Metal Matrix Composites

ANNEALED aluminum/silicon carbide (Al/SiC) composites exhibit a relatively high density of dislocations, which are frequently decorated with fine precipitates, in the Al matrix. This high dislocation density is the major reason for the unexpected strength of these composite materials. The large difference (10:1) between the coefficients of thermal expansion (CTE) of Al and SiC results in sufficient stress to generate dislocations at the Al/SiC interface during cooling. In thisin situ investigation, we observed this dislocation generation process during cooling from annealing temperatures using a High Voltage Electron Microscope (HVEM) equipped with a double tilt heating stage. Two types of bulk annealed composites were examined: one with SiC of discontinuous whisker morphology and one of platelet morphology. In addition, control samples with zero volume percent were examined. Both types of composites showed the generation of dislocations at the Al/SiC interface resulting in densities of at least 1013 m-2. One sample viewed end-on to the whiskers showed only a rearrangement of dislocations, whereas, the same material when sectioned so that the lengths of whiskers were in the plane of the foil, showed the generation of dislocations at the ends of the whiskers on cooling. The control samples did not show the generation of dislocations on cooling except at a few large precipitate particles. The results support the hypothesis that the high dislocation density observed in annealed composite materials is a result of differential thermal contraction of Al and SiC. The SiC particles act as dislocation sources during cooling from annealing temperatures resulting in high dislocation densities which strengthen the material.

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An in situ HVEM study of dislocation generation at Al/SiC interfaces in metal matrix composites

Despite successful applications of diffusion bonding, there is uncertainty as to the dominant mechanisms which operate when the two surfaces to be joined are brought together under an applied pressure at an elevated temperature. This paper describes a theoretical model for solid-state diffusion bonding, the aim of which is to understand how the various competing mechanisms operating at the bond interface are affected both by process variables (time, temperature, pressure) and by the properties of the materials being joined. Thus in the model the rate-controlling mechanisms governing the elimination of the interface between the two surfaces are identified, their relative kinetics predicted, and their individual importance in the various stages of diffusion bonding assessed. The approach adopted bears some similarity with that used when modelling the pressure sintering of powders, although the geometry at the interface of a diffusion bond results in differences.

Theoretical model for diffusion bonding

Modelling solid-state diffusion bonding

An understanding of the precise correlation between the presence of secondary phases and material damping has eluded investigators, partly as a result of the fact that often there are various mechanisms involved. As a step towards the clarification of damping phenomena in metals and alloys, this paper provides a systematic review of the studies that have been completed on the damping mechanisms present in metals and alloys, with particular emphasis on precipitation. The damping mechanisms associated with secondary phases in metals and alloys have been subdivided into four categories, namely interface damping theory, thermal mismatch-induced dislocation damping theory, interaction damping theory and the rule of mixtures damping theory. A number of alloy systems are discussed to demonstrate the applicability of the four types of theory and the level of understanding of these complex mechanisms. As an extension of precipitation damping theory, the damping behaviour and mechanisms in particle-reinforced metal matrix composites are extensively discussed.

Effects of secondary phases on the damping behaviour of metals, alloys and metal matrix composites

The rate-limiting step in the diffusion bonding of materials such as Ti−6 A1−4V is the complete elimination of porosity from the bondline. Here a model for predicting the time to complete bonding based on the mechanisms of porosity removal is developed. Bonding is pictured as a two-stage process: in the first stage long-wavelength surface asperities are flattened by plastic flow, and in the second stage voids are reduced in size by a combination of plastic flow and vacancy diffusion. Both stages have been treated analytically, and the resulting predictions of bonding time are in general agreement with the times usually required for industrial diffusion-bonding operations. In addition, a limited number of experiments have been conducted to verify some basic predictions of the model. As expected, the calculations have shown that the bonding die must be carefully designed so that the local bonding pressure is never unreasonably low. The results also demonstrate that the presence of long-wavelength surface asperities—a result of most surface preparation procedures —can significantly lengthen the bonding times.

