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Aiming to improve thermoelectric properties of half-Heusler compounds MNiSn (M = Ti, Zr, Hf), phase equilibria in the (Ti,M)NiSn systems have been investigated focusing on the half-Heusler phase separation between TiNiSn and ZrNiSn, TiNiSn and HfNiSn. Solubility limit of an element M in each half-Heusler phase was determined by XRD and EPMA. Nearly single phase alloys were fabricated by the directional solidification using optical floating zone melting method to evaluate intrinsic thermoelectric properties of the alloys. Lattice thermal conductivity can be reduced in (Ti,M)NiSn alloys by phonon scattering due to the solid solution effect of M site substitution. Moreover, electrical properties can be enhanced by the Ti addition in (Ti,Zr)NiSn alloys. Among present MNiSn alloys, (Ti0.15,Zr0.85)NiSn alloy has the highest power factor of 5.3 mWm-1K-2 at around 745 K.

The evolution of microstructure in an Fe-0.67%C steel used for railway wheels has been investigated. To elucidate the mechanism of the ultrafine microstructure which is formed on the railway wheels tread surface, we have experimentally reproduced the same microstructure using uniaxial compressive deformation and subsequent annealing at 873 K. The deformation conditions required for ultrafine microstructure formation are the initial strain rate of 1 (=100) s-1 and total strain of 0.7. The mechanism of microstructural refinement is not the primary recrystallization but continuous recrystallization, i.e., the recovery process associated with the rearrangement of accumulated dislocations during deformation and annealing at temperatures between 773 K and 873 K. The recovery process never occurs at temperatures lower than 773 K. Primary recrystallization which involves the nucleation and growth of new ferrite grains takes place at temperatures higher than 873 K.

Half-Heusler compound TiNiSn is one of the most promising candidates of thermoelectric materials which can be used to directly convert the waste heat to clean electric energy at high temperatures (around 1000 K). Thermoelectric power generation is an appealing approach for conserving energy and preserving the global environment. Half-Heusler compounds have the cubic C1b type ordered structure and show semiconducting behavior when their valence electron count (VEC) is around 18. TiNiSn is the most attractive one not only because it has excellent thermoelectric properties but also it consists of eco-friendly elements which are neither toxic nor costly. However, TiNiSn has a bothersome problem that fabrication of single phase TiNiSn alloy is quite difficult. We have found that TiNiSn phase forms by the ternary peritectic reaction. Thereby, inevitable non-equilibrium solidification results in the formation of impurity coexisting phases which tend to decrease thermoelectric properties. In the present work, to establish the basis of new fabrication processes for TiNiSn alloys, we have started from the investigation on the diffusion paths which are closely related to the formation of TiNiSn phase. The diffusion behavior was evaluated using solid/liquid diffusion couples composed of the binary Ti-Ni intermetallic compounds and Sn liquid phase, where we have selected TiNi, TiNi3 and Ti2Ni as solid phases for instance. The most interesting result is that the single-phase TiNiSn phase layer forms at the TiNi/Sn(L) interface during annealing at 1073 K for only 1 h. Moreover, faceted grains of TiNiSn single-crystal grow at the interface toward the liquid Sn phase. We have confirmed two interesting microstructural features using EBSD analyses. One is that most of these TiNiSn single-crystals have the same crystallographic orientation, and the other is that TiNiSn phase layer formed on the TiNi side of the interface consists of very fine sub-microns grains. While TiNiSn solely forms at the TiNi interface, Heusler TiNi2Sn also forms with TiNiSn at the TiNi3 interface and Ti6Sn5 tends to coexist at the Ti2Ni interface.

Aiming to improve thermoelectric properties of half-Heusler (Ma,Mb)NiSn alloys (Ma,Mb = Hf, Zr, Ti), phase equilibria in the (Ma,Mb)NiSn systems were investigated focusing on the phase separation of TiNiSn from ZrNiSn and HfNiSn while (Zr,Hf)NiSn forms all proportion miscible solid solution. Diffusion couples consisting of liquid Sn and solid (Ti,Zr)Ni were used to examine the partitioning behavior which is associated with T-rich and Ti-poor half-Heusler phase separation during the reaction at the interface. Thermal conductivity can be reduced in (Ma0.5,Mb0.5)NiSn and (Ti0.13,Zr0.87)NiSn alloys due to the solid solution effect of M-site substitution. (Ti0.13,Zr0.87)NiSn alloy has high potential as a ecological thermoelectric material.

In order to evaluate the thermoelectric properties of n-type half-Heusler compound NbCoSn, single-phase NbCoSn alloy was prepared using directional solidification based on the phase diagram information of the Nb-Co-Sn ternary system which was investigated in this work. The isotherm at 1273 K and the reaction scheme together with the projection of liquidus surface were determined. Thermoelectric properties of NbCoSn based alloys were evaluated by Seebeck coefficient, electrical resistivity and power factor, which were measured in a temperature range from room temperature to 1073 K. NbCoSn has excellent values of Seebeck coefficient exceeding -250 μV/K at around 900 K and shows the metal-like temperature dependence of electrical resistivity.

