Revealing atomic-scale distributions of energy and stress in defective or complex systems, based on the behavior of electrons, should contribute much to materials science and engineering, while only few practical ab initio methods were developed for this purpose. Thus, we developed computational techniques of local-energy and local-stress calculations within the plane-wave PAW (projector augmented wave)-GGA (generalized gradient approximation) framework. This is natural extension of ab initio energy-density and stress-density schemes, while the inherent gauge dependency is removed by integrating these densities in each local region where the contained gauge-dependent terms are integrated to be zero. In this overview, we explain our scheme with some details and discuss the concepts or physical meanings of local energy and local stress via the comparison with related schemes using similar densities, LCAO (linear combination of atomic orbitals) methods, Green’s function formulation implemented by multiple scattering or TB (tight-binding) methods, or EAM (embedded-atom method) potentials. We present recent applications to metallic surfaces, grain boundaries (GBs) with and without solute segregation, tensile tests of metallic GBs, local elastic constants of microstructures in alloys, and machine-learning based GB-energy prediction, where the local-energy and local-stress analyses provide novel aspects of phenomena, deep insights into the mechanism, and effective data for novel machine-learning based modelling. We discuss unsettled issues and future applications, especially for large-scale metallic systems.
A heat flow switching device was developed using semiconductors characterized by very small lattice thermal conductivity. We selected Ag2Ch (Ch = S, Se) which possesses semiconducting electron transport properties and very small lattice thermal conductivity, and tried to control their electron thermal conductivity using bias voltage. The samples were prepared by means of self-propagating high-temperature synthesis under vacuum atmosphere, and mechanically rolled into ribbons of 10 µm in thickness. For making the capacitor-type device, amorphous Si and Mo were deposited on the rolled films using RF-sputtering. We compared thermal conductivity with and without bias voltage by means of the AC heating method. As a result, we succeeded in observing a 10% increase of heat flow in the capacitor type heat flow switching device.
This Paper was Originally Published in Japanese in J. Thermoelec. Soc. Jpn. 16 (2019) 73–76.
The fine structure of a Zr80Pt20 amorphous alloy was evaluated by anomalous X-ray scattering (AXS) coupled with reverse Monte Carlo (RMC) simulation. Featureless short-range ordered structures were preferentially formed by the Zr component, similar to those described by dense random packing (DRP), while the Pt atoms adopted a somewhat different structure. The Pt atoms adopted Zr-rich coordination and an icosahedral local atomic arrangement, in unique chemical short-range ordering (CSRO) as well as topological short-range ordering (TSRO). The present AXS-RMC analysis also suggested that the pre-peak signal at 17 nm−1 in the X-ray diffraction profile was largely attributed to the correlation between the Pt–Pt pair with a separation of approximately 0.4–0.5 nm. The geometrical and chemical features of the common neighbors of the middle-range Pt–Pt pairs indicate the unique CSRO and TSRO, where both chemical and density fluctuations were observed in two distinct regions: a Zr-rich region with high packing density and a Pt-rich region with low packing density.
Fig. 1 Experimental interference functions, QΔiPt(Q), QΔiZr(Q), and Qi(Q), and corresponding values obtained from RMC simulation.
TiNb-based alloys with prevailing β-phase and BCC crystal structure are currently being investigated as hydrogen storage materials due to their relatively high absorption capacities that can be reached at moderate temperatures. In this paper, low cost TiNbFe alloys were prepared by arc melting starting from the elemental powders of the pre-alloy Nb68.1Fe30.4 with the addition of pure Ti following the proportion of Nb68.1Fe30.4 + X wt% Ti (X = 23, 33, 41, 47). The crystal structure including the phase’s evolution and absorption kinetics at 30°C were evaluated in detail as a function of Ti addition. XRD results in combination with SEM observations showed the presence of β-phase (Nb-rich) with a BCC type-structure in all compositions beyond that TiFe, NbFe and α (Ti-rich) phases. After a simple activation step, the samples containing 41 and 47 wt% of Ti showed very fast absorption kinetics at 30°C but with different hydrogen storage capacities of 1.95 and 1.37 wt% of H2, respectively. Upon hydrogenation, the β-BCC phase is partially converted into FCC hydride TiH1.9−X beyond that TiFeHX and NbHX hydrides. These hydrides resulted in different steps of decomposition as indicated by the DSC curves. The current study confirmed the significance of TiNbFe alloys for hydrogen storage applications at low temperatures.
