The importance of controlling grain boundary (GB) segregation is increasing, especially with the strengthening of steel nowadays. In this study, a theoretical prediction method for the amount of GB segregation for a solute element in polycrystals is established. This prediction method entails the development of a nano-polycrystalline GB model for simulating GBs in polycrystals, and the segregation energy of a solute element is comprehensively calculated 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 based on its distance from the GB center and averaged to determine 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 the results with experimental findings, it is determined that this prediction method can deliver excellent prediction accuracies.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 84 (2020) 237–243.
First-principles calculations were employed to evaluate the trapping energy of a H atom in a screw dislocation and an edge dislocation in Al. After obtaining the dislocation core structure in the absence of H, we calculated the trapping energy of H at several tens of possible trapping sites in the dislocation core and its vicinity. The maximum trapping energies were 0.08 and 0.15 eV/atom for the screw and edge dislocations without the zero-point vibrational energy correction, while they were 0.11 and 0.18 eV/atom for the screw and the edge dislocations with the correction. The calculation conditions employed in the present work correspond to the line density of approximately 1.0 H atom/nm along the dislocation line, which is sufficiently low to exclude H-H interaction.
Antiperovskite manganese nitride Mn3.1Zn0.5Sn0.4N was composited with Al by compressive torsion processing (CTP). This class of nitrides is anticipated for use as a thermal-expansion compensator because of giant negative thermal expansion (NTE). It is difficult to form a composite with molten metal that causes a chemical reaction. To date, only limited success has been achieved. Composites formed by CTP at 573 K exhibit lower thermal expansion and electrical resistivity than those produced by electric-current sintering. That achievement is attributable to suppressed chemical erosion of the filler by treatment at a lower temperature while the strong mechanical energy strengthens the interface between the filler and the matrix and reduces pores in the material. Compressive torsion processing is a promising method for forming metal matrix composites containing giant NTE fillers.
Pure iron and Fe–M (M = Mo, Si, Mn, Cr, Al, V) binary ferritic alloys were nitrided at 843 K for various times. Phase constituent, hardness distribution and growth rate of the compound layer are investigated by means of X-ray diffraction, EBSD, EPMA, 3DAP and nanoindentation. In pure iron, ε and γ′ (beneath ε) form on the surface, and voids form along the interface. The hardness of γ′ is higher than ferrite matrix, while void formation causes softening of compound layer. Although alloying effects on the hardness of compound layer are small, Si, V and Al additions suppress the formation of voids, and therefore the softening is also contained. All the elements investigated increase growth rate of compound layers. Especially the Si, V and Al additions significantly accelerate their growth rate due to the presence of excess nitrogen in compound layer. However, in the case of Al-added specimen, the growth layer becomes sluggish at longer nitriding time, presumably because of the extensive precipitation of AlN into the diffusion layer.
This Paper was Originally Published in Japanese in Netsu-shori 59 (2019) 336–343.
In this study, we examined the distribution of nanostructures in the interdiffusion layer of Al–Mg/Al–Zn/Al–Mg multilayer composites with different layer width via microbeam small-angle X-ray scattering (SAXS) measurements and direct observation using transmission electron microscopy (TEM). The changes in scattering profiles across the graded interfacial layers reflected the spatial change in the volume fraction, average size of precipitates, and their distribution in the sample. Microstructural parameters obtained from the SAXS analysis were used to explain the local hardness change in the interface area. Within the single interdiffusion layer of Al–Zn–Mg alloy, both of the Orowan and cut-through mechanisms were observed. TEM imaging was used with the same projection of SAXS to analyze the various shapes of the precipitates to explain the anisotropy in the 2-dimensional scattered intensity measured via SAXS. The present results revealed the spatial distribution of nanostructures with averaged parameters and explained the local mechanical properties within the interface region of the multilayer composite sheets.
