The relaxation processes correlated to atomic defects were investigated by the internal friction method using a multifunctional internal friction apparatus for Ti–Mo alloys. The microstructures of the Ti–Mo alloys with different Mo content and heat treatments were observed using an optic microscopy and a scanning electronic microscopy. Their phase constitutions were detected by X-ray diffraction (XRD). The β phase increases and α phase decreases in volume with increasing Mo content for the furnace-cooled alloys. Similarly, the βM phase volume increases when Mo content is increased. The two relaxational internal friction peaks, named as P1 for the low temperature peak and P2 for the high temperature peak, respectively, are found on the internal friction temperature dependent curves in the alloys. The two peaks always appear in the present experimental specimens with different Mo content and different heat treatments except for the water quenched Ti–24 Mo alloys. The P1 and P2 peaks are all influenced by Mo content and heat treatments and related to phase constitutions and microstructures. The P1 peak height increases with increasing Mo content for the water quenched alloys and the P1 peak temperature is not changed with Mo content. The P2 peak height increases also with increasing Mo content and the P2 peak temperature is raised when Mo content is increased but for the water quenched Ti–3Mo and Ti–5Mo alloys. Relaxation parameters of P1 peak are Hwq1 = 1.56 ± 0.1 eV and τ0wq1 = 2.5 × 10−16±0.1 s and those of P2 peak are Hwq2 = 2.3 ± 0.1 eV and τ0wq2 = 2.1 × 10−17±0.1 s for the water-quenched Ti–5Mo alloy, respectively. The P1 peak is attributed to the stress-induced short-range ordering of oxygen complexes around Ti atoms in the Ti–Mo alloys. The increase of the P1 peak height with increasing Mo content is attributed to the increase of β phase in volume. The P2 peak is resulted from the interaction of Mo–O atoms or the reorientation of Mo–Mo atomic pairs by means of vacancies. The Mo–O interaction is strengthened when Mo content is increased, which results in the increase of both the peak temperature and the activation energy of the P2 peak.
The P1 and P2 peak-temperatures in water quenched Ti–5Mo alloy increase with increasing the vibration frequency.
We investigated the structural phase transitions of Fe–Ni alloys under hydrostatic pressure conditions at 298 K by examining the X-ray diffraction patterns of polycrystalline samples with Ni contents of 5%–31.6%, and pressure–composition phase diagram was established for the pressure range of 0–15 GPa. The diagram comprised three structural phases: bcc, fcc, and hcp, and these all coexisted in 25% Ni at 11 GPa during compression and in 23% Ni at 7 GPa during decompression. During both processes, low- and high-pressure phases coexisted. During the pressure-induced structural transitions, significant hysteresis was also observed. The 27% Ni alloy showed the two-stage transition: bcc–fcc–hcp, and the fcc phase was quenched to ambient pressure. For the 5–23% Ni alloys, a bcc–hcp transition was observed at high pressure. As the Ni content increased, the transition pressure decreased, and the volume reduction of the bcc–hcp transition also decreased due to the increase in the atomic volume of the hcp phase. The transition pressure from fcc to hcp rapidly increased with the Ni content. Compression curves of 29.9% Ni and 31.6% Ni alloys exhibited an anomaly at 2.0 and 2.9 GPa, respectively, and the compression anomaly was attributed to the magnetic transition.
Fig. 4 The pressure–composition phase diagram of the Fe-rich portion for the Fe–Ni alloy system under compression (blue) and decompression (red) at 298 K. Lines are guides to the eye.
The roles of mechanical size and chemical bonding effect of segregated elements in grain boundaries (GBs) on the interactions between the GBs and dislocations are still not well understood. Because a twist GB tends to have a higher GB energy than a tilt GB owing to its random structure, the mechanical and chemical effects of solute elements on the dislocation-GB interactions in a twist GB are generally different from those in a tilt GB. In addition, dislocation emission from a GB in hcp metals is more complex than that in fcc metals because of the intense plastic anisotropy in hcp metals. In this study, interactions between basal 〈a〉 edge dislocations and a twist GB were studied in non-segregated and Al- and Fe-segregated twist Mg GBs by molecular dynamics simulations. The simulations showed that a prismatic dislocation was emitted from the GB after two basal dislocations were adsorbed into the GB. Dislocation adsorption into the GB was enhanced by Al and Fe segregation, but dislocation emission from the GB was suppressed by the segregation. Analyses of the GB width and potential energy suggested that the dislocation adsorption was mainly determined by mechanical effects, while dislocation emission strongly depended on chemical effects.
