Cu fine-particle paste is a promising material to form a low-cost interconnect for flexible electronics devices. It has been reported that Cu particles can be sintered at low temperature (well below the half of the melting point) through two-step heat treatment processes of oxidation and reduction. However, the mechanism of the low temperature sintering is not clear yet. In this study, we investigated the oxidation sintering process of Cu fine particles by thermal gravimetric analysis (TGA) in the temperature range of 200-300℃, X-ray diffraction (XRD), and microstructural observation. It was found from TGA that the oxidation process was initially rate-controlled by surface reaction and then by Cu diffusion at grain boundaries of Cu₂O. Transmission electron microscopy observation revealed the formation of a core (Cu)-shell (Cu₂O) structure during the oxidation process. The adjacent Cu₂O shells were bonded to each other resulting in a cross-linked structure. The subsequent reduction process led to the formation of a porous structure by oxygen removal, but the cross-linked structure was maintained, which would make the low-temperature sintered Cu body as robust as solidified solder and sintered Ag paste.

Friction stir welding (FSW) was performed under the two welding conditions (rotation speed-traveling speed) of 150 rpm-100 mm/min and 200 rpm-400 mm/min using 6 mass％Ni-0.63 mass％C steels. The slightly lower peak welding temperature and significantly higher cooling rate was obtained under the condition of 200 rpm-400 mm/min. Texture analysis for retained austenite and martensite revealed that the parent austenite had simple shear texture, which lead to the formation of {110}<111> texture in transformed martensite. Moreover, material flow behavior as a function of distance from the top surface in the stir zone was analyzed based on textures obtained by EBSD measurement. A concentric material flow, which has smaller radius as the far from the tool shoulder, was formed under the condition of 150 rpm-100 mm/min. On the other hand, a heterogeneous material flow with the center shifted to the retreating side was formed under the condition of 200 rpm-400 mm/min. In addition, a different vertical component of material flow was predicted to occur under each welding conditions.

A large step on the initial magnetization curve has been widely observed in Nd-Fe-B hot-deformed magnets. This behavior has been considered as that the first- and latter-half parts of the initial magnetization curve correspond to the magnetization reversals of multi- and single-domain grains, respectively. In this study, the detailed magnetic domain structure of a Nd-Fe-B hot-deformed magnet has been studied for the initial magnetization and demagnetization processes by using a soft X-ray magnetic circular dichroism (XMCD) microscopy. The observed magnetization reversal behavior for the initial magnetization process is quite different from that previously considered. The multi-domain states move from grains to grains through the gradual displacement of domain wall passing through many grains, and then the massive domain wall displacement takes place. The latter part of domain wall displacement is similar with that found in the demagnetization process near the coercivity. Moreover, we found that there are several strong pinning sites which are identical for both the initial magnetization and the demagnetization processes.

The effect of one-pass strain, |±Δε|, on grain refinement was systematically investigated by cyclic - HPT straining with a repetitive deformation process in which positive and negative shear strain are introduced. The steady-state grain size, d_ss, depended on |±Δε| rather than the given total strain, Σ |±Δε|. The unstable dislocation cell walls formed by positive strain, +Δε, was discomposed by negative strain, −Δε. The stability of dislocation cell wall increased as the number of dislocations introduced by applying |±Δε| in a grain, n, increased. The decrease in n was caused by decreasing +Δε and grain size. It was found that n affected the stability of dislocation cell walls and was an important factor in determining d_ss.

In tensile tests, α-Mg/C14-Mg₂Ca eutectic alloy with a lamellar structure is plastically deformed above 473 K but ruptures before yielding at temperatures below 423 K. This study investigates the effect of the α/C14 interface on the creep strength of α-Mg/C14-Mg₂Ca eutectic alloy at 473 K under 40 MPa stress. The creep curves of the alloy exhibited three stages: a normal transient creep stage, minimum creep-rate stage, and accelerating stage. The minimum creep rate was proportional to the lamellar spacing, indicating that the α/C14 lamellar interface plays a creep-strengthening role. In high-resolution transmission electron microscope observations of the specimens after the creep test, a dislocations appeared within the α-Mg lamellae and were randomly distributed on the α/C14 interface. It was deduced that the α/C14 interface presents a barrier to dislocation glide and does not annihilate and/or rearrange its dislocation caused by the creep test.

