The atomic and electronic structures of the Al Σ=9 tilt grain boundary with segregated impurity atoms have been calculated by the first-principles pseudopotential method based on the local density functional theory. Effects of impurities of group I (Na), group II (Ca), group IV (Si) and group VI (S) have been examined. For the Na and Ca segregation cases, the impurity-Al interactions seem to have metallic characters. However, the boundary expands substantially and the charge density decreases significantly over the boundary. Thus these impurities should cause weaker intergranular adhesion. For both the Si and S segregation cases, the charge density increases around the impurity atom. The Si atom forms the covalent-metallic character mixing bonds with neighboring Al atoms. Such strong and directional bonds should prevent the rearrangement of atoms under stresses. However, the S atom forms such a strong bond with only one neighbor, differently from the Si case. It can be said that each impurity has various effects on the local atomic and electronic structure of an Al grain boundary according to the nature of each species, which seems to dominate the mechanism of embrittlement.
Amorphous GeSe2, Ge3Se4 and Ge4Se5 have been prepared by mechanical milling (MM), and their structural changes have been investigated by X-ray diffraction, Raman scattering and EXAFS measurements. Amorphous Ge4Se5 was obtained at the milling time of 50 h in the present experiment, which has never been obtained by the rapid quenching from the melt. The short-range order in GeSe2 remains almost unchanged during the milling. Since the configuration of tetrahedral units in the crystalline phase becomes rapidly less ordered with increasing the milling time, the amorphization is complete at a short milling time. On the other hand, Ge3Se4 and Ge4Se5 samples, both of which are eutectic mixtures of intermediate compounds of GeSe2 and GeSe, need longer milling times for the amorphization, as they are accompanied by the change in local structures. The short-range order in these Ge–Se alloys is described by covalent 4(Ge)–2(Se) folded coordination structure. The structural feature of crystalline GeSe phase with a partial ionicity, i.e. 3(Ge)–3(Se) folded coordination, disappears not only in amorphous Ge3Se4 but also in amorphous Ge4Se5, where the concentration ratio of Ge/Se is close to that of the stoichiometric composition of GeSe. The bond-lengths of Ge–Se and Ge–Ge in these amorphous alloys are determined to be 0.236 and 0.248 nm, respectively, which are consistent with the corresponding covalent distances.
Using a quasi-chemical method coupled with Miedema’s semi-empirical method, we have estimated critical glass-forming compositions for bulk metallic glasses of metal-metal type alloys such as La–Ni–Al, Cu–Zr–Ti and Zr–Ni–Al that exhibit maximum temperature spans of their supercooled liquid region, ΔTx (=Tx−Tg), reaching 37–77 K. We applied Cowley’s chemical short-range order (CSRO) parameters ηij for specific constituent i-j atomic pairs calculated by the quasi-chemical method to obtain the short-range order parameter γ of the amorphous phase, which is used as a constant in the calculation of formation enthalpies by Miedema’s method. In this study, the constant γ is treated as a function of the composition of atomic pairs such as γij(ηij). From the calculated results, it appears that the most negative value of ηij corresponds to a highly stable liquid state because of the occurrence of chemical short-range order, implying that bulk metallic glasses can be produced. Stability and instability of the liquid phase have been discussed in terms of the compositional dependence of Cowley’s CSRO parameters and enthalpy difference parameters. The calculated results predict fairly well the critical glass-forming compositions for the various bulk metallic glasses, even predicting the asymmetrical composition dependence of the glass-forming composition ranges. Furthermore, the relation of ηij (=η12) with interaction parameters ε12 yields the characteristics of interaction between dissimilar solutes in 0-1-2 ternary metallic solutions.
This study examined the sintering phenomena of two different tungsten heavy alloys, W–8Mo–7Ni–3Fe (mass%) and W–22.4Mo–7.8Ni–3.4Fe (mass%). Experimental results revealed that extending the isothermal hold caused increase in the concentration of Mo in the liquid phase. A high concentration of Mo in the liquid phase tended to trigger the precipitation of a MoNi-type intermetallic phase at the interface between the W–Mo grains (solid solution of W and Mo) and the matrix phase. The approximate chemical formulation of this intermetallic phase is (W4Mo6)(Ni7Fe3). It was difficult to suppress the precipitation of this intermetallic phase by rapid cooling, because the phase transformation temperature of matrix phase into this intermetallic phase (1628 K) is very close to the solidification temperature of the liquid phase (1645 K).
Phase field models have been known as one of the most adequate deterministic models for directly simulating the dendrite growth morphology, nevertheless, it was not clear how the grain morphologies were influenced by the constant parameters in the models. In this paper, it was studied quantitatively that the connection between the growth morphologies and some parameters including undercooling, the coupling coefficient between the temperature field and the phase field, the anisotropic coefficient and so on. The formation and transformation between the dendrite morphology and seaweed morphology were discussed. Besides, the relations between the tip growth speed and the parameters were studied, the theory of the growth tip branching-off and the theory of side-branching were analyzed.
