Shigekazu Morito

Synthesis of BaTi2O5 Nanopowders by Sol–Gel Method and the Dielectric Properties of the Ceramics

A sol–gel process using barium diethoxide and titanium tetraisopropoxide as the sources for Ba and Ti, respectively, was employed to synthesize BaTi2O5 nanopowders and ceramics. BaTi2O5 nanopowders in the range of 20–50 nm in size were obtained by calcination of the gel precursors at 650 °C. By a further firing at 1000 °C, some BaTi2O5 nanopowders were grown to 200 nm in size. Conventional sintering at 1150 °C using these BaTi2O5 nanopowders yields BaTi2O5 ceramics of 80–85% relative density. A dense BaTi2O5 ceramic with a relative density of above 95% was fabricated by spark plasma sintering (SPS) at 1000 °C. Furthermore, by SPS combined with the hot-forging technique, we obtained a certain degree of grain orientation along the b-axis in the dense BaTi2O5 ceramic and observed an increase in the dielectric constant reaching 580.

Dielectric and Piezoelectric Properties of 10% KF-Doped BaTiO3 Ceramics

10% KF-doped barium titanate powders, Ba0.9K0.1TiO2.9F0.1, were synthesized through a sol–gel process. The powders, calcined at 650 °C, consist of cubic crystalline particles of ~70 nm in length; the particle size increases to ~200 nm as the firing temperature increases to ca. 800 °C, above which F2 begins to evaporate. Dense ceramics were fabricated by the spark plasma sintering (SPS) method; the average grain size is ~2 µm in lengths. The ceramics, well annealed at 1,000 °C in an O2 gas flow, have good dielectric and piezoelectric properties; the piezoelectric d33 value is 230 pC/N at room temperature. At the ferroelectric Curie temperature TC = 47 °C, the dielectric constant and loss tan δ are 10,000 and <5% at 10 kHz, respectively. The Curie–Weiss relation holds in the fully disordered cubic and ordered rhombohedral phases, showing the second order 1:2 relation. Below 10 kHz, large dielectric dispersion caused by a domain-wall motion appears at the temperature range of -50 to 107 °C. Some discussions are made for these dielectric properties of the ceramics.

Effect of Cooling Rate on Morphology and Crystallography of Lath Martensite in Fe-Ni Alloys

The development of blocks and subblocks in the lath martensite was observed with SEM/EBSD and TEM/Kikuchi diffraction patterns analyses. Morphology of lath martensite formed below Ms temperature was observed using step quenching method in 18 mass% Ni maraging steel. The development of lath martensite structure can be described as follow; lath groups with single K-S variant start to form at the austenite grain boundary. Next, other sub-blocks appear adjacent to the lath groups formed first and these lath groups form macroscopic blocks observed in optical microscopy. The morphologies of the lath martensite in Fe -23 mass% Ni alloy cooled at different cooling rates after austenitization were also observed. The packet size and block thickness decreases with increasing cooling rate, although the sub-block thickness do not change. A packet locally contains small packets with different relationship of close packed planes, and the amount of small packets increases with increasing the cooling rate.

Comparison of Deformation Structure of Lath Martensite in Low Carbon and Ultra-Low Carbon Steels

The microstructural development of cold-rolled lath martensite structure in the low carbon steels and ultra-low carbon steels are studied and compared. In low carbon steel of as-quenched specimens, very thin austenite films exist at boundaries of adjacent laths, but do not exist in ultra-low carbon steel. After cold rolling for the low carbon steel, the lamellar dislocation cells, irregularly bent laths and kinked laths regions are frequently observed and, in some instances, the disappearance of initial lath boundaries is observed. The existence of retained austenite films suggests that the lath boundaries rarely disappear during cold-rolling in the low carbon steel.

Effect of Titanium Carbide Inclusions on Morphology of Low-Carbon Steel Martensite

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.

Microstructural Observation on Materials of the Japanese Sword under Fold-Forging Process

The Japanese sword is produced from a special kind of steel called tamahagane, using a forge-fold operation repeated several times. The purpose of this study is to clarify the development of microstructures with successive forge-fold operations using tamahagane. Specimens under several stages of sword making have been investigated with optical microscopy, scanning electron microscopy and electron probe micro analysis methods. Microstructures have been found to be a combination of ferrite and pearlite with a lot of nonmetallic inclusions. The ferrite bands become finer and among other inclusions Fe2SiO4 takes a spherical shape with increasing recurrence of forge-fold operations.

Evidence of Lath Martensite in High-C Japanese Sword Produced from Tamahagane Steel by Tatara Process

Field Emission Scanning Electron Microscopy with Electron Back-Scattering Diffraction (SEM-EBSD) and Optical microscopy were used to point out the microstructural features of a Japanese sword prepared from tamahagane steel using traditional method. A lath martensite structure, which is usually characterized by packet and block in a prior austenite grain, existed both on the surface and the cross-section of the sword. SEM-EBSD study revealed that the development of prior austenite grain and packet were not much distinctive but the blocks within the packets were fairly observed. It was found that the packet size increased with the prior austenite grain size but the increment was small. Vickers micro-hardness measurement revealed that the sharp end was comparatively harder than other sections of the sword. EPMA study showed that the average carbon content of the sword was around 1 mass% along with a variety of non-metallic inclusions. Formation of lath martensite structure in such high carbon steel is remarkable but comparable to 0.6 mass% carbon ordinary steel. It was realized that the traditional method of preparation using tamahagane as well as the higher content of carbon provided the extraordinary features to the Japanese sword different from the ordinary steel.

Three-Dimensional Approach to Observing Growth of Blocks and Packets in Fe-18Ni Maraging Steel

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.

Study of Metallurgy and Mechanical Property on Japanese Sword

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.

Characterization of Substructure Evolution in Ferrous Lenticular Martensite

This study investigated the substructure evolution in lenticular martensite. The substructure of lenticular martensite changes from fine transformation twins in the midrib and twinned region to a high density of dislocations in the untwinned region during growth. On the basis of careful observation of the morphology and substructure of midrib and examination of the stress-induced growth behavior of thin plate martensite, we concluded that the midrib in lenticular martensite is thin plate martensite itself. Tangled and curved dislocations appeared near the martensite-austenite boundary of the untwinned region in Fe-33Ni and in the entire untwinned region in Fe-31Ni, because the martensite inherited the accommodation dislocations in the surrounding austenite. The difference of Ms temperature causes the difference in the substructure between Fe-33Ni and Fe-31Ni. The higher Ms temperature of Fe-31Ni induces the plastic deformation of the surrounding austenite at an earlier stage of transformation, resulting in the appearance of tangled and curved dislocations in the entire untwinned region.