Fig. 4 Absorption kinetics curves at 30°C under 20 bar of hydrogen pressure for the samples of Nb68.1Fe30.4 with 41 wt% and 47 wt% of Ti.
It was tried to find the optimal condition to prepare the metallic niobium powder with minimal oxygen content by atmospheric magnesium-gas reduction of niobium pentoxide (Nb2O5) powder at 1073–1223 K, which are industrially moderate and low temperature ranges until maximally 80 h inside the chamber held under the argon circumstances of 110 kPa. Magnesium oxide of the by-product of the reduction, was dissolved and removed fully by dissolving in a 10% aqueous hydrochloric acid solution. The particle size of the niobium powder reduced for 20 h was slightly increased within the range of 200∼600 nm according to increase of reduction temperatures. And such fine particles were further coarsened to near 1 µm by increase of reduction times until 80 h at 1173 K, which is thought to be the most suitable for magnesium-gas reduction to be applied in industry. The reduction time satisfied for a maximal reduction effect was found to be 60 hours as the oxygen content was then minimally saturated to about 0.42 wt.%. Furthermore, the hydrogen contamination due to acid leaching of 0.28 wt.% was fully removed by dehydrogenation, which was a heat treatment performed under vacuum at 827 K for 2 h; this resulted in the formation of metallic niobium powder.
Surface damage induced by mechanical polishing of cold-rolled and annealed Pd specimens was examined by cross-sectional electron backscatter diffraction (EBSD) measurements. Fine grains with high-angle grain boundaries were detected in the outermost layer in both specimens. Less granular but layered gradation of crystallographic orientation was detected in the sub-surface layer of the annealed specimen. In the cold-rolled specimen, a lot of elongated grains were detected in the entire inner layer. The formation of the sub-surface layer seemed to be prevented in the cold-rolled specimen by pre-introduced microstructures. In the annealed specimen, the depth of the surface damage layer was dependent on the crystallographic orientation of the matrix grain. This study clearly demonstrated the application of cross-sectional EBSD analysis for evaluating surface damage in metallic materials.
Water accelerates the deformation and failure of rock and hence deteriorates the stability of rock structures on and under the ground. However, most of the previous studies examined the mechanical properties of rocks in air-dried and water-saturated conditions or the effect of water saturation on compressive strength. In this study, Brazilian tension tests were conducted with tuff, sandstone, and andesite in seven water saturation conditions between almost completely dried and water saturated. These conditions were controlled by varying the time the specimens were submerged in water and then dried. The test results showed that the Brazilian tensile strength of the tuff and the sandstone increases with a decrease of water saturation and then tends to be constant in very low water saturation conditions. In contrast, the Brazilian tensile strength of the andesite consistently increased with a decrease of water saturation. This trend for each of the rocks was also observed in the Young’s modulus estimated from the load–displacement curves in the Brazilian tension tests as well as the uniaxial compressive strength obtained in the previous studies. The applicability of the Hertz contact theory to the Brazilian tension tests with these rocks in various water saturation conditions was validated by comparing the estimated and measured Young’s modulus and by relating the Young’s modulus and the strength. The results in this study are helpful for estimating the strength and Young’s modulus of rocks in various water saturation conditions and for assessing the stability of underground structures.
The temperature dependence of the mechanical properties of metals is closely related to its crystal structure. In bcc metals, the strength of the material increases with decreasing temperature, and the ductility decreases drastically at low temperatures. The mechanical properties of fcc metals hardly depend on temperature, and thus fcc metals exhibit large elongations even at low temperatures. In this study, the microstructure and low-temperature tensile properties of Cu–50 mass%Fe alloy consisting of fcc (Cu) and bcc (Fe) dual-phase structures were investigated. The Cu and Fe layers were aligned along the rolling direction. The deformation structures remained after annealing at 1023 K for 1.8 ks, while recrystallized ultra-fine grains were formed after annealing at 1123 K for 1.8 ks. At both annealing temperatures, Cu and Fe precipitated in the Fe and Cu layers, respectively. The elongation obtained when the alloy was annealed at 1123 K for 1.8 ks was higher than that at 1023 K for 1.8 ks. The recovery of strain and the formation of ultra-fine grain during annealing at 1123 K were largely responsible for the higher elongation. The tensile strength at 77 K was higher than that at 293 K for both annealing temperatures. Nevertheless, the elongation at 77 K was approximately equal to that at 293 K. Therefore, the fcc and bcc dual-phase structures has an excellent temperature dependence of mechanical properties: high strength in bcc structure and high elongation in fcc structure at low temperatures. A dimple fracture surface appeared at 77 K, indicating that ductile fracture occurred in both phases. Therefore, the Fe phase exhibited significant ductility even at 77 K. The excellent low-temperature tensile properties of the Cu–Fe dual-phase alloy may be due to strengthening by the Fe phase and suppressing of brittle fracture in the Fe phase by the ductile Cu phase.