We have researched and developed a new Heusler-type ferromagnetic shape memory alloy Co2NiGa, which has large magnetostriction caused by the magnetic-field-induced re-arrangement of martensite twin. The ribbon samples produced by rapid-solidification melt-spinning method show strong texture and large magnetostriction ε of about 110×10−6 for an applied magnetic field of 800 kA·m−1 at room temperature. When increasing temperature, the martensite phase of Co52.4Ni22Ga25.6 and Co52.1Ni26.1Ga21.8 ribbons disappears at about 390 K and 330 K, respectively, where the drastic shape recovery arises. Some of the ribbons have good ductility and are not broken even after bending to an angle of 180°, indicating that the Co2NiGa system has a possibility of new ferromagnetic shape memory material with good ductility.
Tantalum in contact with gallium is compressed up to 7 GPa and heated. When the temperature keeps above 50°C of the melting temperature of gallium, the lattice constant of tantalum is approximately constant. The lattice expansion begins to increase with raising the temperature from above 300°C. The expansion becomes about 0.28% after the heat treatment at 700°C for 1 h. It shows the formation of a terminal solid solution phase on the tantalum side of the binary Ta–Ga phase diagram. On the other hand, a solid solution phase on the gallium side is not confirmed. The intermediate Ga3Ta phase appears with the heating at 300°C. The formation of the compound becomes notable after the heat treatments at 600°C and 700°C. The chemical reaction between tantalum and gallium is quite small up to 500°C at 7 GPa. This shows that tantalum is stable against gallium about 400°C over the melting temperature of gallium at the pressure. The property is useful for application as an inactive material against Ga-alloys under high pressure.
Importance of coating adhesion in an aqueous corrosion environment was studied experimentally. Tensile adhesion strength of HVOF sprayed 316L stainless steel and Hastelloy C coatings were tested in as-sprayed condition as well as after immersion in seawater. It was found that the adhesion strength of the stainless steel coatings degraded rapidly whereas that of the Hastelloy coatings remained almost intact. Specimens with an artificial defect were also immersed in seawater. The cross sectional observation after the test revealed that the corrosion at the coating-substrate interface proceeded much faster with the stainless steel coating as compared to the Ni-base alloy coating. A model experiment to simulate the galvanic corrosion of a coating-substrate couple was carried out and no significant difference in the galvanic current density was found between the two coatings when coupled with the steel substrate. The tightness of the coating-substrate interface was then tested with a fluorescent dye penetration test. The dye could penetrate the boundary between the stainless steel coating and the substrate whereas the boundary between the Ni-base alloy coating and the substrate was so tight that no penetration occurred. The penetration behavior of the dye into the micro-gaps at the coating-substrate boundary was discussed from the viewpoint of classical Washburn-Rideal theory applied to a model of capillary flow between a pair of parallel circular disks. It was concluded that such micro-gaps between the coating and substrate must be eliminated for these barrier-type coatings to be used in corrosive environments. Heat treatment was highly effective for suppressing the preferential corrosion at the coating-substrate boundary.
The surface compositions and morphologies of pure chromium after wet polishing, air oxidation and further micro-wetting by distilled water, were investigated with X-ray photoelectron spectroscopy (XPS) and the AC non-contact mode of atomic force microscope (AFM). An organic contaminants/water layer on the chromium oxide/hydroxide layer, was detected by XPS analysis for each surface. The oxide/hydroxide layer became thicker, and the oxide:hydroxide ratio increased, after air oxidation. In ambient air, the AFM showed a thin liquid layer on each surface, which was easily moved by the cantilever of the AFM and can be condensed or evaporated. The inner part of the liquid might be adsorbed water, and the outer part is thought to be organic contaminants since the liquid did not combine with distilled water applied by post-wetting. Micro-droplets of distilled water deposited by post-wetting always occupied positions without or with little of this liquid, which might explain the obtained higher micro- than macro-wettability.
The influence of copper content on the corrosion of iron has been investigated to make possible the use of scrap as a source material for reinforcing steel in concrete structures. An AC impedance technique was used to monitor the corrosion of iron, Fe–0.4 mass%Cu and Fe–3 mass%Cu undergoing cyclic wet-dry conditions with a 1 hour immersion in a pH10 solution of Ca(OH)2 containing 0.01 M NaCl followed by a 3 hour drying at 298 K and 50% RH. The corrosion rate of iron is greatly accelerated by wet-dry cycles. This is because active FeOOH species produced by the oxidation of Fe(II,III)oxide in air during drying act as very strong oxidants that promote corrosion in the wet condition. In each cycle, shortly before the surface dried out, a large increase in the corrosion rate was observed. This can be explained by the acceleration of oxygen transport through the thin electrolyte layer. Fe–0.4 mass%Cu showed a similar trend to iron. On the other hand, Fe–3 mass%Cu showed much lower corrosion rates and the corrosion was not accelerated by wet-dry cycling. Monitoring showed that the addition of 0.4 mass%Cu does not diminish the corrosion resistance of iron and the addition of 3 mass%Cu improves it under these wet-dry conditions. Energy Dispersive Analysis of X-ray (EDX) results indicated enrichment of copper in an inner rust layer after exposure to 56 wet-dry cycles.
