The physical properties of structural as well as functional components are largely governed by the materials microstructure. In polycrystalline materials, the crystallographic phase and preferred grain orientation (texture) can dominate the physical properties. A range of microscopy and diffraction techniques are hence routinely used for their characterization, but are prone to miss local variations across relevant length scales.

Wavelength resolved neutron imaging, in this context often referred to as Bragg edge imaging, has shown its great potential to reveal such local variations. This paper summarizes its capabilities for determining phase fractions deep in the bulk (up to centimeters) of samples and to detect spatial variations of texture. The pedagogic presentation of the experimental results (for a TRIP steel that exhibits an austenite-to-martensite transformation after plastic deformation) may serve as a great starting point for other researchers who want to evaluate if the method can help their own research questions.

Figure 1: Example experimental data for samples of the same material, obtained from two separate stocks: Batch 1 (a-d) and batch 2 (e-h). The transmission profiles are plotted for an area in the center of the sample and on the side. For sample batch 1, the Bragg-edge patterns are very similar for the two sample orientations, indicating a low degree of preferred crystallite orientation. Contrary, sample batch 2 shows strongly varying Bragg-edge profiles, and the radiograph in (e) taken at 4 Å shows that the transmission in the center of the sample is larger than at the side, thus revealing textural heterogeneities. (d+h) Pole figures obtained by neutron diffraction for the complete sample bulk, depicting locations that are probed in above sample orientations by imaging. (i) The imaging geometry, with the relevant scattering vector corresponding to a Bragg edge is illustrated. Rotating the sample around ω probes the circumference of the pole figures in the present coordinate system.

The presented experimental results have been obtained over the last years at different neutron sources and are now contextualized in view of the strength and shortcomings of the technique. The case study compares samples that are non-texured (i.e. possess randomly orientated grains) vs those that are textured. The samples have been measured using a tunable double crystal monochromator at the (now closed) BER2 research reactor in Germany as well as using the time-of-flight technique at the ISIS spallation source in the UK. The results also exemplify the complementary of the two instrumental approaches that offer different wavelength resolutions. Figure 1 shows wavelength-selective neutron imaging of two samples and how the data relates to pole figures that were measured by neutron diffraction.

This work actually concludes the work carried out by the first author during his PhD project, where he e.g. established spectral neutron tomography [1] and investigated complex phase distribution in TRIP steel caused by torsion of rectangular shaped specimens [2]. The work has contributed to further establish neutron imaging as a viable tool for (quantitative) characterization of crystalline materials, which can be expected to further gain popularity as corresponding capabilities have just been added to the imaging beamline NeXT at ILL and will be a core capability of ODIN at ESS.  Within Europe, wavelength resolved neutron imaging is available at ISIS (UK), PSI (Switzerland) and FRM2 (Germany). One current research trend that benefits from these capabilities concerns microstructure control of parts made by additive manufacturing [3].

The successful PhD work of Dr Khanh Van Tran was carried out at the Technical University of Berlin and Helmholtz Zentrum Berlin. The work relied on close collaboration with Rutherford Appleton Laboratory (ISIS Facility), The University of California at Berkeley, The University of Tennessee Knoxville and the European Spallation Source.

Contact:

Robin Woracek

Instrument Scientist

European Spallation Source ERIC, Lund, Sweden