B. Derby

Evaluation of the efficiency of TiB2 and TiC as protective coatings for SiC monofilament in titanium-based composites

The vigorous interfacial reactions in SiC/Ti-6Al-4V composites at elevated temperatures lead to the deterioration of the mechanical properties of the composites. TiB2 and TiC were selected as potential protective coatings for SiC fibres in titanium-based composites. These coatings were deposited on to fibres by the chemical vapour deposition technique. Comparisons and evaluations have been made of the effectiveness of these ceramics as protective coatings for SiC fibres by incorporating the coated fibres into a Ti-6Al-4V matrix using the diffusion bonding method. Emphasis has been placed on the chemical compatibility of the candidate coating with SiC and Ti-6Al-4V by examining the interfaces of the fibre/coating/matrix using microscopic methods and chemical analysis. A stoichiometric TiB2 coating was found to be stable with SiC and has proved an effective barrier to prevent the SiC fibre from reacting with the Ti-6Al-4V. The TiC coating showed no apparent reaction with a titanium-alloy matrix under the conditions studied, but was found to react with the SiC fibre substrate.

The formation of TiC/Al2O3 microstructures by a self-propagating high-temperature synthesis reaction

The effect of processing variables on reaction rate and ceramic microstructure are examined for the self-propagating high-temperature synthesis reaction 3TiO2+4Al+3C → 3TiC+2Al2O3. Reaction controlling methods used are reactant particle size, the use of diluents (to lower the combustion temperature) and the use of reactant preheating (to increase the combustion temperature). The ceramic microstructure has an unusual laminar structure which is generally only observed during unstable combustion wave propagation.

Comparison of interfaces in Ti composites reinforced with uncoated and TiB2/C-coated SiC fibres

Interfaces play an important role in determining the mechanical properties of composite materials. The interfaces established between a titanium‐alloy matrix (Ti‐6Al‐4V) and uncoated and TiB2/C‐coated SiC fibres are analysed by scanning electron microscopy, transmission electron microscopy and X‐ray techniques. Emphasis is placed upon the interfacial morphology and microstructure, identification of reaction products, and the stability of the coating layer. Complex multi‐reaction layers are observed frequently in the interfacial zones. Previous, often contradictory, reports about the interlayers are reviewed. Experimental observation demonstrates that the type and distribution of interlayers vary in a given system, due to prolonged treatment of the samples at temperature. The formation and distribution of the interlayers are discussed further, with respect to these and previous findings. Methods of reducing interfacial reactivity are discussed.

X-Ray Microtomographic Studies Of Metal-Matrix Composites Using Laboratory X-Ray Sources

X‐ray microtomography (XMT) is a non‐destructive technique that allows the internal structure of a material to be imaged by the spatial distribution of its linear X‐ray absorption coefficients. This paper demonstrates the use of XMT to investigate: (1) the distribution of TiB2 reinforcement in composites formed by powder processing; (2) the local void volume fraction as a function of position in highly deformed regions of failed tensile specimens of SiC‐reinforced material allowing a valid damage parameter to be defined at high strains; (3) absorption coefficients measured at different energies simultaneously using a multichannel analyser which can sometimes be used to separate linear absorption changes due to (a) density variations and (b) compositional variations in individual voxels; and (4) the use of sequential sections to provide a three‐dimensional representation of the failed specimens.

IN SITU SCANNING ELECTRON MICROSCOPE STUDIES OF FRACTURE IN PARTICULATE-REINFORCED METAL-MATRIX COMPOSITES

In situ observations of specimen surfaces have been used to characterize the fracture behaviour of particulate-reinforced metal-matrix composites. Composites of silicon carbide particle sizes 3, 10 and 30 μm with volume fractions of 5, 10 and 20% in commercial-purity aluminium and aluminium-1% magnesium matrices were studied. The results of this surface study are compared with complementary metallographic studies of sectioned specimens illustrating behaviour from the bulk. Significant differences between the results of these two studies are outlined. Previous work using these techniques is critically examined and recommendations made for the appropriate interpretation of in situ straining experiments.

