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.

/pdf/the-metal-flux-a-preparative-tool-for-the-exploration-of-4gmt0kl7ri.pdf

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

Attractive elevated-temperature properties and low density make the titanium aluminides very interesting for both engine and airframe applications, particularly in the aerospace industry. The challenge to the materials scientist is to maintain these characteristics while building-in “forgiveness”. The basic phase diagram and crystal structure of both the Ti3Al and TiAl phases are reviewed, followed by a consideration of chemistry-processing-microstructure-deformation/fracture-mechanical property relationships in monolithic material. Conventional and innovative synthesis methods are presented, including use of hydrogen as a temporary alloying element. Composite concepts as a method to enhance not only “forgiveness” but also elevated-temperature behaviour are discussed. Environmental effects are evaluated prior to consideration of present and projected applications of both monolithic and composite material. It is concluded that while the titanium aluminides in monolithic form can be used now in non-demanding applications, much further research and development is required before this material class can be used in critical applications, especially in composite concepts.

Synthesis, properties and applications of titanium aluminides

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

Designing eutectic high entropy alloys of CoCrFeNiNbx

Structure and stability of Laves phases. Part I. Critical assessment of factors controlling Laves phase stability

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

While there are nearly 300 high-melting-temperature intermetallic compounds, numerous factors limit the commercial viability of these materials for structural applications. Once the desirability of a material’s crystal structure has been determined, the engineer must then focus on the compound’s oxidation and corrosion resistance, its cost and any associated environmental hazards that may diminish its practical value. The final engineering limitations are imposed by process capabilities, which must be improved if high-temperature intermetallics are to one day emerge from the laboratory.

Selecting high-temperature structural intermetallic compounds: The engineering approach

On the basis of creep strength, ultimate tensile strength and oxidation resistance, seven intermetallic compounds with melting temperatures above 1600°C have been selected as possible candidate materials for high temperature structural applications in advanced aero-turbines. These compounds, Nb3Al, Cr3Si, Co2Nb, Cr2Nb, MoSi2, Mo5Si3 and Nb2Al, have been evaluated and their properties reported herein. All seven of the compounds displayed excellent creep resistance at 1200°C with Mo5Si3 and Nb2Al being the strongest. Nb3Al, with the precipitation of the niobium solid solution displayed the greatest low temperature toughness. The greatest ultimate tensile strengths were observed for Co2Nb and MoSi2, while MoSi2 was by far the most oxidation resistant.

High Temperature Properties of Refractory Intermetallics

With the ultimate objective of exploiting refractory intermetallics for high-temperature structural materials, several binary and ternary two-phase intermetallics/refractory-metal solid solutions were explored. The ductile solid solution is used to toughen the composite microstructure via in-situ phase separation. While the viability of ductile phase separation in solid state was briefly considered for systems such as Nb 3 Al/Nb, much of the work focused on processing eutectic systems such as Cr 2 Nb/Nb and (Nb,Mo) 5 Si 3 /Nb,Mo). This paper describes results obtained via containerless directional solidification of these high-melting eutectic alloys using an optical float-zone furnace. The observations are explained on the basis of solidification theory and parameters unique to the optical float-zone furnace. It is demonstrated that, by this technique, casting-defect- and macrosegregation-free material, with well-aligned microstructure, can be readily produced. Moreover, the potential to approach sub-micron laminate spacing at high growth rate in alloys with very high melting eutectics has also been established. Room-temperature bend test evaluation of directionally solidified material is discussed in light of prevailing theories of ductile phase toughening. The results of a preliminary exploration of the NbMoCrSiAl multicomponent system are presented, showing the prevalence of eutectic phase separation and the potential for improving oxidation resistance.

In-situ refractory intermetallic-based composites

With the advent of ceramic superconductors, while interest in the A15 intermetallics such as Nb 3 Al is declining, their potential for high temperature structural applications appears promising. The A15 compounds, though considered to have the simplest of the topologically closed packed structures, form a distinct class of intermetallics with a cubic structure and are adjacent phases to the refractory metal solid solutions. This report summarizes the evaluation of microstructures, mechanical properties and cyclic oxidation resistance of Nb 3 Al and Cr 3 Si and their ternary alloys, and to a limited extent binary V 3 Si. The mechanical properties of binary compounds, elastic modulus, ultimate strength and creep behavior, are compared with known data for other A15 compounds. Solubility limits, melting point suppression and enhancement in creep resistance of several ternary alloys based on Nb 3 Al and Cr 3 Si are summarized. The mechanical properties and microstructural observations are discussed from the perspective of developing high temperature structural materials based on A15 intemetallics.

D. M. Shah

Papers

Appraisal of other silicides as structural materials

Selecting high-temperature structural intermetallic compounds: The engineering approach

High Temperature Properties of Refractory Intermetallics

In-situ refractory intermetallic-based composites

Evaluation of refractory intermetallics with A15 structure for high temperature structural applications