Long-range order relaxation and phase transformation in γ-TiAl alloys

Residual electrical resistivity measurements were used for characterizing gamma-TiAl intermetallic compounds and for investigating the kinetics of long-range order (LRO) relaxation. In annealed and slowly cooled materials, a linear relationship between residual resistivity and aluminium content was observed in the composition range from 50 to 56.1 at.% Al. The isothermal relaxation of LRO after small temperature changes was investigated in gamma-TiAl compounds (with 49.8, 50.7, 52.6 and 54.1 at.% Al). The approach to equilibrium could be satisfactorily represented by a sum of two exponential relaxation processes. The slower process was assigned to LRO relaxation. From the investigation of the temperature dependence of relaxation times, an activation enthalpy of 2.97 +/- 0.15 eV was determined for the stoichiometric alloy. This value is close to the activation enthalpy for titanium self-diffusion in TiAl. For Al contents higher than 50.7 at.%, the LRO changes were found to be superimposed on a supplementary phenomenon, which produces a continuous resistivity decrease. A TEM investigation showed that this drift was associated with the precipitation of Ti3Al5, which is a superstructure of the L1(0) structure. (C) 1999 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.