Materials Science Forum Vols. 561-565

Abstract: γ-TiAl has a tetragonal structure that is almost cubic (c/a ≈ 1.02). As a result of this, the electron backscatter diffraction patterns (EBSPs) of orientations related by 90°<100] rotations are almost identical. Reliable orientation measurement during automated EBSD mapping requires distinguishing very small differences in the positions of certain bands within the EBSPs. EBSD was used to measure the micro-texture and bulk texture of γ-TiAl hot-deformed under conditions approximating plane-strain compression. The reliability and accuracy of the indexing was assessed by inspecting the orientation maps for misindexing and by comparing the texture measured using EBSD with that measured using X-ray diffraction (XRD). Using Advanced Fit refinement and high-resolution EBSPs produced reliable and accurate results. Substantial misindexing was present when the EBSPs were indexed using the Hough transform method alone.

Abstract: The phase equilibria among α-Fe, γ-Fe and Fe2Ti phases in Fe-Ti-Ni ternary system at 1473 K and 1373 K with Ni (γ former element) addition were examined, by paying attention to the γ+Fe2Ti two-phase region. The Fe2Ti single-phase region extends toward the equal-titanium concentration direction up to about 30 at% Ni, and the Laves phase becomes in equilibrium with γ-Fe by 12 at% Ni addition, but the γ+Fe2Ti two-phase region is limited because of the formation of liquid phase by further Ni addition at 1473 K. With decreasing temperature, a ternary eutectic reaction (L→γ-Fe+Fe2Ti+Ni3Ti) occurs, making the two-phase region wider just below the invariant reaction at 1373 K, and the region becomes narrower again by the enlargement of the three-phases region toward Fe-rich side.

Abstract: Ni3Al-based alloys possess good oxidation resistance, moderate room and high temperature strength and ductility. Introduction of Cr-carbide particles through a solidification route is attempted to provide higher hardness and wear resistance. The mechanical and physical properties are measured at room temperature for several alloys with various carbon concentrations up to 2.0 wt.%. Hardness and wear resistance as well as compressive strength increase with increasing carbon concentration, while bend ductility decreases. Adhesion between carbides and matrix phase is good, and cracks propagate mainly through carbides. The crystallographic orientation relationships between constituent phases are also attempted using electron back-scattering diffraction (EBSD) technique. The thermal conductivity is found to be less sensitive to the alloy composition.

Abstract: The changes in microstructure and defect structure of two different semiconducting transition-metal silicides, ReSi1.75 and Ru2Si3 with ternary alloying of substitutional elements with a valence electron number different from that of the constituent metal have been investigated in order to see if the crystal and defect structures of these silicides and thereby their physical properties can be controlled through defect engineering according to the valence electron counting rule. The Si vacancy concentration and its arrangement can be successfully controlled in ReSi1.75 while the relative magnitude of the metal and silicon subcell dimensions in the chimney-ladder structures can be successfully controlled in Ru2Si3.

Abstract: High purity molybdenum metal powder is produced commercially from hexavalent molybdenum precursors, viz.: ammonium dimolybdate (ADM) or molybdenum trioxide. One conventional process incorporates first-stage and second-stage flowsheet components, with hydrogen gas serving as reductant. This two-stage strategy is employed in order to minimize the formation of volatile molybdenum species that would otherwise be generated at the high temperature required to obtain molybdenum (Mo) in a single stage conversion of the molybdenum precursor. Although molybdenum powder has been produced commercially for over a century, a comprehensive understanding of the kinetic mechanisms and powder characteristics, e.g. oxygen content and particle morphology, is far from being definitive. In fact, it might be argued that the “art” and engineering, in a commercial context, has advanced ahead of the fine-detail science-derived metallurgical process-engineering. Theoretical contributions presented in this paper are focused primarily on the fundamentals of the conversion process associated with second-stage reduction process – MoO2 to Mo and the factors that contribute to the oxygen content of the molybdenum powder product (1000 to 100 ppm(w) O, range). Thus, equilibrium-configuration details concerning both solid and gas phases are addressed, including the volatile hexavalent molybdenum vapor complexes as well as solubility of oxygen in molybdenum. In regard to the role of a chemical vapor-transport mechanism on powder morphology in second-stage conversion of MoO2 to Mo, it is shown that the partial pressure of the prominent molybdenum hydroxide vapor-complex (MoO2(OH)2) is far too low to support such a mechanism. This contention has been corroborated by employing helium to control the partial pressures of hydrogen and water in the gas phase. Secondarily, a limited assessment of the intrinsic rate-controlling mechanisms that can contribute to the residual oxygen-content of the Mo powder product is also provided. Powder morphology, and its concomitant influence on specific surface-area of the Mo powder product, is found to correlate with the oxygen-content determination of the powder produced during second-stage reduction, and according to the processing strategy employed. Consequently, it has been found cogent to “partition” second-stage reduction into: i) a relatively high-rate Primary Reduction Sequence, and ii) a lower rate Deoxidation Sequence.

