Although thermoelectric materials with a figure of merit, ZT, higher than 1.2 have gained considerable interest in electric power generation, little is hitherto known on the influence of severe plastic deformation on thermoelectric performance. Severe plastic deformation is one of the elegant techniques to furnish ultrafine grained microstructure with a high level of point, linear and surface defects. Particularly the phonon scattering on these defects is used as a main route to reduce thermal conductivity in thermoelectric materials and - as a consequence - increasing ZT.

The present article provides an overview on the achievements of the various techniques of severe plastic deformation to gain high figures of merit in the thermoelectric materials, commonly used in energy conversion devices, such as skutterudites, clathrates, Heusler phases, and bismuth tellurides.

For all skutterudites, high pressure torsion, as one of the major techniques of severe plastic deformation processes, is a great tool to either enhance ZT of hot pressed samples or to directly produce fast and easily high ZT thermoelectric bulks. A still unsurpassed highlight is the enhancement of ZT from 1.6 to almost 2 (Sr_0.09Ba_0.11Yb_0.06Co₄Sb₁₂) at 825 K.

Whilst for thermoelectric clathrates so far little success was reported, high pressure torsion treatment of Heusler and Half-Heusler phases in some cases was able to boost ZT (VTa_0.05Fe₂Al_0.95, rising ZT from 0.22 to 0.3) although the absolute ZT increases are still disappointing. For p- and n-type Half-Heusler alloys (NbFeSb- and TiNiSn-based) at least three temperature cycles are necessary to gain a thermally stable state, indicating that obviously the introduced defects and structure changes are more resistant against heat treatments than in case of skutterudites.

Bismuth tellurides of type V₁(VI)₁ and/or V₂(VI)₃ (V, VI denote the group elements) have been already deformed by high temperature pressure or high temperature extrusion before the SPD methods were known, for the sake of improving ZT at least with temperatures 300–500 K, at most fighting with the condition to achieve a high electrical conductivity because of the strong anisotropy in these materials. By starting with ball milling followed by high temperature pressing at not too high temperatures, not only the electrical conductivity could be kept large but also the lattice thermal conductivity was diminished such that figures of merit up to ZT = 1.4 at T = 373 K were achieved. This value could be reached by many of the ECAP experiments published so far, although only across the sample long axis because of lattice anisotropy. First applications of HPT did not reach that ZT level as either the conditions of texture could be not fulfilled, or, above all, the processing rates and/or temperatures were too high. Recent investigations not having involved SPD found the importance of the lattice defects’ specific phonon scattering efficiencies, especially that of dislocations, and by introducing them in sufficiently high densities, enhancements of p-type Bi-Tellurides up to ZT = 1.9 were possible. These findings recommend the use of SPD methods here, not at least as they have been already applied very successfully by the authors of this review to both p- and n-type Skutterudites increasing the figure of merit up to ZT ∼ 2.

As concerns mechanical properties, the application of SPD methods significantly raises the strength while leaving the elastic moduli unchanged unless new phases have been formed.

Interphase precipitation is a phenomenon in which precipitates are generated on interphase boundaries during phase transformation, creating carbides having a form like bands or fibers in micro-alloyed steels. In isothermal transformation, a lower transformation temperature reduces the carbide diameter and band spacing, and a higher cooling rate reduces them in the continuous cooling process.

Various interphase precipitation mechanisms have been suggested, and some models for the ledge mechanism attempted to explain the regular band spacing in steels containing carbide-forming elements quantitatively. Recently, as the orientation relationship in the grains of the interphase boundary is not consistent with the ledge mechanism, three-dimensional interface structures have been suggested to explain the experimental results of observation by transmission electron microscopy in low carbon steel. The newly-proposed interphase structure model may explain both the ledge and quasi-ledge mechanisms.

Steels with tensile strength of more than 590 MPa, which are manufactured by using interphase precipitated carbides, have been developed and used practically not only in plate and sheet products but also in forged products to improve formability. In steels consisting of ferrite and a second hard phase, interphase precipitated carbides are used to realize high local ductility and to reduce the difference of hardness between the two phases. Ferritic steels strengthened by nanometer-sized carbides are developed to achieve excellent formability, realizing precipitation-strengthening of more than 300 MPa.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 81 (2017) 447–457. In order to provide a more precise explanation of the interphase precipitation phenomenon, the entire article was revised and recently published literature was cited. Figures 1 and 2 in the original paper were omitted to simplify the article.

