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Solid solutioning of Mg2(Si, Sn) has been a promising approach in reducing thermal conductivity and leads to improvement of thermoelectric performance. In addition to the Mg2(Si, Sn) solid solutions, we have noticed a layered structure with a gradient composition, which is formed by nonequilibrium solidification and peritectic reaction process and can provide further reduction of thermal conductivity of the Mg2(Si, Sn) solid solutions. All layers of the layered structure have the same face-centered cubic-based structure but varying Sn/Si concentration ratios in each layer. The interfaces between the layers are semi-coherent, reticulating with different numbers of misfit dislocations. Such an interfacial structure brings large numbers of phonon-scattering sources, resulting in further reduction of thermal conductivity in the Mg2(Si, Sn) solid solutions. Consequently, the undoped Mg2Si0.75Sn0.25 containing a higher density of the layered structure has relatively lower thermal conductivity, 1.9 W m-1 K-1 at 523 K, than Mg2Si0.25Sn0.75 with a much lower density of the layered structure, 2.3 W m-1 K-1 at 523 K.

The {100}-oriented (CaxSr1-x)Si2 thin films have been prepared by co-sputtering method at various deposition temperatures. Constituent phase of the films primarily depends on the deposition temperature and the composition x. Although CaSi2 films consisted of layered structure regardless of deposition temperature, the phase was changed by the deposition temperature: the majority phases of the film deposited at 600°C, 650°C and 700°C were 1T layered structure, 1T layered structure + 2H layered structure and 1T layered structure + 6R layered structure, respectively. When the (CaxSr1-x)Si2 films deposited at 700°C, the α-SrSi2-type phase was mainly confirmed below x = 0.17, which is the most stable phase of SrSi2. However, the main phase of all CaxSr1-xSi2 films deposited at 600°C changed to be 1T-type layered structure. Substitution with Ca below x = 0.50 in the film deposited at 600°C led to the decrease in the electrical resistivity compared with that of pure SrSi2.

Ca-Mg-Si films were firstly prepared on (001)Al2O3 substrates by RF-magnetron sputtering method from Mg disc target together with Ca and Si chips. The composition of the deposited films was controlled by adjusting deposition temperature and Ca/Si area ratio of Ca and Si chips on Mg disk target. Ca0.32Mg0.33Si0.35 film deposited at 610 K consisted of a single phase of CaMgSi and this CaMgSi phase was stable after heat treated at 770 K under an atmospheric Ar with 5% -H2. As-deposited film shows the semiconductor behavior and have a power factor of 50 µW/(mK2) at 670 K, while annealed one showed the metallic behavior and its power factor down below 10 µW/(mK2) at 320-770 K. On the other hand, Ca0.27Mg0. 51Si0.2 film deposited at 590 K showed no obvious crystalline phase but became single phase of Ca7Mg7.25Si14 after heat treatment at 770 K under an atmospheric Ar with 5% -H2. As deposited film had a large power factor of 100 µW/(mK2) at 670 K. However, power factor decreased below 1 µW/(mK2) at 320-770K after the heat treatment at 770 K under an atmospheric Ar with 5% -H2.

Solid solutioning of Mg

Thermoelectric composite alloys consisting of the β-FeSi2 matrix and SiO2 particles dispersion were fabricated by a so-called combined reactions sintering process using reduction and oxidation reactions between eutectoid Si decomposed from α-Fe2Si5 and added Fe-oxide powder. Typical microstructure may include some of residual eutectoid Si particles, intermediate product Fe2SiO4 particles, and/or remaining reduced Fe particles depending on the composite alloy compositions and the process conditions. Partitioning of doping element, n-type Co or p-type Mn, during the process plays an important role to control the optimum carrier concentration of the composite alloys. Thermal conductivity can be reduced, as expected, by the dispersion of SiO2 particles. The solubility of doping elements, Co, Mn, Al, and Ru was evaluated in α-Fe2Si5 at 1373 K and in β-FeSi2 at 1073 K being based on the isotherm determination. It is suggested that suitable dopants for the present process are n-type Co and p-type Mn, since they have sufficiently large solubility around 10 at% in both α-Fe2Si5 and β-FeSi2 phases.