In order to clarify the effects of carbon concentration change on the growth of the compound layer in nitrided steel, the variation with time of the concentration of alloy elements on the surface layer and the phases of the compound layer were investigated in nitrided steels containing various amounts of carbon in the matrix. It was found that the variation with time of the carbon concentration in the compound layer was mainly responsible for the variation with time of the compound layer microstructure. Furthermore, it was discovered that the variation with time of the carbon concentration of the compound layer was caused by both the migration of carbon from the matrix to the compound layer and from the surface of the compound layer to the atmosphere. Thus, the gradient of the chemical potential of carbon in the through-thickness direction of the compound layer and the compound layer microstructure changed with nitriding time.
This Paper was Originally Published in Japanese in J. Japan Soc. Heat Treat. 59 (2019) 61–66. Captions of figures are slightly changed.
The semi-solid slurry of A356 alloy of various solid fractions was fabricated and water quenched to freeze the morphology of the Al grains in the semi-solid state. Two kinds of specimens with different volume (large and small) were prepared to change the cooling rate during the quenching. The difference in the amount of solid growth around the original spherical Al grains during the quenching was investigated by the color etching method with Weck’s reagent, which is sensitive to the solute segregation. It was found that the amount of solid growth during the quenching increased with a decreasing cooling rate. The amount of solid growth of each spherical Al grain also increased for a decreasing local solid fraction, or the amount of residual liquid and the mutual distance between the solid grains. The solid fraction obtained by the color etching method showed a good agreement with the estimated solid fraction from the phase diagram. It was confirmed that the present color metallography was effective to evaluate the correct solid fraction at the semi-solid state.
Fig. 5 Microstructures of the large specimen hold at and quenched from 588°C (a) etched by NaOH, A: the area of solid, (b) etched by Weck’s reagent, (c) the original solid phase eliminating solid growth during water quenching, A consists of A0 (original solid area) and Aq (solid growth during water quenching).
The kinetics for the isothermal carburization of pure iron (Fe) at a temperature of 1073 K (800°C) for times between 1.8 ks (0.5 h) and 32.4 ks (9 h) was experimentally observed by Togashi and Nishizawa. According to the observation, the austenite (γ) phase with the face-centered cubic (fcc) structure is produced on the Fe specimen of the ferrite (α) phase with the body-centered cubic (bcc) structure, and gradually grows into the α phase. The carbon (C) concentration in the γ phase at the moving γ/α interface is greater than that of the γ/(γ + α) phase boundary in a phase diagram of the binary Fe–C system. Although the former one gradually approaches to the latter one with increasing annealing time, their difference hardly vanishes even at the longest annealing time of 32.4 ks (9 h). The molar Gibbs energy of the γ phase was described by a two-sublattice model to evaluate the chemical driving force working at the moving γ/α interface. The evaluation provides that the chemical driving force monotonically decreases with increasing annealing time. This annealing time dependence of the chemical driving was used to calculate the migration distance of the γ/α interface as a function of the annealing time. The observation for the interface migration was satisfactorily reproduced by the calculation. According to the calculation, the migration of the γ/α interface is controlled by the interface reaction at the moving interface in the early stages, but it is governed mainly by the volume diffusion of C across the γ phase and partially by the interface reaction in the late stages. The experimental annealing times mostly belong to the transition stages between the rate-controlling processes of the interface reaction and the volume diffusion.
Fig. 5 The thickness l of the γ layer versus the annealing time t. Open circles show the observation reported by Togashi and Nishizawa,7) a solid curve and a dashed line indicate the calculation from eq. (11), and a thin dashed line represents the extrapolation of the dashed line in the early stages.
Titanium matrix composite materials with high strength and toughness are expected to be obtained from trimodal composites (metal matrix (titanium)/ceramic (TiC)/precipitated metal (titanium) in ceramic) by titanium precipitation in brittle ceramic particles. Titanium carbide (TiC) particles dispersed in titanium matrix composites are prepared by the arc-melting method, and the effect of nitrogen content on the microstructural changes in the TiC particles is revealed. Titanium precipitation in the TiC particles is observed when more than 2 at% nitrogen is added. The mechanism of titanium precipitation is discussed in terms of the crystallographic relationship between titanium and metastable Ti2C.
Fig. 4 Representative high resolution TEM micrograph of the interface between TiC and precipitated Ti for the 5N5TiC (a) and SAD pattern (b) with its key diagram (c).
