Cold neutron diffraction contrast tomography of polycrystalline material

Traditional neutron imaging is based on the attenuation of a neutron beam through scatteririg and absorption upon traversing a sample of interest. It offers insight into the sample's material distribution at high spatial resolution in a non-destructive way. In this work, it is expanded to include the diffracted neutrons that were ignored so far and obtain a crystallographic distribution (grain mapping). Samples are rotated in a cold neutron beam of limited wavelength band. Projections of the crystallites formed by the neutrons they diffract are captured on a two dimensional imaging detector. Their positions on the detector reveal their orientation whereas the projections themselves are used to reconstruct the shape of the grains. Indebted to established synchrotron diffraction contrast tomography, this 'cold neutron diffraction contrast tomography' is performed on recrystallized aluminium for experimental comparison between both. Differences between set-up and method are discussed, followed by the application range in terms of sample properties (crystallite size and number, mosaicity and typical materials). Neutron diffraction contrast tomography allows to study large grains in bulky metallic structures.

Simultaneous neutron transmission and diffraction contrast tomography as a non-destructive 3D method for bulk single crystal quality investigations

Steven Luc X Peetermans

Traditional neutron tomography allows to reconstruct the attenuation cross section, a measure for the material distribution, at high spatial resolution and non-destructively. However, it does not state anything about the ordering structure of the atoms inside this material. Extending the setup with a second neutron imaging detector, diffracted neutrons from the ordered crystal lattice could be captured. Emerging iterative reconstruction techniques allow reconstructing the local Bragg reflectivity in the sample, a measure for the spatial distribution in crystal quality (orientation, homogeneity of phases). Simultaneous acquisition ensures optimal use of the neutron flux and a direct comparison of different sample properties. (C) 2013 AIP Publishing LLC.

Amer Inst Physics2013

Energy-selective neutron imaging for materials science

Steven Luc X Peetermans

Common neutron imaging techniques study the attenuation of a neutron beam penetrating a sample of interest. The recorded radiograph shows a contrast depending on traversed material and its thickness. Tomography allows separating both and obtaining 3D spatial information about the material distribution, solving problems in numerous fields ranging from virtually separating fossils from surrounding rock to water management in fuel cells. It is nowadays routinely performed at PSI¿s neutron imaging facilities. Energy-selective neutron imaging studies the wavelength-dependency of the cross-section by using a beam of reduced wavelength bandwidth instead of averaging out the cross-section over the incident beam spectrum. The range of observed contrasts/image information is than extended and can largely be understood in the context of the Bragg law. Different types of monochromator (mechanical neutron velocity selector, double crystal monochromator, filter materials) are characterized for use in neutron imaging. In polycrystalline samples, sharp Bragg edges are observed as coherent elastic scattering at the (hkl) plane can occur for all wavelengths up to 2dhkl, after which a sharp increase in transmission intensity is observed. Much like diffraction peaks, they contain information on e.g. crystal phase or projected strain. The absence of coherent elastic scattering past the last Bragg edge (Bragg cut-off) allows for quantification. In samples with few grains or even single crystals, all orientations w.r.t. the beam are no longer present and rather than Bragg edges, the cross section now exhibits distinct peaks, the ensemble of which holds information on the crystallite¿s phase, orientation and shape. A spatial variation in contrast appears across the sample, between those grains fulfilling the Bragg condition ¿ scattering and decreasing the transmitted beam intensity ¿ and those that do not. After initial qualitative assessments, recent advances on the quantitative grain orientation mapping are made based on time-of-flight measurements of high energy resolution recorded at the ISIS pulsed neutron source. But where do these scattered neutrons go to? A new set-up was developed to permit simultaneous transmission and diffractive neutron imaging. Capturing the neutrons diffracted by a grain also yields a projection of that grain, with the position on the detector indicative of the orientation. These projections can in turn be used for algebraic reconstruction, which yields a grain volume as well. After feasibility studies on an iron single crystal cube the recent push towards polycrystalline samples will is illustrated with a neutron diffraction contrast tomography (nDCT) of a coarse-grained aluminium strain sample.

EPFL2015

Simultaneous neutron transmission and diffraction imaging investigations of single crystal nickel-based superalloy turbine blades

Steven Luc X Peetermans

The integrity of turbine blades is at the heart of every operating aerospace engine or land-based gas turbine. In the most demanding environments, they are cast as single crystals. Whereas traditional neutron imaging is an excellent tool for the non-destructive testing in search of cracks or residual core material, it is insensitive to variations in crystallographic properties. In this work we show that by simultaneous imaging of the diffracted neutrons, one can also probe variations in crystal orientation. In a matter of seconds, different dendritic growth in two industrial turbine blades could be ascertained. (C) 2016 Elsevier Ltd. All rights reserved.

Elsevier Sci Ltd2016

Energy-selective neutron imaging for materials science

Steven Luc X Peetermans

EPFL2015