In order to develop the technique to predict the surface characteristics of fatigue specimens of austenitic stainless steels by controlling finishing processing of lathe turning and shot peening, the applicability of the response surface models formerly created by the present authors using experimental design of experiments approaches to the fatigue specimens with different types and/or rolling conditions of austenitic stainless steels have been investigated. The conclusive remarks could be summarized as follows: (1) The predicted surface residual stress obtained by the response surface model of the lathe turning created for the round bars of SUS304 agreed with the observed surface residual stress of the fatigue specimen of SUS316. (2) The response surface model of the shot peening created for the flat plate of SUS316 was applicable to the prediction of the surface roughness RSm, the surface hardness and the surface residual stress of the fatigue specimen of SUS316 in spite of the different processing of the shot peening and the rolling conditions of the material.

Rotating bending fatigue tests were performed to investigate the effect of induction hardening (HQI) on the very high cycle fatigue (VHCF) properties of the axle steel. In addition, finite element analyses (FEA) and scanning electron microscope (SEM) observations were conducted to elucidate the fundamental behavior. The results obtained are summarized as follows; (1) It was found that the fatigue strength of the axle steel was improved by almost 300 MPa in VHCF region by HQI, (2) A lot of specimens of the HQI had fractured in VHCF region, therefore, the consideration of VHCF properties should be taken into account in the design of railway axles, (3) Crack initiation sites can be classified into three forms; the surface of the HQI layer in the minimum section, the base metal beyond the HQI layer in the axial direction, and the internal base material in the minimum section. These can be reconfirmed through the comparison between the local fatigue strength distribution and the bending stress distribution obtained by FEA.

A probabilistic model is newly proposed for describing random deterioration of pumping wells, in consideration of application to efficient management of pumping wells using a concept of asset management procedure. First, introducing a health index for quantifying specific capacity, we formulate a differential equation describing mean deterioration of the helth index. Next, it is extended to a random differential equation, in which a Gaussian white noise is introduced for representing intensive fluctuation around the mean deterioration behavior, so that intensively random variation of the health index can be well reproduced. A probability distribution of the health index is then derived in an analytical form. Finally, the obtained probability distribution is compared with actual data of the health index for active eight pumping wells in Joyo area, Kyoto prefecture, Japan. It is clarified that the scatter of health index of actual pumping wells is quite well reproduced by the theoretically derived probability distribution from the newly proposed probabilistic model.

Statistical estimations for probability distributions having tails of special shape, such as a double-mode distri-bution as well as the so-called heavy-tailed or fat-tailed distribution, are quantitatively discussed through virtual experiments using computer simulations. In this paper, a probability distribution describing the damage degree of concrete liners of tunnels in cold region is examined as an example of the double-mode probability distribution. First, virtual data sets of observations are generated by the use of quasi-random numbers for a Pareto distribution as a typical example of the fat-tailed distribution, whereas the actual data set obtained for the damage degree of concrete liners is used for generating virtual data set for the double-mode distribution. Next, statistical estimations are executed by the use of probability papers to identify the probability distribution showing the “best” fitting among supposed plural candidates for probability distributions. Finally, the accuracy of the estimation is quantified by applying the coefficient of determination. The results show that the accuracy of the estimation in the tail region is scarcely improved even if the number of data is increased.

The application of carbon fiber reinforced plastics (CFRP) has been expanding as large structural members, whereas there is a growing trend to expand its use to structural members of small devices such as “drones”. From such background, this study sampled strength data from tensile tests of micro-sized unidirectionally reinforced CFRP in diameter or thickness and discussed the damage and fracture process, as well as the number n_c fiber-break points up to fracture from the data. In our recent paper, the authors estimated the number of fiber-break points from previously obtained sampling data using an inverse analysis model of Markov processes. In this study, furthermore, another inverse analysis model, in which analytical solution can be obtained, was newly proposed. This model was in a good agreement with absorbing state probabilities and n_c obtained from the previous model. However, it was also found that there was a data population for which the fracture probability was limited to less than one. This is because the estimation limit occurs when some of the data are somewhat far from the data population. This problem can be solved to some extent by using the maximum likelihood estimation method for complete or censored data.