Strain softening is the mechanical behavior of soil and rock materials and is important in understanding soft rock foundation. To investigate the mechanical behavior of siltstone, a sedimentary soft rock, consolidation tests using constant-strain rate loading were conducted using the consolidation ring to constrain lateral deformation. Using Quaternary siltstones distributed in the Boso Peninsula, central Japan as specimens, strain softening in the consolidation process was confirmed in some formations using two test machines at Kyoto University and Nagoya Institute of Technology. Just before the yielding, stress decreased suddenly at increasing strain. The stress at the time of the softening differed even for specimens taken from the same formation. Furthermore, micro-focus X-ray CT images taken before and after the tests indicated that the specimens had no macro cracks inside. This suggests that strain softening is not due to brittle failure in local areas but due to the softening of the framework structure of the siltstone itself.
This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 69 (2020) 250–255.
The purpose of the present study is to investigate the effect of component layer geometry on wear resistance in multilayered structures consisting of soft Cu layers and hard nanocrystalline Ni layers, which were fabricated by electrodeposition. Pulsed-potentials were applied only during the nickel layer deposition to obtain the nanocrystalline Ni microstructure. We prepared the multilayers with various component layer thicknesses and various ratios of the Ni to Cu layer thicknesses. Pulse-plated nickel films without Cu layers were also prepared for comparison. The formation of nanostructured grains was recognized in the pulse-plated nickel using transmission electron microscopy (TEM). Among the multilayered films prepared in this study, the multilayer with the nanocrystalline nickel layers of 190 nm thickness and the Cu layer of 10 nm thickness revealed the best wear resistance. The weight-loss rate and the worn depth of this multilayered film were approximately half of those measured in the nanocrystalline nickel film without the Cu layers at the vertical loads of 10 N and 20 N. These results suggest that the periodic insertion of thin Cu layers can improve the wear property of nanocrystalline nickel film.
Weight-loss rates of the nanocrystalline-Ni/Cu multilayered films subjected to sliding wear at different pin loads.
Header stub welds for thermal power plants undergo creep-fatigue damage due to thermal expansion and contraction, which are caused by cyclic startup and shutdown of the plant. In this study, creep-fatigue tests were conducted using header stub mock-up specimens of 47Ni–23Cr–23Fe–7W alloy to investigate the creep-fatigue damage process. It was discovered that the cracks were initiated from the outer surface of the tube near the bond line, and they propagated toward the inner side in both fatigue and creep-fatigue tests. Transgranular cracks were observed in fatigue tests, whereas cracks were found to be progressed along the grain boundary in creep-fatigue tests. The failure mechanism of 47Ni–23Cr–23Fe–7W alloy header stub mock-up specimens was composed of two steps. At first, cracks progressed on the surface near the bond line, and then progressed toward the inner surface. Crack initiation and propagation behaviors of the 47Ni–23Cr–23Fe–7W alloy header stub mock-up specimen were the same as those of the 2.25Cr–1Mo steel header mock-up specimen.
This Paper was Originally Published in Japanese in J. Soc. Mater. Sci., Japan 68 (2019) 136–141.
Low carbon steel sample S15C was nitrided by active screen plasma nitriding (ASPN) and direct current plasma nitriding with screen (S-DCPN) using a chromium screen to form simultaneously chromium nitride coating/nitrogen-diffusion layer on the sample surface. The sample was placed on the sample stage in a floating potential and a cathodic potential. A chromium screen was mounted on the cathodic stage parallel to the top of the sample. Plasma nitriding treatments in a floating potential (ASPN) and a cathodic potential (S-DCPN) were performed in a nitrogen–hydrogen atmosphere with 75% N2–25% H2 for 120 min at 773 K and 873 K under 200 Pa. After nitriding, the nitrided microstructure was examined with a scanning electron microscope, glow discharge optical emission spectroscopy and X-ray diffraction studies. In addition, the hardness and corrosion properties were also measured. The nitrided layer formed by S-DCPN consisted of a Cr-concentrated layer (∼750 HV) followed by a nitrogen-diffusion layer whereas that formed by ASPN consisted of deposited layer of chromium nitrides and iron nitrides without nitrogen-diffusion layer. Corrosion resistance of the samples treated by S-DCPN was better than that of untreated sample and sample treated by ASPN.
This Paper was Originally Published in Japanese in J. Jpn. Soc. Heat Treatment 59 (2019) 344–349.
