Cyclic deformation behaviour and dislocation structure of Ti-5 at.% Al single crystals oriented for double prism slip

Abstract

Cyclic deformation behaviour of Ti-5at.%Al single crystals oriented for [21 10] was investigated in the total (i.e. elastic + plastic) axial strain amplitude from 0.195% to 0.798% at room temperature in air. The crystals displayed an initial cyclic hardening followed by a striking softening period, and then a saturation stage was reached at different cyclic strain amplitudes, except at the lowest of 0.195% at which the initial hardening stage was absent. The cyclic stress-strain curve in Ti-5 at.% Al single crystal with double prism slip contains a clear plateau with a saturation shear stress of 86–89 MPa over the complete range of applied strain amplitudes. (1010) and (1100) double-prism-slip systems can be distinguished from the traces on the (0001) surface by trace analysis with an optical microscope. Slip marking is planar and becomes denser as the cyclic strain amplitude increases. Two groups of planar dislocation lines lying along the (1010) and (1100) prism plane traces were determined from the (0001) reflection by transmission electron microscopy. The typical dislocation configuration is planar dislocation bands parallel to the [0001] direction in a foil with a (1010) normal; however, long straight dislocations are formed in a (1100) foil. Both the width and the spacing of the slip bands decrease with increase in the cyclic strain amplitude in the (1010) primary slip plane. Finally, the relationship between cyclic plateau behaviour and the saturation dislocation structure of Ti-5 at.% Al single crystals is discussed.