It is very important to suppress the coarsening of dispersed particles in heat resisting alloys not to deteriorate the high temperature strength. However, the theoretical systematization has not been given adequately. The purpose of this article is to find out general rules of minimization of the coarsening of dispersed particles on the basis of the thermodynamical equilibrium and the generalized Ostwald ripening equation. It is concluded that the most stable combination is made in the binary alloy in which (1) the main component of the matrix has the least chemical binding force for the formation of dispersed particles and the main component of the dispersed particles has the greatest chemical binding force and (2) the sufficiency ratio of the main metal component of the dispersed particles to the whole dispersed particles is 1/2. These rules agree with the experimental results of some heat resisting alloys.
In order to determine the deuterium diffusivity in metals, we have developed a new technique, ion driven permeation, in which implantation of energetic deuterium ions is utilized in stead of gaseous charging or electrochemical charging for deuterium permeation. The deuterium permeation rate caused by the ion implantation is generally similar to those of the latter two techniques. The steady state permeation rate and the diffusion coefficient, however, tend to decrease with increasing duration of implantation (or with increasing implanted fluence) because of modification of surface and/or subsurface region by the ion bombardment. Therefore, the values of diffusion coefficient determined from initial permeation rise for a fully annealed specimen are reprpoducible and reliable. The diffusion coefficiens thus determined are (This article is not displayable. Please see full text pdf.) \
oindentfor deuterium in Cu and Ni at 500 K-1100 K, respectively. Both are in good agreement with those values determined by the gaseous permeation technique. Appling this technique to Al, for which the value of hydrogen diffusion coefficient has been widely scattered due to the surface oxide, we can get the deuterium diffusion coefficient as (This article is not displayable. Please see full text pdf.) \
oindentSince energetic deuterium goes directry into the bulk, the determined values seem not to be influenced by the surface oxide.
The relationship between delayed failure occurring in air environment and precipitation behaviors was studied on maraging steels by sustained load test and electrical resistivity measurement. The formation of low temperature precipitates associated with the presence of titanium was found to have a close relation to the delayed failure occurring on cobalt-free maraging steels containing titanium as a main hardening element. In case of 18%Ni maraging steels, the delayed failure in air environment was scarcely affected by the precipitation of Mo rich low temperature precipitates which played an important role in the hardening at low temperature aging, but the delayed failure began to be markedly revealed depending on the titanium content, when titanium was added in the 18%Ni maraging steels. It was estimated from the activation energies that the delayed failure of 18%Ni maraging steels was also significantly affected by the formation of the low temperature precipitates relevant to titanium. Since it is assumed that the delayed failure is controlled by interaction between dislocation motion cutting the coherent precipitates and hydrogen introduced by dissociation of water adsorbed on the steel surface, the reason why the delayed failure does not occur by the formation of Mo low temperature precipitates only would be that Mo rich low temperature precipitates do not promote the dissociation of water adsorbed on the steel surface unlike the low temperature precipitates relevant to titanium.
Creep of three CrMoV steels containing different amounts of chromium was analyzed using the following formula as a constitutive equation for the creep curve: (This article is not displayable. Please see full text pdf.) \
oindentwhere εo, A, B and α are constants to be experimentally determined. The second and third terms represent work-hardening and weakening, respectively. Minimum creep rates of the steels have higher activation energies than that for self-diffusion, and their values vary depending on the material. In contrast with the inconsistency in the minimum creep rates, the rate constant α and the strain factor A showed systematic trends. α of all the three steels took the same activation energy as that for the self-diffusion and did not vary with changing chromium content, being in accord with the fact that chromium does not change the diffusivity in iron. A was always independent of temperature, and there was little difference in their values among the steels. The higher activation energies and the variation with materials observed in the minimum creep rates were ascribed to temperature and material dependence of another strain factor B related to the weakening.
The oxidative leaching reactions of cobalt sulfides are interesting in connection with a hydrometallurgical treatment of cobalt sulfide as well as complex cobalt sulfide ores such as cobaltite (CoAsS), carrollite (CuS,Co2S) and pentlandite (Ni,Fe,Co)S. The chemical compositions of the complex sulfide ores vary over a wide range, and thus there still remains some ambiguity in the leaching mechanism. The synthetic three kinds of cobalt sulfide, whose chemical compositions were CoS, CoS2 and Co3S4, were used in this study. The oxidative leaching reactions of those cobalt sulfide samples in aqueous nitric acid solution are discussed from both a thermodynamic and a kinetic aspect based on the experimental results obtained on the leaching rates, the stoichiometry of the reaction and X-ray analysis of the reaction products. From the experimental results, the following conclusion is obtained. The oxidative leaching reaction is a simultaneous one of the formation of sulfate ion and elemental sulfur, and general equations are expressed by the following formlas. (This article is not displayable. Please see full text pdf.) \
oindentThe overall reaction of the oxidative leaching of the samples used is controlled by a chemical surface reaction step, and the rate equation is given by the following formula. (This article is not displayable. Please see full text pdf.)
