The mechanical behavior of a Mo-TiC30 vol.% ceramic-metal composite was investigated over a large temperature range (25°C to 700°C). High-energy X-ray tomography was used to reveal the percolation of the hard titanium carbide phase through the composite. Using a polycrystal approach for a two-phase material, finite element simulations were performed on a real 3D aggregate of the material. The 3D microstructure, used as starting configuration for the predictions, was obtained by serial-sectioning in a dual beam Focused Ion Beam (FIB)-Scanning Electron Microscope (SEM) coupled to an Electron Back Scattering Diffraction system (3D EBSD, EBSD tomography). The 3D aggregate consists of a molybdenum matrix and a percolating TiC skeleton. As most BCC metals, the molybdenum matrix phase is characterized by a change in the plasticity mechanisms with temperature. We used a polycrystal model for the BCC material, which was extended to two phases (TiC and Mo). The model parameters of the matrix were determined from experiments on pure molydenum. For all temperatures investigated, the TiC particles were considered as brittle. Gradual damage of the TiC particles was treated, based on an accumulative failure law that is approximated by an evolution of the apparent particle elastic stiffness. The model enabled us to determine the evolution of the local mechanical fields with deformation and temperature. We showed that a 3D aggregate representing the actual microstructure of the composite is required to understand the local and global mechanical properties of the studied composite.
The interaction of matrix screw dislocations with twin boundaries has been simulated by computer for different h.c.p. models representing α-titanium and magnesium. The atomic structure of the screw core in these two models is appropriate for crystals that slip predominantly on the prism and basal planes, respectively, and this behaviour is summarised in the first part of the paper. Then the movement under three different components of applied strain of the 1/3〈 11–0〉 matrix screw dislocation (in both its prism and basal forms) into the boundary of the 10–12 and 10–11 twins is described for the geometry where the screw is parallel to the interface. The screw crosses the 10–12 boundary by cross-slip, onto either of the two slip systems, but the 10–11 boundary usually absorbs the screw by a process of decomposition into two twinning dislocations. This behaviour and the glide resistance are discussed in terms of the interfacial structure of the twins and the properties of twinning dislocations.
The presence of hydrides in the microstructure can substantially reduce the tensile ductility of Zr and Ti alloys. For treating hydride-induced embrittlement in these alloys, a fracture model has been developed by considering the hydrides to crack readily under tensile loading so that an array of microcracks form in the microstructure. Interaction of the plastic fields of the microcracks leads to fracture of the matrix ligaments, and a loss in the tensile ductility. Application of the proposed model to Zircaloys reveals that hydride-induced embrittlement depends on the hydride size, morphology, and distribution, as well as the continuity of the hydride network, in accordance with experimental observations.
Polymer composites with 3D woven graphite fiber reinforcement (3D interlock weaves) have been tested in compression-compression fatigue under load control. As under monotonic loading, the principal mechanism of failure is kink band formation in the primary load bearing tows. Observations of kink bands and microcracking in sectioned specimens suggest that fatigue progresses by the accumulation of damage to the resin within individual tows. It is conjectured that resin damage leads to failure by lowering the critical stress for kink band formation on a single cycle. If resin damage is assumed to accumulate at a rate proportional to some power of the local axial shear stress in a misaligned tow, then a simple formula follows for the cycles to kink band formation. Under load control, only a few kink bands are required for specimen failure. Then the formula is also the basis for estimates of fatigue life. Fatigue life data and measured misalignment angles, which determine the local axial shear stress, support the fatigue model.
