The effects of the presence, amount and difference in distribution of bubbles in a material and the grain boundary mobility on the time law of normal grain growth are investigated using a Monte Carlo computer simulation technique. The simulation evolves the microstructure with a mixed grain size when the bubbles are nonuniformly distributed or when the microstructure consists of grain boundaries with a different mobility in the absence of bubbles. The presence of bubbles causes the time exponent for grain growth n(=dlogD⁄dlogt) to increase from 2.5 to 2.9-3.8, retarding the grain growth. However, n is not affected very much by the difference in the distribution of bubbles. It becomes clear that the width of grains represents the dependence of 1⁄f and 1⁄\sqrtf on the area fraction of bubbles f only when the individual bubbles or the arrays of bubbles are randomly distributed, respectively.
Duplex stainless steels become brittle at medium temperatures, owing to the phase decomposition of a ferrite into a Cr-rich α′ phase and an Fe-rich α phase. A cast CF3M duplex stainless steel containing 25% volume fraction of ferrite was aged at 723 K for time periods up to 10000 h to investigate the phase decomposition mechanism and to evaluate the composition of the decomposition products using Mössbauer spectroscopy at room temperature. The half width of the internal magnetic field distribution curve estimated from the experimental Mössbauer spectra increases in the initial stage of aging, suggesting compositional fluctuation due to spinodal decomposition. The ferrite phase finally decomposes into the Fe-rich α and Cr-rich α′ phases after prolonged aging. The composition of the Fe-rich α phase is estimated to be Fe-11 at%Cr-5 at%Ni by numerically analyzing the internal magnetic field distribution based upon the local environmental effect of solute atoms on the central Fe nucleus. The result is consistent with the one previously obtained by atom-probe analysis. The isomer shift of 57Fe in the α′ phase is estimated to be −0.114±0.002 mm/s at room temperature by decomposing an overlapped paramagnetic absorption peak into component peaks. The Cr content of the phase is estimated to be about 85 at% by plotting the isomer shift on an experimental isomer shift versus Cr content diagram.
A-C:H films were annealed in an attempt to investigate the stability of a-C:H films at high temperature. A T.T.T. (Time-Temperature-Transformation) diagram could be drawn up using the results of annealed a-C:H films under hydrogen ambient. According to the T.T.T. diagram, the annealed a-C:H films could be classified into three regions; an amorphous phase region, a reconstruction region and a crystallizing region. In the amorphous region (<873-1073 K), the effusion of hydrogen from the a-C:H films was observed but the structure change of the a-C:H films was not observed even though the annealing time was up to 1 Ms. The structure change of the a-C:H films occurred in the reconstruction region (873 K-1123 K). However, the crystal grains were not observed in this region. In the crystallizing region (>1073-1173 K), the crystal grains which have a graphitic structure were formed in the matrix.
Softening of cold-drawn wires of a tough pitch copper containing a trace amount of lead has been reported to be enhanced when hot-rolled rods of the copper are preheated at 600°C before cold-drawing. This study was undertaken to clarify the reason for the softening-enhancement by means of electrical resistivity measurements, structural observations and EDS analyses of precipitates appearing during preheating. It was found that the softening was attributable to the decrease in the amount of impurities dissolved in the copper matrix due to the formation of Pb-S complex precipitates during preheating. Nucleation sites and chemical compositions of these precipitates were closely examined and discussions were made on thermodynamic stability of these precipitates.
The effect of dispersed particles on grain growth has been studied by computer simulation using a two-dimensional lattice model. The relation between \barR and \barr, the mean radii of matrix grains and dispersed particles, is approximated by \barR=[(π⁄2αΦ)⁄f]1⁄2·\barr, where f is the area fraction of dispersed phase, α is the correction factor for adjusting the pinning force, and Φ is the fraction of particles being located on grain boundaries. The parameters, α and Φ, have been evaluated by computer simulation, and it was concluded that Zener’s relation in two dimensional structure; \barR=(π⁄4)·\barr⁄f is to be modified to \barR=(5.0⁄\sqrtp)\barr⁄\sqrtf, where p is a parameter representing the pinning effect in this simulation.