Electron vortex beams are considered as probes for next generation electron beam instruments with unprecedented measurement capabilities because vortex beams carry intrinsic orbital angular momenta. In order to clarify the performance of vortex beams, we generated vortex beams by using fork-shaped gratings manufactured by focused ion beam instruments. In vortex beam generation experiment, we found that shapes and sizes of grating openings were superimposed on rings of diffraction spots, typical behavior of vortex beams. This paper discusses experimental results on the generation and control of vortex beams by changing grating opening shapes and sizes.

The charged domain walls in ferroelectric materials exhibit intriguing physical properties. We examine herein the charged-domain-wall structures in Ca_3−xSr_xTi₂O₇ using transmission electron microscopy. When viewed along the [001] axis, the wavy charged domain walls are observed over a wide range (>5 µm). In contrast, short charged-domain-wall fragments (from 10 to 200 nm long) occur because they are intercepted and truncated by the conventional 180° domain walls. These results reveal the unusual charged domain structures in Ca_3−xSr_xTi₂O₇ and will be useful for understanding their formation process.

The electric potential distribution within a commercial multilayer ceramic capacitor under an applied voltage was determined by electron holography. We demonstrated that equipotential contour lines present essential information about the electrical conductivity of submicrometer-scale precipitates formed in commercial BaTiO₃ multilayer ceramic capacitors. Considerable changes observed in the equipotential contour map of precipitates could be explained by simulations taking into account the electrical conductivities of the precipitates and BaTiO₃ matrix. This method can lead to a deeper understanding of the relationship between the complex microstructure and the material functionalities in capacitors widely used in industry.

The accumulation and distribution of electron-induced secondary electrons around epoxy resin are studied using electron holography. The distribution of secondary electrons is determined to be sensitive to the surface conditions of the epoxy resin, particularly to the presence of conductive materials on the surface that are introduced during specimen preparation processes. These results provide a deeper understanding of the charging and discharging mechanisms for epoxy resin, and the behavior of secondary electrons around these materials. This study also provides a new perspective for the visualization of various forms of electron behavior around insulating materials such as epoxy resin through the control of their surface characteristics.

Recently, the effect of dynamical electron diffraction on the phase shift in electromagnetic field analysis using transmission electron microscopy has become increasingly important. In the present study, we investigated the effect of dynamical electron diffraction on the phase shift in electron holograms recorded from a wedge-shaped specimen of single-crystal Si around a Bragg diffraction condition. The results show that the effective inner potential depends on the direction of the incident electron beam, especially near Bragg conditions. The characteristic phase shift was analyzed using dynamical electron diffraction theory (Bethe method).

The microstructural evolution in different regions on the cross section of AZ91D vacuum die castings during solution treatment, especially during the early stage, was investigated. Optical microscope and scanning electron microscope were employed for the microstructure observation. The results exhibited that the supersaturated α-Mg solution located in the grain boundaries first dissolved. The β-Mg₁₇Al₁₂ particles in the surface layer had a slower dissolution speed when compared to those in the central region, and the remaining β-Mg₁₇Al₁₂ particles in the surface layer could effectively restrain the grain growth during the early stage. In the central region, due to the mergence of neighbouring dendrite arms of the same ESC, most remaining β-Mg₁₇Al₁₂ particles were encapsulated in the relatively large near globular α-Mg grains evolved from the ESCs. The shrinkage porosities concentrated in the segregation band could effectively restrain the grain growth, causing the segregation band maintained a fine microstructure during the solution treatment.

A novel composite consisting of silica nanoparticles (SiO₂) and a typical organic ferroelectric of diisopropylammonium bromide (C₆H₁₆NBr, DIPAB) was prepared at different composition mass ratios. Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) techniques were utilized for characterization of the composite. Experiments for testing ferroelectricity was conducted from room temperature to 165°C under a weak electric field (5 V·cm⁻¹) at a low frequency of 1 kHz. The results revealed a reduction of phase transition temperature with increasing SiO₂ content. The Landau theories developed for bonded and isolated ferroelectric particles were used to explain the obtained anomalies.

To decrease car body weight and improve crash safety, tailor-welded blanks are often used for the car body materials. In the past, little research has been conducted on the formability of the weld metal (WM). This study focuses on the bendability of the WM and evaluates it by the bending test. The main results are as follows. (1) The bendability of the laser-welded specimens was better than that of the plasma-welded specimens. (2) The bendability of the weld line (WL) was related to the homogeneity of the WM structure, and it increased with increasing homogeneity. (3) The surface roughness of the WL has no effect on the bendability of the WL. (4) The deformation of the bent surface of the WL changed with the WL width, and the bendability increased as the WL narrowed because the bent surface showed increasing uniaxial stretching deformation.

