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Deformation mechanisms in a Mg-Al-Ca alloy reinforced with short alumina fibres were studied by acoustic emission and neutron diffraction method (ND). The microstructure after deformation was observed by electron microscope (SEM). The fibres plane orientation with respect to the loading axis was found to be a key parameter, which influences the active deformation processes, such as twinning and dislocation slip. In-situ neutron diffraction tests were performed at room temperature (RT) and 100 °C. The relative twin volume in the matrix as a function of the applied compressive load were evaluated from the ND tests. Acoustic emission results indicate a larger twinning activity and more frequent fibre cracking in sample with perpendicular fibre plane orientation. Both of these mechanisms increase in magnitude at 100 °C. [ABSTRACT FROM AUTHOR]

The stress-induced martensitic transformation in tensioned nickel-titanium shapememory alloys proceeds by propagation of macroscopic fronts of localized deformation. We used three-dimensional synchrotron x-ray diffraction to image at micrometer-scale resolution the grain-resolved elastic strains and stresses in austenite around one such front in a prestrained nickel-titanium wire. We found that the local stresses in austenite grains are modified ahead of the nose cone–shaped buried interface where the martensitic transformation begins. Elevated shear stresses at the cone interface explain why the martensitic transformation proceeds in a localized manner. We established the crossover from stresses in individual grains to a continuum macroscopic internal stress field in the wire and rationalized the experimentally observed internal stress field and the topology of the macroscopic front by means of finite element simulations of the localized deformation. [ABSTRACT FROM AUTHOR]

Abstract: Neutron strain scanning was employed to investigate the internal stress fields in steel plate coupons with embedded prestrained superelastic NiTi shape memory alloy inserts. Strain fields in steel were evaluated at T =21°C and 130°C on virgin coupons as well as on mechanically and thermally fatigued coupons. Internal stress fields were evaluated by direct calculation of principal stress components from the experimentally measured lattice strains as well as by employing an inverse finite element modeling approach. It is shown that if the NiTi inserts are embedded into the elastic steel matrix following a carefully designed technological procedure, the internal stress fields vary with temperature in a reproducible and predictable way. It is estimated that this mechanism of internal stress generation can be safely applied in the temperature range from −20°C to 150°C and is relatively resistant to thermal and mechanical fatigue. The predictability and fatigue endurance of the mechanism are of essential importance for the development of future smart metal matrix composites or smart structures with embedded shape memory alloy components. [Copyright &y& Elsevier]

Abstract: Microstructural changes taking place during the heat treatment of cold-worked NiTi alloy are of key interest in shape memory alloy technology, since they are responsible for setting the austenite shape and functional properties of the heat-treated alloy. In this work, microstructural evolution during non-conventional electropulse heat treatment of thin NiTi filaments was investigated in a unique high-speed in situ synchrotron X-ray diffraction experiment with simultaneous evaluation of the tensile force and electrical resistivity of the treated wire. The in situ results provide direct experimental evidence on the evolution of the internal stress and density of defects during fast heating from 20°C to ∼700°C. This evidence is used to characterize a sequence of dynamic recovery and recrystallization processes responsible for the microstructure and superelastic functional property changes during the electropulse treatments. [ABSTRACT FROM AUTHOR]

The amount of telomeric C-heterochromatin was investigated in 20 inbred lines of try and their open-pollinated source cultivars, in this short communication only the banding pattern of the chromosome in the lines is presented. The inbred lines had an amount of heterochromatin similar to the source cultivars. Both groups carried some chromosomes with small relomeric C-bands. The individual source cultivars were composed of 2--4 karyotypes having different amounts of telomeric heterochromatin. Most karyotypes were also identified in the respective inbred lines. Results suggest that the inbreeding process was not related to a reduction of telomeric C-heterochromatin in try chromosomes. [ABSTRACT FROM AUTHOR]

