It is meaningful to understand the relation between the activity of powder evaluated by physicochemical methods and its initial sinterability. In the present work, the influences of powder properties on moldability, initial sintering behavior and microstructual development of sintered body were investigated. The initial sintering of three kinds of silicon nitride powder, LC 10 (code L), H1 (code H) and A-100 (code T), added with 3wt% BeO was performed under nitrogen atmosphere without pressure. The results obtained are as follows: (1) The moldability of the angular shaped powder (T) consisting of aggregated particles was low. Between the two fine powders (L, H) consisting of primary particles of irregular shape, the powder (L) having more ultra-fine particles was slightly behind in moldability. (2) If the tendency of densification depend qualitatively on two parameters, i.e., the moldability and the activity for sintering which each powder is possessing, a sequence of the activity per unit particle for sintering was as follows: L>H>T. The sequence agreed with that of specific surface area of powders. However, the densification rate in the initial stage was concluded as follows: H>L>T. (3) The bulk density of sintered body was highest at ca. 1660°C for all Si3N4. The addition of BeO promoted the α→β phase transformation and the order of transformation rate accorded with that of initial sinterability. (4) The bulk density of green body increased linearly with an increase of compaction pressure within the range of 3ton/cm2, and the compaction pressure had no connection with the α→β transformation.
Changes in the crystalline phase and microstructure on the surface resulting from low-temperature annealing of tetragonal zirconia polycrystals doped 2, 3 and 4 mol% Y2O3 were investigated under humidity controlled conditions at 0-7.35kPa of water vapor pressure and 100-600°C for 1-50hr. The tetragonal phase ZrO2 transformed to the monoclinic structure by annealing at low temperature conditions such as 100-500°C. The phase transformation caused microcracks on the surface and resulted in significant loss of fracture strength. The critical temperatures of the phase transformation of ZrO2-2 mol% Y2O3, ZrO2-3 mol% Y2O3 and ZrO2-4 mol% Y2O3 were 500, 400 and 350°C, respectively. The phase transformation proceeded rapidly on the surface but quite slowly inside the body. The rate of the phase transformation on the surface was first order with respect to the concentration of the tetragonal ZrO2 and increased with increasing water vapor pressure by Langmuir type relation. The appearent activation energy of the phase transformation was about 85kJ/mol.
Studies were made on the synthesis of transition metal nitrides (TiN, ZrN, HfN, NbN and TaN) by the combustion reaction of metal powder compacts under 0.1-25MPa nitrogen pressures. Under low nitrogen pressures (≅0.1MPa), stoichiometric nitrides were not obtained in all cases due to incomplete supply of nitrogen gas into the powder compacts. Under high nitrogen pressures (≅10MPa), it was found that the nitriding ratio of product depends on the adiabatic combustion temperature and melting point of nitrides. In the cases of Ti, Zr, Hf and Nb, which have quite high adiabatic temperatures compared with the melting points of the corresponding nitrides, the nitriding ratio was low (<80%) because of coalescing of metal particles by melting. On the other hand, in the cases of Ta and other metals with their nitride additions (0.5Ti+0.5TiN, 0.5Zr+0.5ZrN and 0.7Nb+0.3NbN), which have the adiabatic temperatures close to the melting points of the corresponding nitrides, the stoichiometric nitrides were produced.
In order to develop MgO base ceramics having higher thermal shock resistance for MHD plant use, ceramic composites of MgO reinforced with 15 and 20wt% of SiC whisker were fabricated by the hot-pressing at 1800°C and 1900°C under 200kg/cm2 for 30min. The effective fracture energy (γeff) of the hot-pressed composites and MgO were measured by work of fracture technique using chevron notched specimens over the range from room temperature to 1400°C. Up to 1000°C, the γeff values of the composite ceramics were slightly lager than those of the hot-pressed MgO. At temperatures above 1000°C, the γeff values of the composites increased remarkably, and were about 3-7times higher than those of hot-pressed MgO at 1400°C. The mechanism of the increase in γeff values were discussed on the basis of the load-deflection behavior in bending fracture, the chemical reaction of SiC whisker with MgO matrix and the fracture surface morphology.
0.6 BaO-0.4 Fe2O3 glass was prepared by twin-roller quenching method and its heat treatment process was examined by ESR and Mössbauer spectroscopy. Fe3+ ions were found to occupy distorted tetrahedral sites in this glass. BaFe2O4 and Ba2Fe2O5 were precipitated through the heat treatment at a temperature of 440°C. The former is an antiferromagnetic crystal having Néel temperature of 880K. Before the crystallization occurred, the linewidth of ESR spectrum decreased monotonously although neither isomer shift nor quadrupole splitting changed. It was considered that in this region Fe3+-O2--Fe3+ bond angle dominantly changed as a result of thermal relaxation of structure. When the crystallization occurred, both isomer shift and quadrupole splitting decreased significantly. These facts indicated that Fe3+-O2- distance was longer in the glass than in the precipitated crystals and that the coordination state of O2- ions around Fe3+ was more distorted in the glass. Hyperfine field increased with increasing heat treatment temperature. This phenomenon was explained by considering the size of crystallized BaFe2O4 particles.