The constituent phases, electrical conductivity, and Seebeck coefficient of Ca-Si films deposited on (001) Al2O3 substrates by a radio frequency magnetron sputtering method using a Mg disk target with Ca and Si chips are investigated. X-ray diffraction analysis indicates that the films consist of a single phase of CaSi2, CaSi or Ca5Si3 that are deposited together with the films consisting of a mixture of CaSi2 and CaSi. Films with a CaSi2 or CaSi single phase exhibit a metallic behavior. In contrast, films with a Ca5Si3 single phase show p-type conduction and their Seebeck coefficient reaches 90 μV/K at 400°C.

The microstructure and thermoelectric properties of an off-stoichiometric quaternary (Ti 0.2 ,Zr 0.8 )Ni 1.1 Sn half-Heusler (HH) alloy was investigated in three heating cycles. A high density of coherent nanoprecipitates with an average diameter of ∼13 nm and an interprecipitate spacing of ∼6 nm was observed in the alloy. Formation of the extremely fine nanoprecipitates, most likely to be the ‘full’ Heusler (FH) phase, was not only strongly related to the degree of excess Ni concentration ratio in the alloy, but also appeared to be affected by the Ti–Zr substitutions. We noticed the behaviour of both electrical resistivity ( ρ ) and Seebeck coefficient ( S ) of the alloy was closely associated with the microstructure evolution of the FH-nanoprecipitates, which depended on their phase instability at elevated temperature and the cyclic heating process. The ρ and S reduced after the 1st heating cycle and stabilised thereafter in the subsequent heating cycles. Despite of the presence of the metallic FH-nanoprecipitates, the stabilised S maintained similar magnitudes to S of the ZrNiSn (without FH-nanoprecipitates) and did not show degradation of S as previously seen in the ZrNi 1.1 Sn containing relatively much larger FH-nanoprecipitates. The high density of FH-nanoprecipitates and the presence of Ti–Zr point defects were responsible for the significant reduction of thermal conductivity ( κ ) of the alloy, about 30% and 20% less than κ of the ZrNiSn and ZrNi 1.1 Sn alloys, respectively. Moreover, further reduction of κ was noticed due to formation of the diffuse HH/FH interfaced FH-nanoprecipitates from the cyclic heating process. Consequently, the alloy has shown a maximum dimensionless figure of merit ( ZT ) up to 0.81 at 870 K.

Magnesium silicide (Mg2Si) thick films with (110) orientation were fabricated on (001) sapphire substrate using radiofrequency magnetron sputtering. Stoichiometric Mg2Si films with composition Si/(Mg + Si) = 0.33 were achieved over a range of vacuum from 10 mTorr to 140 mTorr and 300°C. On postannealing the film at 500°C, the out-of-plane lattice parameter shifted to lower values and the electrical conductivity increased by two orders of magnitude. A room-temperature Seebeck coefficient of 517 μV K−1 was observed and found to decrease with increasing temperature; the Seebeck coefficient remained at a constant positive value of 212 μV K−1 at 500°C. This can be related to the possibility of p-type conduction in this temperature range.

The microstructures of thermoelectric TiNiSn half-Heusler alloys have been studied in detail. For concentration ratios that are slightly rich in Ni, a high density of Heusler-phase nanosized precipitates tended to precipitate within a half-Heusler matrix. The morphology and average size of the Heusler nanoprecipitates were very sensitive to the Ni concentration ratio in the half-Heusler matrix of the alloys. Smaller Heusler nanoprecipitates with coherent ellipsoidal ( HH orientations. Interfacial defects between the Heusler and half-Heusler phases, as well as lattice point defects, Ni antisites and vacancies, were found to be closely related to the formation of the Heusler nanoprecipitates. A mechanism has been proposed in this study to describe the coarsening of the Heusler nanoprecipitates via the formation of lattice point defects and interfacial defects.

