This review highlights the use and great potential of liquid metals as exotic and powerful solvents (i.e. fluxes) for the synthesis of intermetallic phases. The results presented demonstrate that considerable advances in the discovery of novel and complex phases are achievable utilizing molten metals as solvents. A wide cross-section of examples of flux-grown intermetallic phases and related solids are discussed and a brief history of the origins of flux chemistry is given. The most commonly used metal fluxes are surveyed and where possible, the underlying principal reasons that make the flux reaction work are discussed.

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The metal flux: a preparative tool for the exploration of intermetallic compounds.

This paper reviews the advantages and disadvantages of various thermal barrier coating (TBC) systems, with the aim of custom designing a TBC system to be both strain tolerant and have a low thermal conductivity. Methods of heat transfer within zirconia based ceramics are discussed, including the influence of coating microstructure and ceramic composition. It is shown the addition of dopant atoms (colouring) is effective in reducing ‘phonon’ transport and that layered microstructures are effective in reducing ‘photon’ transport. Advanced processing, using EB-PVD coating methods has allowed both coloured and layered ceramic coatings to be produced. Measured thermal conductivities of 1.0 W mK −1 have been achieved using these methods, much lower than current commercial EB-PVD coatings at 1.5–1.9 W mK −1 .

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Methods to reduce the thermal conductivity of EB-PVD TBCs

Combustion synthesis is an attractive technique to synthesize a wide variety of advanced materials including powders and near-net shape products of ceramics, intermetallics, composites, and functionally graded materials. This method was discovered in the former Soviet Union by Merzhanov et al. (1971). The development of this technique by Merzhanov and coworkers led to the appearance of a new scientijc direction that incorporates both aspects of combustion and materials science. At about the same time, some work concerning the combustion aspects of this method was also done in the United States (Booth, 1953; Walton and Poulos, 1959; Hardt and Phung, 1973). However, the full potential of combustion synthesis in the production of advanced materials was not utilized. The scientijc and technological activity in thejeld picked up in the United States during the 1980s. The signijcant results of combustion synthesis have been described in a number of review articles (e.g., Munir and Anselmi-Tamburini, 1989; Merzhanov, I990a; Holt and Dunmead, 1991; Rice, 1991; Varma and Lebrat, 1992; Merzhanov, 1993b; Moore and Feng, 1995). At the present time, scientists and engineers in many other countries are also involved in research and further development of combustion synthesis, and interesting theoretical, experimental, and technological results have been reported from various parts of the world (see SHS Bibliography, 1996). This review article summarizes the state of the art in combustion synthesis, from both the scientijc and technological points of view. In this context, we discuss wide-ranging topics including theory, phenomenology, and mechanisms of product structure formation, as well as types and properties of product synthesized, and methods for large-scale materials production by combustion synthesis technique.

Combustion Synthesis of Advanced Materials: Principles and Applications

The present paper describes the potential application of ceramic matrix composites to aero-engine components by reviewing the related published papers and our experience in this field. It contains the material requirements for aero-engines, trends in aero-engine materials use, Japanese projects related to ceramic matrix composites (CMCs) and potential application of CMCs to aero-engines, such as combustors, nozzle flaps, bladed disks and others. From the point of application to aero-engines, the remaining research and development issues are discussed to some extent. Material developments, particularly of the interface and fibers for high temperature, are still required and stressed.

Potential application of ceramic matrix composites to aero-engine components

Intermetallic-matrix composites—a review

With the recognition of oxidation-resistant MoSi 2 as the ideal matrix material for high temperature structural composites, there has been a growing interest in “other” silicides. This paper is an attempt to place such interest in proper perspective with a comprehensive review of recent activities. The “other” silicides are appraised in terms of diverse strategies for the development of structural materials, which may be broadly grouped as monolithic, in situ composites and artificially reinforced composites. With alloying as an underlying common theme, it is argued that a broader and a more fundamental understanding of the silicides is necessary. The formation of silicides is reviewed with the focus on the disilicides, 5-3 silicides and monosilicides, as the three principal useful groups. For the refractory metal based disilicides, the relationship between the crystal structures C11 b , C40, C49 and C54 is examined in terms of stacking sequences and contrasted in relation to the structures of aluminides. The role of interstitial elements and the in situ composite approaches are emphasized for the complex 5-3 silicides and the monosilicides as being the most adjacent phases to refractory metal solid solutions. For most silicides with a non-cubic crystal structure, the effect of associated anisotropy of the coefficient of thermal expansion (CTE) on the mechanical integrity through processing and application of the material, is brought forth as a potentially critical issue. A qualitative model is proposed to rationalize the pronounced occurrence of “pest” disintegration in terms of the anisotropy of CTE, the nature of the grain structure and the ductile-brittle transition temperature of intermetallics.

Appraisal of other silicides as structural materials

The durability and performance of coatings in gas turbine and diesel engines

MoSi2-base composites are currently being developed as structural aerospace materials for use at temperatures up to 1400 °C. Work is being carried out on improving high-temperature creep resistance and room temperature fracture toughness by continuous fiber reinforcement. Candidate fibers consist of high-strength ceramic fibers, such as SiC and single-crystal Al2O3, and ductile high-strength molybdenum and tungsten alloy fibers. Critical issues being addressed concerning development of refractory-metal-fiber-reinforced MoSi2-base composites include identification of diffusion barrier fiber coatings that prevent reaction between the fiber and matrix, identification of fabrication techniques that do not result in loss of fiber ductility and fiber coating integrity, and oxidation resistance. A key issue regarding the development of ceramic-fiber-reinforced MoSi2-base composites is identification of debond fiber coatings that provide necessary fracture toughness and damage tolerance. Additions of SiC and Si3N4 particulate to MoSi2 are effective in lowering matrix thermal expansion to match more closely that of reinforcing fibers. Mechanical properties and environmental resistance of composite systems will be discussed.

Development of continuous-fiber-reinforced MoSi2-base composites

In this paper, we describe a framework for the processing of niobium-reinforced MoSi 2 composites. As a part of the program, composites containing coated and uncoated niobium reinforcements were produced. Chemical compatibility between the coatings and the matrix was studied and the effect of the interface modification by the coating on the fracture toughness of the composites was investigated via four-point bending tests on chevron-notched samples. The results indicated that the coatings have a significant effect on the debonding at the reinforcement-matrix interface, which in turn can affect the damage tolerance of the composite. Also observation of the crack propagation in the composites suggests that the matrix failed at the early stages of loading. The results are discussed in terms of the mismatch between the elastic constants of the composite constituents.

Processing and mechanical properties of niobium-reinforced MoSi2 composites

A method is taught for modifying the coefficient of thermal expansion of a molybdenum disilicide matrix material, so as to permit preparation of refractory metal fiber reinforced matrices having improved high temperature strength, creep resistance, toughness, and resistance to matrix cracking during thermal cycling.

Ralph J. Hecht

Papers

Appraisal of other silicides as structural materials

The durability and performance of coatings in gas turbine and diesel engines

Development of continuous-fiber-reinforced MoSi2-base composites

Processing and mechanical properties of niobium-reinforced MoSi2 composites

Method for improving refractory metal fiber reinforced molybdenum disilicide composites