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Ca-Mg-Si ternary films were deposited at 300–335°C on (001)Al2O3 substrates by a radio-frequency magnetron sputtering. Amorphous films were obtained for a wide range of compositions but not for single-phase CaMgSi. For all compositions, the electrical conductivity of the as-deposited films increased with the increase in temperature up to 400°C. The conduction type was controlled mainly by changing the Si/(Ca + Mg + Si) ratios of the films, and films with p- and n-type conductions were observed respectively with Si/(Ca + Mg + Si) ratios below 0.6 and above 0.7 along a fixed Ca/(Ca + Mg) ratio of about 0.50. A high thermoelectric power factor above 140 μW/(m K2) with p-type conduction was obtained at 400°C for an amorphous-phase film.

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.

A method for controlling the conduction-type in Mg2Si films without doping is investigated. Mg2Si films exhibit p-type conduction after a post-heat treatment up to 500 °C in atmospheric He. However, covering the films with Mg ribbon during a subsequent heat treatment at 500 °C converts the conduction to n-type, demonstrating that the heat treatment atmosphere can control the conduction type. Based on the reported first principles calculations suggesting that interstitial Mg and Mg vacancies in Mg2Si are the origins of n-type and p-type conduction, respectively, the post-heat treatment in He induces Mg vacancies due to the evaporation of Mg from the film, resulting in p-type conduction. The subsequent heat treatment when the film is covered with Mg ribbon fills the Mg vacancies and the additional interstitial Mg is incorporated, resulting in n-type conduction. These observations differ from the reported data for heat treatment of stable n-type conduction in non-doped Mg2Si-sintered bodies and may realize a novel control method for the conduction type in Mg2Si films.

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 new concept of thermally power generation system, a sensitized thermal cell (STC) based on a dye-sensitized solar cell, had been suggested. In this STC, thermally excited charge carriers in a semiconductor were used instead of photon excited charge carriers in a dye. In the previous repot, the β-FeSi2 used as the semiconductor included impurities and the battery characteristics was low. Here, to obtain high purity β-FeSi2, the combination method of arc melting and drop cast was used. As a result, the battery characteristics was improved. The present results show that the properties of the semiconductor contribute to the performance of the STC.

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.

Mg2Si thin films were deposited at 320 °C on (001)Al2O3 and (100)CaF2 substrates by radio-frequency magnetron sputtering. Both films showed a preferential (111) out-of-plane orientation with an in-plane random orientation irrespective of post-heat treatment. Mg2Si films on (001)Al2O3 substrates were under in-plane tensile strain, while those on (100)CaF2 substrates were under in-plane compressive strain both before and after heat treatment. Heat-treated films showed p-type conduction up to 500 °C. Their electrical conductivity and Seebeck coefficient were almost independent of the kind of substrate within the limit of the present study, from 0.22% compressive strain to 0.34% tensile strain at room temperature.

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.

Single crystals of E21 (L12) Ni3AlC1-x were prepared by the unidirectional solidification using the optical floating zone melting method to determine their mechanical properties. Particularly the effects of interstitial carbon atoms on mechanical properties were evaluated by compression tests at room temperature. Operative slip system of E21 Ni3AlC is {111} type which is the same as that of L12 Ni3Al. Strength of Ni3AlC single crystals increases with carbon concentration due to the solid solution effect, though the stress relief of yielding behavior is enhanced at the intermediate carbon content at around 3at%. A large gap appears in the carbon concentration dependence of critical resolved shear stress (as well as yield stress) at almost the same carbon content. This discontinuity in strengthening is attributed to the interaction between multiple solute carbon atoms and mobile dislocations.

The n-type thermoelectric properties of the half-Heusler compound ZrNiSn can be converted to p-type by the addition of Co and Ir. We found that Co and Ir atoms preferably occupy the vacancy sites instead of substituting at Ni sites. This implies that the phase stability of the compound gradually changes towards that of the Heusler compound Zr(Ni,M)2Sn, where M is Co and/or Ir. The occupation of vacancy sites by Co and Ir atoms leads to a drastic reduction in lattice thermal conductivity owing to the enhancement of phonon scattering by the solid solution effect.

