A 3D fourth order fabric tensor approach of anisotropy in granular media

We propose a micromechanical approach for granular media, with a particular account of the texture-induced anisotropy and of the strain localization rule. The approach is mainly based on the consideration of a fourth order fabric tensor able to capture general anisotropy which can be induced by complex distribution of contacts. Incorporation of this fourth order fabric tensor in a suitable homogenization scheme allows to determine the corresponding macroscopic elastic properties of the granular material. For this purpose, in addition to the classical Voigt upper bound, a new kinematics-based localization rule is proposed. It generalizes the one formulated by Cambou et al. [B. Cambou, Ph. Dubujet, F. Emeriault, F. Sidoroff, Eur. J. Mech. A/Solids 14 (1995) 225–276] in the case of an isotropic contact distribution. The results of the complete model compare well to numerical simulations results when available [C.S. Chang, C.L. Liao, Appl. Mech. Rev. 47 (1 Part 2) (1994) 197–207] (case of isotropic distribution of contacts). Finally, the interest of the fourth order fabric tensor based approach combined with the proposed localization rule is shown for different distributions of contacts by comparing its predictions to those given by a second order fabric tensor approach.     

Crystallographic phase transition, electrical and magnetic properties of stoichiometric La(Mn,Rh)O3

Temperature-induced phase transitions, electrical conductivity, and some magnetic properties of stoichiometric LaMn_1−xRh_xO₃ (0.1≤x≤0.5) have been studied. The phase transitions of LaMn_0.9Rh_0.1O₃ were investigated by high-temperature X-ray powder diffractometry and orthorhombic-pseudocubic-rhombohedral transitions were observed. The electrical conductivity measurement shows semiconducting property above room temperature. Breaking of slope in the temperature dependence of the conductivity bring the evidence in support of the phase transition for LaMn_0.9Rh_0.1O₃. At low temperature, the compounds show ferromagnetic behavior but the ferromagnetic intensity increases with the increase of the oxygen nonstoichiometry.     

Texture development, microstructure and phase transformation characteristics of sputtered Ni-Ti Shape Memory Alloy films grown on TiN<111>

Near equiatomic Ni-Ti films have been deposited by magnetron co-sputtering on TiN films with a topmost layer formed by < 111> oriented grains (TiN/SiO₂/Si(100) substrate) in a chamber installed at a synchrotron radiation beamline. In-situ X-ray diffraction during Ni-Ti film growth and their complementary ex-situ characterization by Auger electron spectroscopy, scanning electron microscopy and electrical resistivity measurements during temperature cycling have allowed us to establish a relationship between the structure and processing parameters.A preferential development of < 110> oriented grains of the B2 phase since the beginning of the deposition has been observed (without and with the application of a substrate bias voltage of −45 and −90 V). The biaxial stress state is considerably influenced by the energy of the bombarding ions, which is dependent on the substrate bias voltage value applied during the growth of the Ni-Ti film. Furthermore, the present work reveals that the control of the energy of the bombarding ions is a promising tool to vary the transformation characteristics of Ni-Ti films, as shown by electrical resistivity measurements during temperature cycling.The in-situ study of the structural evolution of the growing Ni-Ti film as a consequence of changing the Ti:Ni ratio during deposition (on a TiN<111> layer) has also been performed. The preferential growth of < 110> oriented grains of the Ni-Ti B2 phase has been as well observed despite the precipitation of Ti₂Ni during the deposition of a Ti-rich Ni-Ti film fraction. Functionally graded Ni-Ti films should lead to an intrinsic “two-way” shape memory effect which is a plus for the miniaturization of Ni-Ti films based devices in the field of micro-electro-mechanical systems.     

AsNCa3 at high pressure

A constant pressure optimization scheme is applied to the study of ternary calcium nitrides under pressure. The enthalpy is minimized with respect to the electronic configuration, the positions of the atoms and the cell metric (i.e. the lattice parameters). Symmetry corrections can be performed during the relaxation towards the equilibrium structure in order to be able to investigate the compound in a certain proposed symmetry. We obtain excellent agreement with experiment for the zero-pressure structural parameters of the cubic anti-perovskite structure BiNCa₃ and of the distorted anti-perovskite structures AsNCa₃ and PNCa₃. For AsNCa₃ the structural parameters, band gap energies, etc. are investigated as a function of the pressure. A new cubic phase is predicted to have a lower enthalpy than the orthorhombic phase for pressures above 59 GPa.     