Fracture energies of Al O /Nb interfaces and MoSi /Nb interfaces with and without A1 O coating were measured using sandwich-type chevron-notched specimens. The relations between the mechanical properties, microstructures, types of bonds at the interface and processing routes were explored. The fracture energy of the Al O /Nb interface was determined to be 9 J/m and changed to 16 J/m when Nb was pre-oxidized before the formation of the Al O /Nb interface. The fracture energy of the MoSi /Nb interface could not be determined directly because of the formation of the interfacial compounds. However, the fracture energy at the MoSi /Nb interfacial region was found to depend on the interfacial bond strength, roughness of interfaces and microstructure of interfacial compounds. The interfacial fracture energies of A1 O with silicides, MoSi , Nb Si , or (Nb, Mo)Si were estimated to be about 16 J/m , while the interfacial fracture energies between two suicides or between Nb and a silicide were larger than 34 J/m . The measured fracture energies of the various interfaces are discussed in terms of the interfacial microstructures and types of bonds at the interfaces.
An experimental study has been made of a laminate consisting of monolithic thin alumina plates alternating with unidirectional carbon/epoxy (C/E) prepreg tapes. The main advantages of this system over the traditional means of reinforcing ceramics, are the avoidance of large flaws due to processing, which occur in fiber reinforced brittle matrix composites, and the nearly isotropic behavior under biaxial loading. In addition, the multiple fracture mechanism occurring in the system gives rise to pseudo ductile behavior and enhanced strain energy dissipation. The mechanical behavior of the laminate is explored. The effects of the number of layers, volume fraction and transverse properties are also investigated. The loss of stiffness with increase of the applied strain is estimated using a simple shear lag theory, which includes the plastic behavior of the interface.
The coalesced γ″ precipitate in the aged nickel-base superalloy has been investigated using a high resolution electron microscopy. The encounter of γ″ precipitates during aging formed the coalesced precipitate that consists of several domains which keep the original orientation before encountering. The thickness of boundary in the coalesced γ″ precipitate was in the range of zero to a few atom layers.
Internal stresses in planar random alumina fibre reinforced aluminium with a range of fibre volume fractions have been studied theoretically and in tensile tests and cyclic Bauschinger experiments at room temperature and 77 K. The Eshelby S tensor for a planar random array of fibres is calculated, which allows the mean field model to be used to predict the internal stresses. The conventional Orowan-Wilson method of analysing cyclic Bauschinger experiments is modified, enabling the plastically and thermally induced matrix mean stresses to be separated. This analysis is applied to experimental results and the plastic mean stress and the initial magnitude of the thermal mean stress in the matrix are measured. The results for the thermal matrix mean stress are compared with measurements from monotonic flow curves and generally good agreement is observed. The measured thermal matrix mean stress in these composites is approximately independent of fibre volume fraction.
Considering first the extreme cases of either a perfectly soft or perfectly rigid isolated fibre, the investigation of the effect of the transverse Young's modulus ratio on the stress field resulting from the applied transverse loading is then extended to intermediate fibre matrix and interphase matrix transverse modulus ratios representative of real composite materials. Regarding the effect of the randomness of the fibre distribution, the simplest arrangement of two isolated fibres is considered, the reference situations being those of either an isolated fibre or of a regular arrangement. In the case of the two isolated fibres, three values of the angle between the direction of the fibre alignment and that of the applied transverse loading are taken into consideration (0, 90 and 45°), this last critical situation being of essential interest, due to the “alignment effect” which tends to rotate the fibre pair towards the direction of the applied loading, thereby inducing a particular stress field. Simple analytical formulae are used to determine the stress field resulting from the applied transverse loading in the simplest case of an isolated fibre, i.e. a two-phase (fibre matrix) system, or a simplified (perfectly soft fibre) three-phase (fibre-interphase matrix) system. In the general three-phase system, and in the spatial fibre arrangement of either a fibre pair or a regular distribution, a global finite element numerical calculation is performed; thereby directly taking into account