Single crystals of Ti-Al alloys containing 1.4, 2.9, 5 or 6.6wt%Al were oriented for <a> slip on either basal or prism planes or were loaded parallel to the c-axis to impose a non-basal deformation mode. Most of the tests were conducted in compression between 77 and 1000K. Trace analysis of pre-polished surfaces enabled identification of the twin or slip systems primarily responsible for deformation. Increasing the deformation temperature, Al content, or both, acted to inhibit secondary twin and slip systems, thereby increasing the tendency toward strain accommodation by a single slip system having the highest resolved stress. In the crystals oriented for basal slip, transitions from twinning to multiple slip and, finally, to basal slip occurred with increasing temperature in the lower-Al-content alloys, whereas for Ti-6.6%Al, only basal slip was observed at all temperatures tested. A comparison of the critically resolved shear stress values for basal and prism slip as a function of Al content showed that prism slip was favored at room temperature in pure Ti, but the stress required to activate these 2 systems became essentially equal in the Ti-6.6%Al crystals over a wide range of temperatures. Compression tests on crystals oriented so that the load was applied parallel to the c-axis exhibited extensive twinning in lower Al concentrations and <c+a> slip at higher Al concentrations, with a mixture of <c+a> slip and twinning at intermediate compositions.A few tests also were conducted in tension, with the load applied parallel to the c-axis. In these cases, twinning was observed, and the resolved shear for plastic deformation by twinning was much lower that that for <c+a> slip observed in compression loading.

Deformation Behavior of HCP Ti-Al Alloy Single Crystals. J.C.Williams, R.G.Baggerly, N.E.Paton: Metallurgical and Materials Transactions A, 2002, 33[3A], 837-50