Dislocation character and operative slip systems in α-Nb 5 Si 3 were examined by transmission electron microscopy. Two-phase alloys comprised of (Nb) and α-Nb 5 Si 3 were used in this study. Although few dislocations were present in the α-Nb 5 Si 3 phase of a pre-deformed alloy, many developed after 15% of compressive deformation at 1673 K. Two types of the Burgers vectors were identified for α-Nb 5 Si 3 : 5 Si 3 at 1673 K were determined as {011)

To evaluate the effect of the substitution of the M-site atoms Ti, Zr, and Hf on thermoelectric properties, nearly single-phase (M0.5a,M0.5b)NiSn alloys (Ma, Mb = Hf, Zr, Ti) were fabricated by directional solidification. The lattice thermal conductivity can be most effectively reduced in the (Ti0.5,Hf0.5)NiSn alloy, probably due to the solid-solution effect, since the difference in atomic size and atomic mass are maximized between Ti and Hf. Moreover, we have found the phase separation between Ti-rich and Ti-poor half-Heusler phases in (Ti0.5,Hf0.5)NiSn and (Ti0.5,Zr0.5)NiSn alloys through observation of microstructure, chemical concentration measurement, and x-ray diffractometry.

Development of primary recrystallization was studied in a steel containing 0.4 vol.% of fine TiC precipitations with an average size of 12 nm. After sufficiently large cold strains, the recrystallization developed readily upon annealing at temperatures above 600 °C. An increase in the cold strain as well as the annealing temperature resulted in the acceleration of recrystallization kinetics. However, a certain amount of cold worked microstructures of about 15 vol.% remained unrecrystallized even after annealing at a rather high temperature of 700 °C. The unrecrystallized portions were composed of grains with the 〈0 0 1〉 crystallographic direction parallel to the compression axis. Both the low stored energies in these grains and the pinning of recrystallizing grain boundaries by the dispersed carbides were discussed as crucial factors that resulted in the incomplete recrystallization.

Mechanisms of microstructure evolution during annealing after cold working were studied in an Fe-15%Cr ferritic stainless steel, which was processed by bar rolling/swaging to various total strains ranging from 1.0 to 7.3 at ambient temperature. Two types of recrystallization behavior were observed depending on the cold strain. An ordinary primary (discontinuous) recrystallization developed in the samples processed to conventional strains of 1.0–2.0. On the other hand, rapid recovery at early annealing resulted in ultrafine-grained microstructures in the larger strained samples that continuously coarsened on further annealing. Such annealing behavior was considered as continuous recrystallization.

Aiming for the design of simple thermal coatings including Ir as a diffusion barrier layer with B2 aluminides bond coat such as CoAl, diffusion behavior and phase equilibria at the Ir/CoAl interface have been investigated to understand thermal and chemical stability of the interface. Diffusion couples were prepared with various conditions of Ir, plate, powder and film deposited to examine the effects on diffusion behavior. Moreover, the interdiffusion coefficient of binary Ir-M in the Ir solid solution, DIr-M was determined using Boltzmann-Matano method, where M is selected form elements used in heat resistant alloys. The potential of Ir as a diffusion barrier has been evaluated particularly against diffusion of Al.

Various investigations have been attempted to improve the low temperature ductility of Nb-silicides by microstructural control while they show superior high temperature strength. Present authors have focused on the microstructure evolution through the eutectic and eutectoid reactions in Nb-rich portion of Nb-Si binary system, and with small amounts of additives (Zr or Mg) alloys large Nb grains with fine silicide (α-Nb 5 Si 3 ) particles have been obtained, which is attractive for high temperature use. For further understanding of this phenomenon, the present study has two objectives; one is to apply the advanced solidification technique for further microstructure control, and the other is to investigate the effect of co-existence of Zr and trace amount of Mg on the microstructure evolution during the eutectoid reaction in terms of the interfacial energy between phases. EBSD analysis revealed that uni-directionally solidified alloy show the same crystallographic orientation relationship (O.R.) between Nb and α-Nb 5 Si 3 with that in arc-melted alloy having the same composition. On the other hand, Mg-doped alloy containing Zr shows an O.R. which was not observed in previous works. This implies that Mg doping is effective to control the interfacial energy between Nb and α-Nb 5 Si 3 even in Nb-Si-Zr alloys. Two-step heat-treatment is found to be effective to obtain finer microstructure, and a further investigation on the controlling factors of eutectoid decomposition will provide a proper route to well-controlled microstructures.