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)

Focusing on the durability of carbon steel railway wheels, so-called tread thermal cracks, one of severe damages on the tread surface, were investigated to understand the cracks generation processes involving initiation and propagation. We have demonstrated that tread thermal cracks can be experimentally reproduced under the condition of concurrent loading of continuous rolling contact with rails and cyclic frictional heat from brake blocks, through the experiments using an actual railway wheel. Relations of residual stress, cracks configuration and fracture surface were examined to consider the crack generation process. Plastic deformation and thermal stress near the surface result in substantial tensile residual stress, which causes the generation of cracks.

Associated with the durability of railway wheels, microstructure development of high-carbon steel (0.67% C) during uniaxial compressive deformation and after subsequent annealing was studied using scanning electron microscopy and electron back scattering diffraction. The spheroidization of cementite is accelerated due to compressive deformation depending on the temperature. Annealing at 600 °C leads to the formation of ferrite grain boundaries due to continuous recrystallization and results in an ultrafine microstructure with a ferrite grain size of less than 1 μm. Formation of high angle boundaries due to the dislocation accumulation on low angle boundaries results in an equiaxial fine microstructure after deformation at 700 °C. Annealing at 600 °C or higher leads to the coarsening of spheroidized cementite, which reduces the pinning effect of the particles to the migration of high angle boundaries. The compressive yield stress increased approximately 25% through microstructure development, along with a decrease in the work hardening rate.

The microstructure development of a high-carbon steel (0.67 pct C) for railway wheels as they are affected by rolling contact with rail tracks and by cyclic frictional heat from braking is studied in the vicinity of the contact surface by scanning electron microscopy (SEM) and electron backscattered diffraction. An ultrafine microstructure consisting of ferrite grains with a size of less than 1 μm and spheroidized cementite particles is formed in the region up to 100 μm below the contact surface. The generation of low-angle sub-boundaries associated with the rearrangement of accumulated dislocations involved in continuous recrystallization of ferrite microstructure contributes to the microstructure refinement at temperatures lower than A1 temperature (1000 K). Fine spheroidized cementite particles with uniform distribution obstruct the migration of high-angle grain boundaries, by which the dislocation density is maintained sufficiently high for the formation of sub-boundaries. The formation of a texture, corresponding to the surface texture typically formed in low-carbon steels by hot rolling without lubrication at ferritic temperatures, is observed in the ultrarefined microstructure region. The results drawn from this study strongly indicate the occurrence of “in-situ microstructure control” under service conditions.

Mechanical tests have been carried out on Fe–15%Cr ferritic stainless steel with various microstructures. Ultrafine-grained microstructures with grain sizes of 0.2–0.3 µm were developed by large-strain cold-working and light annealing. The effects of severe deformation on the mechanical behaviour of as-processed and recovered steel were evaluated with reference to the same material having conventional work-hardened and recrystallised microstructures. Despite the low dislocation density in the fine grain interiors in the as-processed state, the samples with strain-induced submicrocrystalline structure were characterised by high internal stresses that resulted in a higher strength than could be expected from simple grain-size strengthening. These internal stresses were associated with a non-equilibrium state of strain-induced grain boundaries after severe deformation.

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.