A prediction method for grain boundary segregation using a nano-polycrystalline grain boundary model is applied to the grain boundary segregation of Cr and Mn in bcc-Fe polycrystals for which experimental results exist, and the validity of the prediction method is verified. In this prediction method, focusing on the fact that the atomic structure of grain boundaries is almost independent of grain size, grain boundaries of polycrystals with grain sizes of the order of micrometers are modeled as grain boundaries of nano-polycrystals, for which structural relaxation calculations by molecular dynamics calculations are possible. For this grain boundary model, the grain boundary segregation energy of each site is calculated exhaustively using the interatomic potential. In addition, the average amount of grain boundary segregation in the polycrystal is calculated from the obtained grain boundary segregation energy. With this prediction method, the average amounts of grain boundary segregation and segregation energies of Cr and Mn in bcc-Fe polycrystals can be calculated and compared with the experimental results. Calculated results for both elements reproduced the experimental results well, suggesting that this prediction method is also effective in predicting the grain boundary segregation of other solute elements.

A plasticity resistance by a grain boundary was evaluated by nanoindentation measurement in the vicinity of the grain boundary for austenitic stainless steels. Converting a load (P) - displacement (h) relation to a P/h - h curve, the plasticity resistance can be evaluated in the parameter α with hardness dimension obtained as the slope on the loading curve. The slope α is constant until the plastic zone underneath the indenter reaches to the grain boundary at a certain penetration depth (stage I), and then turns into a higher value in the deeper penetration depth range (stage II). The lower α in the stage I (α_I) means the deformation resistance in the grain interior, and that in the stage II (α_II) includes the deformation resistances of both the grain interior and the grain boundary. Therefore, Δα = α_II − α_I could correspond to the plasticity resistance by the grain boundary. It has been found that the average of Δα is higher for the nitrogen-added sample than that for the nitrogen-free one, suggesting a nitrogen effect including segregation on the plasticity resistance by a grain boundary.

The importance of controlling grain boundary (GB) segregation is increasing with the strengthening of steel. In this study, a theoretical prediction method for the amount of GB segregation for a solute element in polycrystals, is established. In this prediction method, a nano-polycrystalline GB model for simulating GBs in polycrystals is developed, and the segregation energy of a solute element is calculated comprehensively for all atomic sites constituting the GB model by using an interatomic potential. From the obtained segregation energies, the segregation amount of the solute element at each atomic site is determined. Subsequently, each atomic site is classified for based on its distance from the GB center, and averaged to calculate the segregation profile of the solute element for that distance from the GB center. By applying this method to the GB segregation of P in bcc-Fe and comparing its results with experimental findings, it is determined that this prediction method can attain a good prediction accuracy.

To investigate the reason why low-carbon steels with carbon-clusters shows the maximum strength during low-temperature aging, interactions between an edge dislocation and carbon clusters are performed through molecular dynamics (MD) simulations. Carbon clusters are modeled based on atom probe tomography (APT) observations. To express a transition process of carbon configurations from solid solution state to carbon cluster state to precipitation state during aging process, we reduce a carbon presence area with a fixed number of carbon atoms, i.e., the carbon concentration can be continuously increased. The MD simulations can represent the age hardening/softening tendency observed in the experiment and the carbon cluster state shows the maximum strength where the dislocation passes through the carbon cluster not by the Orowan but by the cutting mechanism. The MD analysis found that partial clusters in the carbon cluster act as the main resistance to dislocation passage; the biased distribution of carbon atoms is also confirmed in the actual observed carbon clusters by APT. A new interaction mechanism between dislocation and carbon clusters is developed based on the phenomena in the MD simulations and the availability is discussed.

