The Nb-Hf-Si ternary phase diagram: Liquid-solid phase equilibria in Nb- and Hf-rich alloys

Liquid-solid phase equilibria in ternary Nb-Hf-Si alloys have been investig ated using scanning electron microscopy, X-ray diffraction, and electron be am microprobe analysis. A range of alloys was directionally solidified usin g cold crucible Czochralski crystal growth and six phases were identified: Nb(Hf)(5)Si-3, Hf(Nb)(5)Si-3, Nb(Hf)(3)Si, Hf(Nb)(2)Si, beta(Nb,Hf,Si), and alpha(Nb,Hf,Si). A range of compositions was investigated for Si concentra tions up to 35.0 %. Microstructural and microchemical evidences provided a clear definition of the Nb-Hf-Si liquidus surface and indicate that the met al-rich end of the ternary phase diagram possesses three transition reactio ns: L + Nb(Hf)(5)Si-3 --> Nb(Kf)(3)Si + P(Nb, Hf, Si) L + Hf(Nb)(5)Si-3 --> Hf(Nb)(2)Si + Nb(Hf)(5)Si-3 L + Nb(Hf)(5)Si-3 --> Hf(Nb)(2)Si + beta(Nb, Hf, Si) The first of these transition reactions occurs at a composition of approxim ately Nb-21Hf-16Si, and at a temperature of similar to 1850 degrees C. The second of these transition reactions oc curs at a composition of approximat ely Nb-27Hf-22Si, and a temperature of similar to 2040 degrees C. The third transition reaction occurs at a composition of approximately Nb-26Hf-14Si, and a temperature of similar to 1840 degrees C.