The effect of machining on the oxide film of 304L and 316L austenite stainless steels (SS) were investigated by X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM), X-Ray electron probe micro-analysis (EPMA), and transmission electron microscopy (TEM). The composition, microstructure, and corrosion resistance of the oxide film formed in 300°C high temperature water environment was analysed. The results shown that several kinds of metal oxides were formed on surface such as Fe2O3, Cr2O3, MoO3, and Ni(OH)2. The polished specimen of 316L SS can form integral oxide film after immersion for 20 days in 300°C high temperature water. However, the oxide film was loose and porous of milled specimen tested under same conditions. The corrosion resistance of the oxide film formed on polished specimens was better than those formed on milled specimens. The enrichments of Cr and Ni were detected at the interface of oxide film and matrix. The related diffused mechanism during the oxidation process was analysed.
Fig. 8 TEM morphologies of D120 specimen: (a) cross section morphology of oxide film, (b) and (c) were corresponding magnified figures.
Photoluminescence (PL) emission from anodic aluminum oxide (AAO) formed by etidronic acid anodizing was investigated via fluorescence spectroscopy. Highly pure aluminum plates were anodized in an etidronic acid solution under various operating conditions. PL emission from the typical AAO film was identified with an approximately 250–275-nm range in excitation and a 375–450-nm range in emission, and this distribution was greatly different from the AAO film formed by typical carboxylic acid treatment. The PL intensity increased with the decrease of the concentration of etidronic acid solution. The intensity also increased with anodizing time and subsequent thermal treatment, whereas excess anodizing and high temperature treatment caused an intensity decrease. The AAO film consisted of an outer oxide containing incorporated etidronate anions and a thin honeycomb inner oxide without anions, and the oxygen vacancy localized in the AAO film and anion distribution strongly affected the PL intensity. The appropriate operating condition through anodizing in 0.05 M etidronic acid for 40 h and subsequent thermal treatment at 773 K for 2 h caused the maximum enhancement of the PL emission.
Under the primary conditions of a pressurized water reactor, the intergranular stress corrosion cracking susceptibility of a Ni based alloy is improved by chromium carbide precipitation in grain boundaries. The effects of chromium carbide precipitation treatments are explained by the grain boundary strengthening mechanism, the intergranular corrosion prevention mechanism, and so on. However, few studies have demonstrated these mechanisms, and the effects on intergranular stress corrosion cracking susceptibility are not completely understood. Therefore, for the purpose of demonstrating the change in the internal stress distribution with or without grain boundary carbide precipitation treatment in the 600 alloy, in situ measurements were performed in this study using the X-ray diffraction technique with the BL-28B2 beamline at the SPring-8 synchrotron radiation facility under tensile stress. The results confirmed that there is a difference in the strain distribution in grain boundaries and trans-grains. Intergranular stress corrosion cracking is suppressed by preventing stress concentration in grain boundaries owing to the carbides precipitated at the grain boundaries.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 83 (2019) 54–58.
Fig. 6 Difference equivalent stress, dσe, of a difference from the equivalent stress without the applied stress and the equivalent stress ratio, α = σe(GB)/σe(TG), as a function of the applied stress at the grain boundary (GB) and trans-grain (TG). (a) Untreated specimen and (b) thermally treated specimen.
This study investigates the interaction between E. coli bacteria and various surface states of Cu. Pure Cu was oxidized at 298 K, 673 K, and 1273 K, and its surface chemical state was characterized using X-ray photoelectron spectroscopy. The oxidized specimens were immersed in both bacteria-free and bacteria-containing solutions, and the release of Cu ions from each specimen was evaluated. As a result, the specimens, which existed as mainly Cu0 or Cu+ on the surface, exhibited the same corrosion behavior and promoted the elution of Cu ions due to the presence of E. coli in the immersion solution. In contrast, the release of Cu ions from the specimen that existed mainly as Cu2+ did not change in the presence of bacteria. According to the XPS analyses before and after immersion in bacteria-free and bacteria-containing solutions, the generation of Cu2+ on the surface was inhibited by the presence of E. coli, and promoted Cu ion elution. This study proposes that the specific corrosion behavior of Cu due to bacteria is beneficial for developing its antibacterial properties.
