Critical Inclusion Size and Void Growth in Dual-Phase Ferrite-Bainite Steel During Ductile Fracture
Ferrite-bainite dual-phase steel is widely used in the automotive industry owing to its high strength and excellent ductility.The impact of inclusions and void growth behavior in dual-phase steel is a major concern among researchers seeking to achieve better mechanical properties.To investigate this,a cross-length-scale multimodal method was employed to study the influence of local microstructures on void growth during ductile fracture of a dual-phase ferrite-bainite steel.During tensile testing,laboratory X-ray computed tomography(XCT)was used to measure the evolution of void volume.3D-electron back scatter diffraction(3D-EBSD)provided information about the voids nucleated at both inclusion particles and bainite phases or their boundaries.Carefully controlled,broad-focused ion beam excavation was per-formed to reveal a new interface at a specific depth of the voids.Results showed that voids resulting from large inclusions are significantly bigger than either small inclusions or the bainite phase.Large inclusions lead to large voids even when the strain correlated with the growth of those voids is lower.An investiga-tion of the dislocation densities surrounding the voids suggested that they may be related to the strain gradient around the different inclusion sizes.A critical inclusion size estimated to be around 1.85-2.86 μm was found below which nucleation occurs but with limited growth.The elevated rate of local dislocation multiplication due to local deformation gradient effects can impede the growth of smaller voids.The growth of voids is heterogeneous,and their shape correlates well with the deformability of the surround-ing grains,as indicated by a Schmid factor weighted using the grain size.This weighted Schmid factor ex-plains not only the shape of the voids but also sheds light on the ease of void coalescence based on the microstructures separating the voids.
dual-phase ferrite-bainite steelvoid nucleation and growth3D-EBSDX-ray computed tomographysize effect
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