Appraisal of other silicides as structural materials

Abstract

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.