Deformation behavior of Nb–Si–Zr alloys is investigated at various temperatures ranging from R.T. to 1670 K. The master alloy ingots composed of Nb–18.1 at% Si–1.5 at% Zr doped with Mg are Ar-arc-melted. The ingots contain Nb rods (radius: 1 μm) in Nb 3 Si matrix formed by eutectic reaction. Alloys are subjected to heat treatments at 1923 K for 4–100 h to obtain a large Nb network structure with small silicide (α-Nb 5 Si 3 ) particles by decomposing Nb 3 Si matrix into Nb and Nb 5 Si 3 through a eutectoid reaction. Compression tests are conducted at room temperature in air and at elevated temperatures in Ar atmosphere. At 1471 K the maximum strength is 500 MPa and compressive ductility is higher than 10% with a strain rate of 1.0 × 10 −4 s −1 , while at room temperature the maximum strength is over 1500 MPa and compressive ductility is about 1.5%. The high-temperature deformation obeys a power-law type equation. The stress exponent n is evaluated to be 4.8 and the apparent activation energy is 350 kJ/mol. The Vickers indentation at room temperature revealed that the crack propagation at room temperature is suppressed effectively by ductile Nb. This suggests that the Nb aggregate in the network structure acts as a large Nb grain containing fine Nb 5 Si 3 particles, which might be beneficial for ductility at low temperatures.

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.

15%Cr ferritic stainless steel was machined in rectangular samples and then processed by multiple forging to a total cumulative strain of 7.2 at an ambient temperature. The large strain deformation resulted in almost equiaxed submicrocrystalline structure with a mean grain/subgrain size of 230 nm and about 2.2×1014 m-2 dislocation density in grain/subgrain interiors. The annealing at a relatively low temperature of 500oC did not lead to any discontinuous recrystallizations. The grain/subgrain size and the interior dislocation density slightly changed to 240 nm and 2.1×1014 m-2, respectively, after annealing for 30 min, while the Vickers hardness decreased from 3140 MPa in the as-processed state to 2900 MPa. This annealing softening was attributed to remarkable release (by 50%) of internal stresses, which are associated with a non-equilibrium character of strain-induced grain/subgrain boundaries.

Deformation microstructures of an Fe–15%Cr ferritic stainless steel during cold working to strains above 7 were studied. Two processing methods, unidirectional working consisting of bar rolling followed by swaging and multidirectional working involving multiple forging through three orthogonal axes, were used. The unidirectional deformation resulted in the evolution of a ribbon-like microstructure consisting of highly elongated grains. The mean grain size gradually decreased to about 0.4 μm during rolling/swaging to strains above 7. In contrast, the multiple forging resulted in the development of almost equiaxed fine grains with an average grain size of 0.45 μm at relatively small strains of about 4 followed by a slight grain refinement during further processing. On the other hand, both processing methods were characterised by almost the same strain dependence for the transverse grain and subgrain sizes, which were measured crosswise to the metal flow direction.

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.

Unidirectional solidification experiments were performed on the (Nb)/Nb3Si eutectic alloys in the Nb–Si binary and Nb–Ti–Si ternary systems. The effect of solidification rates on microstructure development was studied using an optical floating zone melting furnace within the solidification rates from 10 to 200 mm/h. For the binary invariant eutectic alloy, a planar eutectic microstructure forms at 10 mm/h, while cellular eutectic microstructures develop at the higher solidification rates. The orientation relationship between Nb and Nb3Si formed upon planar eutectic solidification is determined as: {1 1 0}Nb//{1 1 0)Nb3Si, and 〈1 1 2〉Nb//〈0 0 1]Nb3Si. For the ternary univariant eutectic alloys, the formation of cellular eutectic microstructures is prevailing for all over the rates of solidification. The growth of Nb3Si is faster than that of (Nb). The size distribution of (Nb) particles spreads widely in an asymmetric manner. The cell size variation against solidification rates is in a reasonable agreement with the theoretical prediction.

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.

Mechanical properties of E2 1 based Co 3 AlC single crystals and single-phase polycrystalline Co 3 AlC and Fe 3 AlC alloys have been evaluated by compression tests performed in a wide temperature range from room temperature to 1373 K. Effect of the ordering of C on the phase stability and mechanical properties has been investigated and discussed with the anti-phase boundary (APB) formation. Temperature dependence of critical resolved shear stress (CRSS) has been determined to consider the compressive mechanical behavior of Co 3 AlC single crystal. Yield strength anomaly appears in Co 3 AlC single crystal as we have expected from the similarity between E2 1 ′ and L1 2 . Schmid law is only applicable in a high temperature range between 1173 and 1273 K. Crystallographic analysis and microstructure observation were conducted using transmission electron microscopy (TEM). It was found that disordering of C atoms could be induced by the irradiation damage of electron beam after high-resolution transmission electron microscopy (HRTEM) observation.