A class of intermetallics of TiNiSn half-Heusler compound with MgAgAs structure type is currently of interest as a potential high temperature thermoelectric material. The ternary TiNiSn compound has showed promising thermoelectric properties, a high Seebeck coefficient and low electrical resistivity. The present study reports the effect of Hf alloying on Ti site, Pt and Pd alloying on Ni site, and Sb doping on Sn site for the optimization of thermoelectric properties of TiNiSn-based compounds. Also, to achieve a low thermal conductivity, a powder metallurgy technique is used for the fabrication of the compounds. These efforts result in the dimensionless figure of merit, ZT as 0.78 for the hot-pressed (Ti0.95Hf0.05)Ni(Sn0.99Sb0.01) sample at 770 K with a large power factor (4.1 mW/mK2), which makes these materials very attractive for potential power generation applications.

The (Nb)/Nb3Si eutectic alloys in the Nb–Si binary and Nb–Ti–Si ternary systems were unidirectionally solidified in an optical floating zone melting furnace. The effect of solidification rates on microstructures of the alloys is investigated. The coupled growth of (Nb) and Nb3Si is observed in the binary alloy solidified at 10 mm/h. The ternary alloys exhibit the development of discontinuous (Nb) particles within the Nb3Si matrix. The unidirectionally solidified alloys show improved fracture toughness and enhanced high temperature compressive strength as compared with the arc-melted counterparts. The role of an aligned eutectic microstructure in toughening is discussed from the existing models based on the crack bridging and the crack deflection.

Phase equilibria in Co–Al–C ternary and Co–Ni–Al–C quaternary systems were investigated to establish the basis for designing new class of α(Co) based heat resistant alloys strengthened by the E2 1 type intermetallic compound Co 3 AlC. Phase stability of E2 1 (Co, Ni) 3 AlC was examined from the viewpoint of magnetic properties such as Curie temperature and saturation magnetization. The possibility of two-phase separation is indicated between E2 1 (E2 1 ′) (Co,Ni) 3 AlC and E2 1 (L1 2 ) (Ni,Co) 3 Al(C) in the Co–Ni–Al–C quaternary system, where we denote E2 1 ′ as standing for the ordered crystal structure of (Co,Ni) 3 AlC 0.5 formed by the extra ordering of carbon atoms. Phase diagrams information was determined by means of electron probe microanalysis and microstructural observation for the T–Al–C (T: Co, Ni, Rh, Ir) and T–Al–B (T: Rh, Ir) systems to examine the phase equilibria in each alloy system focusing on the existence of E2 1 T 3 AlC and T 3 AlB. The existence of E2 1 Ir 3 AlB (E2 1 ′ Ir 3 AlB 0.5 ) phase has been revealed in the Ir–Al–B system by diffraction analysis of transmission electron microscopy and electron probe microanalysis.

Ferritic heat-resistant steels involving intermetallic Laves phase have drawn a growing interest for the enhancement of creep strength, while the brittleness of Laves phase may lower the toughness of the alloy. We believe it is possible to modify the morphology of Laves phase precipitates by controlling the α-Fe matrix microstructure. In order to make clear the influence of matrix microstructures on age-hardening, the precipitation behavior of Laves phase was investigated by transmission electron microscopy (TEM). The matrix of the Fe–10Cr–1.4W–4.5Co (at%) alloy is controlled by heat treatments so as to provide three types of microstructures; ferrite, ferrite+martensite, and martensite. Alloys with ferrite and ferrite+martensite matrices show age-hardening behavior comprised of two hardness peaks. At around the first hardness peak, it is revealed by TEM observation that fine particles precipitate coherently within the ferrite matrix. In the martensite matrix, most of R-phase and Laves phase precipitates exist on laths and dislocations.

The crystallography of a Nb/Nb5Si3 lamellar microstructure formed by Nb3Si decomposition has been investigated by transmission election microscopy. Upon solidification of a Nb–10 at.% Ti–20 at.% Si alloy, the high-temperature phase Nb3Si is retained at room temperature. During annealing at 1673 K, the Nb3Si phase transforms into the Nb and Nb5Si3 (D8l) phases with a lamellar microstructure through a eutectoid reaction. The orientation relationship between Nb and Nb5Si3 is determined by selected-area diffraction analyses as , and [133]Nb//[111]Nb5Si3. The present orientation relationship is explained by good atomic matching on the close-packed planes along the near close-packed directions.

