S. R. Nutt

An analysis of the effect of residual stresses on deformation and damage mechanisms in AlSiC composites

Abstract The effect of thermally induced residual stresses on the mechanical properties and ductility of Alue5f8SiC composites was investigated numerically. The predicted behavior in uniaxial loading was calculated with and without the inclusion of the residual stresses which result from the mismatch in thermal expansion between aluminum and SiC. In this analysis, void nucleation by interfacial debonding at the whiskers ends was assumed to be the limiting failure mechanism. Two cases, both with a fiber volume fraction of 20% and fiber aspect ratio of 4, but with different fiber spacings, were considered. The residual stresses had a small effect on the predicted ductility of the composite, even when a relatively weak interface strength was assumed. The residual stresses are shown to redistribute as interfacial failure is approached. A close end-to-end fiber spacing gives a greater flow strength in compression than in tension and the residual stresses which arise during thermomechanical processing tend to enhance this effect.

An analysis of residual stress formation in whisker-reinforced AlSiC composites

Abstract The effects of fiber spacing, volume fraction and aspect ratio on the residual stresses in metal-matrix composites are analyzed numerically. The composite is modeled as a periodic array of cylindrical cells, each consisting of the matrix alloy with a whisker embedded in the ceter. Account is taken of thermoelasticity both in the fiber and in the matrix and of temperature-dependent plasticity in the aluminum matrix. A general formulation, valid for finite strains and rotations, is discussed. Quenching is simulated by imposing a temperature history obtained from a macroscopic solution of the heat equation for a cylindrical bar to the surface of a cell. The resulting residual stress fields are calculated. The results show that the side-to-side spacing of fibers is the most important microstructural parameter affecting the distribution of residual stress and plastic deformation in the matrix. The overall level of plastic deformation in the matrix, measured by the volume average of effective plastic strain, depends primarily on fiber volume fraction. The fiber aspect ratio has little effect, apparently because the residual fields become essentially independent of axial position a short distance from the fiber corner.

Interface structures in beta-silicon carbide thin films

Interface structures in monocrystalline beta‐silicon carbide thin films grown on (001) silicon substrates have been studied by high‐resolution electron microscopy of cross‐sectional specimens. Despite a large lattice mismatch, there is a periodic registry of {111} atom planes across the SiC‐Si interface. Planar defects on SiC {111} planes are grown‐in and arise primarily from lattice and thermal expansion mismatch. Thermal oxidation in wet atmospheres results in preferential attack of the SiC film at sites where planar defects intersect the film surface, whereas oxidation in dry atmospheres does not.

TERRACE GROWTH AND POLYTYPE DEVELOPMENT IN EPITAXIAL BETA -SIC FILMS ON ALPHA -SIC (6H AND 15R) SUBSTRATES

Epitaxial β-SiC (3C) films were grown on (0001) 6H-SiC and 15R-SiC substrates by chemical vapor deposition (CVD). TEM characterization revealed that films on both substrates exhibited large areas of atomically flat, coherent interfaces. However, when 3C-SiC films were grown on 6H substrates, double position boundaries (DPBs) were frequently observed, and islands of 6H were occasionally embedded in the predominantly 3C film. In contrast, films of 3C-SiC grown on 15R substrates exhibited relatively few DPBs and only occasional islands of 15R. A model of interlay er interactions in SiC was applied to predict the atomic structures at both 3C/6H and 3C/s15R interfaces, and these predictions were consistent with experimental observations of the interfaces by TEM. The observed interface structures and defect distributions were attributed to a microscopic kinetic mechanism of terrace growth. Consideration of step energies and growth kinetics led to the prediction that DPBs can be avoided by growing 3C-SiC films on 15R-SiC substrates.

Interfaces in nickel aluminide/alumina fibre composites

Metal matrix composites based on the intermetallic alloy Ni3Al and fibres of Al2O3 were fabricated by hot-pressing nickel aluminide powders and alumina fibres. Two matrix alloys were used in this investigation: Ni3Al microalloyed with boron and Ni3Al alloyed with 8 at% chromium and smaller amounts of zirconium and boron. The materials were studied using optical and transmission electron microscopy with particular emphasis placed on the characteristics of the matrix-fibre interface. The base Ni3Al/Al3O3 composite displayed no evidence of chemical reaction at the interface, an intimate bond between matrix and fibre was observed, and the material exhibited 10% ductility at room temperature. Composites with the more complex matrix alloy were brittle, a phenomenon attributed to the formation of zirconia particles at the interface.

