Magnesium‐Based Alloys

This chapter reviews and summarizes the key advances made during the development of continuous fiber metal matrix composites (MMCs) for the aerospace industry from the early 1960s to the present day. The materials and their processing routes are explained with a view to showing how component design influences the final material system choice. Many significant technical problems have been overcome during the lengthy development history and large benefits have been predicted, however, the realization of these benefits through to production applications in the aerospace sector has to date been limited. The difficulties encountered in escaping the development “valley of death” are discussed and ways to improve the exploitation of MMCs in the future suggested.

4.19 Development of Continuously-Reinforced Metal Matrix Composites for Aerospace Applications

Interfacial control and macroscopic failure in long-fibre-reinforced and laminated inorganic composites

The interfacial fracture energy of Ti/Al2O3 composites was measured with and without a diffusion barrier at different bonding temperatures by using four-point bending tests. It was found that the interfacial fracture energy increases with increasing bonding temperature up to 950 °C. When the bonding temperature was further raised to 1000 °C, the interfacial fracture energy drops. The decrease of the interfacial fracture energy is due to the formation of the continuous intermetallic compound, Ti3Al, at the interface between Ti and Al2O3. By using a diffusion barrier, the interfacial fracture energy decreases from 25.4 to near O J/m2 and 32.9 to 8.7 J/m2 for applied bonding temperatures of 800 and 900 °C, respectively. This is because the diffusion barrier reduced the diffusion of Al across the interface and into the Ti, thereby preventing a strong chemical bond at the interface. For the composite bonded at 900 °C, the crack propagation was found to occur at the interface between the Ti and Al2O3. The interfacial failure was found to be in the Ti3Al reaction layer for the composite processed at 1000 °C. With a diffusion barrier, the crack propagation path follows several interfaces. Evaluation of the processing temperature on the mechanical properties of the Ti was also obtained by using a nanoindentation technique.

Interfacial reactions and adhesion strength of metal/ceramic composites

Chapter VI – Creep and Creep Rupture

An examination has been made of the chemical stability of SiC monofilaments, with and without sputtered coatings intended to produce diffusion barrier layers of Y2O3, in contact with matrices of Mg Li alloy (up to 400°C) and Ti (up to 1000°C). Even very thin layers were found to offer some protection in the Mg Li alloy, under conditions such that the uncoated fibres suffered catastrophic embrittlement by penetration of Li into the grain boundaries. Yttrium-coated fibres in a Ti matrix were found to exhibit only marginally improved stability when compared with uncoated fibres. The probable explanation for this has been identified as a tendency for Y to penetrate into the SiC fibre before a stable Y2O3 layer could form, although high hydrogen levels in the Ti matrix (absorbed during composite fabrication) may also have impaired the interfacial stability in much of the material examined. Fibre preoxidation prior to Y coating was found to inhibit this Y penetration into the fibre material, allowing a Y2O3 barrier layer to form in situ. This barrier layer has been shown to offer considerable fibre protection.

Sputter deposited barrier coatings on SiC monofilaments for use in reactive metallic matrices—III. Microstructural stability in composites based on magnesium and titanium

A potential advantage of sputter deposition as a process for producing protective coatings is that there is a degree of control over the stress state of the deposit. The factors affecting this are briefly discussed. Some experimental data are then presented for X-ray diffraction by {103} planes in a yttrium coating on SiC monofilaments. In order to interpret these data, it was necessary to set up a model describing the expected contributions of radial and hoop strains to the observed spectra. The stress state could then be deduced. Combination of these results with predictions from a stress analysis model allowed the differential thermal contraction stresses to be subtracted and the stress introduced during the deposition process to be estimated. An equal biaxial stress of about -800 MPa was obtained in this way. for a coating produced under conditions designed to favour a large compressive stress. This is consistent with observations that these coatings exhibit good mechanical stability.

Sputter deposited barrier coatings on SiC monofilaments for use in reactive metallic matrices—II. System stress state

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Sputter deposited barrier coatings on SiC monofilaments for use in reactive metallic matrices—III. Microstructural stability in composites based on magnesium and titanium

Sputter deposited barrier coatings on SiC monofilaments for use in reactive metallic matrices—II. System stress state