The effects of the microstructure upon the tensile properties and deformation behavior of γ-phase Ti-48Al were studied by means of transmission electron microscopy. The microstructure was found to have a marked effect upon the tensile properties; with the grain size and lamellar volume fraction exerting related roles. Acoustic emission studies during tensile testing revealed spikes whose amplitude and frequency increased with increasing volume fraction of lamellar grains. The analysis of failed samples indicated that micro-cracking was the main cause of the spikes; with twinning making a minorcontribution in near-lamellar materials. The most important factor which controlled the ductility was the grain size. The ductility, yield stress and work-hardening rate went through maximum values at volume fractions (of the lamellar constituent) of between 0.50 and 0.60. A high work-hardening rate which was associated with the low mobility of the dislocations was deduced to be the probable cause of the low ductility. In near-γ and duplex structures, slip via motion of the ½<110] unit dislocations, and twinning, were the main deformation modes at room temperature. Twinning was more common in near- and fully-lamellar structures. The occurrence of twinning was largely dictated by the Schmid factor. The ½<110] dislocations were common even for grain orientations where the Schmid factor was higher for <101] super-dislocations. The latter were observed in favorably oriented grains. The activity of both systems led to a higher ductility at room temperature, as compared to Al-rich single-phase γ-alloys. A higher twin density was observed in lamellar grains, but the propagation depended upon the orientation and geometry of the individual γ lamellae. An increase in ductility at high temperatures was related to an increased activity of ½<110] dislocations (including climb motion), and twin thickening.

Microstructural Effects on the Tensile Properties and Deformation Behavior of a Ti-48Al Gamma Titanium Aluminide. G.B.Viswanathan, M.J.Mills, V.K.Vasudevan: Metallurgical and Materials Transactions A, 2003, 34[10], 2113-27