First-principles study of phase transition and elastic properties of XBi (X = Ce, Pr) compounds

The pressure-induced phase transitions of CeBi and PrBi compounds were investigated by using full-potential linearized augmented plane-wave (FP-LAPW) method. The calculations indicate that the transition pressure for CeBi compound from the NaCl-type (B1) structure to the body centered tetragonal (BCT) structure are 11.53 GPa from total energy (E)-volume (V) data and 6.48 GPa from equal Gibbs free energy (G). For PrBi compound, the same phase transition sequence occurred at 10.94 GPa obtained from the slope of the common tangent of E-V curves and 6.04 GPa from the equal G. The detailed structural changes during the phase transition were analyzed. From the elastic constants at zero pressure, we can conclude that B1 phase of XBi (X = Ce, Pr) compounds are mechanical stable, consistent with the experimental observations.     

Shrinking patterns and phase coexistence of polymer gels

We have investigated the shrinking phase transition of slightly ionized poly(N-isopropylacrylamide) (NIPA) gels. The macroscopic conformation change was observed on the heating process in two different methods; a continuous heating process and an isothermal process. It was found that the macroscopic behavior can be characterized by several conformation changes, the phase coexistence (or ‘linked-dumplings’), the grain pattern, the bubble pattern, and the opaque phase. Those have correlations with the phase transition velocity. The heating conditions to determine the characteristic conformations and the stability of the phase coexistence were qualitatively discussed in terms of the classical phase separation model of nucleation and spinodal decomposition.     

Micromechanical formulation and 3D finite element modeling of microinstabilities in composites

A 3D micromechanical formulation and a FE-model of fiber micro-buckling in materials with isotropic and transversal isotropic fibers in compression is presented. Three variants of geometrical modeling of the characteristic cell are proposed and compared. An appropriate one is then selected. An eigenvalue analysis of a characteristic cell is performed. The results show that the fiber anisotropy reduces significantly the critical loads and must be taken into account.     

First-principles study of phase transition and structural properties of AlAs

We have investigated the phase transition and structural properties of AlAs in three crystallographic structures, i.e., B3 (zinc blende), B1 (rocksalt), and B8 (nickel arsenide), at high pressures using the full-potential linearized muffin-tin orbital (FP-LMTO) scheme within the generalized gradient approximation correction (GGA) in the frame of density functional theory (DFT). For B8 structure, it is found that the c/a ratios kept nearly constant (0.2% fluctuation) corresponding to V/V₀  0.7–1.05 (V is the primitive cell volume and V₀ is the experimental equilibrium volume of B3 structure), which is in full agreement with experiment, but the c/a ratios increase linearly with the values of V/V₀ decreasing corresponding to V/V₀  0.4–0.7. This indicates under low pressure the compression along c-axis and a-axis is the same, but the compression along c-axis is more difficult than along a-axis under higher pressure. Based on the condition of equal enthalpies AlAs is found to undergo a structural phase transition from B3 to B8 at 5.34 GPa, in agreement with the experimental value of 7 ± 5 GPa, and is speculated to undergo the B3–B1 transition at 6.24 GPa.     

Safety performance of traffic phases and phase transitions in three phase traffic theory

Crash risk prediction models were developed to link safety to various phases and phase transitions defined by the three phase traffic theory. Results of the Bayesian conditional logit analysis showed that different traffic states differed distinctly with respect to safety performance. The random-parameter logit approach was utilized to account for the heterogeneity caused by unobserved factors. The Bayesian inference approach based on the Markov Chain Monte Carlo (MCMC) method was used for the estimation of the random-parameter logit model. The proposed approach increased the prediction performance of the crash risk models as compared with the conventional logit model. The three phase traffic theory can help us better understand the mechanism of crash occurrences in various traffic states. The contributing factors to crash likelihood can be well explained by the mechanism of phase transitions. We further discovered that the free flow state can be divided into two sub-phases on the basis of safety performance, including a true free flow state in which the interactions between vehicles are minor, and a platooned traffic state in which bunched vehicles travel in successions. The results of this study suggest that a safety perspective can be added to the three phase traffic theory. The results also suggest that the heterogeneity between different traffic states should be considered when estimating the risks of crash occurrences on freeways.     