Deformation microstructures of a low carbon steel with ferrite-pearlite structures have been examined with tritium autoradiography and thermal desorption spectroscopy. The amount of non-diffusible tritium at room temperature increases with strain. When deformed at room temperature which is in the ductile fracture region, the distribution of tritium is fairly uniform within a grain, while it is inhomogeneous when deformed at −80°C which is in the brittle fracture region. In the latter case, the accumulated sites are distributed regularly in arrays and localized as bands. Comparison with slip band traces indicated that the distribution is likely along slip bands. The amount of non-diffusible tritium which desorbs at 180°C is remarkably large when deformed at −80°C. Desorption at 180°C disappears when the deformed sample is annealed at 500°C, suggesting that the defects as the accumulation sites are microdefects presumably point defects or their clusters.
The relationship between a depth of undercut and a depth of nonpropagating crack has been investigated experimentally and systematically under the condition of bending load. Though the fatigue strength depends on a length of nonpropagating crack in the direction of plate thickness, many fatigue tests were usually carried out by using rotatory bending fatigue test and surface cracks were observed. Welded joints are more often loaded with axial stress or bending stress than rotatory bending stress. Furthermore the fatigue failure of welded joint may occur at the toe of weld at which undercut exists and grain size of weld is larger than that of base metal. Grain size of material used (SM400) was increased by using heat treatments 1273 K-7200 s and 1073 K-14400 s. The depth of artificial undercut was varied from 0 to 250 μm. The stress ratio R was zero. After fatigue testing, nonpropagating cracks lying on the section of a specimen were observed by using an optical microscope. All nonpropagating crack existed within two grains from the bottom of the undercut of which the depth was varied from 0 to 250 μm. A crack within the first grain was the first stage crack and a crack within the second grain was the second stage crack. The depth of nonpropagating crack depended on the size of two grains at the tip of the undercut. The shorter the total depth of undercut and nonpropagating crack was, the higher the fatigue strength was. In the case that the difference between undercut depths of two spacimens was smaller than one grain size, the fatigue strength was similar.
Local electronic states around Si ions in molten SiO2 have been investigated using DV-Xα molecular orbital calculation. Bond order and density of states were utilized to clarify the strength of the covalent bond between Si ion and O ion. It was found that the partial densities of states of 3s, 3p and 3d of the Si ions overlapped widely with the partial densities of states of 2p and 3s of the O ion. As a result, the bond order between the Si and O ions becomes very high, that is, the strength of the covalent bond becomes high. This is probably the reason for the formation of network structure in the molten SiO2. It is therefore considered that the molecular orbital calculation is useful in investigating the local electronic states of the molten SiO2.
An estimation of the surface tension of metal at far higher temperatures than the melting point has been done on the basis of the following assumptions: (i) The one third law is valid for liquid metal. (ii) The free random walk of atoms yields the density distribution near the liquid-gas interface. Based on above assumptions the following expressions of surface tension are obtained: (1) The surface tension is given by γ=A(Tc−T)B from the melting point to the critical point, where Tc is the critical temperature and B is the critical exponent. (2) The critical exponent B is theoretically estimated to be 11/9. (3) The coefficient A is given by A=α(θtVt−2⁄3)ν, where θt and Vt are the reduced temperature and the atomic volume at the triple point, respectively. The matters bearing similarity (the metals in a group and non-polar liquids) have a definite set of (α, ν). (4) Tc is expressed as Tc=T+Bγ⁄s, where s is the surface entropy. Tc should be calculated from γ and s at the melting point.
The oxidation characteristics of Ti-14Al-21Nb (mass%) have been studied over a temperature range 1000∼1300 K in a flow of purified oxygen under atmospheric pressure. The mass gain due to oxidation at 1000 K for 100 ks is negligibly small, though a very thin scale is formed. The oxidation at and below 1200 K follows nearly parabolic rate laws. The scale consists mainly of an outer rutile layer and a thin inner layer which is a porous mixture of small grains of rutile, alumina, and niobium oxide. On the other hand, above 1200 K parabolic manner of oxidation is disturbed by the occasional appearance of acceleration periods. This acceleration is attributable to the local fracture of the scale resulting in multilayer scales. A model for the development of scale structure is presented on the basis of the detailed metallographic examinations.
