The morphological characteristics of alumina cluster inclusions acting as precipitation sites for MnS in steel were studied by 3D-SEM observation and model calculation. It was found that notches at the bonding parts of alumina particles with an apex angle of less than 70° remarkably act as precipitation sites for MnS.

It had been reported that Ti-29Nb-13Ta-4.6Zr (TNTZ) alloy forms a dense oxide layer by high-temperature oxidation whereas CP Ti forms a multilayered oxide consisted of rutile monolayers and void layer. This morphological change is supposed to be mainly caused by Nb addition in Ti since the dense oxide layer of TNTZ consists of multiple oxide phases, at least with rutile TiO₂ and TiNb₂O₇. In this study, high-temperature oxidation at 1273 K for 3.6 ks in the air of Ti-xNb alloys (x=1, 5, 7, 10, 13, 15, 18, 20, 23, 26, 28, 30 and 32 mol%) was investigated to discuss the effect of Nb addition to Ti on its high-oxidation behavior, and on its oxide microstructure. From the results of the SEM observation, an oxide layer with a void layer was formed on Ti-xNb substrate from 1 mol%Nb up to 10 mol%Nb. However, densification of the oxide layer was confirmed at Ti-13Nb. Then, the dense oxide layer was formed up to 32 mol%Nb. XRD results indicated that only rutile-type TiO₂ was identified from 1 mol%Nb up to 10 mol%Nb, then both TiO₂ and TiNb₂O₇ were formed from 13 mol%Nb to 32 mol%Nb. These results indicate that dense oxide layer formation attributes to phase separation from TiO₂ to TiNb₂O₇. Until 10 mol%Nb, the thickness of oxide layer was suppressed by Nb addition, whereas the layer thickness increased with increasing Nb content from 13 mol%Nb. The maximum exfoliation resistance of the oxide layer was obtained at 20 mol%Nb. The results of oxide growth rate at each Ti-xNb alloys suggested that Nb diffusion in Ti may rate-determining process of the dense oxide layer formation.

In the present work, we investigated the alternating current (AC) etching of aluminum wires with the aim of fabricating catalyst supports. This support material was found to possess a spongy surface layer that could be subsequently filled with γ-alumina by a combination of hydration and calcination. These support wires are easily fabricated at low cost, and could be mass produced continuously. The integrated structure of the catalyst produces a strong interconnection between the wire substrate and a thick catalyst layer on the wire. This work also demonstrated that the diffusion inside the catalyst layer can be controlled by varying the etched structure of the support layer. A micro-structured catalytic wall reactor concept was developed using these wires, situated parallel to one another within a tubular device, such that microchannels were present between the wires. The steam-reforming of methanol was assessed in this type of microreactor.

Mater. Trans. 58（2017） 782-789に掲載

In this study, rolled Mg-Ca alloys were processed by high-temperature pre-annealing at 500℃ and subsequent warm rolling at 200℃. The rolled alloys processed by repetition of the above rolling procedure were finally annealed at 350℃. This process was effective rolling procedure for suppression of basal texture formation of Mg-Mn and Mg-Ce alloys. The rolled Mg-0.1Ca alloy exhibited a significantly increased Erichsen value of 7.1, which was much larger value compared with commercial Mg alloys. On the other hand, excessive calcium addition more than 0.3 mass% deteriorated room temperature formability. Basal texture intensity of Mg-Ca alloys had much smaller value, when calcium concentration was set more than 0.1 mass%, indicating that the suppression of basal texture formation mainly contributed to the enhanced stretch formability of Mg-0.1Ca alloy. The deteriorated formability of Mg-0.3Ca alloy was attributed to the formation of coarse precipitates in the alloy. Retardation of dynamic recrystallization during warm rolling was suggested to be one of the reason for suppression of strong basal texture formation for Mg-Ca alloy, when calcium concentration was set to more than 0.1 mass%. The internal friction of the Mg-0.1Ca alloy was found to be almost the same as that of pure magnesium at certain strain amplitude.

The behavior of the martensite transformation of Ti-42 mass%Nb alloy was investigated by electrical resistivity measurement. The β phase was fully retained upon quenching from 1050°C. In the cooling and heating process below room temperature, hysteresis due to the martensite transformation was clearly observed, in contrast to previously reported behavior. The M_S and A_f temperatures of the sample prepared from the outer layer of a quenched plate were lower than those of the inner layer. Treatment of the solution under a low vacuum condition also lowered these temperatures and the hysteresis loop became much smaller. The effect of the solution temperature above β-transus on the transformation behavior was negligible. By heating to 400°C, the whole resistivity curve shifted upward, and the ω_S temperature increased, but the hysteresis loop still appeared at a low temperature. In situ TEM observation revealed that acicular martensite was formed at a low temperature and disappeared with heating, and that the reflection intensities of ω and α” in the β matrix increased with cooling. However, such behavior was hardly observed in a region irradiated by an electron beam during cooling.

Deformation and recrystallization behavior of pure niobium was investigated in order to clarify the origin of its low hydro-formability despite its high ductility comparable with pure iron. Pure niobium exhibited lower strain hardening in cold rolling compared with pure iron. Furthermore, in post-deformation annealing, the hardness of niobium decreased monotonously with increasing temperature, and a typical sharp drop by recrystallization was not evident. This softening behavior was contrasted with the pure iron. It is suggested that niobium exhibits the so-called in-situ recrystallization because of its low elastic modulus and low accumulative plastic strain energy. The low hydro-formability of pure niobium sheets or tubes can be caused by its low strain hardening and in-situ recrystallization which is associated with this recovered residual rolled texture.

In this study, we evaluated the osseoconductivity of Type 316L stainless steel with self-organized nanopores of three different average diameters (26, 90, and 177 nm), formed by anodic polarization. The proliferation, alkaline phosphatase activity, and morphology of MC3T3-E1 mouse osteoblast-like cells, cultured on the self-organized nanopores were evaluated. The cell densities on samples with the nanopores were higher than those on mechanically finished surfaces that were mirror-polished or ground with #2000 SiC paper. In particular, the highest cell density and alkaline phosphatase activity were obtained on the nanoporous sample with the smallest diameter of 26 nm. Cells on the samples with 26 nm nanopores extended further and spread more filopodia compared with cells on samples with the other surface morphologies. Therefore, we concluded that self-organized nanopores with an optimal diameter (e.g., 26 nm) on Type 316L stainless steel could enhance long-term cell activity.

Mater. Trans. 57（2016） 2065-2071に掲載