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.

Effects of pre-strain on the compressive mechanical behavior were investigated on the alloys with an aligned lamellar microstructure consisting of C11 b MoSi 2 and C40 TaSi 2 prepared through optical floating zone (OFZ) method and annealing in the two-phase MoSi 2 /TaSi 2 region. Single crystals of C40 TaSi 2 were successfully grown at solidification rate of 5 or 10 mm/h, and well-aligned lamellar microstructure can be achieved by selecting an MoSi 2 –17 mol% TaSi 2 alloy. Firstly, compression tests were conducted to examine the effect of the angle ϕ between the aligned lamellae and the loading axis on strength and ductility. Results indicate that ϕ = 0, lamellae parallel to the axis, is a hard-orientation and ϕ = 54 and 40 (identically 45) are soft orientations. The alloy with ϕ = 0 shows higher strength but lower ductility than those with ϕ = 54 and 40 where ductility is evaluated in terms of brittle-to-ductile transition temperature (BDTT). Then effects of pre-straining at 1773 K on the mechanical behavior at 1573 K were investigated using soft-oriented lamellar alloys with ϕ = 40 whose BDTT is determined at least lower than 1673 K. Pre-straining to a few to several percent at 1773 K improves ductility of the alloy at 1573 K and also raises 0.2% flow stress, compared with the absence of the pre-strain. We believe that dislocations can be generated and stored in the alloy at 1773 K, and these dislocations are mobile even at 1573 K, although the analyses of operative slip systems and characteristics of dislocations remain as future works.

We focused on half-Heusler compounds MPtSn, where M is Hf, Zr and Ti, to seek for half-Heusler compounds which intrinsically show excellent p-type thermoelectric properties in a wide temperature range up to around 1000 K. Nearly single-phase MPtSn alloys were fabricated by directional solidification using optical floating zone melting method to evaluate thermoelectric properties as properly as possible. We have found that HfPtSn and ZrPtSn show p-type thermoelectric properties in a measured temperature range from 300 to 1100 K as it is expected from our previous work. It is interesting that HfNiSn and ZrNiSn with the same valence electrons count of 18 are well-known to show n-type thermolectric properties. HfPtSn shows quite high values of p-type thermoelectric power around 250 muV/K in low and intermediate temperature ranges, while ZrPtSn shows much smaller maximum value of about 70 muV/K. On the other hand, TiPtSn exhibits very large n-type thermoelectric power of around 500 muV/K at ambient temperatures though it decreases drastically at elevated temperatures. High electrical resistivity is a major drawback that all three MPtSn compounds have in common. HfPtSn has the lowest thermal conductivity among MPtSn though the values are relatively high. The lattice contribution is supposed to dominates the thermal conduction because of high electrical resistivity and low carrier concentration measured as 2.17 times 1025 m-3

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).

It has been shown that a fine lamellar structure composed of Nb solid solution, (Nb), and Nb5Si3 is formed through eutectoid decomposition in the Nb-Si binary system and its ternary derivatives. Such alloys would exhibit a high strength at over 1400 K, yet showing room-temperature toughness of over 10 to 20 MPa m1/2 if a proper lamellar spacing is chosen. In the present work, effects of processing on the microstructure evolution and mechanical properties are investigated on the Nb-18 at. pct Si alloys prepared by hot pressing (HP) and spark-plasma sintering (SPS). The powders used in the present work are of pure Nb and Nb5Si3 in order for the fabrication to become possible at temperatures higher than the melting point of Si and to reduce the formation of SiO2. The results show that the SPS yields more uniform two-phase microstructure but the alloy fabricated through HP tends to provide higher elevated temperature strength.

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.