Attainment of full interfacial contact during diffusion bonding

The influence of heat treatment on the microstructure of a Ni-45.5 wt pct W directionally solidified eutectic composite has been studied. Four types of microstructural conditions have been examined using light and thin foil electron microscopy techniques. The as-grown composite consists of W fibers in a matrix which contains some WNi4 precipitates. Quenching the composite from above the peritectoid temperature results in W fibers in a Ni-W solid solution matrix. Subsequent aging results in precipitation of coherent WNi4. Long time aging at temperatures just under the peritectoid temperature promotes the peritectoid reaction leading to envelopment of the W fibers by a layer of WNi4. The envelopment reaction is accompanied by a change from faceted to round fiber morphology. The presence of W fibers in the Ni-W solid solution influences aging in two ways when compared to a solid solution alloy. First, the fiber/matrix interfaces act as dislocation sources on quenching; these dislocations are absorbed into the WNi4 precipitate/matrix interfaces leading to early loss of coherency. Second, the WNi4 precipitates undergo a preferential coarsening due to elastic interaction between the fibers and the WNi4 precipitates; this leads to a mosaic structure, the occurrence of which is uniquely dependent on the presence of W fibers.

Microstructural studies of a Ni-W directionally solidified eutectic composite

Unlike many eutectic composites, the Ni-W eutectic exhibits extensive ductility by slip. Furthermore, its properties may be greatly varied by proper heat treatments. Here results of studies of deformation in both monotonic and fatigue loading are reported. During monotonie deformation the fiber /matrix interface acts as a source of dislocations at low strains and an obstacle to matrix slip at higher strains. Deforming the quenched-plus-aged eutectic causes planar matrix slip, with the result that matrix slip bands create stress concentrations in the fibers at low strains. The aged eutectic reaches generally higher stress levels for comparable strains than does the as-quenched eutectic, and the failure strains decrease with increasing aging times. For the composites tested in fatigue, the aged eutectic has better high-stress fatigue resistance than the as-quenched material, but for low-stress, high-cycle fatigue their cycles to failure are nearly the same. However, both crack initiation and crack propagation are different in the two conditions, so the coincidence in high-cycle fatigue is probably fortuitous. The effect of matrix strength on composite performance is not simple, since changes in strength may be accompanied by alterations in slip modes and failure processes.

Monotonic and fatigue deformation of Ni-W directionally solidified eutectic

Curved ingots of Al, Al-6.1 pct Ni, and Al-33.2 pct Cu were grown by rotating a mold on a radius arm through an induction coil. The specimens were examined to determine their microstructural morphology and crystallography. In both eutectic alloys the minor phase (Al3Ni or CuAl2) curves to follow the shape of the mold. The lamellae in Al-CuAl2 and the fibers in Al-Al3Ni are morphologically identical to those in linearly grown samples. In both cases preferred orientations develop between the phases early in the growth process and remain nearly unchanged at various angular positions. In Al-CuAl2 rotations at sub-grain-like faults allow the crystallographic curvature, but no mechanism could be identified in Al-Al3Ni. Observations of the detailed crystallographic relations suggest that a boundary-coincidence model provides a useful description of the Al-CuAl2 interfaces.

Structure and crystallography of curved Al-Al 3 Ni and Al-CuAl 2 directionally solidified eutectic alloys

The interfacial structure of Ni3Al-Ni3Cb directionally solidified eutectic composites has been investigated by transmission electron microscopy. These interfaces contain at least three distinguishable arrays of features. Two of the arrays, misfit dislocations, have been discussed previously by Nakagawa and Weatherly. The third set, ledges which can fulfill both structural and kinetic growth functions, may interact with the dislocation arrays through strain-energy mechanisms. The interaction is manifested both as a local alteration of the line vector of the dislocation in certain circumstances, and as a change in the response of the dislocation image to ±g electron-microscope image-contrast experiments. A simple model of the strain field of a ledge based on that of an edge dislocation is formulated to rationalize the behavior of a misfit dislocation lying in close proximity to a ledge. The interaction of ledges and dislocation segments is expected to have significance in physical processes of practical interest such as production of matrix slip dislocations, misfit dislocation rearrangement, boundary sliding, and coarsening, and these processes are discussed in some detail.

G. Garmong

Papers

Attainment of full interfacial contact during diffusion bonding

Microstructural studies of a Ni-W directionally solidified eutectic composite

Monotonic and fatigue deformation of Ni-W directionally solidified eutectic

Structure and crystallography of curved Al-Al 3 Ni and Al-CuAl 2 directionally solidified eutectic alloys

Interfacial ledge structures in Ni3 Al-Ni3Cb eutectic composites