To evaluate the stability of the α-Mg/C14–Mg₂Ca lamellar microstructure, the aging treatment was carried out for the Mg–14.8 mass％Ca near-eutectic alloy at 573-723 K for 1-150 h. The spacing of the lamellar structure obtained by the eutectic reaction L → α-Mg＋C14–Mg₂Ca after casting is approximately 250 nm. It was found by the HRTEM observation that the α/C14 interface is composed of terraces and steps, and the terrace is parallel to the (1101)α pyramidal plane of the α-Mg lamellae. The lamellar microstructure is stable below 573 K. By contrast, the lamellar spacing (λ) continuously increases with the increasing aging time (t) above 573 K, and the increase of λ can be described as λ²−λ₀² = k_T t, where λ₀ is the lamellar spacing for the as cast specimen and k_T is a constant depending on aging temperature. The activation energy for the coarsening of the α/C14 lamellar microstructure was evaluated as 112 kJ/mol, which is close to the activation energy for the inter-diffusion of Ca in Mg. The hardness of the lamellar region decreases with the increasing λ, which is indicative that the α/C14 interface acts as an obstacle to the basal slip of dislocations in the α-Mg lamellae.

The surface morphological change of electro-deposited Pt–75 at％ Cu (Pt–75Cu) nanoparticles under potential cycling were investigated by identical-location scanning electron microscopy. Electro-deposited nanoparticles consisted of numerous nuclei (～3 nm in diameter) that agglomerated to form larger secondary particles (30-50 nm). In the initial immersion in the sulfuric acid, Pt-shell formed on the surface due to the selective dissolution of Cu from Pt-75Cu. Although around 40％ of Cu dissolved, noticeable surface morphological change does not appear. Then Pt-75Cu was subjected to potential cycling between 0.05-1.0 V, surface smoothing of the nanoparticles slightly progressed due to surface diffusion of Pt. On the other hand, when Pt-75Cu was potential-cycled between 0.05-1.4 V, a particle diameter of the nanoparticles drastically decreased, and the nucleus on the surface completely disappeared owing to Pt dissolution and re-deposition. The heat-treated nanoparticles formed numerous pores on the surface during the immersion and the initial stage of the potential cycle. However, the final morphology was similar to the non-heated one. The formation of pores can be explained by the coarsening of nuclei by heat treatment.

The formation of texture through thermomechanical treatment was investigated in Mg-18.8 at％ Sc shape memory alloy to enhance its superelasticity at room temperature. The samples were cold rolled in an α phase or in a β phase and then finally heat treated at 690℃ followed by water quenching to obtain a β phase. In the case of cold rolling in the α phase, a basal-plane texture was formed, while no preferential texture was observed along in-plane direction. After the final heat treatment, {011}<uvw>_β transformation texture was obtained, according to Burgers relationship, indicating no improvement of the superelasticity along in-plane direction. In the case of the cold rolling in the β phase, a weak {111}<011>_β recrystallization texture was obtained. The sample showed about 0.65％ superelastic tensile strain along rolling direction, while that along transverse direction (//~<113>β) showed only about 0.43％. This trend is in good agreement with the orientation dependence on the transformation strain, but, the obtained superelastic strain was much lower than the expected value, which is due to the weak texture and suggests the existence of a strong grain constraint in the Mg-Sc shape memory alloy.

The effect of Ti addition on the oxidation resistance of high-purity 19%Cr ferritic stainless steels has been investigated during isothermal heat treatment at temperatures between 1073 K and 1273 K in air. The microstructures of scale and the scale/metal interface were investigated in detail by means of SEM-EBSD and FE-TEM together with μ-EDS.