In the present work neutron diffraction has been applied for ex situ investigation of residual stresses in Mg-4%Al-1%Ca (AX41) magnesium alloy reinforced with short Saffil fibers. Samples were deformed in compression at room temperature. Two types of fiber arrangement were investigated. In both samples the fibers were homogeneously distributed and arranged in parallel planes with a random fiber orientation. In the first sample these planes were parallel with the loading axis and in the second one they were perpendicular to the loading direction. Significant dependence of both the mechanical properties and residual strains on the fibers orientation was observed. Sample with parallel fiber arrangement showed higher hardness and lower ductility. Further the increment of residual tensile lattice strain in the matrix with a macroscopic deformation is much higher than in the other case. It was found that the residual strain evolution strongly depends on the orientation of grains in the matrix. [ABSTRACT FROM AUTHOR]

Motivated by an assumption that the instability of the cyclic tensile superelastic behavior of NiTi polycrystal is linked to its fatigue performance (number of cycles till failure), the instability was investigated by high resolution in situ synchrotron X-ray diffraction method. NiTi wires were cyclically deformed in tension at room temperature while X-ray diffraction patterns were recorded in three preselected states along the superelastic stress–strain curve, analyzed and interpreted in terms of the gradual evolution of microstructural state during cycling. It is found that the cyclic instability is due to the gradual redistribution of internal stresses originating from the accumulation of incremental plastic strains accompanying the stress induced martensitic transformation in constrained polycrystalline environment. The degree of cyclic instability increases with the increasing involvement of slip in the hybrid slip/transformation process, which depends on initial microstructure (grain size, defects, precipitates), martensitic transformation (crystallographic incompatibility between transforming phases), temperature and parameters of the cyclic loading (strain rate, amplitude, stress state, type of loading etc.). [ABSTRACT FROM AUTHOR]

Abstract: Superelastic deformation of thin Ni–Ti wires containing various nanograined microstructures was investigated by tensile cyclic loading with in situ evaluation of electric resistivity. Defects created by the superelastic cycling in these wires were analyzed by transmission electron microscopy. The role of dislocation slip in superelastic deformation is discussed. Ni–Ti wires having finest microstructures (grain diameter <100nm) are highly resistant against dislocation slip, while those with fully recrystallized microstructure and grain size exceeding 200nm are prone to dislocation slip. The density of the observed dislocation defects increases significantly with increasing grain size. The upper plateau stress of the superelastic stress–strain curves is largely grain size independent from 10 up to 1000nm. It is hence claimed that the Hall–Petch relationship fails for the stress-induced martensitic transformation in this grain size range. It is proposed that dislocation slip taking place during superelastic cycling is responsible for the accumulated irreversible strains, cyclic instability and degradation of functional properties. No residual martensite phase was found in the microstructures of superelastically cycled wires by TEM and results of the in situ electric resistance measurements during straining also indirectly suggest that none or very little martensite phase remains in the studied cycled superelastic wires after unloading. The accumulation of dislocation defects, however, does not prevent the superelasticity. It only affects the shape of the stress–strain response, makes it unstable upon cycling and changes the deformation mode from localized to homogeneous. The activity of dislocation slip during superelastic deformation of Ni–Ti increases with increasing test temperature and ultimately destroys the superelasticity as the plateau stress approaches the yield stress for slip. Deformation twins in the austenite phase ({114} compound twins) were frequently found in cycled wires having largest grain size. It is proposed that they formed in the highly deformed B19′ martensite phase during forward loading and are retained in austenite after unloading. Such twinning would represent an additional deformation mechanism of Ni–Ti yielding residual irrecoverable strains. [Copyright &y& Elsevier]