The anodic dissolution characteristics of Al2Cu intermetallic compounds which is stoichiometric or Al-rich specimens are investigated by the measurements of an electrochemical method such as anodic polarization curves and potentiostatic dissolution tests, immersed corrosion tests and scanning electron microscopic observations of corroded surfaces for the specimens in a dilute HClO4 solution. The main results obtained are summarized as follows: (1) The anodic polarization curves of Al2Cu intermetallic compound specimens in a dilute HClO4 solution show a typical active-passive transition, and the limiting anodic current density of the specimens increases with an increase in the concentration of HClO4 and the solution temperature irrespective stoichiometries of the specimens. However, the anodic dissolution rate of the Al-rich specimen is larger than that of stoichiometric ones under the same conditions. (2) The activation energies of anodic dissolution of Al2Cu are 27.6 kJ/mol for the Al-rich specimen and 48.5 kJ/mol for the stoichiometric ones. (3) The proposed anodic dissolution mechanism for Al2Cu intermetallic compound specimens in the dilute HClO4 solution under the potentiostatic condition is shown in the following equations: The selective anodic dissolution of Al species from the stoichiometric Al2Cu intermetallic compound: (This article is not displayable. Please see full text pdf.) \
oindentThe selective anodic dissolution of Al species from the eutectic phase in the Al-rich Al2Cu intermetallic compound specimen: (This article is not displayable. Please see full text pdf.)
The metal powder injection molding (MIM) process enables extremely complex parts to be net-shape-molded. However, the need for producing complex-shape products with high dimensional accuracy brings about problems of dimensional accuracy and nonuniform deformation. To quantify the nonuniform deformation during sintering, and thus improve the dimensional accuracy, the measurement of two- and three-dimensional sintering shrinkage is required by all means. However, the conventional dilatometric measurement techniques enables only one-dimensional deformation in a simple shape specimen. The authors have developed an “in-situ monitoring system of sintering shrinkage” that uses digital image processing and enables the measurements of two- and three-dimensional changes during sintering on a non-contact and real time basis. In this paper, the nonuniform and anisotropic sintering shrinkage of two types of complex-shaped powder compacts prepared by metal injection molding was measured with MR-DICM (Multi-Reference Digital Image Correlation Method), which was an improvement of the digital image correlation method and newly developed for the computation of sintering shrinkage. The distribution of shrinkage in complex-shaped powder compacts of stainless steel (SUS304) was precisely determined from room temperature to 1543 K.
Size-restricted Ni powder particles were plasma sprayed on the flat SUS304 substrate surface, and the effect of substrate temperature on the splatting behavior of the particles was evaluated. The results obtained are summarized as follows: 1) In the splat behavior of Ni powder on the heated SUS304 substrate, intense splashing of the powder was observed in the substrate temperature range up to 573 K. The splashing behavior, on the other hand, did not occur in the substrate temperature range over 623 K. 2) Transition temperature, Tt, was defined in this study, on which the powder’s splat pattern changed to the form without splashing from the one with splashing. Tt was about 600 K in the case of Ni splat on the SUS304 substrate. 3) From the observation results of splat behavior on both the heat-treated substrate at room temperature and the Au coated substrate at elevated temperatures the transition behavior of splatting seemed to depend not on the oxide layer formed but on the other factors relating to substrate temperature.
Ni-base superalloys were prepared by machanical alloying of Ni, Cr powders with a few minor elements in a planetary ball mill in order to examine the effect of an addition of minor elements. Milled powders were CIPed, sintered and HIPed, and then they were oxidized by cyclic heating to 1273 K from room temperature. Oxidation behavior of the specimens has been studied by means of mass gain measurements, scanning electron microscopy, X-ray diffraction, grazing incidence X-ray diffraction, electron probe microanalysis and energy dispersive X-ray spectroscopy. In the process of cyclic oxidation, scales formed on the NNi-20Cr alloy were spalled markedly; here NNi indicates Ni powders containing 11 mass%Fe in each particle. On the other hand, the alloy containing 1.0 mass%Y2O3 showed no spalling of scales, the surface oxides being spinel, Cr2O3 and YCrO3. The outer layer of the oxides consists mainly of spinels, and Cr2O3 and YCrO3 are located in the inner layer. The other alloys containing Al, Ti and Y2O3 or specimens in which NNi is replaced by pure Ni(INi) were investigated in the same way as described above. It is concluded that the Ni-Cr superalloys which contain NNi powders with the addition of 1.0 mass%Y2O3 improve the high-temperature oxidation resistance considerably.