In the relation to the transfer of Si and S from coke ash and slag to metal, via the volatile species, in the blast furnace, the reduction of the silicate melts containing Cas by a solid carbon has been investigated. At temperatures between 1873 and 2073 K and under argon atmosphere, the silicate was melted in a graphite crucible as a reductant. The mass loss measured by means of the thermo-balance is due to the following reactions: (This article is not displayable. Please see full text pdf.) \
oindentThe evolution rate of SiO increases linearly with decreasing the slag basicity and with increasing the activity of SiO2. The activation energy varies from 340 to 580 kJ/mol with the slag composition. The evolution rate of SiO from CaO-SiO2 and CaO-SiO2-Al2O3 slags is retarded by the presence of the small amount of S, but the generation of SiO from SiO2 and SiO2-Al2O3 melts is markedly accelerated by the addition of CaS. This result corresponds to the wetting of the melt with the graphite crucible. The reduction rate of SiO2, which is equal to the sum of the evolution rates of SiO and SiS, is increased by the addition of CaS to the calcium silicate slags.
Dense square plates of pentlandite, Fe4.5Ni4.5S8, were oxidized in a mixed 20 vol%O2-N2 gas stream at 783, 823 and 863 K. Iron was preferentially oxidized, and a duplex oxide was formed on the sample surface; the outer and inner oxides were Fe2O3 and Fe3O4, respectively. A small amount of nickel was contained in the Fe3O4 layer. A very small amount of SO2 gas was evolved in the initial hundred seconds of oxidation, and no SO2 gas was detected thereafter. The analysis of elements using EPMA in the inner sulfide core revealed, on the other hand, that sulfur was oxidized slowly during the oxidation for 72 ks. The thickness of Fe3O4 layer increased rapidly at the initial stage of oxidation, and then both layers of Fe3O4 and Fe2O3 grew following the parabolic rate law. The activation energies for the formation of Fe3O4 and Fe2O3 layers were 170 and 160 kJ·mol−1, respectively.
The carbothermic reductions of bauxite alone and that of mixtures of bauxite and CaO or Fe2O3 were investigated by thermogravimetry (TG) in CO atmosphere. Two types of briquette were prepared; Type A was well-mixed pellets of bauxite, coke and CaO or Fe2O3, whereas type B consisted of coke particles and mixed pellets of bauxite and CaO or Fe2O3. The results obtained by TG, elemental analysis, etc. may be summarized as follows: (1) The composition of A type product did not differ so much from B type product. (2) The Ca component noticeably volatilized when an Al-Fe-Si(-C) melting phase existed. (3) Addition of CaO showed to prevent the volatilization of Al, Si and Fe; it was as effective as the addition of Fe2O3. (4) Addition of CaO resulted in a rise of the reducing temperature avobe 20-30 K, decreasing the velocity of reduction. (5) The Fe components volatilized were greater in amounts than those expected from equilibrium estimations.
Passivity and its breakdown on titanium have been studied in NaCl solutions of various concentrations, pH’s and temperatures. The corrosion rate of commercially pure titanium depends on pH and temperature of the solution. Depassivation pH where active dissolution begins to occur with decreasing pH is found from the relationship between pH and polarization resistance at constant temperatures. The concentration of Cl− does not affect the breakdown of passive film on titanium unless pH decreases. The titanium hydride layer is formed on the titanium surface when the active dissolution reaction takes place. Titanium dissolves in the solution to produce Ti3+ through the titanium hydride layer. The investigation of the corrosion behavior of titanium in these solutions of high concentrations of chloride and hydrogen ions is useful for a better understanding of crevice corrosion of titanium.