the mechanical interaction between the fibres. The representative mechanical quantities thus determined are discussed in relation with both the possible fundamental mechanisms of deformation and fracture and the actually observed phenomena. Abordée par l'analyse de la situation de référence d'une fibre isolée, dans les cas extrêmes d'une fibre infiniment souple, de module travers nul, ou, au contraire, infiniment rigide, de module travers infini, l'étude de l'effet du rapport des modules d'Young travers sur le champ résultant d'une force appliquée transversalement est ensuite étendue au cas de valeurs intermédiaires du rapport des modules fibre matrice ou interphase matrice, les valeurs retenues correspondant à des matériaux composites réels. En ce qui concerne l'effet de la distribution aléatoire des fibres, c'est l'arrangement le plus simple de deux fibres isolées qui sera pris en compte, les situations de référence étant celles d'une fibre isolée ou d'un arrangement périodique. Dans le cas de deux fibres isolées, trois valeurs de l'angle que fait la direction d'alignement des centres avec l'axe des efforts ont été retenues (0, 90 et 45°), cette dernière configuration étant particulièrement digne d'intérêt puisque très critique, étant donné “l'effet d'alignement” tendant à amener par rotation les deux fibres suivant l'axe des efforts et conduisant ainsi à une répartition des contraintes particulière. Le champ de contrainte résultant de la force transversale sera déterminé à l'aide de formules analytiques simples dans les cas les plus élémentaires d'un système à deux phases (fibre matrice) ou trois phases (fibre-interphase matrice), mais dans lequel la fibre est de raideur négligeable. Dans le cas général du modèle à trois phases, ou dans l'étude de l'effet de la distribution spatiale des fibres (deux fibres, ou arrangement périodique), le problème est alors abordé par un calcul numérique global par éléments finis, permettant ainsi de prendre directement en compte l'interaction élastique entre les fibres. Les grandeurs mécaniques pertinentes ainsi déterminées sont alors discutées en fonction des mécanismes de déformation et de rupture possibles et des phénomènes effectivement observés.
The logarithmic decrement, δ, was measured in Nb-V-O alloys with the V content of 0.5, 1.0, 1.9, 7, 12, 20 and 50 at.% using a low frequency ( = 3-7 Hz) inverse torsion pendulum in the temperature range from 20 to 800°C. The complex oxygen atom Snoek peaks were analysed with the help of a computer and individual Debye form constituent peaks- were extracted. The parameters of these peaks were used to derive information of two sorts: (a) distribution of oxygen atoms over octahedral interstices which differ in the number of V atoms in the nearest neighbour lattice sites (static parameter) and (b) the potential barriers for oxygen atoms diffusing from -positions (dynamic parameter). The first was used to evaluate the binding energies of oxygen with vanadium atoms in V-O complexes, the second made understandable the structure of potential barriers for diffusion of the oxygen atoms from positions . The differences between measurements of static and dynamic characteristics are discussed.
The cubic titanium trialuminide alloys studied previously show limited ductility and toughness, which can be related to the difficulties of dislocation emission, multiplication and mobility and to the excessively high energies of faults associated with the dislocations. The present study examines several new alloys chosen in an attempt to reduce these fault energies and thereby improve the mechanical properties. The region of single phase L1 material is unfortunately so limited that only minor changes in composition are possible before second phases form and precipitation hardening occurs. Over the range of compositions of the L1 phase evaluated there are generally only small changes in fault energies. These changes, as well as the changes in dislocation configurations seen, may be rationalized in terms of the instability of the matrix towards the particular second phase forming.
The strength of the coupling between heat flow and microstructure formation during solidification depends on the kinetics of the microstructural phenomenon. Strong coupling makes it necessary to solve the equations governing heat flow and those governing microstructure formation simultaneously in order to predict the evolution of temperature field and the microstructure. If the coupling is weak, uncoupling the equations to some extent and simplifying the calculation procedure does not affect the predictions of the calculations. In this work the strength of the coupling in the case of equiaxed eutectic grain formation is investigated in order to see to what extent the governing equations can be uncoupled. It is found that the coupling is so strong that full coupling of the equations is essential to get reliable results.