Interlamellar boundaries govern cracking

Abstract Contrary to previous researches of cracking behaviors in lamellar materials, a novel phenomenon has been found in polysynthetic twinned (PST) TiAl single crystals that the specimens with crack divider (type II) own much higher toughness than those with crack arrester (type I). By combining microscopic characterization and theoretical calculation, we reveal that this contrary is caused by the coherency of interlamellar boundaries. For PST TiAl single crystal with coherent interlamellar boundaries, delamination in type I specimen is suppressed by the coherent boundaries and the crack tends to penetrate lamellae along the deformation twinning planes. While sessile dislocations produced in type II specimen hinder the further motion of other dislocations and induce the passivation at crack tip. As a result, the specimen with type II owns higher fracture toughness than that with type I. A toughness ratio, RT=Ttype I / Ttype II, is proposed to evaluate the cracking behavior with different levels of boundary coherency in lamellar materials. In previous studies, RT > 1 because their boundaries are not coherent, which promotes the delamination and crack tip passivation. The present work sheds light on the relationship of interlamellar boundaries and crack propagation, which provides a new perspective for the development of lamellar materials.