Phase equilibria and quenched phases in the Ti–Al–Mo systemThursday (29.09.2016) 09:30 - 09:45 Part of:
Materials based on the ternary Al-Mo-Ti system are promising candidates for high-temperature applications. In order to overcome their brittleness at ambient temperature, the positive effect of lamellar structure on the mechanical properties can be utilized. This has already been shown for binary Mo3Al + Mo3Al8 two-phase alloys that are formed by the eutectoid decomposition of the AlMo phase . Due to the difficulties caused by the retention of the AlMo phase even after water quenching, contradictory information concerning the crystal structure of this high-temperature phase and the formed phase equilibria can be found in the literature. In the present work, the phase equilibria in the binary Al–Mo and the ternary Ti–Al–Mo system were studied experimentally at 1673 and 1773 K. The binary AlMo phase was successfully quenched. With this it has been demonstrated that the beta continuous solid solution and the binary high-temperature AlMo phase are one and the same phase, which is in accordance with the experimental work of Nino et al.  and the latest thermodynamic description for the Ti–Al–Mo system performed by Cupid et al. . Moreover, the structural investigations using XRD confirm the disordered nature of the AlMo phase (bcc/A2) which was already proposed by Rexer  in an early work. In addition, TEM/SAED micrographs revealed diffuse scattering indicative for a trend to form the omega phase in quenched A2-type AlMo, whereas superstructure reflections indicative for occupational ordering in the A2 structure were not detected. Thermal analysis measurements in combination with XRD and TEM investigations were performed to study the decomposition of the metastable quenched beta phase. The results show a two-step decomposition process from beta over an intermediate state to the equilibrium phase assemblage with an eutectoid microstructure. Based on the obtained results, partial isothermal sections at 1673 and 1773 K and an isoplethal section at 50 at.% Al were constructed. The phase equilibrium studies at 1673 K also showed the existence of a new phase at compositions around 62Al-24Mo-14Ti. Therefore, the two- and three-phase equilibria involving this new phase were studied in the ternary Ti–Al–Mo system using complementary experimental methods such as SEM/BSE, SEM/EBSD, XRD and EPMA.
 R. Nino, S. Miura and T. Mohri, Intermetallics 9(2) (2001) 113
 J. Rexer, Int. J. Mater. Res. 62 (1971) 844
 R. Nino et al., Intermetallics 11(6) (2003) 611
 D.M. Cupid et al., Intermetallics 18(6) (2010) 1185