The corrosion degradation of the steel sheet pile material accelerates in recent years. The extremely accelerated corrosion induces the thickness loss and decreases the mechanical characteristics. It is essential to evaluate the corroded situations for the design and maintenance of the infrastructures. In this study, the spatial characteristics of the corrosion loss in service steel sheet pile canals were evaluated by the geostatistics with the variogram and kriging. In experimental procedures, steel sheet pile thicknesses were measured in service drainage canals. The thickness sampling was conducted in the air, tidal, water, sludge and soil zones. In analytical procedures, the variogram analysis is conducted for modeling of the spatial structures of the corrosion loss distribution. The ordinary kriging based on the theoretical variogram interpolates the no-sampling area. As a result, the thickness profile shows the largest corrosion loss in the tidal zone. The kriging interpolation shows the acceleration of the corrosion in the flanges of the tidal zone.

This paper describes study on the structure and magnetic properties of submicron Fe-B amorphous spherical particles and particle chains synthesized by liquid phase reduction method in order to realize new noise suppression sheet (NSS) toward 5G (3.5 GHz/4.7 GHz) applications. In case of Fe-B amorphous spherical particles, their peak frequencies originating from magnetic resonance can be observed and their values were about 3-4 GHz. On the other hand, the peak frequency values of Fe-B amorphous particle chains were a little lower than those of Fe-B amorphous spherical particles, but their peak values in imaginary part of the complex permeability (µ ”) at 3.5 GHz/4.7 GHz were almost as same as those of Fe-B amorphous spherical particles. Also, the intrinsic permeability of Fe-B amorphous particle chains were 2-4 times higher than those of Fe-B amorphous spherical particles owing to decrease in the demagnetizing field. Therefore, these results suggest that submicron Fe-B amorphous particle chains are one of the candidates as not only NSS for 5G applications.

Metallic glasses exhibit a high mechanical strength at room temperature and superior formability in a supercooled liquid region, which make them to become promising materials for engineering applications. However, two major drawbacks, one is difficulty in fabricating bulk samples and the other is their intrinsic brittle properties during fracture, highly limited their practical applications. With respect to the former, we focused on fabricating micrometer size products and a Fe-based metallic glassy micro gear was successfully developed through our newly proposed microparts fabrication process. For the latter, a concept of amorphous/glassy state gradient is introduced, intending to improve a brittleness by controlling the crack propagation path depending on a created gradient state. The temperature/thermal history gradient is thermally introduced within a single monolithic Zr-based metallic glass, and it was found that a brittleness presumably improved by thermally created amorphous/glassy state gradient.

Studies on dissimilar metal welding and mechanical fastening using metallic glasses in Utsunomiya University have been introduced. Metallic glass ribbons placed between the two dissimilar metal plates become supercooled liquid with low viscosity by resistance welding and promote supercooling at the nuggets. As a consequence, the residual stress at the welding boundary and the formation of brittle intermetallic compounds are suppressed, resulting in excellent dissimilar resistance welding with high joint strength and high reproducibility. Mechanical fastening of two metal plates is also demonstrated by resistance heating and viscous flow forming of metallic glass rivets. Producing metallic glass rivets using bulk metallic glasses by a resistance heating is now under examination.

A new model of the medium-range ordered structure in metallic glasses is proposed. The molecular dynamics simulations revealed that there are two different types of basic clusters formed in metallic glasses. One is the icosahedral cluster (I-cluster) formed around the smaller-sized elements, and the other is the Z14, Z15, and Z16 Frank-Kasper clusters (Z-clusters) formed around the bigger-sized elements. The I- and Z-clusters are mainly connected by volume-sharing type connection and form a network extended to the whole glassy phase. Among them, the connection between Z-clusters corresponds to the dicsclination line proposed by Nelson. The topological nature of the network formed by the Z-clusters can be understood by the continuous random network model.