Fig. 7 (b) Plots of different strengthening mechanisms on the specimen after aged at 393 K for 5 min, 3 h. Distribution of Mg is not shown.
A commercial hot-extruded AZ80Mg alloy was multi-directionally forged (MDFed) to a cumulative strain of ΣΔε = 5.6 at maximum under decreasing temperature conditions from 623 K down to 433 K at an initial strain rate of 3.0 × 10−3 s−1. In addition to the above MDFing method, a simplified MDFing method, i.e., MDFing under two-step decreasing temperature conditions, was proposed and applied. These MDFing led to homogeneous grain refinements to have average grain sizes down to 0.6 µm. The MDFed Mg alloys were further extruded to form plates and tubes at temperatures between 453 K and 523 K at various initial strain rates from 3.0 × 10−4 s−1 to 1.0 × 10−2 s−1. Extrusions of the MDFed AZ80Mg alloys were successfully carried out even at such relatively low temperatures. Under some conditions, the strain-rate sensitivity of the flow stress exceeded 0.3, which suggested occurrence of superplastic deformation. By the above warm extrusion employing superplasticity accompanied by work hardening, high-strength AZ80Mg alloy tubes with tensile strength over 400 MPa could be successfully fabricated.
The phase component, microstructure and compressive properties of a novel high entropy alloy, Al0.5CuFeNiV in as-cast and annealed conditions were investigated. Its phase structure is composed of FCC+B2 (ordered BCC) phase in both states. The ordered BCC phase is the major dendritic phase depleted with Cu, while the FCC phase is enriched with Cu, forming a netlike framework. Moreover, the alloys in both states possess a promising trade-off in strength and ductility. Specifically, the yield strength and the elongation of the as-cast alloy are 1456 MPa and 10.0%, respectively. After annealing, the strength decreases to 861 MPa, while the elongation increases to 15.5%. The combination of phase constitution, microstructure morphology, and solid solution strengthening was conducted to explain their mechanical behavior.
Fig. 5 Engineering stress vs. engineering strain compression curves of the as-cast and annealing Al0.5CuFeNiV HEAs (a), instantaneous strain hardening index, n* = d(ln σ)/d(ln ε), vs. true strain curves of the as-cast and annealed alloys (b).
Creep behavior of the weldment of a new type of martensitic heat-resistant steel 12Cr10Co3W2MoNiVNbNB joined with vacuum electron beam welding is studied employing nano-indentation technique at room temperature. Different load-displacement curves are obtained for base metal, heat affected zone (HAZ) and weld metal of the welded joint. The creep displacement exhibits a sharp rise at first, then the creep rate continuously decreases with time, and is approaching zero at late steady-state creep. The creep rate of the weld is smaller than HAZ, than base metal for late creep stage. Overall, the indention stress decreases with increasing dwelling time for all the regions, and weld metal yields the greatest indention stress while the base metal has the lowest value. Weld metal exhibits the smallest creep strain rate sensitivity value, indicating it has the best room temperature creep resistance, probably due to the newly formed wide lath martensite. The weldment yields various microstructure for base metal, HAZ and weld metal, and wider martensite laths with a high dislocation density are found in the weld seam.
Fig. 4 Micro-hardness of the 12Cr10Co3W2MoNiVNb NB weldment.
Fatigue properties of parts fabricated by laser powder bed fusion, a kind of additive manufacturing, are inferior to those of conventional materials. Defects and inclusions are the causes of low fatigue properties, but their effect is not clarified. Inconel 718 specimens were fabricated by laser powder bed fusion. Cross-sectional observation and fatigue tests were performed. Gas porosity, cracks, and inclusions were observed in cross-section. TiN inclusions were observed in the fatigue fracture origins for specimens with hot isostatic pressing. To enhance the fatigue properties of Inconel 718 fabricated by laser powder bed fusion, it was suggested to remove entirely TiN inclusions within the powder.
