Acta Metallurgica et Materialia最新文献

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 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.

The evolution of two-dimensional shapes to equilibrium shapes is investigated for two kinetic mechanisms, surface diffusion and surface attachment limited kinetics. Qualitative differences are found that may be used in experiments for easy distinction among the two mechanisms, and find topological changes not expected for the corresponding isotropic problems. We take advantage of the mathematical developments for surface evolution and equilibration problems when surface energy anisotropy is “crystalline”, so extreme that crystals are fully faceted. We confirm the prediction that with this anisotropy these problems are more easily solvable than for lesser anisotropies, and the techniques developed may even be useful for approximating isotropic problems.

The martensitic transformation of Cu-Zn-Al single crystals has been followed in situ and in real time by synchrotron X-ray topography. We have shown that at the M_stemperature, the transformation proceeds by nucleation and growth of transformation variants associated by self-accommodating pairs. The nucleation of these variants is triggered by the crystal substructure. When the martensitic transformation is multivariant the behaviour of the crystal substructure and the development of elastic stresses inducing a reversible curvature of lattice planes has been displayed.

Fracture energies of Al₂O₃/Nb interfaces and MoSi₂/Nb interfaces with and without A1₂O₃coating 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 Al₂O₃/Nb interface was determined to be 9 J/m²and changed to 16 J/m²when Nb was pre-oxidized before the formation of the Al₂O₃/Nb interface. The fracture energy of the MoSi₂/Nb interface could not be determined directly because of the formation of the interfacial compounds. However, the fracture energy at the MoSi₂/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 A1₂O₃with silicides, MoSi₂, Nb₅Si₃, or (Nb, Mo)Si₂were estimated to be about 16 J/m², while the interfacial fracture energies between two suicides or between Nb and a silicide were larger than 34 J/m². The measured fracture energies of the various interfaces are discussed in terms of the interfacial microstructures and types of bonds at the interfaces.

The fracture toughness is predicted for an interface between niobium and alumina. It is assumed that voids at the triple points of grain junctions on the interface are the sites of cleavage decohesion. These voids are treated as microcracks and the plastic zone is modelled by a cohesive zone of constant shear stress. The interaction between the microcrack and the main crack is studied using an integral equation approach. The fracture toughness is calculated via various fracture criteria and compared against experimental data. Scatter in the experimental data is related to the variability in microdefect size.

The theory of thermal grooving under stress is extended to explain the formation of holes and hillocks in very thin films undergoing thermal cycling. Previous experimental work and attempts to model holes and hillocks are first reviewed. A theoretical model based on the review is then developed to quantify the surface topological evolution at triple junctions in thin films with large columnar grains. The surface profiles are computed for various applied stresses and angles of intersecting grain boundaries, and the model is applied to describe a 120° triple junction in a Cu thin film through an annealing cycle from room temperature to 325°C. The simulation, which uses experimentally determined stress and materials property data, illustrates how this model can predict the failure of films undergoing a given thermal cycle.

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.

The effect of heat treatment variables such as initial microstructure, isothermal reaction time and cooling rate on the phase transformations occurring at 1420°C in Ti-48A1 and Ti-48Al-2Mn-2Nb alloys was studied. The main effect of the initial microstructure, which comprised either lamellae of α₂and γ or equiaxed γ grains, was to alter the kinetics, through a change in the chemical driving force of the phase transformations. Therefore, the equiaxed γ grains transformed to α much faster than the lamellar structure and in the initially lamellar structure, growth of α resulted in the delineation of the initial dendritic structure formed during solidification. The effect of the rate of cooling from the heat treatment temperature on the final morphology of these alloys was drastic and resulted in a change in morphology from lamellar grains obtained on furnace cooling to a feathery and mottled morphology obtained on water quenching. TEM analysis of water quenched Ti-48Al-2Mn-2Nb revealed complex morphologies including a structure which consisted of equiaxed γ grains and residual α₂and abutting colonies of γ and α₂. Based on the TEM results, the early stages of formation of γ from α were studied and mechanisms of nucleation and growth discussed. The relative importance and the coexistence of massive and martensitic transformation products is also discussed.