TEM study of silicon carbide whisker microstructures

AbstractΒ-SiC whiskers produced by a number of manufacturers have been examined in the transmission and scanning electron microscopes. In all cases defective microstructures were found with high densities of planar defects such as stacking faults and microtwins. Two distinct types of defective whisker can be identified. The first contains regions of very closely spaced twins on {111} planes arranged perpendicular to the whisker axis [111], these were sometimes separated by defect-free regions. In these whiskers a rough surface profile was normal with the roughness closely associated with the highly defective regions of the whisker. The second type of whisker contained stacking faults spaced relatively widely also on {111} planes but now on the planes inclined to the [111] axis of the whisker. This leads to a characteristic chevron contrast in the TEM. This second type of whisker had a much smoother surface profile than the first type with perpendicular defects. No whisker contained both defect types but some batches of whiskers contained populations of both types of whisker. The first type of whisker is shown to have defects similar to those reported as common during vapour-liquid-solid whisker growth. This is also consistent with the higher impurity content and the presence of voids found in these whiskers. The second type may be indicative of a different growth mechanism possible under certain conditions of SiC whisker synthesis.

Characterization of microstructural damage during plastic strain of a particulate-reinforced metal matrix composite at elevated temperature

The evolution of damage in a SiC-reinforced 2618 AI alloy during plastic strain has been investigated by elastic modulus reduction and direct observations of the microstructure at room temperature and temperatures up to 220 °C. Particle fracture increases as a function of strain at all temperatures but the total number of fractured particles at any given strain is lower at higher test temperatures. The dependence of fracture on particle size and aspect ratio was recorded. Normalized elastic modulus measurements decrease as a function of strain at the same rate for tests at 25,110 and 220 °C with an anomalous set of measurements at 165 °C showing a reduced damage rate. There is no universal correlation between the number of damaged particles and reduced modulus with each test temperature showing a different relation. This indicates the different temperature dependence of void nucleation and subsequent growth. The results are used to interpret different models of load sharing between reinforcement and matrix during straining.[/p]

The compatibility of TiB2 protective coatings with SiC fibre and Ti-6Al-4V

TiB2 coatings have been studied as prospective protective layers to inhibit the interfacial reaction between SiC fibres and Ti‐alloy matrices. This protective coating has been deposited onto SiC monofilament fibres using a chemical vapour deposition (CVD) technique. The fibre‐matrix compatibility of these TiB2‐coated SiC fibres in Ti‐6Al‐4V composites was evaluated by incorporating the coated fibres into Ti‐6Al‐4V using a diffusion bonding technique. The interfaces of this composite were characterized by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and electron probe microanalysis, to evaluate the interfacial microstructures, chemical stability and the efficiency of TiB2 as a protective coating for SiC fibres in Ti‐alloy matrices, and to study the effects of deposition temperature on the interface of the coated fibre. Results show that stoichiometric TiB2 coatings are stable chemically to both SiC fibres and Ti‐6Al‐4V and hinder the deleterious fibre‐matrix reactions effectively. Boron‐rich TiB2 coatings should be avoided, as they lead to the formation of a needle‐like TiB phase at the fibre–matrix interface. These findings provide promising evidence for the value of further exploration of the use of stoichiometric TiB2 as a protective coating for SiC fibre in Ti‐based composites.

Processing and microstructural characterisation of RBSiC-TaSi2 composites

Reaction bonded silicon carbide (RBSiC) ceramics typically contain 10 vol% silicon inherent to the reaction bonding process. However, the relatively low melting point (1410°C) of the silicon phase is a limiting factor in the high temperature use of RBSiC materials. The application temperature can potentially be enhanced by replacing the silicon with more refractory metal disilicide phases. In this paper we report the infiltration of SiC-graphite compacts with alloyed Si-Ta melts in an attempt to precipitate TaSi2 (Tm=2040°C) in place of the residual silicon. High density RBSiC-TaSi2 ceramics with virtually no porosity were readily produced, but subsequent XRD and SEM examination revealed that the silicon phase was not completely removed. In addition, the materials possessed complex, inhomogeneous microstructures and were susceptible to various types of crack formation phenomena.

Microstructure and fracture behaviour of particle‐reinforced metal–matrix composites

The fracture behaviour of particle‐reinforced metal‐matrix composites is shown to be controlled by a period of damage nucleation and evolution prior to final failure. The nucleation of damage can be by reinforcement fracture or decohesion and the mode of damage is shown to be controlled by the size of the reinforcement and segregation of alloying elements from the matrix. The nucleation and growth of damage can be monitored by a number of techniques. Acoustic emission and tomography are used here and the results are found to be consistent with simple models of void growth.