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  Microstructure of Duplex-Phase NbSi2 (C40) / MoSi2 (C11b) Crystals Containing a Single Set of Lamellae



A new super-high temperature structural silicide for space plane has been developed. Microstructures of duplex-phase C40/C11b silicides from combinations of NbSi2, VSi2 or TiSi2 with MoSi2 were investigated focusing on formation and alignment of lamellae. The lamellar structure was formed by zone melting and subsequent appropriate heat treatment for all combinations, but oriented lamellae were obtained only in NbSi2/MoSi2 duplex crystals. The oriented lamellar structure satisfied the crystallographic relationship of (0001)C40//(110)C11b, <20] C40// [10] C11b and <100] C40 // [001]C11b, resulting in formation of three variants of the C11b phase and two kinds of boundary: a type of C11b/C11b grain boundary and a type of C11b/C40 phase boundary. Further annealing induced coarsening of both phases and changed the crystallographic relation due to relaxation of misfit strain at boundaries.  

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