PHASE-EQUILIBRIA AND TRANSFORMATIONS IN A TI-ZR-SI SYSTEM

The phase equilibria in the titanium-zirconium rich region of the tern ary Ti-Zr-Si system have been studied in the temperature range from 14 73 to 1323 K, together with microstructures present in the as-cast sta te. DTA, microscopy, X-ray diffraction (XRD), and chemical analysis by X-ray energy-dispersive spectroscopy have been employed. Whereas Ti5S i3 nucleates heterogeneously and grows rapidly in titanium matrices, T i3Si exhibits very slow precipitation kinetics. It also is displaced b y (Ti, Zr)(5)Si-3 and a ternary silicide in ternary and more complex s ystems, explaining its absence in commercial alloys. Zirconium exhibit s substantial solid solubility in Ti5Si3 in ternary alloys, while Ti e xhibits only limited solubility in Zr2Si, Zr3Si, and Zr5Si3. Additiona lly, a new silicide of general formula (Ti, Zr)(2)Si is formed in tern ary alloys with a Ti:Zr ratio ranging from 2.26 to 0.68. Isothermal se ctions for the Ti-Zr-Si system are presented for temperatures of 1473 and 1323 K.