Creep deformation of alumina-SiC composites

Abstract Composites of alumina reinforced with SiC whiskers have been creep tested in bending and in compression at 1200–1400 °C in an air ambient. The flexural creep data follow a power law constitutive relation with two distinct stress exponents that depend on the level of applied stress. Crept specimens were examined by transmission electron microscopy to determine the mechanisms of creep deformation and microstructural damage. The primary mechanism of creep deformation under these conditions is grain boundary and interface sliding resulting from diffusion. At high stress levels, the sliding is often unaccommodated, resulting in cavitation at grain boundary-interface junctions. Cavitation is associated with an increase in the stress exponent for flexural creep.

Defect structures in single crystal TiC

Abstract A transmission electron microscopy study is presented of sub-boundary defect structures in single crystal TiC. Sub-boundaries consisting of organized dislocation arrays and hexagonal networks were observed. The Burgers vectors of dislocations in the sub-boundaries were determined to be 1/2 , and the dislocations were approximately on {111} planes. Dislocation nodes in hexagonal networks provided preferential sites for nucleation of triangular planar defects. These defects were identified as platelet precipitates of TiB 2, although it was difficult to distinguish between precipitates and impurity-stabilized stacking faults during the initial stages of the precipitation. The precipitates were bounded by Shockley partial dislocations and resided on {111} habit planes. Segregation of impurities to sub-boundaries resulted in unusual platelet configurations involving beak-like extensions on intersecting {111} planes. In such cases, the beak-like extensions were bounded by two uncommon partial dislo...

Creep behavior of an SiC-reinforced XDTM MoSi2 composite

Abstract The compressive creep behavior of an MoSi 2 ue5f8SiC composite and a base alloy was investigated at 1050–1300°C. Creep experiments performed in air showed power-law type constitutive behavior with a stress exponent of approximately 3.5. Both materials showed some effect of thermal oxidation during creep. Microstructural observations by transmission electron microscopy indicated that deformation occurred primarily by dislocation glide and that the rate-controlling process was recovery by dislocation climb. In the composite material, SiC particles were effective impediments to glide, as were numerous strain centers, tentatively identified as coherent precipitate particles. Cavitation occurred at SiC-matrix interfaces and at grain boundaries within polycrystalline SiC particles, and the process was apparently facilitated by the accumulation of glassy phase at these sites during creep. High resolution electron microscopy observations indicated the glassy phase originated both from thermal oxidation during creep and from non-crystalline material that pre-existed in the as-fabricated composite.

Fracture toughness and fatigue crack growth behaviour of an Al2O3-SiC composite

The fracture toughness and fatigue crack growth characteristics of an Al2O3-SiC whisker composite were investigated. Quasi static fracture experiments were conducted on double edge-notched tension specimens and on four-point bend specimens containing a through-thickness Mode I crack which was introduced under uniaxial cyclic compression. The toughness results obtained using this procedure are more reproducible than those derived from the indentation technique and the notched bend bar method. The fracture toughness of the composite is about 60% higher than that of the unreinforced matrix material. Crack growth characteristics at room temperature were also investigated in notched plates of Al2O3-SiC subjected to fully compressive far-field cyclic loads. In the presence of a stress concentrator, this composite is found to be highly susceptible to fatigue crack growth under cyclic compressive loads.

Heteroepitaxial growth of β'-SiC films on TiC substrates: Interface structures and defects

Thin epitaxial films of β-SiC were grown by CVD on (100), (111), and (112) TiC substrates. TEM observations of the resulting interfaces revealed that island nucleation prevailed in the early stages of deposition for all three substrate orientations. Films grown on (111) and (112) TiC were monocrystalline, while SiC films deposited on (100) substrates were polycrystalline and not epitaxial, a phenomenon attributed to the poor match of atomic positions in SiC and TiC on their respective (100) planes. The (111) interface was abrupt and atomically flat, while the (112) interface exhibited {111} facets and steps. Simulated images of the stable (111) interface were calculated based on several possible atomic configurations, and the atomic structure of the interface was deduced from comparisons between the simulated images and phase-contrast TEM images.