Papers by Keyword: Neutron Diffraction

Abstract: We report on experimental investigations of a Ni_2.36Mn_0.64Ga Heusler alloy, which transforms to tetragonal martensite at cooling below M_s ≈ 271°С. The evolution of lattice constants was tracked by in situ neutron diffraction measurements. It was found that the martensite tetragonality c/a gradually decreases during heating from room temperature to austenite transition start temperature A_s ≈ 272°С. The phenomenon of martensite stabilization was investigated by differential scanning calorimetry utilizing three different protocols of the martensite aging. It was found that the martensite aging at a constant temperature T = 255°С merely shifts the reverse transformation to higher temperatures, while the reverse transformation temperature interval (A_f – A_s) remains the same (≈ 30°C) independently of aging time. On the other hand, a multistep aging at different temperatures starting from T = 255°С not only shifts the reverse transformation temperature, but makes the transformation temperature interval narrower down to A_s – A_f ≈ 10°C.

Abstract: Laser-welded structures are often subjected to dynamic service loads ranging from cyclic fluctuations to completely random ones. The laser-welded lap joints suffer from defects resulting in notch effects, surface cracks, residual strains and stresses. The fatigue strength of the laser-welded lap joints is reduced significantly because of the presence of these defects, and the size of the welded joints is small. Therefore, the mechanical strength of laser-welded structures must be defined in terms of the fatigue strength and residual stress of the obtained joints or assemblies. To analyze the above-discussed effects, this paper proposed two approaches: numerical and experimental methods. The originality of the work is to weld the rolled sheet in three directions (0°, 45°, 90°). The residual stresses before and after low cyclic tensile tests of assemblies obtained from overlapped thin DP600 steel sheets were calculated by ABAQUS. The obtained results were compared to the experimental data by neutron diffraction. The presented results in terms of residual stresses curves, spatial distributions of residual stresses obtained at the end of laser welding and low-cycle fatigue. It showed the relaxation effect of residual stresses and the direction effect of welding. These results have been explained by several factors.

Abstract: Focusing 3-axis diffractometer set-up equipped with bent perfect crystal (BPC) monochromator and analyzer offers the sensitivity in determination of strains Dd/d < 10^-4 in polycrystalline materials which is about one order of magnitude higher with respect to that of conventional 2-axis neutron scanners. It also offers possibility of line profile analysis for reasonable sample volumes and counting times. In this paper, the feasibility of using the 3-axis set-up even for measurements of rather large bulk polycrystalline samples with an acceptable resolution is presented. As the 3-axis set-up exploits focusing in real and momentum space, by a proper adjustment of the curvature of the analyzer, a high-resolution determination of the lattice changes can also be achieved even on large irradiated gauge volumes, though with a slightly relaxed resolution. It can be successfully exploited namely, in the strain/stress measurements on samples exposed to an external load, e.g. in tension/compression rig, in aging machine etc. In addition to the original performance where the analysis is carried out by rocking the BPC analyzer and the neutron signal registered by a point detector, a new alternative is offered which uses a fixed rocking angle position of the analyzer and the detector signal is registered by a one-dimensional position sensitive detector (PSD). This trick makes possible in some cases the elastic strain/stress measurements considerably faster and thus reduces the drawback of the time consuming step-by-step analysis.

Abstract: The increasing demand on thermo-mechanical strength, lightweight and formability in engineering applications require metallic materials with high sophisticated properties. Such functional alloys consist of heterogeneous composite-like microstructures, which are responsible for their stability in demanding service conditions (high temperature strength, low fatigue and creep resistance). External loads are distributed in between the phases of the alloys introducing high micro stress gradients, responsible for elastic and plastic deformation at the interfaces and micro crack initiation. Thus, the properties of such materials depend mainly on their phase shapes and 3D architectures leading to high stress gradients and elasto-plastic deformation under service conditions.This manuscript describes experimental studies on phase strain distribution for different heat treatment conditions in an AlMg4Si10 alloy. Neutron diffraction was used for strain measurement at an angle dispersive strain scanner with in-situ tensile test setup. Strain evolution under load and after unloading was measured to show elasto-plastic deformation behaviour in between the ductile α-Al matrix and stiff reinforcing Mg₂Si and Si phases. The degree of plastification, its effect on micro stress gradients and its influence on crack initiation could be discussed and comparisons to other composite materials could be drawn.

