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Stretch-flangeability evaluated using hole-expansion testing represents the ability of sheet materials to resist edge fracture during complex shape forming. Despite a property imperative for automotive part applications of advanced high-strength steels, factors governing stretch-flangeability are not yet well understood. In this study, the mechanical properties of a selected group of materials with different microstructures were investigated using tensile, fracture toughness, and hole-expansion tests to find the factor governing the stretch-flangeability that is universally applicable to a variety of metallic materials. It was found that the fracture toughness of materials, measured using the fracture initiation energy, is a universal factor governing stretch-flangeability. We verified that fracture toughness is the key factor governing stretch-flangeability, showing that the hole-expansion ratio could be well predicted using finite element analysis associated with a simple ductile damage model, without explicitly taking into account the microstructural complexity of each specimen. This validates the use of the fracture toughness as a key factor of stretch-flangeability.

A martensitic NiTi alloy was separately processed by equal-channel angular pressing, repetitive corrugation and straightening by rolling and cold rolling. Measurements by dilatometry revealed a significant increase in the austenitic phase transformation temperature and martensitic stabilization after severe plastic deformation. The release of an excess volume upon recrystallization of the deformed NiTi alloy was measured using a high-precision difference-dilatometer employing non-isothermal and isothermal procedures. The kinetics of the recrystallization process was analyzed according to the Johnson–Mehl–Avrami–Kolmogorov theory and the average effective Avrami exponent was estimated as n ≈ 1. It is concluded that the recrystallization behavior is essentially similar for all three processing modes.