Theory and finite element computations of a unified cyclic phase transformation model for monocrystalline materials at small strains

After a survey the refined numerical treatment and verification is presented for a rate-independent macroscopic unified PT material model (including mass conservation with respect to phase fractions and covexified free energy) by Govindjee and Miehe (Comput Methods Appl Mech Eng 191:215–238, 2001) for describing SME and SE effects within a linear kinematic setting. Special attention is given to temperature dependent PTs. The material model was implemented into ABAQUS via the UMAT material interface in 2004. Validation of this PT model is carried out with experimental data supplied by Xiangyang et al. (J Mech Phys Solids 48:2163–2182, 2000), using 3D finite element computations. Experimentally gained material data from different sources are used and numerical results of energy barriers for PTs are given. Another feature is the simulation of suppressed shape memory effects by quasiplastic temperature induced PT. Furthermore, a plane strain problem is treated with comparisons of butterfly shaped expansions of martensitic PT and plastic deformation, correspondingly.     

Phase contrast micro-tomography and morphological analysis of a short carbon fibre reinforced polyamide

The mechanical properties of components made of short fibre reinforced composites, generally obtained by injection moulding, are strongly linked to fibre orientation. Therefore, it is of great importance to be able to verify the results of manufacturing process simulations obtained by commercial software. From the experimental point of view, the definition of carbon short fibre structure within a polymeric matrix in a micro-tomography is a non-trivial task, as the X-rays absorption properties of the two phases are very similar. This paper presents how this problem was overcome by using phase contrast imaging techniques. High resolution fibre structure reconstructions could therefore be obtained. The reconstruction of a large sample volume by overlapping of successive tomographies was also discussed. Moreover, this work shows that the anisotropy identification techniques based on morphological parameters, previously introduced by some of the Authors for short glass fibre reinforced polymers, can also be adopted for fibre arrangement identification in this type of materials.     

Extending the scope of ‘in silico experiments’: Theoretical approaches for the investigation of reaction mechanisms, nucleation events and phase transitions

The investigation of the atomistic mechanisms of processes in complex systems constitutes a major challenge to both theory and experiment. While experimental studies offer a wide variety of insights at the macroscopic scale, the atomistic level of detail often remains elusive. On the other hand, molecular simulation approaches may easily achieve microscopic resolution and hence appear particularly suited for detailed mechanistic analyses. However, the computational effort is typically quite considerable and in many cases special simulation strategies are needed to make simulations possible. This review is dedicated to special approaches for tackling the time/length-scale problem inherent to molecular dynamics simulations. Employing these techniques opened a series of new perspectives. The latter are illustrated with the example of recent simulation studies of the atomistic mechanisms involved in complex processes like crystal nucleation, phase transitions and reactions in solution. Along this line, we discuss the reaction mechanisms for He insertion into C₆₀ fullerenes, nucleation events and domain morphogenesis in pressure-induced phase transitions in solids and ion aggregation from solution.     

Influence of erbium doping on phase transition and optical properties of strontium barium niobate

The optical properties of erbium impurities in strontium barium niobate are investigated measuring optical absorption and emission in the visible and near infrared spectral region. For the main fluorescence band at 1.55 μm, an anomalous dependence of the fluorescence decay time on dopant concentration is found which, however, can be consistently explained by reabsorption effects. A Judd–Ofelt analysis of the absorption spectra together with an appropriate analysis of the reabsorption yields a radiative quantum efficiency of approximately 60%. In addition, erbium dopants are shown to efficiently influence the phase transition temperature of strontium barium niobate.     

Determination of the phase transition in Pb0.88Ln0.08Ti0.98Mn0.02O3 (Ln=La, Sm, Eu) piezoceramics based on the Stefan–Boltzmann law

A method for determination of the phase transition in piezoelectric ceramic based on the relationship expressed by the Stefan–Boltzmann law is reported, i.e., by means of the radiation that the piezoelectric ceramic emits when it is subjected to different temperatures. The experiment is performed in piezoelectric ceramic based on PbTiO₃ modified by the partial substitution of rare earths for Pb in the Pb_0.88(Ln)_0.08Ti_0.98Mn_0.02O₃ system (Ln=La, Sm, Eu). From the measured emitted radiation, the value of the emissivity is calculated for each type of piezoelectric ceramic.     