Submicron grinding of BaTiO3 powder was studied by a ball mill with small balls. The balls used were 2mm and 5mm in diameter, and their specific gravity were 2.77 (glass), 3.33 (TiO2), 3.60 (Al2O3), 3.95 (ZrSiO4), 5.70 (PSZ), and 7.71 (steel). In each case, it was shown that submicron grinding of BaTiO3 powder was possible in short time. The grinding effect of 2mmφ balls was more efficient than that of 5mmφ. The following experimental equation was obtained. a=0.816ρ+0.569 (2mmφ) a=0.654ρ+0.316 (5mmφ) where a is the grinding rate, ρ is the specific gravity. The concentration of BaTiO3 powder in the slurry was varied from 20 to 50wt%. The lower the concentration of BaTiO3 powder, the higher the effect of grinding. In the case of low concentration, it was also possible to obtain submicron powder in short time.
Statistical characteristics of pitting corrosion observed under tubacular on carbon steel used for fresh water pipes are discussed. Thirty three cases were analysed using the statistical method. The Gausian distribution was applied for the distribution of pit depth in carbon steel. The mean and standard deviation of pit depth had no dependence on duration of use in practice. The standard deviation of pit depth rather than the mean is a good indicator for the estimation of potential occurrence of penetration. The index, m+3σ, is a good indicator for the occurrence of perforation. The standard deviation and mean of pit depth showed good correlation. It is difficult to estimate the life time of pipes using statistical analysis of pit depth because the law of pit growth is unclear.
Previously, the authors have proposed several methods to estimate corrosion fatigue strength at room temperature. In the present study, a method to deduce the corrosion fatigue strength from the fatigue strength of corroded specimens, was proposed and applied to estimate the corrosion fatigue strength of nickel-base heat resisting alloy for gas turbine blades in a high temperature sulfidation environment. The constants in the equation for estimation were obtained from the high temperature fatigue strength of the specimens corroded by the burner-rig test, and the estimated strength was in good agreement with the experimental data. It is made clear that this method can be applied to the estimation of corrosion fatigue strength at high temperatures.
For the purpose of establishing a quantitative evaluation method for the lifetime of sensitized stainless alloys which may experience the intergranular stress corrosion cracking, laboratory accelerated tests were carried out in a high-temperature, high-purity water environment. The probability distribution of the lifetime obtained was approximated by an exponential distribution. Various acceleration factors were discussed in terms of distribution parameters. Based on these studies, the method of evaluating the factor of improvement was proposed and applied for the estimation of the lifetime of alternate materials with respect to that of the reference material.
In order to compare the failure time distribution of a laboratory accelerated test with the field failure time distribution of stress corrosion cracking of Type 304 stainless steel, the effect of CaCl2 concentration on the probability distribution of the failure time Tf, crack initiation time Ti and crack propagation time Tp for smooth plate specimens with or without crevice under a constant condition of 373K and 200MPa has been analyzed. Two parameter Weibull distribution fitted approximately to all the distributions obtained. The apparent shape parameter mi' for Ti of both type of the specimens with or without crevice was found unity, indicating that a chance failure mode was operating. On the other hand, mf' and mp' for Tf and Tp show a maximum value of 4 or 6 at an intermediate concentration range of 30 or 40mass% CaCl2, corresponding to a wearout failure mode, and decrease to unity with a decrease of CaCl2 concentration. This fact means that SCC fracture and propagation at a low concentration range take place by a change failure mode. Since the same chance failure mode had been reported for the field failure time distributions of Type 304 and 316 stainless steel heat exchanger tubes, an approximate acceleration factor of the laboratory test conducted under the condition of 25 mass% CaCl2, 373K, 200MPa with crevice was estimated to be ca. 104 by comparing the mean failure time of both distributions of the field and the accelerated test.
A study on pitting corrosion of carbon steel heat exchanger tubes was carried out to evaluate the effect of corrosion inhibitors and pretreatments of tube surface in several water qualities. The effect of inhibitors on the prevention of pitting corrosion tends to increase in the order: phosphonate based inhibitors<zinc-phosphonate based inhibitors<zinc-chromate based inhibitor. The distribution of pit depth varied with time. However, it was found that the maximum pit depths fit the extreme value distribution and the statistical analysis of extreme values is useful in evaluation of pitting corrosion. The scale parameters (α) were almost independent of the treatments, while the location parameters (λ) varied with the respective treatments. An addition of zinc ions and increasing the concentration of M-alkalinity decreased the pit depth, and consequently λ. Pretreatments of tube surface also influenced the pitting processes and λ. In comparison with no pretreatment and degreasing, pickling could markedly decrease the pitting tendency.
The parameters of extreme value statistical analysis are calculated by several methods, for example, the method of least squares (MLS), minimum variance linear unbiased estimator method (MVLUE), and maximum likelihood estimator method (MLE). We have examined difference in value of the parameters estimated by the above three methods and compared the calculated maximum value with the actual value of corrosion rate for several cases in chemical process plants. In conclusion, MVLUE was the best method for estimating the maximum value of corrosion rate independent of kind of machines, atmosphere, types of corrosion and corrosion positions. The following relation was observed in the estimated values of scale parameter (α), MLS>MVLUE>MLE but no difference in the estimated value of location parameter (λ) was recognised among the above three methods.