The effect of pre-annealing upon the yield stress peak of the Al-rich γ-phase was demonstrated by arranging the available experimental data according to the associated thermal histories. It was found that the yield stress of the alloys was affected by phase transformations at 2 levels. An effect due to interstitial atoms was able to shift the yield-stress peak upwards by 200 to 300C. Therefore, the upper part of the yield stress anomaly (beyond about 600C) was likely to involve a marked extrinsic contribution. It was noted that pinned ordinary screw dislocations were a leading feature of the deformation microstructures of Ti-rich lamellar alloys. The present analysis indicated that it might not be wise to rationalize the mechanical behaviour of the Al-rich γ-phase only in terms of the thermally activated intrinsic locking of dislocations in a single-phase L10 lattice. It was suggested that the elucidation of the yield stress anomaly in the γ-phase was more complicated than was expected on the basis of a direct analogy with other ordered alloys (such as Ni3Al) which were not subject to phase transformation.

Extrinsic Factors Influencing the Yield Stress Anomaly of Al-Rich γ-TiAl. F.Gregori, P.Penhoud, P.Veyssière: Philosophical Magazine A, 2001, 81[3], 529-42