Naoko Sato

Topological Analysis of Martensite Morphology in Dual-Phase Steels

Martensite morphology such as connectivity or dispersivity in ferrite (F)/martensite (M) dual-phase (DP) steels was investigated from topological viewpoint to reveal the effect of the martensite morphology on the mechanical properties. Topological analysis permits evaluation of the microstructural connectivity and dispersivity by measuring the number of handles, independent bodies and genus, etc. The topological analysis was performed on three-dimensional (3D) reconstructed images of the microstructure with different connectivity, dispersivity, volume fraction and hardness of martensite in DP steels that were prepared by changing the intercritical annealing temperature. The connectivity and the volume fraction of martensite increased while the dispersivity and hardness of it decreased with increasing annealing temperature. The effect of connectivity and dispersivity as well as volume fraction and hardness, in particular, on work hardening behavior was individually evaluated at a given strain.

3D Image-Based Stereology

The features present in 3D structure have geometric properties that fall into two broad categories: topological and metric. Metric properties are generally the more familiar; these include volume, surface area, line length and curvature. Equally or even more important in some applications are the topological properties of features. The two principal topological properties are number per unit volume and connectivity. In the present study, a change in morphology of pearlite and dual phase microstructures was examined from differential geometry and topology viewpoint. 3D images of eutectoid pearlite and dual phase steels were obtained by reconstructing serial sectioning images. Their metric and topological features were evaluated using The Euler Poincare formula and The Gauss-Bonnet Theorem. In addition, newly developed fully-automated serial sectioning 3D microscope “Genus_3D” will be also introduced.

Development of Fully Automated Serial-Sectioning 3D Microscope and Topological Approach to Pearlite and Dual-Phase Microstructure in Steels

Using a newly developed fully automated serial-sectioning three-dimensional (3D) microscope, Genus_3D, and a conventional dual-beam SEM, we examined ferrite-martensite dual-phase and eutectoid pearlite microstructures. In particular, we consider the topology and differential geometry. Genus, Euler characteristics, Gaussian curvature, and mean curvatures were obtained from 3D reconstructions. A variation in the martensite morphology in dual-phase steel, i.e., connectivity, was examined to understand the ductile fracture mechanism. In addition, we investigated the 3D morphological variation of lamellar cementite in pearlite during spheroidizing. This 3D observation revealed many holes and fissures in cementite lamellae, which potentially accelerate the spheroidization. The disintegration of lamellar structure into particles was discussed with respect to surface area change per unit volume and local surface morphology (i.e., curvature).