Papers by Keyword: Single Crystal

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Authors: Fei Fang, Fang Cheng Zhang, Wei Yang
Abstract: In-situ observation of the fatigue crack growth and 90o domain switching was carried out for BaTiO3 ferroelectric single crystals under alternating electric field. It is shown that during the electric cycling, the crack propagates continuously. Parallel lines of 90o domain boundaries can be seen and they flip at each reversal of the alternating electric field. The width of the 90o domain switching zone grows with the number of cycles and its frontal always lies ahead of the crack tip. It is suggested that the cyclic stress field induced by the repeated 90o domain switching at the crack tip, as well as the stress field caused by the electrically activated material between the electrode and the material under the electrodes contribute to the observed fatigue crack growth.
Authors: W.H. Liu, Xin Ming Zhang, Jian Guo Tang, Yu Xuan Du
Abstract: Void growth in aluminum single crystals is simulated using the finite element method, to illustrate the effect of grain orientation on void growth, a rate dependent crystal plasticity constitutive theory is implemented as a user-defined plasticity subroutine. A three-dimension unit cell including a sphere void was employed using three-dimensional 12 active slip systems. The computed results for several grain orientations are compared, which have shown that crystallographic orientation has significant influence on growth behavior of void. And the void growth direction and shape significantly depend on the crystallographic orientation. Due to plastic flow localization and anisotropic behavior, void which has an initial sphere shape, develops an irregular shape and some corners.
Authors: Fabio Sorbello, Peter E.J. Flewitt, A.G. Crocker, Gillian E. Smith
Abstract: It is well established that within the lower-shelf temperature range of Fe2-3Si polycrystalline steels, the brittle fracture occurs predominately by transgranular cleavage, unless subject to embrittling heat-treatments. The cleavage fracture develops on the well established {001} planes of the bcc structure. In this paper we revisit the growth, of these cleavage cracks by considering crack propagation in single crystals of Fe2Si steel. Three point bend specimens manufactured from oriented crystals have been tested by impact loading at a temperature of -196°C. High spatial resolution focused ion beam imaging combined with ion milling is used to examine in detail the crack propagation path and has provided a new insight into the mechanisms involved. In particular it has been established that within the process zone of the propagating cracks local strain is accommodated by the formation of {112} twins. The results are discussed with respect to the overall crack propagation mechanism.
Authors: A. Ma, Franz Roters, Dierk Raabe
Abstract: Crystallographic slip, i.e. movement of dislocations on distinct slip planes, is the main source of plastic deformation of most metals. Therefore, it was an obvious idea to build a constitutive model based on dislocation densities as internal state variables in the crystal plasticity. In this paper the dislocation model recently proposed by Ma and Roters (Ma A. and Roters F., Acta Materialia, 52, 3603-3612, 2004) has been extended to a nonlocal model through separating the statistically stored dislocation and geometrically necessary dislocation densities. A nonlocal integration algorithm is proposed, which can be more easily used in conjunction with commercial software such as MARC and ABAQUS than the model proposed in the work of Evers(Evers L.P., Brekelmans W.A.M., Geers M.G.D., Journal of the Mechanics and Physics of Solids, 52, 2379-2401, 2004).
Authors: Afaf Saai, Laurent Tabourot, Christophe Déprés, Herve Louche
Abstract: In this paper, we present a fundamental model of FCC single crystal behaviour at room temperature: this model includes kinematic work hardening derived from the elementary description of the collective dislocations density evolution during cyclic loading. This kinematic work hardening is then coupled with the isotropic work hardening mechanism. Using this original model, a simulation of a tensile test on a single crystal sample is carried out in the case of an initial crystal orientation that promotes single glide even at rather large strains. The evolution of resolved shear stresses on the primary and secondary slip systems are interpreted by means of the interaction between the evolution of isotropic and kinematic work hardening variables. The evolution of the model state-variables including applied resolved shear strain, dislocation densities, and critical shear stresses are represented as functions of the evolution of crystalline orientation during plastic deformation.