E21 Co3AlC might be used as a strengthener to develop new Co-based heat-resistant alloys. To establish the basis of microstructure control for further improving mechanical properties, phase equilibria in the CoAl-C ternary system were investigated and phase diagram information particularly related to E21 Co3AlC and liquid phase were evaluated and reconsidered. Single crystals of E21 Co3AlC have been grown successfully for the first time using optical floating zone (OFZ) melting. Extra ordering of carbon atoms taking place in E21 Co3AlC was confirmed using single crystals by transmission electron microscopy. As a result of this ordering, E21 ′ Co3AlC0.5 accompanies formation of antiphase boundaries. A wide variety of the two-phase Co3Alc/α(Co) microstructures were prepared by unidirectional solidification using OFZ.

Isotherms of the Ir-rich portion of the Ir–Al–X (X: V, Nb, Ta) systems composed of fcc-Ir solid solution ((Ir) ss ), Ir-based L1 2 intermetallic compounds Ir 3 X and B2–IrAl are presented as a second report of a series of investigations on the phase equilibria in the (Ir or Rh)–Al–X (X: Ti, Zr and Hf, or, V, Nb and Ta) ternary systems. In both Ir–Al–Nb and Ir–Al–Ta systems fcc/L1 2 /B2 three-phase equilibrium is found. In Ir–Al–V system, Ir solid solution and Ir 3 V have close compositions each other and fcc/L1 2 /B2 three-phase equilibrium is expected. No ternary compound is observed in all of the isotherms. Solid solubility of each phase is confirmed by chemical analysis by wavelength dispersive X-ray spectroscopy (WDS). The direction of the solubility lobe of L1 2 –Ir 3 X with Al addition is explained in terms of the nearest neighbor interactions. Liquidus surfaces are also established by taking into account the microstructure of as-cast alloys, differential thermal analysis (DTA) data, and the Ir–Al and the Ir–X binary phase diagrams reported previously.

Atomic diffusion and phase equilibria have been investigated at the interfaces of Ir/CoAl and Ir/Nb5Si3 to evaluate the suitability of a diffusion-barrier layer of Ir between an oxidation-resistant layer of B2-CoAl and a base material Nb5Si3. Diffusion couples were prepared by hot pressing and annealed at 1573 K for up to 178 hours. Diffusion layers of (Ir, Co) solid solution and B2-(Ir, Co)Al were formed at the Ir/CoAl interface. The concentration of Al dramatically dropped at the interface, which indicates that the Ir layer effectively works as the diffusion barrier against the inward diffusion of Al. To quantitatively evaluate the potential of Ir as a diffusion barrier, the Boltzmann-Matano analysis was employed to determine the diffusion coefficient of Al using Ir-8 at. pct Al/Ir diffusion couples annealed at temperatures of 1573, 1673, and 1773 K. For instance, an extremely low value of 7.0×10−19 m2/s is evaluated for Ir-4 at. pct Al at 1573 K. At the Ir/Nb5Si3 interface, the intermetallic phases Ir3Si and Ir3Nb are formed on the Ir side and the Nb5Si3 side, respectively. The formation of Ir3Si is controlled by the diffusion of Si through Ir3Nb in which the solubility of Si is limited quite small.

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 0.3at%Si. It is intended to provide basic information for the alloy design of ferritic heat resistant alloys strengthened by intermetallic compounds. In the alloy containing Si, icosahedral quasicrytalline phase (I-phase) is found to precipitate during aging at 873K. 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 873K, and the Laves phase precipitates in both alloys during aging at higher temperature, 973K. The Laves phase formed at 973K transforms to the I-phase in the Si-added alloy but to the R-phase in the Si-free alloy during subsequent aging at 873K. The factors in controlling the phase stability of I-phase, R-phase and Laves phase precipitates in Fe-based alloys are discussed by the atomic size ratio and electron concentration factor (e/a).

Aiming for further improvement of mechanical properties of Co3AlC-based heat resistant alloys, microstructure control was conducted using optical floating zone (OFZ) melting. Unidirectional solidification was performed to align Co3AlC/a(Co) two-phase eutectic microstructure. Co3AlC single phase poly-crystal alloys were successfully fabricated for the first time by taking advantage of OFZ. Mechanical properties were evaluated for selected alloys by compression tests at ambient temperature, 1073 K and 1273 K. Excellent elevated temperature strength is achieved in Co3AlC single phase alloys and ductility is sufficiently improved in Co3AlC/a(Co) two-phase alloys.

