Effect of tin addition on Nb–Si-based in situ composites. Part I: Structural modifications

This paper reports the effect of adding 2 to 8 at.%Sn on the microstructure of Nb–25Ti–8Hf–2Cr–2Al–16Si. The samples were synthesised by casting and were heat treated for 5 days at 1200 °C. The as-cast and heat-treated microstructures were characterised using X-ray diffraction, scanning electron microscopy and microprobe analysis. The results revealed strong microstructural changes when Sn content exceeded 2 at.% Sn. The two-phase (Nb,Ti)SS/γ-M5Si3 composite evolves towards a quaternary phase equilibrium: (Nb,Ti)SS saturated with Sn/α-M5Si3/(Nb1−xTix)3(Sn1−yTiy)/γ′-M5Si3. The γ′-M5Si3 differs from the γ-M5Si3 through its Hf and Ti contents. The (Nb,Ti)SS fraction decreases strongly, benefitting the (Nb1−xTix)3(Sn1−yTiy) fraction. Therefore, adding greater than 2 at.% Sn addition may dramatically affect the mechanical properties of Nb/Nb5Si3 composites because the solid solution (Nb,Ti)SS has been shown to dominate the toughness of such materials.