This Paper was Originally Published in Japanese in J. Jpn. Soc. Powder Powder Metallurgy 67 (2020) 419–423.
Hydrogen embrittlement in aluminum alloys occurs due to local hydrogen accumulation during deformation. Investigating hydrogen diffusion with plastic deformation can help researchers further understand hydrogen embrittlement behavior in terms of its occurrence condition. We assessed hydrogen accumulation behavior in Al–Zn–Mg alloys under strain by combining tensile testing with Kelvin force microscopy. Additionally, the effects of microstructures on hydrogen accumulation were analyzed in detail. Hydrogen accumulated around specific grain boundaries, and it was likely that accumulated hydrogen was repartitioned to the generated vacancies, due to deformation, and the precipitate interfaces.
Al100−xSix (x = 2, 4, 6, 8, 10) amorphous alloy models are constructed by using molecular dynamics (MD) simulations. In order to investigate the local rearrangement during the tension deformation, Honeycutt-Andersen (HA) method and Voronoi tessellation method are performed for the initial state, The HA method and the atomic local shear strain are used for the deformed states of the models. The structure of AlSi alloys is mainly amorphous structure. It is found that the Si element enhanced glass-forming ability. Moreover, the amorphous structure breaks and transforms into other tiny complex structure during the tension deformation. In addition, it is observed that the amorphous regions which preferentially undergo atomic rearrangement in the elastic stage and early plastic stage.
Fig. 1 A simulation sample (Al94Si6) used for the present work.
The physical gelation of an aqueous methylcellulose (MC) solution in response to temperature change was evaluated using a quartz crystal microbalance (QCM), which is an extremely sensitive mass balance that measures changes in mass per unit area from nanogram to microgram level. Then, the potential use of QCM for interfacial selective viscoelasticity measurements was investigated. The viscosity changes accompanying gelation were observed as resonance frequency shifts. The gelation temperature determined from the temperature dependence of the resonance frequency shifts showed good agreement with the gelation temperatures obtained by visual inclination observation and rheology measurements. Furthermore, MC molecules were adsorbed, and the local concentration increased at the interface with hydrophobic quartz units due to the surface properties. We believe that QCM enables the evaluation of interfacial viscoelasticity.
This Paper was Originally Published in Japanese in J. Japan Inst. Met. Mater. 85 (2021) 23–29. Captions of all Figures and Tables are modified.
The relationship between chloride ion concentration and pH on de-passivation of steel rebar embedded in concrete was investigated by a two frequencies impedance measurement using probe electrodes. A saturated calcium hydroxide solution was used for the electrolyte solution as a simulated concrete pore solution. The pair of probe electrodes, which simulates the steel rebar, was immersed in the electrolyte solution prepared by an arbitrary concentration of chloride ions, and the two frequencies impedance measurement was performed. Air was supplied to the electrolyte solution to simulate the neutralization of concrete during the measurement. The charge transfer resistance related to the electrode/solution interface, Rct, was estimated from the two frequencies impedance measurement of probe electrodes in each electrolyte solution. This demonstrated that the Rct was drastically reduced by decreasing the pH of the electrolyte solution, namely, the de-passivation of steel rebar. The corrosion conditions of steel rebar were categorized as “passive state,” “de-passivation,” and “corrosion” based on the changes in the monitoring of Rct and pH in each electrolyte solution. These results were plotted on the two-dimensional diagram composed of chloride ion concentration and pH of the electrolyte solution. The conditions for the de-passivation of the passive film on steel rebar were discussed.
This Paper was Originally Published in Japanese in Zairyo-to-Kankyo 68 (2019) 303–308.
Fig. 2 Schematic of probe electrodes in the electrolyte solution and equivalent circuit between two electrodes. The Rct is the charge transfer resistance at the electrode/solution interface, the Rsol is the solution resistance, and the Cdl is electric double-layer capacitance at the electrode/solution interface.