We previously reported that IrSb3-based ternary compounds showed good electrical properties and that relatively high thermal conductivity prevented the enhancement of the efficiency of these compounds.1 Recently, we have investigated the high-temperature thermoelectric properties of La-filled IrSb3-based skutterudite compounds from the viewpoint of a decrease in lattice thermal conductivity. It has been confirmed from Rietveld analysis that the La ions are partially placed in cages of La-filled and Ge-charge-compensated LaYIr4Ge3YSb12−3Y compounds and 53% filling of La ions is obtained. Owing to the rattling effect of La ions in cages, the La-filled and Ge-charge-compensated LaYIr4Ge3YSb12−3Y compounds exhibit tremendously decreased lattice thermal conductivity at room temperature, 1.8 W/mK from 10.2 W/mK of binary IrSb3.

Phase constitution and microstructure of Fe-corner of the ternary Fe–Si–Cr system are investigated particularly on the multiphase fields involving bcc disordered phase, A2 phase, and bcc ordered phases, B2 and D0 3 phases. The (A2 + D0 3 ) two-phase region is found to exist in compositions between 12 and 15 at.% Si at 10% Cr at 873 K.

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 IrSb3-based skutterudite compounds have a potential for thermoelectric applications because of high Hall mobility, Seebeck coefficient, and relatively low thermal conductivity. In the present study, polycrystalline p- and n-type IrSb3 compounds are prepared by powder metallurgy techniques. The effect of doping on thermoelectric properties has been investigated in binary and ternary IrSb3 compounds using Ru, Ge, Pd, or Pt as a dopant. It is shown that the electrical properties depend strongly not only on the kinds of doping impurities but also their levels. Our theoretical analysis suggests that the effective mass is significantly affected by doping impurities and the levels.

Half-Heusler-type compounds have gained increasing attention as promising thermoelectric materials. In the present work, a focus is placed on TiNiSn with additions of Hf, Zr, Si, or Pt. Nominally stoichiometric TiNiSn alloys were prepared using arc melting and subsequent annealing at 1,073 K for 2 weeks. The thermoelectric properties, such as thermoelectric power, electrical resistivity, and thermal conductivity, were measured in a temperature range from 300 K to 1,000 K. As-cast materials show metallic transport properties, while annealed ones exhibit semiconductor behavior. Microstructures of TiNiSn alloys basically consist of nonequilibrium four-phase; half-Heusler TiNiSn, Heusler TiNi2Sn, metallic Ti6Sn5, and Sn solid solution. The volume fraction of the half-Heusler TiNiSn phase significantly increases by annealing. It is revealed that coexisting metallic phases degrade the thermoelectric properties of half-Heusler TiNiSn. Alloy additions strongly affect not only thermoelectric properties but also phase stability. The thermal conductivity of TiNiSn alloys with alloy additions decreases because of the point-defect phonon scattering.

In order to make clear the effect of matrix substructures on the Laves phase precipitation in Fe-Cr-Nb-Ni system, age hardening behavior was examined during isothermal aging up to 1 000 h at 873 and 973 K and the precipitation morphology of the Laves phase was investigated by transmission electron microscopy (TEM). The microstructures of Fe-10Cr-1Nb and Fe-15Cr-1Nb (mol%) alloys with ferrite matrix contain high density of rod-shaped Laves phase which precipitates within the ferrite grains during aging. On the other hand, in the Fe-10Cr-1Nb-1Ni alloy with massive ferrite matrix, most of the Laves phase precipitates in subgrain boundaries. Additionally the deformed ferrite matrix has been prepared for examining the effect of dislocation density on morphology of the Laves phase. It is found that the difference of matrix substructures between the ferrite and deformed ferrite provides the different number and distribution of nucleation sites for precipitation of the Laves phase. Mechanical properties were evaluated by tensile tests conducted at room temperature, 873 and 973 K.