Mechanical properties of metallic glasses depend on their microstructure. Rejuvenation recently has attracted much attention because it is a potential tool for controlling the microstructure and properties of metallic glasses. Here I explain molecular dynamics studies on microstructure control and deformation behaviors of metallic glasses. From atomistic simulation approaches, I revealed the thermal rejuvenation process in the heterogeneous microstructure of metallic glasses, the formation of nanocrystals in thermal processing for rejuvenation, and the effects of rejuvenation and nanocrystallization on deformation behaviors.

The yield surface, the mechanical properties and our experimental methods for Zr₅₅Al₁₀Cu₃₀Ni₅ metallic glass are explained. The deformation behavior and the yield stress of this material changes for environmental temperature because of its obvious temperature dependency. The effect of normal stress on its yield can be revealed with increasing temperature and the yield surface closes a typical shape based on the Mohr-Coulomb yield criterion. This material also has an obvious strain rate dependency. The experimental study of the influence of deformation speed on the yield surface and function of this material will be important for a practical application of this material under dynamic load in hot environment.

Amorphous alloys are expected to be useful for various applications because they have superior mechanical properties such as strength, hardness and corrosion resistance. So far, the only method for production of amorphous metals is to quench from the liquid state. When materials are exposed to high-energy ions, the lattice structures of the materials tend to be metastable or non-thermally equilibrium, and in some cases, they lose the crystallinity, resulting in amorphization. However, the alloy types and irradiation conditions that cause amorphization by irradiation have never been well clarified yet. In order to study the amorphization by particles irradiation as one of the material processes such as strength control, we have studied the structural changes and the accompanying hardness changes of the bulk crystalline alloys due to various irradiation conditions. In this paper, we discuss the correlation between amorphization and hardness change when heavy ion irradiation is applied to several intermetallic compounds.

Additive manufacturing (AM) can realize the rapid solidification for the formation of ultra-fine solidification microstructures and/or crystal growth reaching the marginal stability condition. The TEM observation of the nucleation and growth behavior of BCC dendrite embedded in Fe-based metallic glasses offers an unique opportunity for clarifying the morphology of the ultra-fine dendrite for the development of AM. The present study reported the network tele-microscopy observation of BCC-Fe dendrites embedded in an Fe-Si-B amorphous matrix obtained by the in-situ heating of TEM specimens using the ultra-high voltage electron microscope Hitachi H-3000.

The structure and catalytic properties of porous CeO₂ prepared from amorphous alloys by a dealloying method were investigated. The ligament shape of CeO₂ could be controlled by using amorphous Ce-Al alloys and crystalline alloys with different atomic arrangements as precursor alloys. The prepared samples showed excellent catalytic properties in soot combustion reactions.

To understand the relation of the glass-forming ability (GFA) to the atomic configurations of Pd_42.5Ni_7.5Cu₃₀P₂₀ (PNCP) metallic glass having the best GFA at present, the local structures were investigated by combining data obtained anomalous x-ray scattering, x-ray and neutron diffraction and applying reverse Monte Carlo modeling. By comparing the results of PNCP with Pd₄₀Ni₄₀P₂₀ and Pd₄₀Cu₄₀P₂₀ having a slightly and much worse GFAs, respectively, characteristic features were observed in the hyper-ordered atomic structures around the secondary metals of Ni and Cu, such as the concentration inhomogeneity, the fraction of pure and slightly distorted icosahedra, and the existence of large persistent homology rings. The structural heterogeneity for the excellent GFA of PNCP would be considered by an incompatible mixture of covalent-like Pd-P configurations and icosahedral clusters around the secondary metals.