Abstract: Transmission electron microscopy, electrical resistivity measurements and mechanical testing were employed to investigate the evolution of microstructure and functional superelastic properties of 0.1mm diameter as-drawn Ni–Ti wires subjected to a non-conventional heat treatment by controlled electric pulse currents. This method enables a better control of the recovery and recrystallization processes taking place during the heat treatment and accordingly a better control on the final microstructure. Using a stepwise approach of millisecond pulse annealing, it is shown how the microstructure evolves from a severely deformed state with no functional properties to an optimal nanograined microstructure (20–50nm) that is partially recovered through polygonization and partially recrystallized and that has the best functional properties. Such a microstructure is highly resistant against dislocation slip upon cycling, while microstructures annealed for longer times and showing mostly recrystallized grains were prone to dislocation slip, particularly as the grain size exceeds 200nm. [Copyright &y& Elsevier]

Abstract: Structure, room temperature- and elevated temperature mechanical properties of the Al–5.7Cr–2.5Fe–1.3Ti alloy are studied in the presented paper. The alloy was prepared by the powder metallurgy including melt atomization into rapidly solidified powder followed by hot extrusion. The alloy shows a fine-grained structure (average grain size of 1μm) and a relatively high tensile strength of 380MPa. Variations in structure, hardness and tensile properties due to the long-term annealing at 400°C/200h are negligible, suggesting the excellent thermal stability of the alloy. Moreover, its tensile properties at 350°C exceed the commercial Al–Si alloy commonly used in elevated temperature applications. [Copyright &y& Elsevier]

Dielectric relaxation measurements on maltose were performed at ambient and increasing pressure. The loss spectra collected below glass transition of this disaccharide revealed presence of two well separated secondary relaxations. Activation energies determined for both modes are Ea=73 kJ/mol and 47 kJ/mol for the slower (β) and faster (γ) relaxation, respectively. From high pressure measurements activation volume ΔV=15.6 ml/mol for the slower secondary relaxation was estimated. Both quantities: activation energy and activation volume for α-process derived from dielectric data, were compared to those obtained from the conformational calculations with use of density functional theory (DFT). We found out satisfactory agreement between both quantities for the molecular motion related to the rotation of the two monosaccharide units around glycosidic linkage in this disaccharide. [ABSTRACT FROM AUTHOR]

A novel original method of determination of stresses and critical resolved shear stresses (CRSSs) using neutron diffraction was proposed. In this method, based on the crystallite group method, the lattice strains were measured in different directions and using different reflections hkl during uniaxial deformation of magnesium alloy AZ31. The advantage of this method is that the stresses for groups of grains having similar orientations can be determined directly from measurement without any models used for data interpretation. The obtained results are unambiguous and do not depend on the models assumptions as in previous works. Moreover, it was possible for the first time to determine the uncertainty of the measured CRSS values and local stresses at groups of grains. The used methodology allowed for the determination of stress partitioning between grains having different orientations and for an explanation of the anisotropic mechanical behaviour of the strongly textured alloy. Finally, the CRSS values allowed for the validation of the type of intergranular interaction assumed in the elastic–plastic self-consistent model and for a significant reduction of the number of unknown parameters when the model is adjusted to the experimental data. [ABSTRACT FROM AUTHOR]

Abstract Recent macroscopic experimental and theoretical evidence on the stress-strain-temperature behavior of NiTi beyond the strain recoverability limits (large strain, high stress, high temperature), where reversible martensitic transformation tends to proceed together with irreversible plastic deformation processes, is reviewed. Model predictions on the transformation – plasticity coupling are laid out based on the mathematical theory of martensitic microstructures and the crystal plasticity theory. A particular attention is paid to the strain compatibility at moving phase interfaces that may have a direct impact on the plasticity accompanying the martensitic transformation. It is suggested that strong transformation-plasticity coupling shall be expected during the reverse martensitic transformation. Macroscopic models from the literature capable of simulation of thermomechanical responses of NiTi polycrystals due to coexisting martensitic transformation and plastic deformation are reviewed. Dedicated thermomechanical loading experiments on superelastic and actuator NiTi wires aimed at improving our understanding of the coupling between martensitic transformation and plasticity are presented. Based on the results of in-situ studies during thermomechanical loading experiments (electric resistance, synchrotron X-ray diffraction, surface strain by DIC, relaxations) and characterization microstructures in deformed wires by TEM, it is shown that: (i) microstructures and consequently functional properties of annealed NiTi wires can be purposely manipulated by thermomechanical processing, (ii) shape setting of NiTi can be performed at relatively low temperatures (<300 °C), (iii) strain drift of NiTi actuators can be brought under control utilizing the knowledge derived from the presented experiments. [ABSTRACT FROM AUTHOR]

