Pseudo-twin boundary improves flow stress and cyclic stability of TiAl single crystal

Polysynthetically twinned (PST) TiAl single crystal with lamellar structures exhibits great mechanical properties at room temperature. Therein twin boundaries (TBs) are important for achieving optimized ductile and fatigue performance of PST TiAl, but their role and the associated mechanism are elusive. Herein, we decipher the role of true TB (TTB) and pseudo TB (PTB) by a combined atomistic simulation and mesoscopic modeling, and find that PTB could remarkably improve room-temperature flow stress and cyclic stability of TiAl single crystal. It is revealed that dislocations pile up at PTB while unobstructedly traverse TTB. The emergency of back stress and the movement of dislocations along PTB contribute to the strengthening mechanism. The flow stress of TiAl single crystal with PTB is 34% higher than that with TTB. It is further found that as the twin thickness decreases, the flow stress of TiAl single crystal with TTB initially increases and then decreases (i.e., inverse Hall–Petch like behavior), whereas that with PTB always increases owing to the extra back stress and interfacial stress (i.e., Hall–Petch like behavior). Atomistic-informed mesoscopic theoretical models are then proposed to describe the flow stress as a function of twin thickness. Under cyclic loading, PTB is found to facilitate strain delocalization of TiAl single crystal during plastic deformation and thus noticeably improve the cyclic stability. These findings should shed light on achieving strong TiAl alloys with enhanced fatigue performance by the introduction and design of PTB.