The influence of isothermal annealing on mechanical properties of the Cu-clad Al (CA) wire was experimentally examined to better understand the annealing mechanisms that occur in the CA wire. In the experiment, the CA wire was prepared by drawing to decrease the diameter d from 10 mm to 2.9, 1.5 and 0.5 mm. The CA wires with these three different diameters were isothermally annealed at temperatures of 423–603 K (150–330°C) for various periods between 10.8 ks and 3456 ks (3 and 960 h). At room temperature, tensile tests were performed on the CA wire using an Instron type testing machine, and hardness tests were made on the Cu layer and the Al core of the CA wire utilizing a micro Vickers hardness testing machine. For the CA wire without annealing, the ultimate tensile strength su is 231, 215 and 198 MPa for d = 0.5, 1.5 and 2.9 mm, respectively, and the elongation eu is 0.4, 0.4 and 1.2% for d = 0.5, 1.5 and 2.9 mm, respectively. Due to isothermal annealing, recovery and recrystallization take place in both the Cu layer and the Al core of the CA wire, and an intermetallic layer consisting of various Cu–Al compounds forms at the Cu/Al interface. The intermetallic layer promotes formation of cavity at the Cu/Al interface during the tensile test. As the thickness l of the intermetallic layer reaches to the critical thickness lm, su attains to the minimum value, and eu takes the maximum value. Here, lm = 1–2 µm for d = 0.5–2.9 mm, respectively. For l > lm, however, su is insensitive to l, but eu decreases with increasing thickness l. Thus, the appropriate combination of the annealing temperature and time is essentially important to realize the optimal mechanical properties of the CA wire.
Fig. 9 The results in Fig. 8(b), 8(d) and 8(f) are represented as open circles, triangles and rhombuses, respectively. The corresponding results reported by Hug and Bellido7) are also shown as solid squares.
This paper describes mesh simplification of 3D-scanned objects, as used in discrete-element modelling (DEM). The mesh simplification algorithm parameter (Rp) is conventionally determined by trial-and-error based on the users’ judgment. We proposed a new criterion to quantify a balance of shape approximation accuracy and a quantity of meshes. A satisfactory Rp value was uniquely determined through minimization of the criterion. The proposed method was applied to mesh simplification in 3D-scanned waste electrical and electronic equipment, and successfully determined a satisfactory Rp value. To evaluate the determined Rp value, we performed a series of angle-of-repose DEM simulations using particles with different Rp values. The simulated repose angle converged to a constant value when the Rp was greater than a specific value. Its precise shape approximation reproduced the rotational resistance due to interparticle moment transmission that occurred upon contact of nonspherical particles. The Rp value was sufficient to obtain a constant value of repose angle, which indicated that the proposed method is valid for decision-making with respect to the selection of the Rp value used.
Al–Ga–In–Sn alloys with different amounts of a refining agent (Al–3Ti–0.3C) were prepared via atmospheric pressure casting. The microstructures of the resulting alloys were analyzed via X-ray diffraction, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. With an increase in the Ti content (through Al–3Ti–0.3C) from 0.03 mass% to 0.24 mass%, the grain size of the alloy was refined from 181 to 73 µm. In addition, Al dendrites of several micrometers are observed in the grain of Al alloy.
Our experimental results for the Al-water reaction at different temperatures of water indicate that the hydrogen production rate of the alloy with a Ti content of 0.12 mass% is the highest; in particular, it is about twice that of the alloy without Ti. On further increasing the Ti content, the hydrogen production rate of the alloy-water reaction remained almost unchanged. The hydrogen yield of the alloy decreases from 88% to 66% with an increase in the Ti content from 0.03 mass% to 0.24 mass% at 30°C. However, when the water temperature is increased to 70°C, the hydrogen yield of the alloy increases to more than 90%.
Based on our observations, the mechanism of grain refinement and its effect on the reaction between the alloy and water was proposed.
The effect of grain boundary segregation on plastic deformation was investigated using the Mg–Y solid solution binary alloy. Deformed microstructural observations revealed many traces of prismatic 〈a〉 dislocations as well as basal dislocations. These dislocations were nucleated at grain boundaries with segregation of yttrium element. By comparison of material factors of binary alloy, the alloying elements having low critical resolved shear stress (CRSS) of non-basal plane and large grain boundary (twin boundary) segregation energy led to activation of non-basal dislocations in the vicinity of grain boundaries. The Mg–Ca alloy had similar material factors and showed the same deformed microstructures as those of the alloys containing rare-earth element, which indicate that calcium element is an alternative alloying element.
Fig. 1 Deformed microstructures in the vicinity of grain boundaries in the Mg–0.3Y alloy; (a) low-angle annular dark-field image including nano-EDX line profiled analysis, (b), (c) the weak beam bright- and dark-field images taken under two-beam diffraction conditions (g-3g) using different diffraction vectors, g = (0002) and (d), (e) g = (10-10). White arrows in Fig. (e) indicate 〈a〉 type prismatic dislocations.