The isotherms of the Rh–Al–C and Ir–Al–C ternary systems were determined at 1373 K for the transition metal rich corner. It has been revealed that the three-phase field consisting of primary solid solution of a transition metal, B2 type intermetallic compound and graphite extends over a very wide compositional range in these two alloy systems. There exists no carbide in related transition metal rich regions. The phase constitution in the Rh–Al–C and Ir–Al–C ternary alloys prepared by arc melting is basically the same as that by spark plasma sintering (SPS) except the presence of Al 2 O 3 particles. The reaction scheme and liquidus surface projection in the transition metal corner of the Rh–Al–C and Ir–Al–C ternary systems were estimated by deliberating on the solidification sequence, as-cast microstructures and invariant reactions on the binary edges.

We have proposed microstructural modification through lattice misfit control on the γ-Fe/E2 1 –(Fe, Mn) 3 AlC x two-phase alloys, aiming for the improvement of mechanical properties. Since it is quite difficult to independently control lattice parameters of both γ-Fe and E2 1 , we took the approach to educe the E2 1 lattice parameter based on two strategies, lowering carbon content in E2 1 by optimizing Al content of alloy composition, and the addition of Ni intending to reduce the phase stability of the E2 1 relative to the L1 2 . The former attempt effectively decreases the lattice parameter of E2 1 and increases that of γ-Fe, by which lattice misfit is reduced. The latter idea is effective for reducing the lattice parameter of γ-Fe, while the lattice parameter of E2 1 slightly decreases only with the first 2 at.% Ni addition and remains almost constant. Consequently, lattice misfit increases, against our expectation.

A new class of heat resisting ferritic steels is investigated in the Fe–Cr–Nb and Fe–Cr–Nb–Ni systems, in which the major strengthener is the Fe2Nb Laves phase. Ferritic steels strengthened by Laves phase are expected to show excellent high temperature strength, while the brittleness of Laves phase may lower the toughness of the alloy. The α-Fe/Fe2Nb two-phase microstructure is selected to improve mechanical properties through changing volume fraction and morphology of Laves phase. Effects of Nb and Ni contents on the microstructure and mechanical properties of the alloys, fixed at 10 at.% Cr, have been systematically investigated. Mechanical properties were evaluated by tensile tests conducted at room temperature, 873 and 973 K. The tensile test revealed that the room temperature ductility decreases with increasing Nb content. The alloys with 1.0–1.5 at.% Nb are found to exhibit a good balance between room temperature ductility and high temperature strength.

Compression tests at very low temperature down to 4.2 K and conventional transmission electron microscopy were conducted to investigate the twinning behavior of the C15 HfV2+Nb Laves phase. We chose two phase C15/bcc alloys to improve the ambient temperature ductility since the single Laves phase intermetallic compound is brittle. Transmission electron microscopy revealed that substantial mechanical twinning takes place in the C15 Laves phase matrix at room temperature and 77 K. A deep minimum in the flow stress, with a drop of nearly 500 MPa, appears at around 77 K in the C15/bcc two-phase alloy with C15 matrix. Since no such anomaly is seen in the bcc phase, we believe that the cause of this can be attributed to mechanical twinning in the C15 Laves phase. Twin bands observed in the C15 matrix of deformed samples at both 298 and 77 K can be classified into three categories by their thickness; coarse twin bands about 10 nm to several hundred nm thick, fine twin bands around 3–10 nm thick, and ultra fine twin bands with average thickness of 1.5 nm. A high density of ultra-fine twin bands is the characteristic feature of twinning in the C15 matrix. They belong to the 〈112〉(111) twinning system and commonly intersect with each other.

The crystallography of the lamellar structure composed of the tetragonal C11b phase and the hexagonal C40 phase which is similar to the well-known TiAl/Ti3Al lamellae was investigated by means of transmission electron microscopy in a MoSi2–15mol%TaSi2 pseudo-binary alloy after homogenized at 1400 °C for 168 h. It was found that the orientation relationship between C11b and C40 phases obeys the coincidence of the equivalent hexagonal atomic planes for the two phases. Three distinct variants of this orientation relationship were identified by electron diffraction. 1 3 1 1 0 Dislocations prevail in the C11b/C40 interfaces to accommodate the lattice misfit between the two phases. On the other hand, networks of 1 3 1 1 0 and 1 3 1 1 3 dislocations are frequently observed in C40 phase. Significance of the 1 3 1 1 3 c-component dislocations for improvement of the plasticity of C40 phase has been discussed.

