Microstructures and mechanical properties of directionally solidified multi-element Nb-Si alloy

Nb-Si system in-situ composites with nominal composition of Nb-21Ti-16Si-7Cr-3Al-2Hf (mole fraction, %) were prepared by directional solidification technology in an optical floating zone furnace at the growth rates of 3, 5, 8 and 12 mm/h. The microstructures and mechanical properties were investigated as a function of the growth rate. The results show that the solidification microstructures consist of Nb solid solution (Nbss), α-Nb5Si3 and β-Nb5Si3. Silicides with high aspect ratio grow continuously and straightly when the growth rate is 5mm/h; lower or higher growth rates are disadvantageous for the formation of favorable directional microstructures. The compressive strength at 1 250 °C and room-temperature fracture toughness of the specimen with the growth rate of 5 mm/h that exhibits excellent mechanical properties are 415 MPa and 22.4 MPa·m1/2, respectively.