Microstructure and plastic deformation behavior in duplex-phase silicides composed of the C40 and C11b phases were examined using pseudo-binary (Nb,Mo)Si2 crystals with a single set of lamellae. Single crystals of the C40 single-phase were grown from the master ingot with a composition of (Nb0.15Mo0.85)Si2 by a floating zone method and duplex-phase microstructure containing a single set of lamellae was obtained by subsequent heat treatment at 1400°C for no less than 6h. During the heat treatment, the C11b phase was precipitated from the C40 matrix by satisfying the crystallographic relationship of (0001)C40//(110)C11b, <1210]C40//C11b and < C40//C11b at the lamellar boundary, while randomly oriented C11b grains also appeared at further annealing. As a result, the duplex-phase silicides with a single set of lamellae contained the C40 phase with a single orientation and three variants of the C11b phase. The lamellar spacing and the volume fraction of their phases depended strongly on annealing time.
In compression tests, yield stress and fracture strain of the duplex-phase silicides depended strongly on angle (φ) between the loading axis and lamellar boundaries, similar to TiAl-PST crystals. At φ=0°, specimens fractured just after showing high fracture stress even at 1400°C. In contrast, at φ=45° where shear deformation in the C11b phase of lamellae occurred parallel to lamellar boundaries, low yield stress and significant plastic strain were achieved at 1400°C.