Dot-dash grain boundary α: A new strategy for improving impact toughness of dual-phase Ti-652 alloy

Conventionally, continuous grain boundary α (GBα) was regarded as a detrimental microstructure for the mechanical properties of dual-phase titanium alloys, therefore, attention has been paid to break off or even eliminate GBα. In this work, Widmannstätten microstructure (LA) with continuous GBα, duplex microstructure (DP) without GBα and hierarchical microstructure (HR) with dot-dash grain boundary α (α_GB-d) were prepared by a fine tuning of the heat treatments. Impressively, HR with α_GB-d possesses a highest impact toughness, which is 56.3 % and 11.1 % higher than those of LA and DP, respectively. To elucidate the fracture mechanisms of α_GB-d, energy controlled Charpy impact tests were carried out by adjusting the impact angle of hammer head, which provides direct evidence of the microstructure evolution during impact deformation. It was found that α_GB-d played two main roles during impact deformation: as a weak micro-zone, α_GB-d is prone to accumulate deformations and becomes the preferred site for crack initiation, in this case, α_GB-d acts as structural element to absorb impact energy; on the other hand, the misorientation between α_GB-d and adjacent α_p hinders dislocation slip transfer and shortens crack propagation path. Thus, comparing with DP and LA, cracks propagating in HR absorb more energy and lead to less catastrophic fracture. This work develops the understanding of grain boundary α, and the findings are expected to provide new design guidance for the mechanical property improvement of titanium alloys.