PHASE-EQUILIBRIA IN THE AL-NB-TI SYSTEM AT HIGH-TEMPERATURES

Phase equilibria in the Al-Nb-Ti system were experimentally determined . From the investigation of diffusion couples and alloys of fixed comp osition by optical microscopy, electron probe microanalysis (EPMA), tr ansmission electron microscopy (TEM) and X-ray diffraction (XRD) two i sothermal sections at 1000 degrees and 1200 degrees C were established . At 1200 degrees C earlier investigations postulated the existence of several intermetallic phases. From the present study the existence of the phases gamma(1) (NbTiAl3, tetragonal), T-1 (''Ti45Al35Nb20'', cub ic B2) and T-2 (''Ti45Al45Nb10'', cubic B2) at this temperature can be ruled out and the stability of the phase alpha (''Ti4Al3Nb'', hexago nal) is at least questionable. At 1000 degrees C a ternary single-phas e field of isolated B2-ordered beta-Nb,Ti is observed which stems from a quasi eutectoid reaction below 1200 degrees C. While there has been only scarce information of the phase equilibria at 1000 degrees C the results presented at 1200 degrees C are discussed in respect to exist ing isothermal sections which have been determined recently at this te mperature. (C) 1997 Elsevier Science Limited.