Plastic deformation behavior of NbSi 2/MoSi 2 crystals with oriented lamellae

Abstract

Plastic deformation behavior and dislocation microstructure were examined in two-phase silicides with oriented lamellae composed of the C40 (NbSi 2) and C11 b (MoSi 2) phases. (Nb 0.15Mo 0.85)Si 2 single crystals with the C40 single phase were obtained by a floating zone method, and C11 b plates precipitated from the C40 matrix during the subsequent annealing at 1400 °C satisfying the crystallographic orientation relationship of < 1 ¯ 2 1 ¯ 0 ] C 40 // [ 1 1 ¯ 0 ] C 11 b and (0001) C40//(110) C11b. Temperature, annealing time and orientation dependences of plastic deformation behavior in the C40/C11 b duplex crystals with the strict orientation relationship were examined in compression. Loading axes ( ϕ) were selected to be 0° and 45° from [ 10 1 ¯ 0 ] on [ 1 2 ¯ 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 C11 b phase, <100] and 1/2<111] dislocations which are dominantly operative in binary MoSi 2 at high temperatures were observed depending on the type of the C11 b variant. Strain continuity at C11 b/C11 b or C11 b/C40 boundaries was also discussed on the basis of continuity of Burgers vector of operative dislocations.