To design and to develop Half-Heusler based high-temperature thermoelectric materials, thermoelectric properties of n-type MNiSn and p-type MPtSn (M = Hf, Zr) were investigated based on two respective strategies. For the n-type (Hf, Zr)NiSn, a combined process of optical floating zone melting and hot-pressing was applied aiming to reduce thermal conduction through the lattice contribution. For the p-type HfPtSn, power factor and hence figure of merit ZT were dramatically improved by the p-type doping of Ir and Co targeting for Pt-site, which effectively lower electrical resistivity. The additions of Ir and Co are expected not only to increase carrier concentration but also to suppress the lattice thermal conduction by substituting for Pt.

We have succeeded to grow almost single-phase of Half-Heusler intermetallic compounds MNiSn, where M = (Hfx,Zr1−x) and x varies from 0 to 1, for the first time by directional solidification using optical floating zone melting (OFZ). Thermoelectric power and electrical resistivity can be dramatically improved since OFZ process effectively reduces solidification defects such as micro-cracks and cavities as well as unfavorable coexisting phases. Dimensionless thermoelectric figure of merit, ZT, of OFZ (Hf,Zr)NiSn alloys can be improved effectively by lowering the lattice thermal conductivity through the solid solution effects due to the substitution of Hf and Zr with each other. The maximum ZT value of 0.9 is achieved in (Hf0.5Zr0.5)NiSn at 963 K.

Targeting to develop E21 Co3AlC based heat resistant alloys, phase stability of E21 Co3AlC and (Co, Ni)3AlC has been investigated together with the magnetic properties of E21’ Co3AlC0.5 which is formed by the extra ordering of carbon atoms accompanying anti-phase boundary (APB). The correlation of ferromagnetism with APB in E21’ Co3AlC0.5 was evaluated using single crystals by high-resolution transmission electron microscopy (HRTEM) and vibrating sample magnetometer. Anti-phase domain (APD) size affects the ferromagnetism: for instance, the saturation magnetization becomes larger as the APD size is smaller. Local atomic configuration at APB was clearly observed by HRTEM image.

Phase stability of E21 (Co, Ni)3AlC was investigated from the viewpoint of magnetic properties in the Co-Ni-Al-C quaternary system. Isotherms of the Co-Al-C and Ni-Al-C ternary systems at 1373 K were revised and evaluated by means of electron probe microanalysis. Magnetic properties measurement revealed that discontinuity appearing on the chemical concentration dependence of Curie temperature and saturation magnetization of E21 (Co,Ni)3AlC indicates the existence of two types of E21 phase in the Co-Ni-Al-C quaternary system.

The development of ultra-high temperature structural materials having superior mechanical properties to the nickel based superalloys has been pursued in the niobium silicide based composites. The unidirectional solidification technique was performed on the (Nb)/Nb3Si eutectic alloy in the Nb-Si binary system. A continuity of Nb phases in the eutectic microstructure increases with decreasing a growth rate, which yields the enhancement of fracture toughness as well as high temperature strength. The mechanisms of microstructural formation, toughening, and strengthening are briefly discussed.

In view of the possibility of the E21-type intermetallic compound as a strengthener in austenitic alloys at high temperatures, mechanical properties and microstructures of the γ / κ two-phase alloys in the Fe-Mn-Al-C system were investigated. Alloy compositions with more than 1.5wt% C were selected in order to stabilize the γ / κ two-phase microstructure in the high temperature regime exceeding 873K. During furnace cooling after the homogenization treatment, κ phase precipitates on the γ grain boundary, which results in the poor tensile ductility. In these alloys, tensile yield strength at 873K and 1073K increases with increasing κ phase volume fraction. Applying ageing treatment at 1073K after the solid solution treatment followed by water quenching causes the formation of a uniform γ / κ lamellar microstructure, and the remarkable improvement of ductility was achieved. The addition of Cr, which is expected to improve the oxidation resistance, reduces the phase stability of κ phase, and leads to the formation of Cr7C3 carbide and β-Mn phase with more than 5.5wt.% Cr addition.

C11b/C40 fully lamellar microstructures, similar to the well-known TiAl/Ti3Al lamellae, were obtained in Ta- and Nb-added MoSi2 polycrystalline alloys in a previous work. In the present study, the crystallography of the lamellar structure is investigated in a MoSi2-15mol%TaSi2 pseudo-binary alloy after homogenized at 1400°C for 168h, in order to provide some useful parameters for microstructural control to improve mechanical properties. The orientation relationship between C11b and C40 phases and its three distinct variants were identified. Coherency of the lamellar interface is analyzed in comparison with the TiAl/Ti3Al lamellae. Approach to modify the C11b/C40 lamellar microstructure to increase its coherency is discussed based on the results obtained.