This Paper was Originally Published in Japanese in J. Japan Inst. Copper 59 (2020) 289–293. All captions are slightly changed.
Fig. 6 Nominal stress and strain curves at 293 K and 77 K in the specimens annealed at 1023 K and 1123 K for 1.8 ks.
By conducting in-situ XRD measurement during tensile deformation while oscillating the tensile tester, it was possible to measure the change in dislocation density of a pure aluminum alloy having coarse grains with the grain size of 20 µm. In the coarse-grained material, the dislocation density during tensile deformation changed through four regions, as in the case of the fine-grained material. Since the dislocation multiplication start stress was very low at 22 MPa, the elastic deformation region was very short. Thereafter, the dislocations multiplied rapidly, but when the stress and dislocation density reached 33 MPa and 1.57 × 1014 m−2, respectively, the dislocation multiplication rate was greatly reduced. This is considered to be due to the low dislocation density required to progress the deformation by plastic deformation in coarse-grained aluminum.
This Paper was Originally Published in Japanese in J. JILM 70 (2020) 274–280.
In this study, the effect of anodization and electroless Ni–P plating on the fatigue strength of commercial A5052-H14 and A2017-T4 aluminum alloys was investigated. The coated aluminum alloys were tested using a rotary bending fatigue testing machine. Anodization led to a slight increase in the fatigue strength of the A2017-T4 alloy of approximately 10% because of the suppression of the generation of fatigue crack, and anodization with a 5-µm thickness for A5052-H14 also led to a slight increase in the fatigue strength. However, anodization with a 20-µm thickness for A5052-H14 led to reduced fatigue strength because of the pits that formed in the film. In addition, electroless Ni–P plating drastically improved the fatigue strength of the A5052-H14 alloy by suppressing the generation of fatigue crack. It also improved the fatigue strength of the A2017-T4 alloy in the high-stress region. However, the fatigue strength in the low-stress region was the same as that of the non-coated specimens. This fatigue strength should have originated from the hydrogen embrittlement by the hydrogen introduced into the specimen during the plating.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 84 (2020) 74–79.
Fig. 7 Relation between stress (σ) and number of cycles to failure (N) for electroless Ni–P plated specimens.
Hydrogen embrittlement of SK85 high-strength steel sheets was evaluated through a three-point bending test. The effects of electroless Ni–P plating and Ni electroplating on hydrogen embrittlement were examined with respect to the hydrogen permeability of the plated films. On the morrow of the plating, electroless Ni–P plating indicates a high degree of hydrogen embrittlement, irrespective of the phosphorus content in the film. However, hydrogen embrittlement of Ni electroplating is further suppressed than that of electroless Ni–P plating, which can be attributed to the excellent hydrogen diffusing ability of Ni electroplating. These results demonstrate that the hydrogen permeability of a plated film is an important factor for hydrogen embrittlement. Hydrogen present on the plated film surface was visualized by employing the hydrogen microprint technique, and the corresponding results reveal that the hydrogen permeability of the plated film is dependent on the film crystal structure. Furthermore, the pits in the film can become a route for hydrogen emission.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 84 (2020) 80–86. The horizontal axis of Fig. 5 was corrected.
Fig. 5 Relationships between the time left standing from the plating and the hydrogen embrittlement rate. (a) Electroless Ni–P, Low P. (b) Electroless Ni–P, Middle P. (c) Electroless Ni–P, High P. (d) Ni electroplating.
