The thermal stability of and nature of damage in Ti alloys subject to high-strain-rate deformation

Samples of Ti-6wt% Al-4wt% V and Timet 550 (Ti-4wt% Al-4wt% Mo-2wt% Sn-0.5wt% Siwt%), which have been deformed at a strain rate of 5-s^-1, were annealed after thinning so that the visibility of dislocations in transmission electron microscopy could be compared before and after annealing. It has been found that imaging with g=0002 produces clear images of dislocations before and after annealing, but that imaging with other diffracting vectors gives reasonable dislocation images only after annealing to at least 700° C. The sharpness of Kikuchi lines in diffraction patterns obtained from fully annealed samples, deformed at 10^-1 and 5 s^-1, has been examined. The lines are sharp for all planes in the fully annealed samples, but become more diffuse in samples deformed at 10^-1s^-1. However, in samples deformed at 5 s^-1 the Kikuchi lines from (0002) planes are sharp but the lines from all other planes are diffuse. These observations are interpreted in terms of the presence of a high density of defects, which do not distort the elastically strong (0002) planes as significantly as they distort all other planes. These observations are discussed with respect to the recent claim that a particular Ti alloy deforms by a mechanism that does not involve dislocations.