Vertical split rim (VSR) cracks are the most common cause for the removal of broken or cracked heavy haul car wheels in North America. This failure mode is characterized by rapid, unstable crack propagation in the hoop or in the tangential direction. To investigate the methods to prevent VSRs, an understanding of how the service loads correlate to their occurrence is necessary. In this study, a finite element analysis is conducted to evaluate the rim stresses below the tread surface during rolling contact between the wheel tread and the rail. Then, the stress intensity factor is calculated using the stress distribution in the wheel rim obtained using the finite element analysis. The results of the analysis show that axial residual tensile stress, which contributes to unstable vertical crack propagation, increases at deeper positions below the tread surface owing to cyclic rolling contact. These results indicate that a vertical crack can propagate rapidly below the tread surface. The fracture toughness of the wheel steel is measured using Charpy impact tests. The results of the test show that Weibull distribution enables the approximation of the relationship between fracture probability and fracture toughness. To evaluate the effect of the fracture toughness of wheel steel on the differences in VSR crack probability, a comparison between high fracture toughness wheels and conventional Class-C wheels is made. The VSR rate can be predicted from the stress intensity factor and Weibull equation for fracture toughness. The prediction results show that the VSR rate of high fracture toughness wheels is only 5% of that of conventional Class-C wheels. Therefore, high fracture toughness steel wheels are considered to have a higher resistance to VSR cracks compared with conventional Class-C wheels. [ABSTRACT FROM AUTHOR]

Three neutron instruments at the Neutron Physics Laboratory (NPL) in Řež near Prague — small-angle scattering (SANS) MAUD, strain scanner SPN-100 and strain diffractometer TKSN-400 — have been modernized recently with new 2D position-sensitive detectors (PSDs) from JINR, Dubna. Here we report on the progress made in relation to the possibilities of the diffractometers due to the improved performance of the detectors. The first part of the paper is dedicated to a detailed description of the hardware and software of the PSDs, as well as its integration with the in-house experimental control software. Then practical examples of neutron scattering experiments for each of the upgraded facilities are presented. [ABSTRACT FROM AUTHOR]

Purpose: To demonstrate a very effective and easy way of stabilization of amorphous indomethacin (IMC) by preparing binary mixtures with octaacetylmaltose (acMAL). In order to understand the origin of increased stability of amorphous system inter- and intramolecular interactions between IMC and acMAL were studied. Methods: The amorphous IMC, acMAL and binary mixtures (IMC-acMAL) with different weight ratios were analyzed by using Dielectric Spectroscopy (DS), Differential Scanning Calorimetry (DSC), Raman Spectroscopy, X-ray Diffraction (XRD), Infrared Spectroscopy (FTIR) and Quantitative Structure-Activity Relationship (QSAR). Results: Our studies have revealed that indomethacin mixed with acetylated saccharide forms homogeneous mixture. Interestingly, even a small amount of modified maltose prevents from recrystallization of amorphous indomethacin. FTIR measurements and QSAR calculations have shown that octaacetylmaltose significantly affects the concentration of indomethacin dimers. Moreover, with increasing the amount of acMAL in the amorphous solid dispersion molecular interactions between matrix and API become more dominant than IMC-IMC ones. Structural investigations with the use of X-ray diffraction technique have demonstrated that binary mixture of indomethacin with acMAL does not recrystallize upon storage at room temperature for more than 1.5 year. Finally, it was shown that acMAL can be used to improve solubility of IMC. Conclusions: Acetylated derivative of maltose might be very effective agent to improve physical stability of amorphous indomethacin as well as to enhance its solubility. Intermolecular interactions between modified carbohydrate and IMC are likely to be responsible for increased stability effect in the glassy state. [ABSTRACT FROM AUTHOR]