In this study, a Ti50Al40V10 single alloying target is used to deposit TiAlVN coatings by a direct current (dc) magnetron sputtering technique. The structural, mechanical, and frictional properties of final coatings deposited at a various N2 content are investigated. It reveals that TiAlVN coating transforms from a single-phase (cubic phase) to a dual-phase structure (cubic and hexagonal phases) with the increase of N2 content. The highest hardness (∼36.5 GPa) and elastic modulus (∼372 GPa), and a good adhesion strength (∼24.4 N) are achieved in the sample deposited with a N2 flow rate of 6 sccm, which are determined by the nano-indentation and scratch test measurements. In addition, TiAlVN coating deposited with a N2 flow rate of 3 sccm has the lowest friction coefficient in the dry and oily conditions.
A quantitative evaluation of the subsequent yield behavior of a polycrystalline magnesium, which is a typical hexagonal metal, based on the crystal plasticity model is presented. The non-normality effect, that is, the difference between the normal to the yield surface and the plastic flow direction under a non-proportional loading condition, is numerically investigated, and it is shown that the non-normality effect of polycrystalline magnesium is stronger than that of cubic metals. Additionally, the intensity of the non-normality of hexagonal metals depends on the amplitude of the offset strain, which is the pre-strain before non-proportional loading, while that of cubic metals is almost constant with respect to the offset strain. The contribution of each slip system to the non-normality effect is investigated. It is clarified that the difficulty of switching the dominant slip system when the strain path changes plays an important role and is the principal mechanism behind the strong non-normality effect of polycrystalline magnesium. The effect of the initial texture on the non-normality effect is also discussed.
Fig. 2 Stress paths on σ11 − σ22 plane. Green solid and red dashed lines indicate directions of plastic deformation rate and the normal to the subsequent yield surface. Stress path for the four different offset strains of ε-p = 0.5%, 1.0%, 5.0% and 10.0% are illustrated.
The effective thermal conductivity (ETC) of graphite flake (GF)/Al composites is significantly influenced by the anisotropic thermal conductivity of GFs and the interfacial thermal resistance between both components. A two-dimensional (2D) image-based simulation was used in this study to investigate the effect of both the orientation of GFs and interfacial thermal resistance on the ETC of the composite. 10 vol% GF/Al and 20 vol% GF/Al composites were fabricated via spark plasma sintering. The microstructure ETC, and the relative density of the GF/Al composites were determined. The experimental ETCs were smaller than calculated ETCs using the rule of mixture. Additionally, the calculated ETC exhibited a decrease of 9.9% due to the effect of GF anisotropic thermal conductivity, and the ETC values decreased by 14.2% due to the effect of the interfacial thermal resistance at the Al–GF interface of the samples, which was determined to be in the range of 5.62–6.41 × 10−8 m2 K W−1.
This work details how thermal-cycling reliability degradation in semiconductor devices encapsulated utilizing a lead-on-chip (LOC) packaging technique is influenced by lead-frame tape structure modification. This work demonstrates through thermal-cycling testing that device pattern reliability is very sensitive to changes in the adhesive layer structure of the lead-frame tape. Particularly, in-situ examinations show that device failures result from fractures in the Si3N4 layer, which comprises the top layer of semiconductor devices, due to the excessive thermal behavior of the adhesive layer during thermal-cycling. This work theoretically proves that, during temperature variation, the Cramer-von Mises stress as well as the shear stress, which has been known to be responsible for Si3N4 damage, is proportionally related to a difference in the coefficient of thermal expansion (CTE) between the lead-frame tape (i.e., the adhesive layer) and the silicon device. Based on the experimental results and theoretical interpretation, this work concludes that, instead of the adhesive layer, an adhesive-filling base layer with a low CTE for lead-frame attachment on the device surface can help semiconductor devices to have better reliability margins in LOC packages.
Fig. 5 Graph showing number of device failures as function of tape structure and thermal cycles.
In this study, a rapid oxynitriding technique based on induction-heating was developed for the titanium alloy Ti–6Al–4V, and its effects on surface characteristics of the alloy were investigated. The surface microstructures of the alloy were characterized by employing a scanning electron microscope, electron back-scatter diffraction, X-ray diffraction and nano-indentation tests. Induction-heating in air for 60 s caused the formation of a hardened layer on the Ti–6Al–4V alloy surface, which can be attributed to the diffusion of oxygen and nitrogen atoms. When water quenching was performed after heating, transformation to an acicular martensitic α′ phase occurred, resulting in increased in substrate hardness. However, cracks were initiated at the surface of the oxynitrided alloy. The cracks were eliminated through fine particle bombarding treatment, whereas the hardened layer formed by the oxynitriding treatment remained. The obtained results thus indicate that the oxynitriding technique developed in this study can enhance the wear resistance and tensile strength of the Ti–6Al–4V alloy within a short period of time.
