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Probing polycrystals' stress The way that a polycrystalline material deforms is in part determined by internal stresses between and within crystal grains. Hayashi et al. developed an x-ray method for mapping the intragranular stresses in a polycrystalline material. They found surprisingly large stresses, which are important for the fundamental understanding of how these materials will fail. This method will work for other materials and provides important information for multiscale deformation modeling. Science , this issue p. 1492 more...

This study is the first report on liquid water and ice imaging conducted at a pulsed spallation neutron source facility. Neutron imaging can be utilised to visualise the water distribution inside polymer electrolyte fuel cells (PEFCs). Particularly, energy-resolved neutron imaging is a methodology capable of distinguishing between liquid water and ice, and is effective for investigating ice formation in PEFCs operating in a subfreezing environment. The distinction principle is based on the fact that the cross sections of liquid water and ice differ from each other at low neutron energies. In order to quantitatively observe transient freezing and thawing phenomena in a multiphase mixture (gas/liquid/solid) within real PEFCs with high spatial resolution, a pulsed neutron beam with both high intensity and wide energy range is most appropriate. In the validation study of the present work, we used water sealed in narrow capillary tubes to simulate the flow channels of a PEFC, and a pulsed neutron beam was applied to distinguish ice, liquid water and super-cooled water, and to clarify freezing and thawing phenomena of the water within the capillary tubes. Moreover, we have enabled the observation of liquid water/ice distributions in a large field of view (300 mm × 300 mm) by manufacturing a sub-zero environment chamber that can be cooled down to −30 °C, as a step towards in situ visualisation of full-size fuel cells. more...

The grain-resolved residual stress (type II) in commercial-quality low carbon steel was observed using scanning three-dimensional X-ray diffraction (3DXRD) microscopy. In this method, grain orientations and lattice parameters are mapped using a monochromatic high-energy X-ray microbeam and 3DXRD-based polycrystalline indexing. Defining the reference lattice parameter a0 as the average value in the entire field of view, grain orientations and lattice parameters are converted into stress tensors, yielding a grain-resolved stress tensor map. The effectiveness of the scanning 3DXRD method was demonstrated by evaluating the residual stress in a cold-rolled low carbon steel sheet using a 50 keV microbeam at SPring-8. The area of the cross-sectional sample was 1×1 mm2, which was sufficiently larger than the grain size of about 20 μm. To produce a two-dimensional map of a circular region with a diameter of 160 μm at a pixel size of 1×1 μm2, the measurement time was about 1 h. From the stress tensor map, differences in residual stress of about 150–200 MPa between some neighboring grains were observed. more...

In other study we examined the plastic behavior for polycrystalline iron by three-dimensional x-ray diffraction (3DXRD) experiment. In this study we analyze the behavior by crystal plasticity finite element (CPFE) analysis, to confirm the validity of application to the deformation analysis of engineering steels of a couple of constitutive models. In the CPFE analysis, the observed microstructure and its crystal orientation are modeled with finite elements to take the inter-granular and intra-granular interactions into consideration. The plastic deformation state of the finite element model was computed by means of CPFE analysis based on the {110} slip system in body centered cubic (BCC) crystal. The experiment showed that the most of the grains rotated toward the preferred orientation along the tensile axis and that intra-granular orientation spread and multi-directionally rotated as the tensile strain increased. These results are reproduced by the CPFE analysis, in which the influence of interaction between neighboring grains is taken into consideration. more...

In situ three-dimensional crystallographic orientation mapping in plastically-deformed polycrystalline iron is demonstrated using a modified three-dimensional x-ray diffraction method. This voxel-by-voxel measurement method enables the observation of intragranular orientation distribution. The experiment is performed using coarse-grained ferrite with a mean grain size of ~ 60 μm and an incident x-ray beam with a beam size of 20 μm × 20 μm. Grains averagely rotate approximately toward the preferred orientation of body-centered cubic uniaxial tensile texture. Intragranular orientation distributions are spread as the tensile strain increases to 10.7 %. Furthermore, intragranular multidirectional rotations are observed in grains near the and corners in the inverse pole figure. more...

