Anisotropy in the room-temperature deformation of α-β colonies in titanium alloys:: role of the α-β interface

The anisotropy in room-temperature critical resolved shear stress (CRSS) has been investigated in single alpha(hcp)-beta(bcc) colonies of a commercial alpha-beta titanium alloy (Ti-6Al-2Sn-4Zr-2Mo-0.1Si ( where the composition is in weight per cent)) oriented for activation of individual basal slip systems. Detailed transmission electron microscopy (TEM) studies of the slip transmission mechanisms through the alpha-beta interfaces have been performed to elucidate the role of these interfaces in determining yield and strain-hardening behaviour. Significant anisotropy in the room-temperature CRSS for the three unique basal slip systems is measured and is attributed to the slip transmission mechanisms active owing to the observed near-Burgers orientation relationship. TEM results indicate that the alpha-beta interface provides little hindrance to slip for the slip system with the smallest misorientation between dislocations in the two phases. For the second slip system, an 11degrees misorientation exists between the primary dislocations, as well as an increased propensity for cross-slip within the beta phase, resulting in a much higher CRSS and increased rate of strain hardening. Extended dislocation pile-ups are observed within the beta laths at the 'exit' alpha-beta interface for this case, together with the formation of residual matrix dislocations. For the third orientation, for which none of the (a/2) (111) dislocations in the b phase are closely aligned for easy slip transmission with the active (a/3)[(11) over bar 20] dislocations in the a phase, large dislocation pile- ups are observed within the a phase at the entrance alpha-beta interfaces. Directly ahead of these pile-ups, direct transmission to b=a[010] dislocations within the beta phase occurs. The b=a[010] dislocations readily decompose into mobile (a/2) (111) dislocations which form complex three-dimensional networks within the beta phase. This process results in an intermediate CRSS and the highest rate of strain hardening. The relative properties for the three colony crystal orientations are discussed in the light of these observations.