Abstract: MoSi2 is a potential high temperature structural materials and shows excellent oxidation resistance at high temperatures. To explore its oxidation behaviour and mechanism, MoSi2 single crystals were prepared and investigated in the present work. The experimental results showed that different from its polycrystalline, MoSi2 single crystal showed good oxidation resistance at both 773 and 1473K. Near parabolic law of oxidation kinetics was followed for all the investigated surfaces of MoSi2 single crystal. The close-packed (110) surface of MoSi2 single crystal showed the best oxidation resistance at both temperatures. After exposure at 773K, the molybdenum oxide formed on the (110) surface of MoSi2 single crystal was found to be Mo4O11 instead of MoO3 formed on the other surfaces. Morphology observation showed a columnar growth of Mo4O11 on the (110) surface. No difference was found on the oxide formed for different surfaces of MoSi2 single crystal at 1473K.

Abstract: Compressive deformation of L10-ordered single crystals of FePd whose c/a ratio less than unity have been investigated from room temperature to 823 K. The results show that the critical resolved shear stress (CRSS) for octahedral glide of ordinary dislocations is smaller than that of super-lattice dislocations in all the temperature range investigated, that is the opposite sense to the case of Ti-56 mol% Al. The CRSS for ordinary dislocations virtually independent to the temperature. On the other hand, the CRSS for super dislocations exhibits a weak positive temperature dependence from room temperature up to 573 K and decreases in higher temperatures.

Abstract: The variations of the crystal structures and thermoelectric properties of the Ru1-xRexSiy chimney-ladder phases were studied as a function of the Re concentration. A series of chimney-ladder phases with a compositional formula of Ru1-xRexSi1.539+0.178x are formed in a wide compositional range, 0.14 ≤ x ≤ 0.76. The composition of the chimney-ladder phase is systematically deviated from the idealized composition satisfying the valence electron concentration rule: VEC=14. Measurements of thermoelectric properties reveal that the chimney-ladder phases exhibit n-type semiconducting behavior at low Re concentrations and p-type semiconducting behavior at high Re concentrations, which are well consistent with the prediction based on the deviation of the composition of the chimney-ladder phase from the idealized composition.

Abstract: Nb-NiTi and Nb-CoTi eutectic alloys were directionally solidified in an optical floating zone furnace. Rod-type eutectic structures with Nb rods aligned parallel to the growth direction are obtained for Nb-41Ni-40Ti grown at relatively slow growth rates below 1.0mm/h, while lamellar-type eutectic structures are obtained for Nb-35Co-34Ti grown at the same condition. The hydrogen permeability for the Nb-41Ni-40Ti DS alloy with Nb rods perpendicular to the membrane surface is 2.60×10-8mol H2 m-1 Pa-1/2 at 673K, which is about 2.5 times higher than that of as-cast sample. No hydrogen embrittlement is observed between 573 and 673K, indicating that the Nb-NiTi rod-type eutectic structure effectively suppresses the hydrogen embrittlement of Nb during hydrogen permeation.

Abstract: A recent developed Ni-Cr-Co-Mo-Nb-Ti-Al type nickel-base superalloy, INCONEL 740, has been selected for the application of USC boilers at the temperature above 750°C. This paper focuses on the structure stability improvement of this alloy. Phase computation by Thermo-Calc has been adopted to study main influencing factors on precipitating phases of the alloy and the results show that the ratio of Al/Ti plays an important role. Four new modified alloys in adjustment of Al and Ti contents and in control of Si level were designed and melted for experimental study. The results indicate that the modified alloys exhibit more stable structure stability at 750, 800 and 850°C long time exposure. The newly developed alloy can be adopted for engineering production and application for USC power plants at temperature above 750°C.