SUMMARY Multiscale mechanical behaviors of ferrite–pearlite steel were predicted using numerical material testing (NMT) based on the finite element method. The microstructure of ferrite–pearlite steel is regarded as a two-component aggregate of ferrite crystal grains and pearlite colonies. In NMT, the macroscopic stress–strain curve and the deformation state of the microstructure were examined by means of a two-scale finite element analysis method based on the framework of the mathematical homogenization theory. The microstructure of ferrite–pearlite steel was modeled with finite elements, and constitutive models for ferrite crystal grains and pearlite colonies were prepared to describe their anisotropic mechanical behavior at the microscale level. While the anisotropic linear elasticity and the single crystal plasticity based on representative characteristic length have been employed for the ferrite crystal grains, the constitutive model of a pearlite colony was newly developed in this study. For that reason, the constitutive behavior of the pearlite colony was investigated using NMT on a smaller scale than the scale of the ferrite–pearlite microstructure, with the microstructure of the pearlite colony modeled as a lamellar structure of ferrite and cementite phases with finite elements. On the basis of the numerical results, the anisotropic constitutive model of the pearlite colony was formulated based on the normal vector of the lamella. The components of the anisotropic elasticity were estimated with NMT based on the finite element method, where the elasticity of the cementite phase was numerically evaluated with a first-principles calculation. Also, an anisotropic plastic constitutive model for the pearlite colony was formulated with two-surface plasticity consisting of yield functions for the interlamellar shear mode and yielding of the overall lamellar structure. After addressing the microscopic modeling of ferrite–pearlite steel, NMT was performed with the finite element models of the ferrite–pearlite microstructure and with the microscopic constitutive models for each of the components. Finally, the results were compared with the corresponding experimental results on both the macroscopic response and the microscopic deformation state to ascertain the validity of the numerical modeling. Copyright © 2011 John Wiley & Sons, Ltd. more...

Single crystal plasticity based on a representative characteristic length is proposed and introduced into a homogenization approach based on finite element analyses, which are applied to characterization of distinctive yielding behaviors of polycrystalline metals, yield-point elongation, and grain size strengthening. The computational manner for an implicit stress update is derived with the framework of a standard multi-surface plasticity at finite strain, where the evolution of the characteristic lengths are numerically converted from the accumulated slips of all of slip systems by exploiting the mathematical feature of the characteristic length as the intermediate function of the plastic internal variables. Furthermore, a constitutive model for a single crystal reproduces the stress–strain curve divided into three parts. Using two-scale finite element analysis, the macroscopic stress–strain response with yield-point elongation under a situation of low dislocation density is reproduced. Finally, the grain size effect on the yield strength is analyzed with modeling of the grain boundary in the context of the proposed constitutive model and is discussed from both macroscopic and microscopic views. more...

The nanoindentation tests on high-temperature oxidized fuel claddings with/without hydrogenation were performed in order to verify the influence of hydrogen on mechanical behavior of the cladding under a loss of coolant accident (LOCA) condition. Hydrogenated and non-hydrogenated Zircaloy-4 cladding tubes were oxidized in steam at the temperature of 1373–1393 K followed by quenching. The nanoindentation test was performed in the area of inhomogeneous structure called as the Widmann–Statten structure, composed of the needle-like α-zirconium grains (α′ phase) and the matrix (α phase). The mechanical properties viz. hardness and elastic modulus were evaluated for α′ phase and matrix α phase, separately. The hardness and elastic modulus of α′ phase are much higher than those of the matrix α phase due to its higher oxygen concentration. There was no significant difference of mechanical properties between the samples with and without hydrogen. more...

The paper clarifies the specific mechanical properties of single crystal YSZ through applying the nanoindentation examination and the finite element analysis with theoretical ultrasonic measurements. The obtained results demonstrate that the lack of precise determination of the Poisson's ratio results in the significant discrepancy between output of nanoindentation and ultrasonic measurements of Young's modulus. It was observed that the anisotropy varied depending on the elastic or plastic measurements. From the variation of the load velocity in nanoindentation, it was asserted that Young's modulus and hardness were expressed by the exponential function of load velocity, i.e., time. more...

Bulk titanium, yttrium, and zirconium hydrides, which have the same structure as that of fluorite-type fcc_C1, were produced and their mechanical properties were investigated. With an increase in the hydrogen content, the lattice parameters of titanium and zirconium hydrides increased, whereas those of yttrium hydride decreased. The elastic moduli of titanium and zirconium hydrides decreased by hydrogen addition, whereas those of yttrium hydride increased. There are linear relations between the electronegativities and hydrogen content dependence of the properties. Therefore, the mechanical properties of the metal hydrides are considered to be determined by a common rule based on the electronegativity. more...

Various shapes of flaw-free bulk titanium hydrides (TiH X : X = 1.53–1.75) were fabricated, and its electrical and thermal properties were studied. The electrical conductivity of titanium hydride was slightly lower than that of titanium metal, and it exhibited metal-like temperature dependence. The Seebeck coefficient of the Ti–H system changed from positive to negative due to hydrogenation, and its absolute value for titanium hydride remained at a low magnitude, which is typical for metallic materials. The thermal conductivity of the hydride was the same as that of the metal and increased slightly with increasing temperature. more...