Abstract: Residual stress is one of the main reasons for failure of automotive cylinder blocks and engine heads. These failures are typically associated with in-service distortion or cracking occurring in engines during operation cycles. The problem becomes more pronounced for engines that are running at elevated operating pressures and temperatures, limiting R&D options in developing and implementing higher-efficiency engines. New aluminum alloys and manufacturing methods have been introduced with varying degree of success, in many cases affected by the stress magnitudes and stress distribution in the component. Therefore, active research is ongoing internationally on finding the most reliable methods of stress analysis as a basis for developing efficient methods for stress mitigation. The current study presents a comparison between two experimental strain measurements techniques: a destructive method that is based on application of strain gauge sensors, and a non-destructive method using neutron diffraction. The results indicate that although the strain gauge method provides an indication of the nature (i.e. compression or tension) of strain within a component, this method should primarily be used for surface measurements and qualitative analyses only. Neutron diffraction remains the superior technique for strain analysis, particularly for engineering components with complex geometries. The results from this study provide the transportation industry with a more comprehensive understanding of the efficacy of utilizing strain gauge sensors, neutron diffraction or finite element modelling for measuring the residual strain in cast components. The results will help manufacturers to develop the next generation of powertrain systems with increased efficiency and improved performance.

Abstract: Characterization of advanced materials by neutron powder diffraction provides information not accessible by other techniques. Thanks to the low absorption of neutrons, the bulk of the material and large-grain samples can be investigated, moreover in situ at elevated temperatures. The neutron diffraction use is demonstrated on two types of technologically important materials: Ti-Zr alloy and Co-Re high temperature alloy. In Ti-Zr alloy, the residual stress relief and microstrain evolution after ECAP was established. Boron influence on TaC strengthening precipitates in Co-Re high temperature alloys was shown not to be significant at the foreseen alloy operation temperatures, although boron content has a strong influence on the matrix phase.

Abstract: The authors have developed the in situ neutron diffraction technique focusing on bainite transformation during austempering. Thanks to the features of time-of-flight type neutron diffraction, textures, phase fractions and lattice parameters can be simultaneously measured at high temperature. In this paper, the design of experimental equipment and analytical approach are mainly described.

Abstract: In order to study the plastic deformation mechanism of AZ31 magnesium alloy, in situ texture measurement during uniaxial tensile deformation is conducted by using neutron diffraction. The specimen is prepared from a rolled sheet so that the deformation axis is parallel to the rolling direction. By increasing strain, the alignment of <10-10> along the tensile axis is strengthened, which is due to the activation of the prism slip system. The basal pole concentration at the prior sheet normal direction is slightly decreased by the deformation and the new texture component is formed at the transvers direction. This can be understood by activation of the {10-12} tension twinning. These results indicate that the tension twinning plays an important role even when the tensile deformation is applied parallel to the basal plane.

Abstract: The structure of MgH₂ samples has been investigated by the neutron diffraction method at room temperature and 5 K. Samples of MgH₂ have been prepared with vacuum extraction technique at high temperature. Obtained neutron data demonstrated that samples contain coexisting Mg and MgH₂ phases in different rate. The distribution of hydrogen atoms in the structure of the samples is comparable at both temperatures. Collected neutron data and results of X-ray analysis show that microstructure of the samples is different at room and at low temperature. Non-stability of partly desorbed MgH₂ samples after low temperature treatment has been discussed on basis of different diffusion of hydrogen atoms in varied microstructure.

Abstract: The work reports the results of neutron diffraction measurements of NiMnGe:T systems where T is Cr or Ti. All investigated compounds have the helicoidal magnetic structure with the propagation vector k = (k_a,0,0). The values of the k_a component decrease with increasing Cr content and increase with increasing Ti content. For all compounds, except the sample with x = 0.18 in Cr-system, the helicoidal order is stable up to the Néel temperature. The obtained data are analysed based on simple model in which the magnetic interactions are described by two exchange integrals J₁ > 0 for first and J₂ < 0 for second neighbouring moments. This model clears up different dependence of k_a component in different systems.