Designing high ductility TiAl alloys based on dislocation nucleation mechanism

The yield strength and ductility of nanoscale biphasic materials are governed by the critical resolved shear stress (CRSS) for dislocation nucleation. Polysynthetic twinned (PST) TiAl single crystals with alternating layers of γ-TiAl and α₂-Ti₃Al exhibit high yield strength and ductility at room temperature. However, the relationship between its high performance and dislocation nucleation mechanism has not been clearly understood. In this work, we investigated the influence of the interfacial dislocations and the normal stress of slip plane on dislocation nucleation in the γ-TiAl/α₂-Ti₃Al alloys via biaxial loading using molecular dynamics simulations. Three types of dislocations were observed in the initial yielding stage, including $\left\{111\right\}<11\bar{2}\text{]}$ twin dislocation and $\left\{111\right\}<\bar{1}01\text{]}$ superlattice dislocation for γ-TiAl, $\left\{1\bar{1}00\right\}<11\bar{2}0>$ prismatic dislocation for α₂-Ti₃Al. The analysis for the yield conditions revealed that there is an approximate linear relation between the resolved shear stress and resolved normal stress for the three types of slip systems, which is consistent with the results of first principles. We proposed a design strategy for high ductility TiAl alloys based on this relationship, which involves introducing stress differences between the CRSS of two phases through pre-stressing. To verify the effectiveness of this strategy in practical applications, we applied pre-compression to the PST TiAl single crystal to introduce the difference in strength between the two phases, which led to crack uniformly occurring in γ phase but limiting in α₂ phase, finally increasing the elongation of PST TiAl single crystal by about 300 %.