To assess the phase stability of actinide oxides with a fluorite structure which has a variable composition (MO 2±x ), thermodynamic modelling has been performed for O-Pu-U-Np quaternary systems on the basis of thermodynamic data available in the literature. Using the thermodynamic data for O-Pu, O-U and O-Np binary systems obtainable in the literature, the thermodynamic modelling for O-Pu-U-Np quaternary system was carried out by a means of CAL-culation of PHAse Diagram (CALPHAD) technique. Using this model, the oxygen potentials for (Pu 0.3 U 0.7 )O 2 , (Pu 0.3 U 0.65 Np 0.05 )O 2 and (Pu 0.3 U 0.58 Np 0.12 )O 3 were calculated between 1123 K and 1973 K and compared with the experimental data. Interaction parameters required for calculation of the Gibbs free energy of the solid solution phase were estimated for O-Pu-U-Np system. 2005 Elsevier B.V. All rights reserved. more...

The terminal solid solubility of hydrogen for pure Zr, Zircaloy-2, Zr–M (M = Fe, Sn, Cr, Ni) was measured, whose dependency on additive element was elucidated in relation to the integrity of fuel cladding. The present experimental method was in good accordance with the previous reports. The terminal hydrogen solid solubility for Zircaloy-2 was larger than that for pure Zr, the reason for which was considered that the solute Sn leads to decrease the free energy of α Zr phase. more...

The various shapes of flaw-free bulk yttrium hydrides could be fabricated and various mechanical properties were obtained. The lattice parameter decreased with the hydrogen content. The yttrium hydride had larger elastic moduli than the metal and the elastic moduli were dependent on the hydrogen content. The microhardness of the hydride was also larger than that of the metal and increased with hydrogen addition. The Debye temperature of the yttrium hydride estimated from the sound velocities and the lattice parameters was larger than that of pure yttrium metal. The hardness to Young's modulus ratio for the yttrium hydride is larger than the ratio for the titanium hydride or zirconium hydride. more...

The mechanical properties of yttrium hydrogen solid solution (0–0.20 H/Y) were studied to reveal the influence of hydrogen on the characteristics of yttrium. The elastic moduli such as Young's modulus and the shear modulus for yttrium hydrogen solid solution increased with increasing of hydrogen content. The microhardness for the solid solution was also increased with increasing of hydrogen content. The electronic structure of the yttrium hydrogen solid solution was evaluated by a first principle molecular orbital calculation. The mechanical properties of yttrium hydrogen solid solution were discussed in terms of the calculation results. more...

Nanoindentation tests were performed for single crystals of TiO 2 , MgO, and YSZ (Yttria Satirized Zirconia) using a Berkovich indenter at room temperature. The indentation tests were performed in the load range from 500 to 1500 μN. The area function calibration was performed by using a standard fused SiO 2 sample. There is no difference among the reduced moduli of TiO 2 , MgO, and YSZ, while the nanohardness of YSZ is larger than those of TiO 2 and MgO. For YSZ, the reduced modulus along a [1 0 0] direction is larger than that along a [1 1 1] direction, which corresponds to the theoretical estimation using the elastic stiffness constants. more...

Various shapes of flaw-free bulk titanium hydride were fabricated and various mechanical properties were obtained. The lattice parameter increased with the hydrogen content. The titanium hydrides have smaller elastic moduli than the metal and the elastic moduli are dependent on the hydrogen content. The microhardness of the hydrides is also smaller than that of the metal and decreased with hydrogen addition. The Debye temperatures of the titanium hydrides estimated from sound velocities and the lattice parameters were smaller than that of pure titanium metal, which was consistent with the Debye temperature obtained from heat capacity measurements. It was found that the IV group metal hydrides have unique mechanical behavior. more...

The creep properties of zirconium hydrogen solid solution were studied to elucidate the influence of hydrogen on the characteristics of zirconium solid solution. The power law creep exponent was found not to change with increasing hydrogen content. The creep rate of a zirconium alloy hydrogen solid solution was discussed in terms of the present result on creep properties and our previous result on the elastic properties. more...

The mechanical properties of titanium–hydrogen solid solution (0–7.84 at.% H) were studied to reveal the influence of hydrogen on the characteristics of titanium. The elastic moduli such as Young’s and shear moduli of titanium–hydrogen solid solution decreased with increasing of hydrogen content. The influence of hydrogen content for decreasing of elastic moduli was independent of temperature. The microhardness for the solid solution was also decreased with increasing of hydrogen content. The electronic structure of the titanium–hydrogen solid solution was evaluated by a first principle molecular orbital calculation. The mechanical properties of titanium–hydrogen solid solution were discussed in terms of the calculation results. more...

An assessment of thermodynamic information for Zr–O–H system was executed using the Calculation of Phase Diagrams (CALPHAD) technique. Phase diagram of Zr–O–H ternary system was calculated using our experimental data and those from literature for Zr–O–H system. Effect of oxygen on phase modification of Zr–H system was also discussed using the assessed thermodynamic information. more...