This Paper was Originally Published in Japanese in J. JSTP 59 (2018) 33–38.

Recently, the realization of the combination of selective laser melting (SLM) and high-strength aluminum alloys (Al alloys) has been discussed in mainly the aerospace industry. However, there have been several problems for the realization, and the one of fundamental problem is the occurrence of microcracks in high-strength Al alloy objects fabricated by SLM. To solve the problem, some researchers have studied addition of alloying elements to high-strength Al alloy. In this study, we focused on the A7075 alloy with additional five% Si (A7075+5Si) and investigated the effects of the Si addition on the processability, microstructure, and mechanical properties. As a result, it was observed that the addition of Si element inhibited the formation of cracks in the A7075 objects. Our result suggests that high density and crack-free A7075+5Si objects had higher mechanical properties than the case of a conventional Al alloy (AlSi10Mg and AlSi12). However, the T6 heat treatment (After solution treatment, water quenching, and artificially ageing) did not increase the mechanical properties of A7075+5Si objects. In order to obtain the A7075+5Si objects with higher strength, it is required that the further studies about suitable heat treatment conditions for A7075+5Si alloy.

This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Powder Metallurgy 66 (2019) 109–115.

In this work, acoustic emission (AE) monitoring was correlated with in-situ optical microscopy observation and electron backscatter diffraction (EBSD) measurements to investigate the evolution of a single stress corrosion crack in SUS420J2 stainless steel subjected to chloride droplet corrosion. A single dominant crack evolution was observed to transition from a slow initiation of active path corrosion-dominant cracking to a rapid propagation of hydrogen-assisted cracking. The initiation-to-propagation was concomitant with a significant increase in the number of AE events. In addition, a cluster analysis of the AE features including traditional waveform parameters and fast Fourier transform (FFT)-derived frequency components was performed using k-means algorithms. Two AE clusters with different frequency levels were extracted. Correlated with the EBSD-derived kernel average misorientation (KAM) map of crack path, low-frequency AE cluster was found to correspond with the location of plastic deformation in the propagation region. High-frequency AE cluster is supposed to be from the cracking process. The correlation between AE feature and SCC progression is expected to provide an AE signals-based in-situ insight into the SCC monitoring.

Commercially pure titanium was subjected to hydrostatic extrusion resulting in formation of an ultra-fine grained microstructure with a strong α-fiber texture and significant improvement of mechanical strength. Anisotropy of the tensile and fracture behavior of the hydrostatically extruded material was studied. It will be demonstrated that the material has significantly higher yield strength along the extrusion direction, while in transversal direction it shows higher work hardening ability related to the α-fiber crystallographic texture. The anisotropy of the fracture behavior in these two directions is less pronounced. A slightly lower fracture initiation toughness and crack growth resistance along the extrusion axis can be related to a lower crack propagation resistance along the boundaries of the elongated grains.

Titanium diboride was sintered at temperatures between 1573 and 1873 K by spark plasma sintering with the addition of FeAl. It was found that the grain growth of TiB₂ was suppressed by sintering with FeAl. A bending strength as high as 1000 MPa and a Vickers hardness of 2700 Hv were obtained for TiB₂ sintered at 1773 K with 10 mass% FeAl. The fracture toughness was estimated to be 12 MPa·m^1/2. On the other hand, considerable grain growth was observed for a specimen sintered at 1773 K without FeAl. The bending strength of the monolithic TiB₂ specimen sintered at 1773 K was 600 MPa and the Vickers hardness was 2200 Hv. These properties deteriorated when the sintering temperature was increased to 1873 K owing to the grain growth of TiB₂.

The effects of surface V species on the hydrogen permeability through V membrane without Pd-catalyst overlayer are investigated by the hydrogen permeation test at 773 K and the XPS measurement. The steady-state hydrogen permeation flux through Pd-coating free V membrane after the redox treatment is more than twice that of the non-treated membrane. The hydrogen flux after 50 hours, however, are almost the same regardless of the redox treatment since the redox-treated membrane is deactivated during the hydrogen permeation test. The fraction of V⁰⁺/V2p_3/2 on the surface of membrane just after the redox treatment is approximately 2%, and the amount of V³⁺ is larger than V⁴⁺. This fine balance of the V species composition on the surface is found to induce the high hydrogen permeability. The composition of V species on the surface of the redox-treated membrane changes to V³⁺ < V⁴⁺ by oxidation of V³⁺ to V⁴⁺ during the hydrogen permeation test for a long time. The loss of the fine balance in the composition of V species is considered to be the cause of the degradation of hydrogen permeability.

