PSI - Issue 74

Pavol Mikula et al. / Procedia Structural Integrity 74 (2025) 56–61 Pavol Mikula / Structural Integrity Procedia 00 ( 2025) 000–000

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Fig. 1. Diffractometer scheme for the strain/stress scanning and the detail of the irradiated gauge volume of the irradiated material.

equipped with a bent perfect crystal (BPC) monochromator which has optimized curvature with respect to luminosity and resolution in a limited range of scattering angles 2 θ S , see e.g. Mikula et al. (1997), Seong et al. (2011). The principle of the measurements of variations of lattice spacing consists in the precise determination of the d hkl -lattice spacing of particularly oriented crystal planes of the sample. In neutron and X-ray diffraction the angular positions of the diffraction maxima are directly related to the values of the lattice spacing through the Bragg equation 2 d ·sin θ = λ ( d = d hkl , θ - Bragg angle, λ – the neutron wavelength) and thus offer a unique non-destructive technique for the investigation of e.g. strain/stress fields. The lattice strain ε is defined as ε = ∆ d / d 0 ( d 0 is the lattice spacing of the standard sample), and by differentiation of the Bragg condition we arrive at ε = ∆d / d 0 = -cot θ · ∆ θ . Therefore, the change of ∆ d in the sample indicates a change in the scattering angle 2 θ by a shift ∆ (2 θ ) of the diffraction peak position for particular reflecting planes. The shift ∆θ permits the determination of the average lattice macrostrain over the irradiated gauge volume. The conversion of strains to stresses is carried out by using appropriate elastic moduli, see Hutchings and Krawitz (1992). The resolution of the conventional scanner is determined mainly by the thickness and curvature of the monochromator, the widths of the slits (usually in the order of 1-3 millimeters), the divergence of the beam diffracted by the gauge volume and the spatial resolution of PSD. The combination of all these uncertainties results in an uncertainty ( FWHM of the gaussian image in PSD) of about roughly (5÷8)x1 0 -3 rad. It appears sufficient for the measurement of ∆ (2 θ hkl ) angular shifts brought about by strains, but not for the measurements of changes of diffraction line profiles imaged by PSD which would permit to carry out microstrain and grain size studies of e.g. plastically deformed polycrystalline samples (see e.g. Delhez et al. (1982), Davydov et al. (2008)). Recently developed the focusing 3-axis diffractometer (see Fig. 3) equipped with the BPC monochromator and analyzer offers the sensitivity in determination of strains ∆ d / d < 10 -4 in polycrystalline materials. The resolution determined by FWHM of the diffraction profiles is about one order of magnitude higher with respect to the conventional 2-axis strain/stress scanners as shown in Fig. 1. This set-up also offers the possibility of the line profile analysis for much larger bulk gauge volumes and reasonable counting times as demonstrated on Ti-Gr 2 samples. 2. Description of the material The goal of our studies was to test the feasibility of the new high-resolution neutron diffraction for obtaining additional experimental results, complementary the ones obtained by conventional methods, namely, in the case of the studies of plastically deformed samples. The choice of the Ti-Gr 2, known as the "workhorse" of the titanium industry, follows from the fact that it is one of the most commonly used grades. We had at the disposal 5 samples: The initial one of the diameter of 8 mm (as prepared), then the sample of the diameter of 7.2 mm submitted ones to the CONFORM-ECAP (Equal Channel Angular Pressing) load and finally 3 plastically deformed samples submitted first to the CONFORM-ECAP (C-E) load for a number pressing routes 1, 2, 3, however, each of them after the (C-E) pressing routes was submitted to the rotary swaging (RS) to the final diameter of 4 mm. There are basically four grades of titanium on the market. Each grade exhibits distinct allowable element limits and strength levels with different characteristics. Knowing these properties is important for engineers, designers, and