In order to study such nanomechanical properties of the zirconium hydride as nanohardness, nanoindentation tests have been carried out in the present study. As reference data, macroscopic mechanical properties of the zirconium hydride such as Vickers hardness have also been measured. The zirconium hydride specimens were fabricated directly from pure zirconium metal in a modified UHV Sieverts apparatus, and had the hydrogen contents with 1.5–1.7 H/Zr. The nanohardness of the zirconium hydride increased with decreasing maximum indentation load, which has been referred to as indentation size effect (ISE). The zirconium hydride had higher nanohardness than crystal-bar zirconium and pure zirconium, which was qualitatively in good agreement with the results of the Vickers hardness tests. The work hardening surface layer due to polishing the samples strongly affected the nanohardness, but barely affected the Vickers hardness. It was considered that the difference between the values of the nanohardness and the Vickers hardness were caused by not only the ISE, but also the difference of the indenter geometry and the testing method between the nanoindentation and the Vickers hardness test. more...

Thermodynamic modelling was performed for O–Th and O–Np systems to assess the phase stability of the MO 2− X phase together with the data in the literature for O–Pu, O–U, and O–Zr systems. Firstly, thermodynamic modelling was demonstrated for O–Th and O–Np systems with thermodynamic data and phase diagram information available in the literature. The assessed data reproduced the general feature of the system with respect to the phase diagrams of the both systems and the oxygen potential in the ThO 2− X and NpO 2− X phases. Secondly the phase stability of the MO 2− X phase was assessed using the obtained sets of data together with those for O–Pu, O–U, and O–Zr. The phase stability of the MO 2− X phase was discussed with respect to the deviation from the ideal solution in oxygen potential and Gibbs free energy. more...

An inverse analysis method based on nonlinear finite element analysis is developed to find an optimized morphology of periodic microstructure for improving the macroscopic mechanical properties in duplex elastoplastic solids. Here a gradient-based computational optimization method and two types of homogenization methods are employed. In this study, the optimization problem is defined as the maximization of the sum of macroscopic external works for several macroscopic deformation modes, enabling us to obtain a high strength material. The morphologic strengthening effect is discussed through a comparison with experiments and classical theories. more...

An analysis, based on fracture mechanics at elevated temperatures, has been carried out for several types of hydrided Zircaloy cladding tubes to elucidate the fracture behavior of high burn up light water reactor fuel cladding during reactivity initiated accidents. Fracture mechanics parameters such as stress intensity factor, J-integral and plastic yield load were estimated by a finite element method analysis, and the material properties of α-phase of zirconium required for the analysis were obtained by tensile tests at elevated temperatures. The failure assessment diagram (FAD) was constructed using the fracture mechanics parameters to estimate the failure stress of the cladding. It was found from the FAD that the predicted failure stress of the cladding qualitatively agreed with the experimental results obtained by burst tests at elevated temperatures for the hydrided Zircaloy cladding tube at Japan Atomic Energy Research Institute. more...

In order to examine the influence of precipitated zirconium hydride on the failure behavior and fracture strength of light water reactor (LWR) cladding tubes, tensile tests were performed at room temperature for non-hydrided and hydrided Zircaloy sheet-type specimens with gauge section of 10.0×5.0 mm and thicknesses of 0.5, 1.0, 1.5, 2.0, 2.5, and 3.0 mm. For specimens with thickness more than 2.5 mm, the ultimate tensile strength of the specimens appeared to be independent of thickness, which implied that plane strain condition was attained. For the specimen with 2.5 mm thickness, the ultimate tensile strength increased slightly with increasing average hydrogen concentration. Through microscopic observation of the hydrided specimen surface by scanning electron microscopy (SEM), it was found that matrix/hydride debonding was not generated but that micro-cracks perpendicular to the axial direction were produced at the hydride layer. more...

Thermal properties of the TiHx (1.53 ⩽ x ⩽ 1.75) were measured in the temperature range from 323 to 673 K. The Debye temperatures for the TiHx compounds were smaller than those for the metal. The heat capacities of the titanium hydrides were larger than that for the metal because of hydrogen vibration. The thermal diffusivities for the titanium hydrides were smaller than that for the metal. The thermal conductivities were estimated. The dependency of thermal conductivity on temperature was slightly changed by hydrogenation. However a significant difference between the values for titanium and the titanium hydrides was not found. more...

The thermal properties of δ-phase yttrium hydrides (YH x : x = 1.72–2.00) were studied in the temperature range from 300 to 773 K. The heat capacities of yttrium hydrides are larger than that of yttrium metal because of hydrogen optical vibration. The thermal conductivities of the yttrium hydrides are larger than that of metal and have strong dependence on temperature. They are not markedly influenced by the hydrogen content. more...