EFFECT OF Si ON MICROSTRUCTURE AND FRACTURE TOUGHNESS OF DIRECTIONALLY SOLIDIFIED Nb SILICIDE ALLOYS

Nb-xSi(x=3,9,16)-22Ti-3Cr-3Al-2Hf (at.%) have been successfully prepared by directional solidification in an optical floating zone furnace. Microstructure analysis and phases identification of the alloys were examined by X-ray diffraction (XRD), Electro Probe Micro Analyzer (EPMA) and Energy Disperse Spectroscopy (EDS). Fracture toughness specimens without pre-crack were prepared, room temperature fracture toughness of alloys was tested by three-point bending method, and fracture mechanism was studied. The results showed that with increasing Si content, Nb5Si3 phase gradually increased and the phase transformed from γ-Nb5Si3 to the stable α-Nb5Si3 phase and β-Nb5Si3 phase. There appeared the Ti-rich Nb5Si3 phase when the Si content is 16 at%. In addition, more micro-cracks generated in the Ti-rich Nb5Si3 phase, which seriously affected room temperature fracture toughness of the alloys.