SiC whisker reinforced composites with 6061 aluminum alloy matrix were fabricated by three different processing routes, high pressure infiltration, high pressure infiltration & hot extrusion and powder metallurgy(P/M) & hot extrusion. The structure, age hardening behavior and mechanical properties were compared on the composites obtained. The extruded composites by the P/M process show less uniform distribution of SiC whiskers, but a higher aspect ratio than those by high pressure infiltration. However, the uniformity of whiskers distribution in the P/M composites is improved by using finer alloy powders for the matrix. Tensile strength and elastic modulus of the composites increased with increasing whisker Vf. At room temperature, the highest tensile strength is obtained in the extruded P/M composites from finer matrix powders. Elastic modulus of the cast and extruded composites is higher than those by the P/M route. At 573 K, the cast composites exhibit higher tensile strength than the hot-extruded composites.
Potassium titanate whisker reinforced pure aluminum and 6061 aluminum matrix composites were fabricated both by powder metallurgy and squeeze casting and their reaction microstructures were evaluated by transmission electron microscopy. No differences in the basic reaction microstructures were observed between the two fabrication methods and between the two different whiskers. The whiskers reacted with pure aluminum and 6061 alloys in the temperature range between 600 and 800°C to form γ-alumina and TiO. TiO was formed inside the whiskers and the whiskers gradually changed their phase from potassium titanate to TiO as the reaction proceeded. γ-alumina was formed on the whiskers as a thin layer. Between the γ-alumina layer and whiskers, a thin inhomogeneous TiO layer with gaps was found. There are the following crystallographic orientation relationships among the whiskers and the reaction products: (This article is not displayable. Please see full text pdf.)
We have studied the effect of nitriding of Nd-Fe-B and Nd-Fe-C alloys by X-ray diffraction, Mössbauer spectroscopy, magnetization measurements and transmission electron microscopy. Samples were nitrided in an atmosphere of flowing NH3 gas at temperatures between 300 and 500°C. For the samples nitrided at 350, 375 and 400°C, interstitial dissolution of nitrogen atoms into Nd2Fe14B is assured by the extension of lattice constant of 0.2∼0.7%. Mössbauer spectroscopy shows that the increment of average hyperfine magnetic field of 57Fe in the Nd2Fe14BNx phase is 2∼4% as compared with that in the Nd2Fe14B phase. This is consistent with the result that the Curie temperature of Nd2Fe14BNx is about 20 degrees higher than that of Nd2Fe14B. The samples nitrided above 400°C are almost amorphous. Similar effects by nitriding, i.e. lattice expansion, increase in Curie temperature and that of hyperfine magnetic field, were also observed for Nd-Fe-C alloys.
Recently, the AB2-type Laves phase alloys and the BCC solid solution alloys have been investigated as next generation hydrogen-absorbing alloys with the high capacity. We propose the new concept of alloy design named “Laves phase-related BCC solid solution” in this work. First, the multi-phase alloy Zr0.5Ti0.5VMn consisting of the BCC solid solution phase and the C14-type Laves phase is designed as a variation of the “Laves phase-related BCC solid solution”. Secondly, the multi-phase alloy Zr63Mn37 consisting of the α-Zr H.C.P.solid solution phase and the ZrMn2 C14-type Laves phase is designed for the control experiments. End members of these alloys such as ZrMn2, Zr93Mn7 and TiV2 are prepared for calculation of the composite law of hydrogen storage capacity. In the pressure-composition isotherm, the composite law of hydrogen storage capacity is successful in the zirconium and manganese binary alloy Zr63Mn37 at each equilibrium pressure between the α-Zr HCP solid solution phase and the ZrMn2 Laves phase. This law, however, does not explain the capacity of the quaternary multi-phase alloy Zr0.5Ti0.5VMn under the simple assumption that this alloy consists of the Ti-V BCC solid solution and the ZrMn2 C14-type Laves phase. In order to understnad this disagreement, the relations of the Ti-V BCC solid solution phase and the ZrMn2 C14-type Laves phase are discussed by investigating the morphology and analyzing the crystal structure of the multi-phase alloy. The morphology is investigated by optical microscopy, scanning electron microscopy and transmission electron microscopy, and the crystal structure is analyzed by X-ray Rietveld refinement and selected area electron diffraction. These metallographical approaches in this work will become the first step of improvement for the hydrogen-absorbing property by the control of the microstructure.