Authors: Nam N. Huynh, Cheng Lu, Guillaume Michal, A. Kiet Tieu
Abstract: This paper proposes a criterion for crack opening in FCC single crystals based on analyses of lattice orientation and interface energy of two adjacent crystals in a crystal plasticity finite element model (CPFEM). It also demonstrates the implementation of the criterion in Abaqus/Standard to simulate crack initiation and propagation in single-edged notch single crystal aluminium samples. Elements in the FEM mesh that have crystalline structures satisfying the crack opening criterion are removed from the mesh at the end of every loading step and FEM analyses are restarted on the new mesh in the next loading step. Removed elements effectively act as voids in the material due to crack nucleation. Similarly, the coalescence of newly removed elements at the end of a loading step with the existent ones simulates crack growth in the material. Two advantages of this approach are noted. Firstly, crack nucleation and its subsequent growth in the material is simulated solely based on lattice evolution history in the material without any presumptions of crack paths or regions where cracks are likely to occur. Secondly, as the criterion for crack nucleation is evaluated based on, and thus changes with, the lattice evolution during loading, a predefined energy criterion for crack opening, which could be erroneous, is avoided. Preliminary results of void nucleation and void growth around the notch tip in Cube and Brass oriented samples using CPFEM modelling appear to agree with molecular dynamics simulations of void growth in FCC single crystals.
Authors: K. Clay, P.N. Quested, Roger Morrell, Ken P. Mingard
Abstract: Control of single crystal orientation in modern gas turbine components is a critical quality control issue for optimum reliable behaviour, and an increasingly important aspect of trade. This paper reviews industrial practice for the determination of primary crystal orientation and for the determination of 'disorientation' (R-value) between sub-grains. Different techniques and measurement conventions are compared, and the errors of measurement have been assessed through a European round-robin. Causes of poor reproducibility in 'disorientation' measurement have been identified by EBSD investigations. Recommendations for unified practice have been made in a Good Practice Guide as a precursor to a new industry-wide standard.
Authors: Michael Ferry, John F. Humphreys
Abstract: Copper single crystals of {110}<001> crystallographic orientation were cold rolled to a true strain of 1.4. Specimens were cut from the as-deformed crystals with all surfaces mechanically ground and deep-etched in concentrated nitric acid to minimise the likelihood of surface nucleation of recrystallized grains during subsequent annealing. The early stages of static recovery were studied by annealing specimens at 300 oC. The crystallographic features of the deformed and annealed microstructures were determined by high resolution electron backscatter diffraction. It was observed that deformation was homogeneous with the microstructure in ND-RD plane exhibiting two complementary sets of intersecting bands at ~+ 35o to ND. Along these bands and in the microstructure, in general, there was an overall spread in orientation about ND towards {110}<112>. However, the orientation spread along these bands was cyclic, that is, sinusoidal orientation gradients were generated about ND with amplitude of up to 20o and wavelength 5-10 µm. Annealing resulted in the preferred growth of cells that have orientations at the edge of the orientation spread of the deformation substructure. This localized coarsening of the microstructure is similar to the discontinuous subgrain growth observed in {110}<001> oriented Al single crystals and indicates that discontinuous subgrain (cellular) growth can also occur in metals of lower stacking fault energy.
Authors: Kate P. Suppel, Jenny S. Forrester, Gregg J. Suaning, Erich H. Kisi
Abstract: Single crystal sapphire in one of four orientations (0001), (11 2 0), (1102) and (1010) was bonded to polycrystalline platinum foil at 1330oC under light load 20 N (0.8 MPa) in argon. The different orientations were used as a means to model individual grains within a polycrystal without the uncertain influence of grain boundaries and glass-forming contamination (e.g. SiO2). Partial bonding was observed in all cases. Optical microscopy, scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used to study the nature of the bond which appears to be physical rather than chemical, in agreement with previous work.
Authors: Hua Ping Xu, Gao Feng Song, Xie Min Mao
Abstract: In this paper, single crystal of CuAlNiBe quaternary shape memory alloy was prepared in a high temperature gradient directional solidification furnace with a selective growing crystallizer. And its shape memory performance characters were systematically compared with other series copper base shape memory alloys. The results show that the single crystal of CuAlNiBe quaternary shape memory alloy has better shape memory properties.
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