An in-field heat treatment furnace utilized for a magnet with a 50 mm room-temperature bore was made to study magnetic field effect on gas-solid reaction. Using this furnace, nitridation of Sm₂Fe₁₇ powder was performed under a 5-T magnetic field and a 0.1-MPa nitrogen gas pressure for the temperature range 623 to 743 K. Applying a magnetic field of 5 T promoted nitridation by approximately 0.6 nitrogen atoms per formula unit compared with zero-field nitridation, and almost fully nitride Sm₂Fe₁₇N_2.9 was obtained at 743 K.

Possibility as an electrode by using vanadium hydride, which is one of typical metal hydrides as hydrogen storage alloy, was examined for an all solid lithium ion battery. The results obtained show vanadium hydride as a negative electrode material by conversion reaction were clarified for the first time in this work. By analyzing the charging/discharging properties and XRD profiles for each process, the reaction mechanism of the conversion reaction of vanadium system and lithiation reaction of hard carbon was clarified.

In this study, we investigated the formation of Ni-based nanowires (NWs) and precise control of their surface morphology via solution synthesis. Ni²⁺ ions were reduced to metallic Ni with hydrazine, and Ni nanoparticles were formed in ethylene glycol solution. The Ni nanoparticles then self-assembled along one direction to form NWs. The surface morphology of the Ni NWs depended on the hydrazine concentration. Needle-like protrusions were formed on the surfaces of the Ni NWs at high hydrazine concentrations, whereas Ni NWs with smooth surfaces were obtained at low hydrazine concentrations. The inclusion of NaBH₄ as a proportion of the reducing agent led to the formation of NWs containing a Ni–B alloy. The catalytic activity of the NWs containing a Ni–B alloy in H₂ generation from NaBH₄ solution was higher than that of the Ni NWs.

In-field heat treatments of an Mn–Ga diffusion couple were performed at 773 K for 12, 24 and 48 h to investigate the phase formation under magnetic fields. Formation of the MnGa₃, Mn₂Ga₅, MnGa, Mn₃Ga₂ and β-Mn phases was confirmed for magnetic fields of 0 T and 5 T. For a diffusion-controlled process, there is a parabolic relationship between the thickness of the phase layers and annealing time for the MnGa and Mn₂Ga₅ phases. The parabolic coefficients of MnGa and Mn₂Ga₅ for a zero magnetic field were determined as 8.7 × 10⁻³ µm²s⁻¹ and 4.6 × 10⁻¹ µm²s⁻¹ respectively. For a magnetic field of 5 T, the parabolic coefficients were evaluated as 11 × 10⁻³ µm²s⁻¹ for the MnGa phase and 4.5 × 10⁻¹ µm²s⁻¹ for the Mn₂Ga₅ phase.

The alloying effect of ZrO₂ on the microstructure of the thermoelectric material, Ti_nO_2n−1, which is a partially reduced rutile phase called the Magnèli phase, is investigated. The microstructure of Ti_nO_2n−1 is characterized by periodically induced planar defects, called shear planes, in the rutile matrix. The configuration of the O atoms around a Ti atom in a shear plane is different from that in the rutile matrix but similar to that in Ti₂O₃. It is speculated that the alloying of ZrO₂, which has an equivalent structure to that of rutile, causes preferential Zr addition in the rutile matrix. From the microstructural observation of the alloying specimen (Ti₁₀O₁₉+6 mol%ZrO₂), it is revealed that the added Zr is distributed homogeneously in the entire microstructure because the valence of the Ti atoms in the matrix rutile and shear plane is similar.

