4D Analysis of Damage Mechanisms in Ductile Fracture of Quenched and Tempered DP Steels Using Synchrotron X-ray Microtomography

This study investigates the ductile fracture mechanisms in as-quenched (As-Q) and quenched-and-tempered (Q-T) dual-phase steels. Using in-situ tensile testing combined with synchrotron X-ray tomography, we performed four-dimensional quantitative analysis of void evolution to reveal how heat treatment fundamentally alters damage behavior. The high-strength, low-ductility As-Q steel exhibited premature failure. Its significant phase-hardness mismatch between ferrite and martensite phases induced continuous and widespread void nucleation via martensite cracking after reaching the ultimate tensile strength. The subsequent rapid, isotropic growth and coalescence of these numerous voids led to early fracture. In contrast, tempering the Q-T steel reduced the hardness mismatch among phases, yielding superior ductility with lower strength. The Q-T steel, in particular, exhibited a two-stage damage process. Throughout most of the deformation, voids grew anisotropically by elongating along the tensile axis. This stable growth, however, gave way to a drastic change just before fracture, characterized by a rapid proliferation of voids oriented perpendicular to the loading direction. This final phase is attributed to damage in the martensite, suggesting the quantitative and qualitative differences in void evolution between the As-Q and Q-T steels.