Abstract: Quenching and partitioning (Q&P) steels represent a key advancement in third-generation advanced high-strength steels (AHSS), offering an exceptional balance between strength and ductility due to their complex multiphase microstructures. However, their application in crash-critical and formability-sensitive components remains limited by an incomplete understanding of their fracture behavior under complex stress states. Therefore, this study aims to systematically investigate the failure mechanisms of Q&P 1000 steel across a wide range of stress states from shear to uniaxial and plane-strain tension using tensile specimens with different geometries. By combining macroscopic mechanical testing with detailed microstructural characterization via scanning electron microscopy (SEM) and electron backscatter diffraction (EBSD), we reveal a transition from ductile fracture, governed by void nucleation and coalescence, to cleavage-dominated fracture under increasing triaxiality. Remarkably, transgranular cleavage fracture features were observed for the first time in Q&P steels even after substantial plastic deformation, which confirms it to be also a failure mechanism of ductile fracture in addition to brittle fracture. Two major damage mechanisms responsible for the failure mode transition were revealed: (i) phase boundary debonding and (ii) martensite cleavage fracture. A stress-based cleavage fracture criterion with a critical stress triaxiality, regulated by the cleavage fracture stress and strain hardening behavior, can well explain and quantify this transition behavior. These results provide new insights into stress-state-dependent failure in Q&P steels and offer guidance for their safe and optimized application in forming and crash-relevant components.

Key words: Fracture mechanisms, Multiphase steel, Stress triaxiality, Cleavage fracture, Phase segmentation, Q&P 1000 steel;