A study of anomalous hardening and deformation by superdislocations in single crystal titanium-56aluminum alloy

Uniaxial compression deformation has been conducted with single crystal gamma-Ti-56Al from 200K to 1073K in three orientations, [1¯ 6 12], [1¯ 2 6] and [2¯ 4 7]. The yield stress and critical resolved shear stress depend on temperature as well as deformation orientation under single slip of <101] superdislocations. A moderate dependence of the stresses on temperature and orientation was observed in the temperature range of RT to 673K while a strong dependence was found to occur in the temperature of 673K to 1073K. The results show that critical resolved shear stress of the single crystal deformed in these orientations is correlated with the Schmid factor for the <101] {lcub}111{rcub} slip systems at intermediate temperatures, and for cube-cross-slip at high temperatures. The Cottrell-Stokes type compression tests performed at 573K/873K show that the flow stress ratio, tau873/tau573, was found to be 1.5 for ascending sequence and 1.0 for descending sequence deformation, respectively. The difference between the two sequences is because the cubecross-slip structure of superdislocations resulted from deformation at high temperatures remains operating at intermediate temperatures. It suggested that the Cottrell-Stokes type compression in single crystal Ti-56Al was not fully reversible. High-resolution electron microscopy and weak beam imaging technique have been applied to the deformed single crystals for investigating of the self-dissociated superdislocation cores. The results revealed that the cores of dissociated superdislocations were partially cross-slipped onto a non-primary octahedral plane at intermediate temperatures forming a "rooftop configuration". The anomalous hardening behavior at intermediate temperatures can be explained by the proposed Interaction Energy Model. The cross-slip mode of the cores is characterized by the cube plane cross-slip at high temperatures. It is believed that Kear-Wilsdorf lock can be utilized to interpret the anomalous hardening at this temperature regime in Ti-56Al single crystal. The energy calculations suggest that "rooftop configuration" and Kear-Wilsdorf lock yield a minimum value of interaction energy between two coupling superpartials in an anisotropic medium. A sudden disappearance of anomalous hardening above their respective peak temperatures in three corresponding orientations was accompanied by the superseding of superdislocations with ordinary dislocations. It may be explained by the fact that the yield stress of ordinary dislocation is low enough to offset the Schmid factor advantage of superdislocation slip in {lcub}111{rcub} or {lcub}010) planes.