It had been reported that Ti–29Nb–13Ta–4.6Zr (TNTZ) alloy forms a dense oxide layer by high-temperature oxidation whereas CP Ti forms a multilayered oxide consisted of rutile monolayers and void layer. This morphological change is supposed to be mainly caused by Nb addition in Ti since the dense oxide layer of TNTZ consists of multiple oxide phases, at least with rutile TiO₂ and TiNb₂O₇. In this study, high-temperature oxidation at 1273 K for 3.6 ks in the air of Ti–xNb alloys (x = 1, 5, 7, 10, 13, 15, 18, 20, 23, 26, 28, 30 and 32 mol%) was investigated to discuss the effect of Nb addition to Ti on its high-oxidation behavior, and on its oxide microstructure. From the results of the SEM observation, an oxide layer with a void layer was formed on Ti–xNb substrate from 1 mol%Nb up to 10 mol%Nb. However, densification of the oxide layer was confirmed at Ti–13Nb. Then, the dense oxide layer was formed up to 32 mol%Nb. XRD results indicated that only rutile-type TiO₂ was identified from 1 mol%Nb up to 10 mol%Nb, then both TiO₂ and TiNb₂O₇ were formed from 13 mol%Nb to 32 mol%Nb. These results indicate that dense oxide layer formation attributes to phase separation from TiO₂ to TiNb₂O₇. Until 10 mol%Nb, the thickness of oxide layer was suppressed by Nb addition, whereas the layer thickness increased with increasing Nb content from 13 mol%Nb. The maximum exfoliation resistance of the oxide layer was obtained at 20 mol%Nb. The results of oxide growth rate at each Ti–xNb alloys suggested that Nb diffusion in Ti may rate-determining process of the dense oxide layer formation.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 82 (2018) 232–239.

A refined fully lamellar microstructure with a prior β grain size of ∼100 µm was obtained for a 0.1 mass percent (%) B-modified Ti–6Al–4V alloy. On the other hand, there were no morphological differences in equiaxed microstructures between the B-free and B-modified alloys; both had α grain sizes of about 8 µm.

The very high-cycle fatigue (VHCF) lifetimes in the regime from 10E+6 to 10E+10 cycles were measured for these microstructures by using hourglass-shaped specimens and an ultrasonic fatigue test machine at a frequency of 20 kHz, while high-cycle fatigue (HCF) lifetimes in the regime from 10E+4 to 10E+7 cycles were measured using smooth cylindrical specimens and a hydraulic servo fatigue test machine at 10 Hz and a fatigue ratio R of 0.1. The VHCF and HCF behaviors of B-modified alloys were found to be highly dependent on both microstructures and the level of applied stress. In the VHCF regime, where applied stress is well below the conventional fatigue threshold, the B-free and B-modified alloys exhibited the same fatigue strength, in other words, the same fatigue lifetime within the same microstructure. A set of fatigue lifetime data for B-free and B-modified alloys with equiaxed microstructures indicated that these alloys had higher fatigue strength than the alloys with lamellar microstructures in the VHCF diagram in the whole cycle range of up to 10E+10 cycles. The overall trend in the HCF lifetime or strength was that the addition of 0.1 mass% B had either a favorable effect or no influence on the fatigue life, depending on the level of applied stress. Above a certain level, which corresponded to a cycle regime up to about 10E+6 cycles for a lamellar microstructure and up to about 10E+7 cycles for an equiaxed microstructure, the fatigue lifetime was remarkably prolonged by the addition of 0.1 mass% B. However, in the regime beyond these cycles, when the stress level decreased, the favorable effect of B was gradually diminished and the HCF lifetime of the B-free and 0.1 mass% B-modified alloys were coincident in both lamellar and equiaxed microstructures; this coincidence was maintained thereafter. The mechanism for the dependence of the VHCF and HCF strength of the B-modified alloy on microstructures and the level of applied stress is proposed and discussed.

The authors have investigated a novel process of platinum recycling using “dry aqua regia”. In our previous work, the recovery of platinum was successfully conducted by solvent leaching, but it yielded a mixture of K₂(PtCl₆) and FeO_x. In this study, a solid–liquid separation process using KCl and NaCl solutions and a precipitation process using NH₄Cl were adopted to obtain pure platinum. Impurities like FeCl₃, FeO_x, and KCl were removed by a solid–liquid separation process, and pure (NH₄)₂(PtCl₆) was obtained by NH₄Cl addition. Then, the obtained (NH₄)₂(PtCl₆) was calcinated to yield pure platinum by thermal decomposition. High-purity platinum was obtained with a recovery ratio of up to 79%. Therefore, the usability of the recycling process using “dry aqua regia” was significantly improved.

This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 83 (2019) 250–255.

A modified cellular automata algorithm is proposed to simulate microstructure evolution during static recrystallization of ultrafine-grained purity copper. The effect of isothermal annealing and the extrusion pass of the equal channel angular pressing on the mean grain size of static recrystallization grains are considered. The results indicate that, during the annealing process, the growth of equiaxed grains is attributed to different nucleation times, grain growth velocities, as well as the interaction of the grains. Simulations comparing with the reported experimental data are included to demonstrate the expected properties of the proposed approach, and the slight differences between the two results are also pointed out due to the effect of non-uniform distribution of the stored energy on the nucleation during static recovery.