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.

Mechanical behavior of multi-phase alloys, consisting of intermetallic compounds B2 type CoAl and E2 1 type Co 3 AlC with primary solid solution (Co), was investigated by compression tests over a wide temperature range from 77 to 1273 K. It has been revealed that E2 1 Co 3 AlC is a good candidate of coexisting phase with B2 CoAl to improve the alloy's ambient temperature ductility and its elevated temperature strength. Most of the multi-phase Co–Al–C ternary alloys exhibit fairly good compressive ductility at whole test temperatures, even at 77 K. All the alloys show the anomalous positive temperature dependence of strength at intermediate temperatures. The deformation mechanism of E2 1 Co 3 AlC is expected to be the same as that of L1 2 intermetallic compounds because of the close resemblance of their crystal structures. The three-phase B2/E2 1 /(Co) alloy, 70Co20Al10C(at%), exhibits excellent ductility of virtually unlimited ambient temperature compressive plastic strain and the highest strength around 340 MPa at 1273 K.

Limited ductility and toughness at low temperatures are serious disadvantages in intermetallics. As in other metallic materials, the two major brittle failure modes in intermetallics are cleavage and intergranular fracture. We focus on cleavage fracture and the corresponding brittle-to-ductile transition mechanism. To prevent cracking, a ‘sufficient number’ of dislocations must be generated at a crack tip, and this ‘sufficient number’ appears to be very large. This event is more like a phase transition than an ordinary thermally activated process. In this paper a new theoretical model, proposed to explain the BDT is summarized. The micro-structural implications of this model are presented and the means of achieving an acceptable combination of strength and toughness through microstructural control are then discussed, along with examples.

A summary is presented of the recent advances in the development of multiphase intermetallic alloys towards improved room temperature ductility investigated by the present authors. The alloy systems of interest are Ni-Al-Be, Co-Al-Ni-Ti, and Co-Al-C, in which two- or three-phase alloys can be designed involving the Ll2, B2 and the primary solid solution, hereby denoted as (Ni) or (Co,Ni) in the first two systems, while the B2, E21, and the primary solid solutions, denoted as (Co) in the latter system. It is shown that by choosing a proper combination of phases with particular morphologies, good room temperature ductility can be obtained not only by compression and bending tests but also by tensile tests. Such a strategy has been so far most successful in the Co-Al-Ni-Ti system, where room temperature ductility of near to 20% is achieved in a B2/Ll2/(Co,Ni) three-phase alloy. It is pointed out that keys to design ductile multi-phase intermetallic alloys would be: (1) refinement of the microstructure utilizing solid state phase transformations such as an invariant reaction: (2) adjustment of amounts and compositions of the constituent phases, which is not possible in the binary system, but is in a multi-component system, and (3) hot fabrication to reduce solidification defects.

Tensile tests were carried out on several alloys based on the B2 intermetallic compound CoAl. The alloys are multi-phase with Co primary solid solution, L1 2 (Co,Ni) 3 Al, or E2 1 Co 3 AlC; they were developed by the present authors and showed some room temperature ductility previously under compression testing. Also, attempts to refine the microstructure of the selected alloys were made using heat treatment and hot or cold rolling. Although no drastic improvement in room temperature ductility is observed, indications of a microstructure favorable for improved ductility are obtained. Also, it is found that some type of solidification defect causes premature failure during tensile tests, but not during compression tests, for which a cure should be sought after further improvement in the mechanical properties.

The CoAlC ternary phase diagram has been experimentally examined for the Co-corner with a particular interest in phase relations among cobalt primary solid solutions, denoted as (Co), the E21 type intermetallic compound Co3AlC, and the B2 type CoAl. Liquidus surface composed with monovariant and contour lines, and corresponding reaction scheme were determined by optical microscopy, differential thermal analysis, X-ray diffraction, and energy dispersive X-ray spectroscopy. Several isothermal sections and isoplethals with respect to a fixed concentration for one of the elements were also evaluated. It is shown that such multiphase alloys as B2/E21, B2/(Co), and B2/E21/(Co) can be produced in the ternary system by proper choices of alloy chemistry and heat treatment.

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