A previous study reported on the relationship between the packing density and the particle size ratio and/or the volume fraction in binary mixture as a part of the investigation for the effect of the particle size distribution on the packing density. In the present study a similar investigation was repeated for the particles which have continuous size distributions, the modified Rosin-Rammler ones. According to the computer simulation using a random packing model the packing density was related to the parameter m (distribution constant) of the modified Rosin-Rammler distribution function. Subsequently the relationship was introduced by using an analytical method based on the binary packing results, and the adequency of the analytical method was also confirmed. The study was further developed onto the packing density of the particle assembly composed of a mixture of two powders with different modified Rosin-Rammler size distributions to investigate the blending effects. The results are summarized as follows: (1) For the powder of modified Rosin-Rammler size distribution the packing density decreases with increase in m. (2) The packing densities of powders with any continuous size distributions can be obtained from the loose binary packing results by an improved analytical method. (3) For the loosely packed bed of two mixed powders with modified Rosin-Rammler size distributions, the packing density was calculated by the above analytical method. The densities change with the parameters, m1 and m2 (distribution constants), De2⁄De1 (ratio of absolute size constants) and X2 (volume fraction). These results give important information on practical use.
The transverse-rupture strength (σm) of P30 cemented carbide (WC-36%βt-8%Co;βt solid solution of WC/TiC/TaC=45/22/33) was studied in relation to structural defects acting as the fracture source. It was found that σm of as-sintered alloy was always affected by the defect of pores, being often accompanied by βt layer or βt segregation. In case of HIP-treated alloy, such defects as cobalt pools surrounded by the βt layer or attached to by the segregated βt appeared, caused by the pores above mentioned. However, in the alloy (s-alloy) prepared by using commercially available spray-dried-powder, pores accompanied by βt layer or cobalt pools having βt layer were not observed. The higher strength of s-alloy was due to this fact. It was found that σm of P30 alloy, whether it was as-sintered alloy or HIP-treated alloy, was closely related to the structural defects peculiarly formed in this alloy.
The joining of magnesia to nickel metal at the temperatures from 1473 to 1673 K has been studied. The technique is an application of the diffusion phenomenon which occurs between magnesia and nickel oxide. Preliminarily, the specimen surface of nickel was oxidized at 1673 K for 0.9 ks in air. Then, a block of magnesia was placed on the preoxidized nickel and was reacted with nickel oxide at the experimental temperatures for 3.6 to 144 ks. Because magnesia and nickel oxide are completely soluble each other, the reaction layers of the joining parts are made of solid solution of these oxides. The advantageous of this method is that the joining can be accomplished under atmospheric pressure without the use of fillers. The fracture strength of the joining parts was measured by three point bending tests. The average values of fracture strength ranged from 30 to 40 MPa. The micro-structures of the diffusion layers and fracture surfaces were observed by EPMA, SEM and EDX.
Microstructures, shape memory effect and mechanical properties of melt-quenched Cu-14 mass%Al-4 mass%Ni alloy foils have been investigated. The size of columnar grains was about 5 to 10 μm, and they grew from the surface of foil. The crystal structure of the foils was identified with a DO3 type which was similar to that of the β1 phase (Cu3Al). The shape memory behavior of the melt-quenched foil was observed between the liquid nitrogen temperature (77 K) and room temperature, because the martensite transformation occurred at between 223 and 123 K. The tensile strength of the β1 phase at room temperature was three times as high as that of the martensite phase at 77 K. The brittleness of the specimen was remarkably improved by addition of elements of IVa, Va and VIa groups of the periodic table. Especially, the tensile strength of a Cu-14 mass%Al-4 mass%Ni-1 mass%V alloy foil was found to be about 400 MPa.
In order to evaluate the quality of composite pipes produced by a centrifugal-thermit process, it is necessary to investigate the process of the production layer formation. Especially, the duration of produced ceramics and metals in melting would be an important factor in the consideration. In the present work, the conditions of heat transfer in the radial direction are varied by setting three kinds of sleeves outside the outer metal pipe; i.e., copper, graphite and graphite covered with refractory cloths. Moreover, silica powder of 6.5 mass% is added in thermit powders, and the sample powders are prepared in two conditions of mixed and separated fillings. Production tests under the above six conditions are performed with a thermit mixture of Al and Fe2O3 system. The temperature changes in the reaction are measured by means of an infrared radiation thermometer, and the produced composite pipes are evaluated by measuring mechanical properties through a squeezing test. As a result of the present work, it is confirmed to be effective to make the duration above the melting point of alumina long and the gradient of the cooling process below the melting point steep for improving the properties of the produced composite pipes.