Investigating phase transitions and strength in single-crystal sapphire using shock–reshock loading techniques

This paper focuses on developing a technique that can probe phase transitions in ceramics and evaluating strength properties of single-crystal sapphire. High strength ceramics that undergo solid-to-solid polymorphic phase transitions can display a wide range of volume changes. Materials with large volumetric changes can be easily detected; however, materials demonstrating small volume changes, pose a different experimental problem. Because of the variety of diagnostics tools (for acquiring data), and differing interpretation of the data, materials that undergo small volume changes, make detection of transformation quite challenging. Different experimental test methods are needed to potentially allow detection and characterization of materials undergoing phase transitions with small volume changes. In addition, these results show that single-crystal sapphire has considerable strength loss at approximately 56 GPa on the Hugoniot. In this study, we report loading profile measurements in the form of particle velocity histories for single-crystal sapphire that is shocked to a given stress level directly or through multiple steps from a shocked state of 56 GPa. We probe the region of suggested phase transformations utilizing the simplest type of off-Hugoniot reshock loading. This also allows an assessment of shear strength in the shocked state of single-crystal c-axis sapphire. The present measurements show no evidence of a phase transition, but do suggest considerable strength loss in sapphire.     

Effects of Sn content on the microstructure, phase constitution and shape memory effect of Ti–Nb–Sn alloys

The effect of Sn content on the microstructure, phase constitution and shape memory effect of Ti–16Nb–xSn (x = 4.0, 4.5, 5.0 at%) alloys were investigated by means of optical microscopy, X-ray diffraction, transmission electron microscopy and bending test. With the increase of Sn content, the β phase becomes stable. The solution-treated Ti–16Nb–4Sn alloy is composed of ″ and β phases at room temperature, whereas the solution-treated Ti–16Nb–5Sn alloy is only composed of β phase at room temperature. TEM observation shows that there is parallel lamellar ″ martensite with the substructure of () type I twin in the Ti–16Nb–4Sn alloy. There exists the dislocation wall inside the single β phase in the Ti–16Nb–5Sn alloy. The shape recovery ratio decreases with increasing the bending strain and the bending temperature, which is in correspondence with the different deformation mechanisms at different temperature ranges. The shape recovery ratio shows a decreasing trend with the increase of Sn content at the same bending strain and temperature. The maximum completely recovery strain is around 4%.     

Structural and microstructural phase evolution during mechano-synthesis of nanocrystalline/amorphous CuAlMn alloy powders

The formation mechanism of Cu–11.5Al–4Mn alloys by mechanical alloying (MA) of pure elemental powders was investigated. During milling, the powder sampling was conducted at predetermined intervals from 1 h to 96 h. The quantitative phase analyses were done by X-ray diffraction and the particles size and morphology were studied by scanning electron microscopy. Furthermore, the microstructure investigation and phase identification were done by transmission electron microscopy. Concerning the results, the nanocrystalline Cu solid solution were formed at short milling times and, by milling evolution, the austenite-to-martensite (2H) phase transformation occurred. Moreover, the formation of considerable amount of amorphous phase and its partial transformation to crystalline phases during the milling process were revealed. It was also found that, by milling development, the powder morphology changes from lamellar to semi-spherical and their size initially increases, then reduces and afterward re-increases.     

Solvothermal-induced phase transition and visible photocatalytic activity of nitrogen-doped titania

Nitrogen-doped titania nanoparticles consisting of pure anatase, pure rutile and bicrystallites (anatase+rutile and anatase+brookite) have been prepared in TiCl(3)-HMT (hexamethylene tetramine)-alcohol solution under solvothermal process. The effect of the solvent type and amount of HMT as pH adjuster on the phase composition of titania and its visible photocatalytic activity for degradation to MO (methyl orange) was investigated. It is found that anatase gradually transferred to rutile with increase of carbon chain using methanol, ethanol, 1-propanol and 1-butanol as solvent. The pure anatase formed at the pH value of 1-2, while bicrystalline titania (anatase+rutile and anatase+brookite) at that of 7-10 in the presence of methanol. The bicrystalline (anatase+brookite) titania have the best visible photocatalytic activity among all the samples. The -(NO) and -(NH) dopants with an N (1s) binding energy of 400 eV may have positive effects on the visible light photocatalytic activity.