3D Fracture Behaviours in Dual-phase Stainless Steel

Single-distance phase retrieval technique was applied to contrast-enhanced phase-sensitive imaging of dual-phase microstructures in ferrite/austenite dual-phase stainless steel. Contrast between the two phases was evaluated, together with noise and spatial resolution, under varying experimental and phase retrieval conditions. Serial sectioning technique was also employed, to validate the shape accuracy of related three-dimensional images. Although the density difference between the two phases was relatively small, the two phases were successfully segmented in the three-dimensional images. The imaging technique was also applied to observe microvoid nucleation and growth behaviour during tensile loading. The loading steps at which microvoids were nucleated were identified by tracking all the microvoids observed at the final loading step, in reverse chronological order, toward the initial unloaded state. Each absorption contrast image was then registered with a corresponding phase-contrast image, in order to classify all the targeted microvoids according to their nucleation site: ferrite, austenite or phase boundary. Premature damage initiation was observed at a relatively early stage at all the nucleation sites; however, subsequent growth was relatively moderate. On the other hand, microvoids initiated later, at fine particles located along the phase boundaries, exhibited enormous growth, thereby inducing the final rupture. It was concluded that the substantial driving force for the growth of microvoids located along morphologically characteristic austenitic particles was the predominant factor in the ductile fracture.