Papers by Author: Taisuke Hayashi

Abstract: The reverse transformation of austenite from pearlite was observed in-situ by using a sophisticated EBSD system. Quantitative information on the nucleation sites and orientation selection of the austenite was obtained. Initially, the nucleation sites were restricted to high-angle grain boundary (HAB) and all the austenite orientations were selected by Kurdjumov-Sachs (K-S) orientation relationship. Latterly, the constraints were relaxed as new nucleation sites were involved. The γ growth was preferentially into the pearlite grains without K-S relation. Nucleation and growth at HAB edge may contribute to γ coarse grains, whereas nucleation at pre-existing γ boundary is important to grain refinement of γ structure.

Abstract: The present study aims to clarify the development of blocks and packets in lath martensite in Fe–18Ni maraging steel using three-dimensional observations. The specimens were step-quenched in order to clarify the sequential development of the three-dimensional morphology in a prior austenite grain. In a prior austenite grain, we found that five independent packets formed during the early stage of martensitic transformation. Four of the packets exist along the prior austenite grain boundaries and one packet grows from the boundary edge into the prior austenite grain. Each packet consists of a single block, although the fraction of martensitic transformation is 50.6%. The observed rules of the block-selection are as follows: (1) the blocks have near Kurdjumov–Sachs orientation relationship with adjacent austenite grains and elongated directions of the laths are parallel to adjacent grain boundaries and (2) transformation shear directions of the laths are parallel to adjacent grain boundaries.

Abstract: Japanese sword has finer grain size and lath martensite in the microstructure of sharp edge amazingly. Nowadays these structures are considered to be one of the ideal structure at which are greatly aimed to strengthen or improve toughness of steels. Though the carbon content of its sharp edge is 0.70 mass %, there are no lenticular martensite and no micocracking in that area. As a result of bending test by actual sword specimen, one sword was finally bent, the other sword was broken. However it is found the sharp edge in Japanese sword has such a large bending strength 2500, 4600MPa respectively as modern, high performance tool steels and the difference of crack propagation under bending depends on the microstructure distribution and the grain size in cross section of Japanese sword.

Abstract: The effect of titanium carbide (TiC) on morphology of low-carbon steel martensite was studied by means of electron backscatter diffraction (EBSD). The nucleation and growth of new morphology subunits such as packet, block and sub-block are observed in the area surrounding of micron-sized TiC particles. The misorientation from a fitted orientation relationship between martensite and austenite near TiC particle is larger than the average misorientation with a localized characteristic. The position of new morphology subunits has a well correspondence with the area in vicinity of TiC particle, which has large misorientation. The micron-sized TiC particle plays a role of stress concentrator in austenite during martensitic transformation which suppresses growth of one martensite variant while stimulates nucleation and growth of another one.

Abstract: Single crystals of (Mo0.8Nb0.2)5Si3, (Mo0.85W0.15)5Si3 and Mo5Si3 were grown and their thermal expansion along the a- and c-axes and single crystalline elastic property were investigated. An anisotropy ratio of coefficient of thermal expansion (ac/aa) is lower for (Mo0.8Nb0.2)5Si3, and higher for (Mo0.85W0.15)5Si3 at RT than that for Mo5Si3. However, the ac/aa for (Mo0.8Nb0.2)5Si3 increases with increasing temperature and the ac/aa of the three compounds is similar at 800°C. The values of isotropic average Young’s (E), bulk (K) and shear (G) moduli for (Mo0.8Nb0.2)5Si3 is the lowest and those for (Mo0.85W0.15)5Si3 is the highest in the three compounds. On the other hand, the difference of E between [100] and [001] increases when partial substitution of Mo by Nb occurs and it decreases when partial substitution of Mo by W occurs. Simply compared with the results of thermal expansion anisotropy, the result shows opposite tendency.