Effect of Alloying Elements on Nb-Rich Portion of Nb-Si-X Ternary Systems and In Situ Crack Observation of Nb-Si-Based Alloys

To find a new route for microstructure control and to find additive elements beneficial for improving high-temperature strength, a systematic investigation is performed on hypoeutectic Nb-15 at. pct Si-X ternary alloys containing a transition element, Fe, Co, Ni, Cu, Ru, Rh, Pd, Re, Os, Ir, Pt, or Au. Information on phase equilibrium is classified in terms of phase stability of silicide phases, alpha Nb5Si3, Nb4SiX, and Nb3Si, and the relationship between microstructure and mechanical properties both at room temperature and high temperature is investigated. All the additive elements are found to stabilize either alpha Nb5Si3 or Nb4SiX but destabilize Nb3Si. A microstructure of Nb-ss/alpha Nb5Si3 alloy composed of spheroidized alpha Nb5Si3 phase embedded in the Nb-ss matrix is effective for toughening, regardless of the initial as-cast microstructure. Also the plastic deformation of Nb-ss dendrites may effectively suppress the propagation of longer cracks. High-temperature strength of alloys is governed by the deformation of Nb-ss phase and increases with higher melting point additives.