Magnesium tin silicide (Mg2Si1−ySny) is a thermoelectric material with a low environmental load and a high power factor of approximately 400°C. High power factor sintering bodies by pressure-less sintering (PLS) have not been produced as Mg contained in the starting material is likely to be oxidized. A certain amount of tin is used as a sintering additive to improve the sintering density and power factor in liquid-phase sintering and the PLS process (LP-PLS). As a result, LP-PLS bodies with a power factor comparable to that of spark plasma sintering (SPS) bodies were successfully fabricated in this work. No significant changes in thermoelectric characteristics were observed even after heat treatment at 250°C for 32 h in air.
In this study, we used neutron diffraction to analyze in a non-destructive method the distribution of internal residual stress in a free-cutting steel bar processed by cold drawing and straightening. Since a change in lattice-plane spacing occurs in a strain-free standard sample used as a reference due to the cold-drawing and straightening processes, it was necessary for the sake of improving measurement accuracy to prepare strain-free standard samples for each individual process. As a result, the residual stresses were successfully measured with excellent stress balance. The residual stresses generated by the cold-drawing process were reduced by subsequent straightening, and the distribution of residual stresses by finite element method (FEM) simulation was consistent with the measured values by neutron diffraction. As a result of the FEM analysis, it is assumed that the rod was subjected to strong tensile strains in the axial direction during the drawing process, and the residual stresses were generated when the rod was unloaded. Those residual stresses were presumably reduced by the redistribution of residual stresses in the subsequent straightening process.
Fig. 12 Distribution of residual stresses after drawing by neutron diffraction (all peaks) and by FEM.
The vermiculation effects of compacted graphite cast iron are seriously affected by the metallurgical and technological factors. The change of characteristic points on the cooling curve is attributed to the interaction of these factors, so it is difficult to establish the evaluation relationship between the vermicularity and the characteristic points of thermal analysis curve. Based on the idea of statistics, a method was studied to evaluate the vermicularity of molten iron by comparing with an established characteristic points representation model of hypothetical ideal metallurgical quality state under the production condition. The results showed that the method can meet the requirements of evaluating the vermicularity of molten iron by multiple characteristic points on the cooling curve. As the matching process among cooling curves was omitted, the calculation and evaluation efficiency can be improved. In addition, it is expected to achieve on-line classification and regulation of vermiculation effect in front of furnace by expanding the sample capacity for establishing the characterization model.
Fig. 1 Illustration of 10 critical characteristic points of vermicularity evaluation of CGI: 7 temperature points of the cooling curve and 3 first order differential points on corresponding cooling rate curve.
Substrate heating effects on the microstructure and magnetic properties of MnAlGe films grown on Si/SiO2 substrate by sputtering system were investigated. The MnAlGe film fabricated by low-temperature substrate heating demonstrated amorphous phase and paramagnetic property. The film of c-axis orientation associated with the Cu2Sb-type structure was obtained by sputtering at a substrate heating temperature of 270°C and it exhibited perpendicular magnetic anisotropy. From the magnetization curves measured at room temperature, the uniaxial magnetic anisotropy energy, Ku, was evaluated to be in the order of 106 erg/cm3, which is consistent with the literature, although the heating processing and temperature are slightly different. Microstructural observation indicated that the c-axis oriented grains were isolated in the matrix of the amorphous phase. The film lost the c-axis orientation at elevated substrate heating temperature, resulting in loss of the anisotropic magnetic property.
SEM image (upper left) and XTEM bright field images of MnAlGe films on Si/SiO2 substrate sputtered at 270°C. Upper right panel is the low magnification image, and lower panels are the high magnification images.
A new CFRP-aluminum joining method has been developed by Cu electrodeposition. Cu deposition was filled into the roughened CFRP surface and the significant anchor effect was attained by the joining method. As a result, CFRP and A6061 Al alloy were successfully joined by Cu electrodeposition and a high joining strength of 67 MPa was obtained.