## 3 Results
### In Situ Monochromatic Imaging  
As shown in Figure 2, martensite and austenite have different attenuation coefficients in the wavelength interval between approx. 0.36 nm and 0.405 nm just below the Bragg-edge wavelengths of both phases. Although the difference of both attenuation coefficients is not as large as for the coefficients between both Bragg edges, a wavelength in this interval is used for monochromatic imaging due to the abovementioned limits of the experimental wavelength resolution. Thus, we present here the result of monochromatic imaging during GTAW at 0.395 nm for the visualization of the phase transformations. For comparison, we additionally measured after the Bragg edge at 0.44 nm using the same welding and imaging parameter. Figure 3 shows the mean value of the transmitted intensity approx. 1 mm underneath the sample top surface as a function of time detected in the rectangular yellow Region-of-Interest (ROI). The welding current was 80A, the welding velocity was 10 cm/min, and  
Figure 2: Calculated (colored) and measured (black) Bragg-edge curves for martensite and austenite at room temperature for sample thickness of 5 mm as used in the experiments. The transmission levels at 0.395 nm for the two phases are shown as colored dashed lines. The theoretical data were calculated with the help of a program library [13]. images were acquired every 2 s with an exposure time of 2 s. The transmission intensity increases drastically as soon as the torch passes by the ROI from approx. 0.42 a.u. to more than 0.56 a.u., i.e., by approx. 60%, and decreases slightly slower back to 0.42 a.u. at t \(\approx\) 150 s. This behavior can be even quantified using the theoretical data presented in Figure 2, where for a neutron wavelength of 0.395 nm the transmission for the martensitic phase is 0.42 a.u. and for the martensitic phase is 0.51 a.u. Thus, this transmission change can be attributed to the transformation from martensite to austenite, where the transmission decrease is due to the almost complete re-transformation to martensite upon cooling. The asymmetry of the transmission curve is due to the different heat fluxes during heating and cooling. Whereas the heat input from the electric welding arc fosters a very high heating rate, the heat flow upon cooling is somewhat slower and limited by the thermal isolation of the sample material. The slope of the transmission curve implies that the martensite deformation is finished within 1-1.5 min. For checking purposes, an additional in situ imaging experiment using the same welding and imaging parameters was performed at a wavelength of 0.44 nm. Since this wavelength is larger than the Bragg-edge wavelengths of both lattice types, we expected that the transmitted intensity remains constant. The result is shown in Figure 4. Indeed, we observed an intensity drop of approx. 15% during passing by of the ROI by the welding arc. Subsequently, the measured intensity increases exponentially within approx. 30 s and reaches the initial transmission of approx. 0.66 a.u., which correlates well with the transmission for the martensitic phase in Figure 2.  
Figure 3: Plot of the transmitted intensity at \(\lambda\) = 0.395 nm measured as mean value from the ROI (yellow rectangle) as a function of time. The insert shows the monochromatic image taken at t \(\approx\) 130 s. The very low neutron intensity results in a poor image quality, showing essentially no visual image contrast in the sample for all times. A possible explanation of the observed drop in the transmitted neutron intensity is the so-called Debye-Waller factor [15; 16]. The Debye-Waller factor describes the scattering of neutrons at an oscillating lattice. The immense heat input by the traveling welding arc melts the material at the surface and excites massive lattice oscillations in the subjacent crystalline material. Since the ROI is located 1 mm underneath the surface, most of the material measured should be solid during analysis.