This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Jpn. 69 (2020) 605–611.
Fig. 1 Results of EBSD analysis (IPF, phase and IQ maps) of W, Q and A materials.
An inorganic-organic composite material, consisting of bioresorbable β-tricalcium phosphate (β-TCP) and a biodegradable polymer, is one of the most important materials for use in artificial bone with good shapability. In this work, to activate bone-forming cells immediately after being implanted in a living body, the ability to release therapeutic ions, such as Ca2+, Mg2+, and silicate ions was incorporated in the composite material. A small amount (8 vol%) of 46.1 SiO2·24.4 Na2O/MgO·26.9 CaO·2.6 P2O5 (mol%) glass particles was embedded successfully into the composite material. When the glass particles containing Na2O were included in the material, the molecular weight of the polymer in the matrix phase was severely reduced and its hydrolysis in Tris-HCl buffer solution was drastically accelerated. On the other hand, when the glass particles containing MgO were included, therapeutic ions were continuously released with almost no change in the buffer solution pH. The released amount of silicate ions was well controlled, to avoid excessive dissolution, compared to those of Ca2+ and Mg2+ ions. Therefore, the glass particle is one of the most promising candidates to be a source in the composite for releasing therapeutic ions that enhance bone formation.
This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Powder Metallurgy 67 (2020) 278–283.
Combined accelerating effects of ε–τ phase transformation by in-magnetic-field annealing and Zn-addition were investigated for Mn53Zn2Al45 in magnetic fields up to 15 T. The in-field annealing for Zn-added sample accelerated the ε–τ transformation 16 times faster than Mn55Al45 at annealing temperature of 623 K. The reduction of magnetization after finishing ε–τ transformation was also more rapid than that of Mn55Al45. However, magnetic-field-induced acceleration of ε–τ phase transformation and suppression of precipitation of β-phase were observed for Zn-added alloy Mn53Zn2Al45 annealed at 573 K. The obtained results suggested that the β-phase stabilized against both τ- and ε-phases by Zn-addition. Therefore, although the ε–τ transformation was efficiently accelerated by in-field annealing of Mn53Zn2Al45, more precisely control of annealing time is required for obtaining high fraction of ferromagnetic τ-phase.
Fig. 6 Annealing time t dependence of M1.5T of ZFA and IFA for Mn53Zn2Al45 at 573 K (a) and 623 K (b). The inset shows t dependence of M1.5T of ZFA and 15T-IFA for Mn55Al45 at 623 K.16)Fullsize Image
Ferromagnetic Mn–Al–C (τ-phase) can be synthesized by a single-route conventional reactive sintering method. The maximum magnetization and coercivity were 75.8 Am2/kg and 57 mT, respectively. The τ-phase fraction was evaluated to be 81 mass% for Mn55Al45C2 annealed at 1273 K. The τ-phase of Mn55Al45C2 can be synthesized at an annealing temperature from 873 to 1273 K, whereas that of Mn55Al45 cannot be synthesized. It was indicated that the phase stability of the hcp-phase (ε-phase) was improved by adding carbon, resulting in an ε–τ transformation.
Fig. 4 Ta dependence of M1.5T of Mn55Al45 and Mn55Al45C2.
We evaluated friction fatigue-induced changes in the microstructure and residual stress of carburized steel. By eddy current testing (ET) and x-ray diffractometry (XRD), the voltage rate (after/before fatigue testing) obtained by ET had a strong correlation with the retained austenite phase and the full width at half maximum of XRD. The results are useful for the analysis of fatigue damage in the microstructure of carburized steel.
Voltage rate (Vr) by eddy current testing (ET) have relation to retained austenite rate (Rγ) and the full width at half maximum rate (Fr) by x-ray diffractometry. On the other hand, it is difficult to detect a friction fatigue-induced change in residual stress rate (σr). Therefore, ET is indicated the possibility of a nondestructive method for fatigue damage.