This article is a commentary on our previous work entitled “Reassessment of oxidation-induced amorphization and dissolution of Nb precipitates in Zr-Nb nuclear fuel cladding tube”, published in Acta Materialia in 2017. The highlight of this 2017 paper is a discovery that solid-state amorphization can occur even when the motion of atoms (or more precisely, all constituent elements of the amorphous material) is not frozen-in. Although judging from the highlight, this work is a fundamental study of amorphous materials, it was originally performed under an engineering research program on nuclear materials, irrelevant to amorphous. The inside story is revealed: how such a fundamental discovery was born out of engineering research.

The influence of thermo-mechanical loading on the mechanical behavior of Zr₅₅Al₁₀Cu₃₀Ni₅ bulk metallic glass was investigated experimentally. Pre-stressed specimen like loaded mechanical parts was subject to external and thermal load by heating. When pre-stress was lower than about 900MPa~1000MPa, during heating, this material showed plastic deformation with softening so that the mechanical tester cannot keep and control constant stress. A higher pre-stress than 1000MPa caused its brittle fracture to Zr₅₅Al₁₀Cu₃₀Ni₅ bulk metallic glass by heating. The temperatures to show these failures induced by heating under prestress to specimen were lower than the glass transition temperature of Zr₅₅Al₁₀Cu₃₀Ni₅ metallic glass measured by differential scanning calorimetry. Therefore, the interaction of the mechanical and thermal load such as external force and environmental temperature appears to decide the yield stress and the thermal properties of Zr₅₅Al₁₀Cu₃₀Ni₅ metallic glass, complementarily. From the result of this study, the direction for the strength design of applications made of this material subjected to various temperature conditions is indicated, and an effective machining method will be suggested.

Neutron irradiation embrittlement behaviors of some Zr-Cu-Al and Zr-Cu-Ni-Al BMGs with various Zr-contents have been examined by Charpy impact tests under neutron fluences of 1~2 x 10²⁴ n/m² at relatively low and high temperatures of about 325 K and 550 K, respectively. In the case of the irradiation at 325 K, Charpy impact strength of Zr₄₅Cu₄₅Al₁₀ BMG increased from 59 kJ/m² up to 92 kJ/m² accompanied with decreasing the hardness. In the Zr-rich BMGs of Zr₅₀Cu₄₀Al₁₀, Zr₅₅Cu₃₀Ni₅Al₁₀, Zr₅₉Cu₃₁Al₁₀, Zr₆₅Cu₂₀Ni₅Al₁₀ and Zr₆₅Cu₁₈Ni₇Al₁₀, impact strengths decreased from about 90 kJ/m² down to about 30 kJ/m² with decreasing the hardness. In the case of the irradiation at 550 K, impact strengths of the Zr-rich BMGs maintained relatively high values of about 60 ~ 90 kJ/m². These results may be explained by amounts of excess free-volume in the neutron irradiated Zr-based BMGs.

For the purpose of developing metallic materials with excellent corrosion resistance under boiling sulfuric acid environment, the coated materials were evaluated for their corrosion resistance and the differences in surface morphology were investigated. As a result, the corrosion resistance of SUS304 coated and sintered eight times by chemical densified coating method (S-ZAC) was maintained at the point after 12h corrosion test, but the corrosion rate rapidly increased after 100h corrosion test. The cross-sectional microstructural analysis of the specimens showed that only a small amount of SiO₂ remained on the surface, while Cr₂O₃ completely disappeared. The mechanism of film delamination in S-ZAC is assumed to be due to the thermal expansion caused by heating, which resulted in curvature of the thick film and cracks at the Cr₂O₃ and SiO₂ interfaces. On the other hand, SUS304 coated with higher density of Cr₂O₃ by repeating the coating and sintering eleven times (MS-ZAC) showed excellent corrosion resistance as well as SiC even after 100h corrosion test. The results of cross-sectional microstructural analysis showed that MS-ZAC had higher Cr₂O₃ concentration, finer SiO₂ grain size, and fewer porosity than S-ZAC. In particular, the mechanism of film maintenance in MS-ZAC is assumed to be the formation of a stacked structure in Cr₂O₃ particle layer, in addition to the thinning of the entire film.