The Ti addition increases the oxidation mass gains but simultaneously improves the critical temperature of the oxidation resistance. The formed scale is mainly Cr₂O₃ regardless of the addition of Ti but Ti addition increases the thickness of Cr₂O₃. Considerable reduction in the grain size of Cr₂O₃ by Ti addition was recognized, which was inferred to increase the oxidation mass gain due to the easy diffusion through the grain boundary. Furthermore, Ti is oxidized beneath the scale/metal interface as internal complex oxides, such as Al₂TiO₅ and Al₂Ti₇O₁₅because of the small amount of aluminum in the steels used. In a somewhat deeper region from the interface, θ-Al₂O₃ forms where Ti cannot be oxidized. The oxygen getter effect by Ti atoms is postulated to be responsible for improving the oxidation resistance.

The purpose of this study is to investigate the influence of Nb content of Ti-xNb-7Al alloys (x=20-35 mass%, abbreviated as xNbA) on β (bcc)→α″ (orthorhombic) transformation with tempering through microstructure, isothermal age-hardening, and shape change of a U-shaped specimen with heating. Full α″-martensite structure was obtained on the quenched 20 NbA. With increasing of Nb content, residual β-phase increased, and single β-phase was obtained in the alloys of more than 25 NbA. From the result, the M_S temperature commonly decreases with increasing of Nb content. The hardness of 20 NbA and 23 NbA rapidly increased in a few seconds by isothermal aging at 450°C, and the hardness of the alloys of more than 25 NbA abruptly increased after an incubation period for 30-60 s. No compositional distribution between α″ and matrix in the aged 23 NbA was found by STEM-EDS analysis, but the obvious distribution of Nb was detected in 35 NbA. U-shaped specimen of 20 NbA exhibited no shape change by heating. On the other hand, the specimen of 23 NbA composed of β+α″ phases showed the shape recovery (SR) first due to work induced α″→β inverse transformation, then deformed toward the bending direction (SA: shape advance) due to β→α″ transformation. The specimens of more than 28 NbA composed of single β-phase exhibited only SA without SR. The starting temperature of SA (M_SA: β→α″) with heating increases with increasing of Nb content. Merging M_S (upper) and M_SA (lower), it is proposed that the whole M_S of xNbA alloy forms a bow shape curve. It is suggested that the low Nb alloys inside the M_S curve transform immediately without atom diffusion by tempering at 450°C, but the higher Nb alloys outside the curve need an incubation period to distribute Nb concentration with atom diffusion, and martensitic α″ transformation will abruptly occur in the local domains less than 25 NbA which is the inside of the curve.

In this study, thermal fatigue tests at maximum temperature 1073 K were performed using 13%Cr-Nb-Si and 18%Cr-Nb-Mo steels as representative heat-resistant ferritic stainless steels for automotive exhaust systems. The changes in the microstructure, the crystal orientation and the hysteresis loop during thermal fatigue in the temperature range from 473 K to 1073 K were investigated. As a result of comparing thermal fatigue life under these conditions, 18%Cr-Nb-Mo steel with high temperature strength was found to have a longer thermal fatigue life than 13%Cr-Nb-Si steel. During the thermal fatigue process, the material was softened by reducing of the amount of solute Nb, and the coarsening of Nb precipitation. By this softening, the form of the hysteresis loops changed with the increase in cycles. By considering the softening of the material, the change in the hysteresis loops could be predicted to some extent. Furthermore, by EBSD analysis, it was recognized that the dynamic recovery and recrystallization accompanied by the uniaxial and fine grain formation occurred during the thermal fatigue process. From the viewpoint of change of the microstructure, the thermal fatigue damage was quantified by the ratio of the low-angle grain boundary, and the change of this index with the progress of the cycle in 18%Cr-Nb-Mo steel had a smaller than 13%Cr-Nb-Si steel. It was thought that this point was caused by the retardation of recrystallization by solute Mo.