via Neutron Diffraction

The impact and fatigue resistance of overlay coatings is significantly influenced by the residual strain (or stress) field induced during coating deposition, post-treatment and in-service loading. Optimization of residual stress field is therefore critical to the life and performance of components. Non-destructive measurement of these stress fields in relatively thinner (300 to 400 micron) thermal spray coatings however poses a challenge because conventional techniques such as deep hole drilling, x-ray diffraction, synchrotron diffraction, and changes in beam curvature either makes these technique destructive, and/or provides only a very near-surface strain measurement. This particularly complicates the strain analysis in cermet coatings, e.g. WC-Co deposited by the thermal spraying process, where the low penetration depth of x- and synchrotron- diffraction rays can only provide a through thickness measurement of stress profile via the destructive layer removal technique. Recent investigations have therefore concentrated on the use of neutron diffraction technique for such analysis, and this paper reports some of the early findings of the comparison of through thickness strain measurements in relatively thin (400 μm) as-sprayed and post-treated WC-12%Co coatings via neutron diffraction technique. Since neutrons are not charged, they do not interact with the electron cloud surrounding the atom (unlike x-ray), hence diffraction results from the interaction with the atomic nucleus. Neutrons therefore have greater penetration depth in most engineering materials and therefore provide a non-destructive through thickness strain measurement. Results of strain measurement are discussed with the structure property relationship and contact fatigue performance, and indicate that post-treatment of these coatings results in harmonization of the strain field within the coating, and at the coating substrate interface. This significantly influences the contact fatigue performance by improving both the cohesive and adhesive strength of these coatings.Copyright

Time-of-flight neutron transmission of mosaic crystals

The energy-dispersive neutron transmission of mosaic crystals presents a series of dips in intensity as a result of reflection in the crystal planes. The positions of these dips can be exploited for the definition of the crystal orientation with a resolution of 1 min of arc. The widths of these dips depend on crystal orientation, on the reflection index, on the mosaicity, and on the incident-beam divergence. The capability of the technique to define the orientation and mosaicity of a Cu crystal has been assessed through time-of-flight experiments. A Cu monochromator has been plastically deformed by uniaxial tension in situ, and the evolution of crystal orientation and mosaicity was tracked using the technique. Several crystal reflections at different locations of the sample were simultaneously studied during the experiment. A linear increase of mosaicity on deformation at an average rate of ∼5 min per percentage of shear strain was observed. The reorientation of the crystal as a result of the applied load showed variations across the specimen.

Texture analysis with a time‐of‐flight neutron strain scanner

A time-of-flight (TOF) neutron strain scanner is a white-beam instrument optimized to measure diffractograms at precise locations within bulky specimens, typically along two perpendicular sample orientations. Here, a method is proposed that exploits the spatial resolution (∼1 mm) provided by such an instrument to determine in a nondestructive manner the crystallographic texture at selected locations within a macroscopic object. The method is based on defining the orientation distribution function (ODF) of the crystallites from several incomplete pole figures, and it has been implemented on ENGIN-X, a neutron strain scanner at the ISIS facility in the UK. This method has been applied to determine the texture at different locations of Al alloy plates welded along the rolling direction and to study a Zr2.5%Nb pressure tube produced for a CANDU nuclear power plant. For benchmarking, the results obtained with this instrument for samples of ferritic steel, copper, Al alloys and Zr alloys have been compared with measurements performed using conventional X-ray diffractometers and more established neutron techniques. For cases where pole figure coverage is incomplete, the use of TOF neutron transmission measurements simultaneously performed on the specimens is proposed as a simple and powerful test to validate the resulting ODF.