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Design of two-phase alloys consisting of niobium solid solution (A2) and α-Nb 5 Si 3 (D8 l ) phases in the Nb-Ti-Si ternary system is pursued for ultra-high temperature structural applications. Compositional and annealing conditions are determined for the formation of A2/D8 l lamellar microstructure via eutectoid decomposition of Nb 3 Si (tP32) phase in the Nb-Si binary and Nb-Ti-Si ternary system. Addition of titanium is found to result in increased room temperature toughness, but decreased high temperature strength. Enhancement of mechanical properties is achieved by applying the directional solidification technique. Mechanical properties of the alloys are discussed from microstructural point of view.

Precipitation behavior of intermetallic phases in ferrite matrix is investigated by transmission electron microscopy (TEM) in Fe-10Cr-1.4W-4.5Co (at%) alloys with and without 0.3 at% Si. It is intended to provide basic information for the alloy design of ferritic heat resistant alloys strengthened by intermetallic eompounds. In the alloy containing Si, icosahedral quasicrytalline phase (I-phase) is found to precipitate during aging at 873 K. It is confirmed that selected area diffraction (SAD) patterns of the precipitates exhibit two-, three- and five-fold symmetry and have diffraction spots in the positions related to the golden section. In the Si-free alloy, the R-phase precipitates instead of I-phase at 873 K, and the Laves phase precipitates in both alloys during aging at higher temperature, 973 K. The Laves phase formed at 973 K transforms to the I-phase in the alloy containing Si but to the R-phase in the Si-free alloy during subsequent aging at 873 K. It is found that the structures among icosahedral quasictystalline and crystalline R- and Laves phases are closely similar by comparing the orientation relationship, precipitate morphology and stability of I-phase with those of R- and Laws phases.

The phase stability of molybdenum disilicide (MoSi 2 , C1 1 b structure) relative to other phases, C40 and C54 phases, in the pseudo-binary systems of MoSi 2 and other types of disilicides of transition metals including Cr, V, Nb, Ta and Ti was investigated by establishing the MoSi 2 -TSi 2 (T=Cr, V, Nb, Ta, Ti) pseudo-binary phase diagrams. It was found that V, Nb, Ta and Ti which substitute for Mo in MoSi 2 strongly stabilize the C40 phase while Cr only shows a weak C40 structure-stabilizing effect. The phase stability was also discussed on the basis of geometrical change in the three phases when composition varies. Change in lattice parameter of each phase indicated that phase stability of C1 1 b , C40 and C54 structures greatly depend on the relative stacking spacing of the equivalent hexagonal atomic plane for the three structures. Based on the results of phase diagram investigation, the present work attempted to design a C11 b /C40 lamellar microstructure, Existence of the equivalent hexagonal stacking atomic plane among C1 1 b , C40 and C54 phases makes it possible to design a coherent C 11 b /C40 two-phase microstructure. Assuming the equivalent atomic planes in C 11 b and C40 phases coincide with each other, high lattice coherency between C 11 b and C40 phases is available in MoSi 2 -CrSi 2 system with a lattice misfit of less than 0.5% and in the other four systems with a misfit of 1.3 to 2.7%, A lamellar microstructure was observed in MoSi 2 -TaSi 2 and MoSi 2 -NbSi 2 systems but no lamellar microstructures were obtained in MoSi 2 -CrSi 2 , MoSi 2 -VSi 2 and MoSi 2 -TiSi 2 systems.

Microstructure and mechanical properties are investigated in the binary Co-AI alloys based on B2 intermetallic compound CoAl. The effort is focused on two-phase alloys of the compound with Co primary solid solution in order to irnpart some ductility to the intrinsically brittle compound. The Co-Al binary phase diagram on the Co-rich side is first examined to confirm the stable phase fields of interest by differential thermal analysis and energy dispersive X-ray spectroscopy. Then the compression tests are carried out on selected two-phase alloys to correlate the 0.2% flow stress as well as compressive ductility with microstructures

Various two- or three-phase alloys were prepared based on the B2 intermetallic compound CoAl in the Co-Al-Ni ternary system, in which the B2 phase forms a continuous solid solution with NiAl of the same ordered structure. Combination of the phases with the B2 phase can be the (Co, Ni) primary solid solution phase being also continuous from Co to Ni-corners, and/or the L1 2 phase based on Ni 3 Al. Then the relationship between microstructure and mechanical properties were systematically investigated by optical and transmission electron microscopy and by micro-Vickers hardness and compression tests. It is found that two-phase alloys of B2/L1 2 show superior room temperature ductility with over 10% plastic strains to those of B2/(Co, Ni). The present work is based on the seemingly ever lasting motivation to provide some ductility to the intrinsically brittle CoAl compound which unless otherwise has attractive properties as a new high-temperature structural material