Plastic deformation behavior of NbSi2/MoSi2 crystals with oriented lamellae

Plastic deformation behavior and dislocation microstructure were examined in two-phase silicides with oriented lamellae composed of the C40 (NbSi2) and C11b (MoSi2) phases. (Nb0.15Mo0.85)Si2 single crystals with the C40 single phase were obtained by a floating zone method, and C11b plates precipitated from the C40 matrix during the subsequent annealing at 1400 °C satisfying the crystallographic orientation relationship of <1¯21¯0]C40//[11¯0]C11b and (0001)C40//(110)C11b. Temperature, annealing time and orientation dependences of plastic deformation behavior in the C40/C11b duplex crystals with the strict orientation relationship were examined in compression. Loading axes (ϕ  ) were selected to be 0° and 45° from [101¯0] on [12¯10] zone in the C40 structure. Yield stress or fracture stress, for example, depended strongly on the angle between the loading axis and lamellar boundaries. The macro-yielding appeared at 1400 °C accompanied by the low yield stress around 300 MPa and an adequate plastic strain at ϕ = 45°, while the crystals deformed at ϕ = 0° failed just after high fracture stress around 900 MPa showing no plastic flow. This suggests that the deformation proceeds more easily parallel to the lamellar boundaries. In the constituent C11b phase, <100] and 1/2<111] dislocations which are dominantly operative in binary MoSi2 at high temperatures were observed depending on the type of the C11b variant. Strain continuity at C11b/C11b or C11b/C40 boundaries was also discussed on the basis of continuity of Burgers vector of operative dislocations.