THE NB-TI-SI TERNARY PHASE-DIAGRAM - EVALUATION OF LIQUID-SOLID PHASE-EQUILIBRIA IN NB-RICH AND TI-RICH ALLOYS

Phase equilibria in ternary Nb-Ti-Si alloys have been investigated usi ng scanning electron microscopy, XRD, and electron beam microprobe ana lysis. Alloys containing up to four phases, Nb(Ti)(5)Si-3, Ti(Nb)(5)Si -3, (Nb,Ti)(3)Si, and (Nb,Ti,Si) mere directionally solidified using c old crucible Czochralski crystal growth, A broad range df Nb and Ti co mpositions were investigated fur Si concentrations up to 35.0 %. Micro structural and microchemical evidence provided a clear definition of t he Nb-Ti-Si liquidus surface and indicate that the metal-rich end of t he ternary phase diagram possesses two transition reactions: L + Nb(Ti )(5)Si-3 --> (Nb,Ti)(3)Si + Ti(Nb)(5)Si-3 L + (Nb,Ti)(3)Si --> (Nb,Ti, Si) + Ti(Nb)(5)Si-3 The first of these transition reactions occurs at a composition of approximately Nb-66Ti-19Si, and probably at a tempera ture between 1600 and 1650 degrees C, The second of these transition r eactions occurs at a composition of approximately Nb-76Ti-13.5Si and a temperature of approximately 1350 degrees C. No ternary eutectic was observed in the compositions that were investigated.