The oxidation behavior of B4C–(25–60 mol%)SiC composites prepared by arc-melting was investigated in the temperature range of 1073 to 1773 K using a thermogravimetric technique. Liquid borosilicate, solid SiO2 and carbon were identified as oxidation products by X-ray diffraction and Raman spectroscopy. Mass gain was observed during oxidation at 1073 K, while mass loss due to the vaporization of boron oxide in liquid borosilicate was observed at temperatures of 1273 K and higher. In situ Raman spectra of the surface of B4C–SiC composites indicated that the silica concentration in the liquid borosilicate increased with increasing SiC content in the composite. Micro-Raman spectroscopy showed that carbon was enriched in the borosilicate layer close to the oxide/composite interface. The parabolic rate constants for B4C–50 mol%SiC composites at 1073 K were proportional to ambient oxygen partial pressures ranging between 30 and 100 kPa. The diffusion of oxygen molecules through the liquid borosilicate layer could be the rate-controlling process. The increase of SiC content in the B4C–SiC composites improved the oxidation resistance in both the mass gain and mass loss regions.
Thick Ni–Al intermetallic coating was fabricated on a spheroidal graphite cast iron by the reaction synthesis processing. In this study, wear property and bonding strength of the coating layer were estimated. Densification of the coating layer was achieved with increasing holding temperature. Wear property of the coating layers was measured by sliding wear test. While coating layers formed with a hot press temperatures from 873 to 1033 K had superior wear property, the layer fabricated at 873 K for 900 s indicated the best wear resistance. This is considered to be caused by higher hardness of Al3Ni2 compound dispersed in the coating layer. Reaction layer was found out at the interface between the coating layer and substrate. The layer was composed of three different phases which were identified as a Al7Fe2Ni and FeAl from quantitative EPMA analyses. These layers well bonded the coating layer to the substrate. Mean shear strength of interface was approximately 73 MPa and increased with increasing holding time for reaction synthesis process.
The formation of metallic glasses by rapid solidification has been studied using an equiatomic substitution technique. This paper reports results on a series of (Ti33Zr33Hf33)100−x−y(Ni50Cu50)xAly glassy alloys with compositions in the range x=10–70 at% and y=10–30 at%, and with a large supercooled liquid region in the range ΔT=Tx−Tg=40 to 124 K where Tx = crystallization onset temperature and Tg = glass transition temperature. The crystallization of the glassy alloys was studied by a combination of X-ray diffraction and differential scanning calorimetry. For (Ni50Cu50)-rich alloys with x=50–70 at%, crystallization took place with a single exothermic reaction, and for (Ti33Zr33Hf33)-rich alloys with x=20–40 at%, crystallization took place with a series of exothermic reactions. Increasing the Al content from y=10 to 30 at% decreased the glass forming ability of the amorphous phase for a wide range of transition metal compositions x=40–70 at%. The most stable amorphous alloy was (Ti33Zr33Hf33)40(Ni50Cu50)50Al10 with a crystallization temperature of Tx=818 K. However the amorphous alloy with the largest supercooled liquid region was (Ti33Zr33Hf33)60(Ni50Cu50)20Al20 with a crystallization-glass transition temperature difference of Tx−Tg=124 K.
Ingots of ferritic stainless steels, Fe–24Cr and Fe–24Cr–2Mo in mass%, were worked to various dimensions for test specimens. Nitrogen was absorbed by the specimens in a furnace filled with nitrogen gas with a pressure of 101.3 kPa at 1473 K to develop a simple and convenient manufacturing process of nickel-free austenitic stainless steels. Changes in the mechanical properties of the alloys with nitrogen absorption treatment are discussed on the basis of the resultant microstructure. Ferritic Fe–24Cr and Fe–24Cr–2Mo were austenitized with nitrogen absorption to a 2-mm depth from the surface. The hardness, tensile strength, 0.2% proof stress, and elongation to fracture increased, and the reduction of area decreased in Fe–24Cr and Fe–24Cr–2Mo by austenitization due to nitrogen absorption. The tensile strength and 0.2% proof stress of these alloys with nitrogen absorption for 129.6 ks were much larger than those of 316L steel, while the elongation to fracture was much smaller than that of 316L steel. Therefore, small devices and parts with a maximum thickness or diameter of 4 mm were manufactured with this process in this study.
Yttria stabilized zirconia (YSZ) films were synthesized at a high deposition rate of 180 nm/s (660 μm/h) by laser chemical vapor deposition (laser CVD) using Zr(dpm)4 and Y(dpm)3 precursors. Morphology of YSZ films changed from columnar to cone structure with increasing deposition rate. YSZ films with the columnar structure showed significant (200) orientation.