Showing papers by "Kouichi Maruyama published in 2012"

Compositional regions of single phases at 1800 °C in Mo-rich Mo–Si–B ternary system

Abstract: Single-phase regions in Mo-rich Mo–Si–B alloys at 1800 °C were experimentally determined using a field-emission (FE) gun-type electron probe micro-analyzer (EPMA) in this study. A quantitative analysis by FE-EPMA was conducted with the calibration method we applied in our previous study to improve the accuracy in the B measurements. The compositional ranges of the determined Mo solid solution, silicide and boride phases were in good agreement with those of Mo–Si and Mo–B binary phase diagrams. On the other hand, the determined solubility of B in Mo solid solution, Mo 3 Si and T 1 (Mo 5 Si 3 ) were quite different from that indicated in previously reported ternary phase diagrams. The compositional region of the T 2 (Mo 5 SiB 2 ) single-phase ranges from 9.7 to 13.3 at% for Si and from 23.5 to 26.8 at% for B. The following two points were made clear by the ternary phase diagram as determined in this study. One is that no stoichiometric T 2 composition is in the T 2 single-phase region, confirming no T 2 single-phase material can be obtained at the stoichiometric composition at 1800 °C. The another is that the T 2 region of this study expands to a Si-rich area from the stoichiometric composition.

Assessment of Titanium Aluminide Alloys for High-Temperature Nuclear Structural Applications

Abstract: Titanium aluminide (TiAl) alloys exhibit high specific strength, low density, good oxidation, corrosion, and creep resistance at elevated temperatures, making them good candidate materials for aerospace and automotive applications. TiAl alloys also show excellent radiation resistance and low neutron activation, and they can be developed to have various microstructures, allowing different combinations of properties for various extreme environments. Hence, TiAl alloys may be used in advanced nuclear systems as high-temperature structural materials. Moreover, TiAl alloys are good materials to be used for fundamental studies on microstructural effects on irradiation behavior of advanced nuclear structural materials. This article reviews the microstructure, creep, radiation, and oxidation properties of TiAl alloys in comparison with other nuclear structural materials to assess the potential of TiAl alloys as candidate structural materials for future nuclear applications.

Strain-induced coarsening of nanoscale precipitates in strength enhanced high Cr ferritic steels

Abstract: Strain-induced coarsening of precipitates, mainly M23C6, has been studied in several types of Gr. 122, Gr. 92 and Gr. 91 steels in a wide range of creep conditions. Strain-induced coarsening of precipitates should be separately evaluated in high stress and short-term creep region (or called “H”), where precipitates and subgrains are thermally stable, and low stress in long-term creep region (or called “L”), where thermal coarsening of precipitates and subgrains appear. Creep plastic deformation does not accelerate the coarsening of precipitates during the primary and secondary creep regions in short-term creep region “H”. However, the coarsening of M23C6 precipitates occurs during the acceleration creep region and increases with decreasing of applied stress. The onset of long-term region “L” is accompanied by the thermal coarsening of M23C6 precipitates as well as strain-induced coarsening of M23C6 precipitates during secondary and acceleration creep regions and maybe in primary creep region under very low creep stress. In long-term region “L”, recovery of the dislocation substructure is controlled simultaneously by creep plastic deformation and pinning force from precipitates. On the other hand, the coarsening of precipitates is due to both thermal coarsening and strain-induced coarsening of precipitates. In fact, the stability of precipitates is the most important factor for the suppression of subgrain recovery in long-term region “L” rather than short-term creep region “H”. In long-term region “L”, greater thermal recovery of subgrains and M23C6 precipitates due to higher creep temperature or/and longer creep rupture life results in greater strain-induced coarsening of M23C6 precipitates.

Development of Oxide Dispersion Strengthened Steels for High Temperature Nuclear Structural Applications

Abstract: Oxide dispersion strengthened (ODS) steels are the most promising candidate materials for high temperature nuclear applications. Mechanical alloying and subsequent thermomechanical treatments are applied to manufacture the ODS steels. Recently improved chemical composition and manufacturing processes have been developed to produce ultrafine grain size with high number-density of nanoscale oxide particles and high dislocation density in the microstructure. Usually, fine grains degrade creep resistance at elevated temperatures. However, the fine-grained ODS steels exhibit not only good radiation resistance, but also superior creep properties. The present paper reviews the chemical compositions, manufacturing processing, microstructural features, thermal creep properties and radiation resistance of recently developed ODS steels. Special attention is paid to the effects of the fine-scale microstructural features on thermal creep and radiation resistance.

Creep Behavior and Microstructures in Grain Boundary Strengthened Mg-Al-Ca Thixomolded® Alloys

Abstract: The microstructures of thixomolded® (TM) Mg-Al-Ca alloys consist of α-Mg and eutectic compounds along grain boundaries. Misch metal (Mm) addition to TM Mg-Al-Ca alloys makes precipitates within α-Mg matrix and their number density and size depend on heat-treatment conditions. The small addition of Mm can keep the network-like grain boundary covering and the improvement of microstructure stability during creep. On the other hand, excessive Mm addition causes the deterioration of creep strength. The grain boundary coverage decreases with increasing Mm content due to the formation of coarse spherical Al-Mm based intermetallic compounds. Creep strength is significantly affected by both of the grain boundary coverage and the morphology of eutectic compounds along grain boundaries.

Role of thermal vacancies on temperature dependence of lattice parameter and elastic moduli in B2-type FeAl

Abstract: Temperature dependence of the lattice parameter and elastic moduli in Fe-40 and -43Al (at.%) was investigated by high temperature X-ray diffractometry (XRD) and the Electro-Magnetic Acoustic Resonance (EMAR) method. The thermal vacancy concentration was estimated from the activation enthalpy and entropy data of vacancy formation previously reported for FeAl. It was found that both the lattice parameter and the elastic moduli of FeAl have a linear relationship with temperature even in the temperature range where thermal vacancy concentration rapidly increases (above 400 °C), thus suggesting that newly generated thermal vacancies at elevated temperature do not make significant influence on the lattice parameter and the elastic properties of B2-type FeAl.