No abstract

A local elongation of 8% for nanoscale precipitated steel was observed via tensile testing, which is higher than that of 5% for bainitic steel. To determine the factor underlying this difference, void nucleation, growth, and coalescence mechanisms in the nanoscale precipitated steel and the bainitic steel were examined using electron backscattering diffraction and subsequent observation by synchrotron radiation X-ray laminography during tensile testing. Synchrotron radiation X-ray laminography analysis of void growth and coalescence revealed that the critical strain and the critical void volume fraction for fracture in the bainitic steel were smaller than those for the nanoscale precipitated steel. Secondary-ion mass spectrometry analyses revealed that C atoms were segregated at grain boundaries in the bainitic steel. Void nucleation sites in the nanoscale precipitated steel were nanoscale precipitates inside the grain and at grain boundaries and coarse precipitates; however, in the bainitic steel, void nucleation sites were entirely at grain boundaries. Nanoindentation hardness measurements showed a larger plastic strain gradient between the grain boundary and matrix in the bainitic steel than in the nanoscale precipitated steel. From these results, the high local elongation exhibited by the nanoscale precipitated steel was concluded to be due to the reduced plastic strain gradient with a uniform hardness distribution between the grain boundary and the grain interior.

High-strength steel is required in many industries. However, elongation decreases as strength increases. Therefore, many types of high-strength steels have been developed in order to improve the balance of these mutually contradictory properties. These representative steel types are dual-phase (DP) steels, comprising mixtures of ferritic and martensitic 1) or bainitic phases, 2) and transformation-induced plasticity (TRIP) 3)-assisted steels, 4) in which the austenitic-martensitic phase transformation is triggered by deformation. Some studies 5,6) have investigated the effects of the microstructures of DP and TRIP steels on the strength and elongation

Synchrotron X-ray laminography was used to examine the time-dependent evolution of the three-dimensional (3D) morphology of micropores in sintered iron during the tensile test. 3D snapshots showed that the networked open pores grow wider than 20 µm along the tensile direction, resulting in the internal necking of the specimen. Subsequently, these pores initiated the cracks perpendicular to the tensile direction by coalescing with the surrounding pre-existing microvoids or with the secondary-generated voids immediately before fracture. Topological analysis of the barycentric positions of these microvoids showed that they form the two-dimensional networks within the ∼20 µm of radius area. These observations strongly indicate that the microvoid coalescence could occur on shear planes formed close to the enlarged open pores or between closed pores by strain accumulation and play an important role in the crack initiation.