Effect of tin addition on Nb-Si-based in situ composites. Part I: Structural modifications
Abstract
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 degrees 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)/gamma-M5Si3 composite evolves towards a quaternary phase equilibrium: (Nb,Ti)(SS) saturated with Sn/alpha-M5Si3/(Nb1-xTix)(3)(Sn1-yTiy)/(y)'-M5Si3. The gamma'-M5Si3 differs from the gamma-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.