In the present work, we investigate microstructures and thermoelectric power of the binary higher manganese silicide (hereafter denoted as HMS) base alloys prepared by ingot metallurgy, i.e., arc-melting in an Ar gas atmosphere. Alloys with 63.5, 64.0 and 64.5 at% Si have a small amount of second phase, either the primary metallic MnSi phase or Si solid solution (hereafter denoted as (Si)). A slight revision is thereby made on the HMS single phase region of the Mn-Si binary phase diagram. The presence of MnSi seems to reduce the thermoelectric power of the HMS, while that of (Si) enhances it. An alloy having the HMS/(Si) eutectic microstructure, 67.9at%Si, shows the highest thermoelectric power in the Mn-Si binary alloys examined. Moreover, effect of doping the p-type elements, Fe and Cr, on the thermoelectric power and microstructural features is also investigated for a 63.5at%Si alloy which consists mostly of HMS phase with a small amount of (Si) phase. It is shown that both elements, especially 0.5 at% Cr addition, enhance the thermoelectric power of the alloys.

Temperature dependence of thermoelectric power and resistivity was measured for the β-FeSi2 based cast alloys on which n-type Co doping was performed with or without Cu addition. The Cu-free alloy shows a homogeneous eutectic microstructure consisting of metallic phases α-FeSi2 and ε-FeSi, while the Cu-doped alloy has a quite heterogeneous microstructure dominated by coarse ε-FeSi dendrites in the α-FeSi2 matrix. Adequate heat treatment condition for the semiconductor β-FeSi2 phase formation has been evaluated as annealing at 923 K for less than 50 h, which is lower in temperature and shorter in duration than previously reported for alloys prepared by powder metallurgy. The resistivity measurement with the aid of microstructure observation has revealed that the β-FeSi2 formation takes place and almost finishes at very early stage of annealing process. Addition of Cu effectively promotes the β-FeSi2 formation rate of the present cast alloys.

To investigate the deformation behavior of the C15 HfV2+Nb Laves phase, compression tests were conducted on the C15/bcc two-phase alloys at very low temperatures down to 4.2 K. Transmission electron microscopy revealed that substantial mechanical twinning takes place in the C15 Laves phase 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 for the C15/bcc two-phase alloy. 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. A high density of ultra-fine twin bands is the characteristic feature of twinning in the C15 matrix. We also investigated the phase relations between the C15 HfV2 Nb and bcc (V, Nb) solid solution in the Hf-V-Nb ternary system in an attempt to understand why we were unsuccessful in our attempts to grow single crystals of the Laves phase.

The Co-Al-C ternary phase diagram has been experimentally examined for the Co-corner with a particular interest in phase relations among B2 type intermetallic compound, the E21 type Co3AlC, and cobalt primary solid solution, denoted as (Co). Reaction scheme, liquidus surface, isothermal sections and isoplethals at constant concentration were determined. Mechanical properties of the B2/E21/(Co) three-phase alloys were investigated by compression tests carried out at a temperature range from 77 to 1273 K, and by tensile tests at room temperature. It has been revealed that both excellent ambient temperature ductility and sufficient high temperature strength can be achieved by proper choice of alloy compositions.

An attempt is made to develop a two-phase alloy consisting of the Ll2 and L21(Heusler) phases in the Co–Al–Ni–Ti quaternary sytem exhibiting a high elevated temperature strength as well as some room temperature ductility as a new class of heat resisting structural materials. The idea behind this approach is expectations for the L21 phase to provide high elevated temperature strength, whereas the Ll2 phase provides some room temperature ductility. Compositional optimization in the room temperature ductility of the Ll2 (Co,Ni)3(Al,Ti) is first carried out and then, based on the result, several L12/L21 two phase alloys are designed. It is found that a few to several per cent room temperature bend ductility is obtained in such two-phase alloys.

Two-phase alloys based on B2 type intermetallic compound (Co,Ni)Al with either Co primary solid solution or L12 type Ni3Al, γ, phase are investigated for their variations in microstructure and mechanical properties. The aim of the present work is to improve ductility and to enhance fracture resistance in extremely brittle B2 CoAl or NiAl by introducing second phase. To start with, microstructure-mechanical property relationship is investigated in two-phase alloys consisting of B2 CoAl and Co primary solid solution in the Co-Al binary system. Subsequently, the similar strategy is extended into the Co-Ni-Al ternary system. Mechanical properties are measured by compression test conducted over the wide temperature range from 77 K to 1273 K at strain rates ranging from 1.4×10−3 to 1.4×10−4 s−1. Micro-structure observations are carried out using both optical and transmission electron microscope.