MECHANICAL-PROPERTIES AND DISLOCATION-STRUCTURES OF TIAL SINGLE-CRYSTALS DEFORMED AT 4.2-293K

Mechanical properties and dislocation structures of Ti-56 at.% Al single crystals have been studied in specimens with [010], [001], [110], A; [132], B; [245] and C; [452] orientations at 4.2, 77, and 293 K in compression. The yield stress and strain rate sensitivity parameter, S, increased rapidly with decreasing temperature. The steep increase of the yield stress with decreasing temperature would be caused by dislocation reactions and dipoles as well as Peierls stress. Slip markings on deformed specimens were straight and rather coarse and slip planes were {111} at all test temperatures. Superdislocations was curved at temperatures below 196 K and seemed not to lie along a given direction. The 1 2 < 112] and < 101] type superdislocations have been observed at all test temperatures. Some of the <101] type superdislocations dissociated into superpartials with the Burgers vector 1 2 < 101] bound by an antiphase boundary (APB) at 77 K. The 1 6 < 112] type Shockley partials bound by a stacking fault (SF) were also observed in the [001] orientation at 4.2 K and the B orientation at 77 K. However, 1 2 < 110] type ordinary dislocations have not been observed below 196 K, which can react very easily with the same and other types of dislocations and change into stair rod dislocations resulting in obstacles to the subsequent dislocations. Therefore, it would be difficult to observe free and reacted portions of the ordinary dislocations because of a small number of the dislocations comparing to the other type dislocations. The serrated flow occurred at 4.2 K due to adiabatic deformation. A new type strengthening mechanism due to dislocation dipoles was proposed. © 1990.