Hierarchically ordered coherent interfaces-driven ultrahigh specific-strength and toughness in a nano-martensite titanium alloy

Precipitates in alloys are traditionally considered as dislocation obstacles that often lead to high stress concentration and even microcracks owing to the strain incompatibility of semi-coherent interfaces, a cause of progressive strain localization and the origin of the strength-toughness conflict in engineering materials, such as titanium (Ti) alloys. Based on the metastability engineering, here we architect hierarchically ordered coherent interfaces for strength-toughness optimization through densely dispersed nanomartensites in a ductile Ti-Cr-Zr-Al alloy with ultrahigh specific-strength and superior fracture toughness. It is unveiled that these ordered coherent interfaces simultaneously serve as dislocation obstacles and sources, leading to a sustainable and self-hardening deformation mechanism via hierarchical nanomartensite-dislocation interactions for ultra-high strength and toughness of Ti alloys. These nanomartensites are thermally stable at elevated temperature less than 400 °C, above which tempering-induced ductile-to-brittle transition occurs due to the decomposition of hierarchically ordered nanomartensites and the spheroidization of prior β lamellae. The design strategy of hierarchically ordered coherent interfaces confers our cost-effective nanomartensite Ti alloys an unprecedented combination of strength, ductility and toughness, which provides a new pathway in the microstructural design for strong and ductile structural materials with superior fracture resistance.