M. Vrána

In situ neutron diffraction study of α–γ Fe–Cr–Ni alloys under tensile deformation

Abstract Neutron diffraction experiments were performed in situ upon tensile loading for five Fe–Cr–Ni alloys with different ferrite volume fractions ranging from 0 to 100%. The diffraction profiles were recorded along with the tensile deformation curves during temporary stops in a deformation machine (5 ks each) with the crosshead fixed. One reflection of each phase, (111) of austenite (γ) and (110) of ferrite (α), respectively, were measured simultaneously by using a position sensitive detector. Evolution of elastic lattice strains and dislocation densities in both constituent phases was evaluated from measured diffraction profiles as a function of external loading. Based on these experimental results, heterogeneous deformation behavior in the α–γ dual-phase alloys is discussed.

Bent perfect crystals in asymmetric diffraction geometry in neutron scattering experiments

Abstract Asymmetric cuts of elastically deformed perfect crystals, although less frequently used in practice, may also be attractive for applications. They offer an additional degree of freedom for optimization of instruments setups in terms of real- and reciprocal-space focusing. Some aspects of an useful employment of cylindrically bent perfect Si crystals in a strongly asymmetric geometry in neutron scattering experiments are presented. Special attention is paid to the limiting case of the fully asymmetric diffraction (FAD) geometry of the bent crystals. Here the absence of real-space focusing (the focus is directly at the crystal) is compensated by the effective mosaicity controlable in a large range, permitting to design instrument configurations offering considerably higher resolution for a given luminosity than the mosaic crystals.

Bragg diffraction optics in high resolution strain measurements

Abstract Using Bragg diffraction optics focusing conditions for a triple-axis setup equipped with a bent Si single crystal monochromator and analyzer for investigation of stress fields in polycrystalline materials are derived. Results of experimental tests of such a setup are presented. Further it is demonstrated that if certain focusing conditions for a bent monochromator are fulfilled the beam diffracted by a polycrystalline sample becomes quasi-parallel which enables high resolution measurements directly with a PSD without the use of a collimator or a crystal-analyzer. In the three axis setup maximum sensitivity in determination of Δd d ≤ 10 −4 can be achieved permitting profile-broadening analysis for reasonable sample volumes and counting times.

Use of multiple Bragg reflections in a bent perfect single crystal for high resolution monochromatization of neutrons

The effectiveness of the method of obtaining a highly monochromatic and collimated beam based on the well-known “Umweganregung” effect may be considerably improved when an elastically deformed single crystal is employed. In such a way a strongly excited multiple reflection effect produces a beam the intensity of which seems to be of practical use. For this purpose a very intensive umweg-effect simulating the 222 forbidden reflection of an elastically bent silicon single crystal at λ=0.156 nm was treated. The variable curvature of such a monochromator enables one to obtain a beam having a precisely fixed wavelength λ with a bandwidth Δλλ in the range from 10−4 to 10−3 and collimation of the order of minutes of arc. Experimental and theoretical treatment has proved that in an elastically deformed crystal the double diffraction simulating a forbidden reflection (Umweganregung) can be assumed as a diffraction on a bicrystal system realized in one crystal.

Multiple Bragg reflections of neutrons in an elastically deformed single crystal

The large positive diffraction intensity changes on forbidden (222) planes, which are produced by simultaneous diffraction, were observed when a perfect Si single crystal was excited into vibration. The excited Umweganregung effect is of the same order as the diffraction intensity obtained on (111) planes under the same experimental conditions. The theoretical consideration is an extension of the lamellae model usually used in the two-wave approximation. Both the theory and the experimental results demonstrate that such an effect may cause very large errors especially in diffraction experiments with non-perfect single crystals.

Residual Stress Distribution of Steel Welded Joints with Weld Metal of Low Martensite Transformation Temperature

Weld metal with a low martensite temperature may reduce the thermal stress in the low temperature region after welding, and consequently may decrease the residual stress in the weld metal and weld toe. Neutron diffraction analysis made clear that the reduction amount of residual stress is about 400 MPa in the case of a welded joint with no fillet and about 200 MPa in the case of that with a fillet. Reduction of 400 MPa brings about high resistance to weld cold cracking. The residual stress reduction of 200 MPa in boxing fillet weld joints increases fatigue strength by about 40 to 70 MPa. It can be said that the increased amount of fatigue strength is almost one forth of the 200 MPa that is the reduction amount of residual stress.

Simultaneous diffraction: Umweganregung peaks in the case of a vibrating single crystal

The positive diffraction intensity changes on forbidden (222) planes of a vibrating single crystal of Si, which are produced by simultaneous diffraction, are experimentally treated for the symmetric Laue case. The neutron diffraction experiment was performed for a flexurally vibrating as well as a non-vibrating perfect silicon bar. Several Umweganregung peaks were observed after exciting the bar into vibration while no Umweganregung peak was observed in the case of a non-vibrating crystal.

Austenite content and dislocation density in electron-beam welds of a stainless maraging steel

Abstract The volume fraction of austenite and dislocation density in electron beam weld joints of a martensitic-austenitic age-hardenable CrNiMoTiAl stainless steel were investigated using neutron diffraction. Experiments were performed on a high-resolution diffractometer which resolved microstrains of the order of 10 −4 . The effect of post-weld heat treatment on austenite content and dislocation density was investigated. As in other maraging steels, the maximum volume fraction of austenite was obtained after quenching and intercritical annealing. Dislocation densities computed on the base of mean-square microstrain values were about 10 11 cm −2 . These densities were systematically higher in martensite than in austenite, with very small differences between the weld metal and base metal.

Neutron diffraction by perfect crystals excited into mechanical resonance vibrations

Abstract Our contribution presents results of investigations concerning neutron diffraction by vibrating perfect crystals of Si and SiO 2 . They were excited in longitudinal, flexural, thickness and thickness-shear modes of vibration with resonance frequencies ranging from 1 to 1500 kHz. In this case, when the acoustic wavelength is much larger than the extinction distance the following phenomena caused by the deformation and Doppler shift were observed: broadening of the perfect crystal rocking curve, enhancement of the integrated reflectivity by up to two orders of magnitude and time modulation of the diffracted beam. When neutron time-off-flight through the vibrating crystal is comparable to the vibration period, multiple reflections having influence on the reflectivity take place. While investigating the forbidden reflections in the crystal excited into vibrations, a strong multiple diffraction phenomena resulting in “umweganregung” effect was observed. Also the vibrating crystals as neutron choppers with 1 μs pulses and a repetition rate of up to 100 kHz were treated.

Correlation of Magnetic Properties and Residual Stress Distribution Monitored by X-Ray and Neutron Diffraction in Welded AISI 1008 Steel Sheets

In this paper, two identical American Iron and Steel Institute 1008 steels sheets were welded together in a butt joint configuration, with the welding line direction parallel to the rolling direction. The sheets were welded using the electron-beam welding technique. Stress distribution was monitored in the surface and the bulk of the welded samples using X-ray Bragg-Brentano and neutron diffraction methods, respectively. Accordingly, surface and bulk magnetic properties, namely magnetic Barkhausen noise and quasi-dc permeability, were monitored in the same area. Both surface and bulk magnetic measurements exhibit good agreement with the surface and bulk stress measurements obtained by the diffraction methods, after a proper calibration procedure. Complementary measurements with scanning electron microscopy and Vickers microhardness tester provide similar good agreement between the microstructural characterization and the residual stress distribution.