Abstract: The present work deals with a non-conventional application of multifunctional materials such as shape memory alloy in engineering components. The concept of active inserts has been adopted in order to redistribute compressive stresses emerged in cutting disc during its operation. According to the present design, the small pre-strained elliptical NiTi elements were placed into openings of steel cutting disc in places with expected maximum stress concentration. To study the stress interaction of the NiTi inserts with steel matrix in detail, the in-situ method of neutron diffraction was employed for residual stress mapping. The diffraction experiments were focused substantially on scan of internal stresses around inserts and their evolution with increased temperature. The performed studies confirm the potential ability of NiTi insert to induce the compressive stress within steel matrix with applied temperature. [Copyright &y& Elsevier]

The main purpose of this work is to develop neutron diffraction methodology in order to determine stresses localised in polycrystalline grains during elastoplastic deformation, directly from experiment. As a result, for the first time, the von Mises stress for chosen grain orientations and Critical Resolved Shear Stresses (CRSS) for active slip systems were unambiguously measured without the help of crystallographic models, which introduce different theoretical assumptions. The stresses measured for groups of grains and the determined CRSS values are important characteristics of a material, which allow to study plastic deformation in textured material at different scales: slip system and grain, which play a key role in mechanical properties and formability of the material. The new method was successfully tested and applied to textured AZ31 alloy subjected to tensile deformation and the components of stress tensor were for the first time determined from measured lattice strains corresponding to chosen orientations of crystallite lattice. The obtained results positively verified hypotheses that, during plastic deformation, a large difference in the hardness as well as in the localised stresses occurs for grains having different lattice orientations. It was found directly from experiment that, the activation of basal glide, having small CRSS, does not lead to significant plastic deformation, and the activation of other non-basal systems (with higher CRSS) induces the development of plasticity at the macroscopic scale. The early plastic deformation occurring due to slip on basal system is small but it can destructively affect fatigue life, limiting applicability of the material for structural components. Finally, the comparison of experimental results with a modified version of Elastic-Plastic Self-Consistent (EPSC) model showed its capability to simulate the mechanical behaviour of such materials. [ABSTRACT FROM AUTHOR]

Rolling contact fatigue failure is one of the very common causes for the replacement of heavy haul car wheels. There are two crack initiation modes in rolling contact fatigue failures, which are surface and subsurface crack initiation modes. Internal defects such as voids and non–metallic inclusions are known to be the possible origins of subsurface cracks. In this study, torsion and axial fatigue tests are conducted on railway wheel steel with artificial defects, and finite element analyses of heavy haul car wheels are performed to study the subsurface crack initiation due to cyclic rolling contact. The fatigue limits of the wheel steel decrease with an increase in the artificial defect size. The shear and axial fatigue limits with different defect sizes agree well the El–Haddad formula. Based on the finite element analysis considering the multiaxial stress state, the critical defect size for subsurface crack initiation is estimated to be 1.1 mm. This indicates that subsurface cracks might be initiated in service conditions as the critical size is less than 1.6 mm, which is the maximum permissible size of an internal defect in the wheel rim as per Association of American Railroads specifications. Therefore, higher–cleanliness wheels would be required to prevent subsurface cracks. • Critical defect size for subsurface crack initiations of railway wheel was studied. • Fatigue limits of wheel steel decreased with an increase of defect size. • Predicted critical defect size was 1.1 mm in actual wheels from multi-axial fatigue evaluation. • Higher cleanliness wheels have superior resistance to subsurface crack initiations. [ABSTRACT FROM AUTHOR]