Solid-state phase equilibria in metal-rich Hf-Ti-Si system at 1350 °C

Knowledge on the phase equilibria at the metal-rich region of Hf-Ti-Si was needed for the development of Nb-silicide based composites. A total of seven Hf-Ti-Si alloys with Si concentrations up to 37.5 at.% were fabricated by directional solidification and annealed at 1350 °C. The microstructure of the annealed alloys was examined using scanning electron microscopy, electron probe microanalysis, and X-ray diffraction analysis. The α(Hf,Ti,Si), β(Hf,Ti,Si), (Hf,Ti)5Si3, and Hf(Ti)2Si phases were identified in the metal-rich region of the Hf-Ti-Si system. The phase relationships between these phases are two three-phase equilibria: β(Hf,Ti,Si) + (Hf,Ti)5Si3 + Hf(Ti)2Si, and β(Hf,Ti,Si) + α(Hf,Ti,Si) + Hf(Ti)2Si, and three two-phase equilibria β(Hf,Ti,Si) + (Hf,Ti)5Si3, β(Hf,Ti,Si) + Hf(Ti)2Si, and α(Hf,Ti,Si) + Hf(Ti)2Si. The solubility of Ti in Hf(Ti)2Si was measured as 9.8 at.%. The Si concentration in β(Hf,Ti,Si) ranges from 0.8 to 3.3 at.%, while that in α(Hf,Ti,Si) is about 2.6-2.7 at.%.