A liquid model has been proposed based on a hypothesis “Small crystal embryos may exist in liquids at temperatures even above the melting point or liquidus temperature. Their amount should decrease as the temperature increases”. Crystallization of Li2O·2SiO2 melt was studied to verify the liquid model. Melts of Li2O·2SiO2 (Tm=1033°C) prepared at various temperatures in the range 1040-1300°C for 3h or 20h were water-quenched and glass samples were obtained. After an isothermal heat-treatment at 600°C, crystals were homogeneously precipitated in the glass. The number density, N, and size, R, of crystals in the glass determined by optical microscopy decreased with increasing melting temperature. Supposing that the crystals might have grown from the preexisting embryos in the melt, the observation can be explained by the above hypothesis. It was recognized that crystallization behaviour is different depending on the thermal history of the melt (3h melting and 20h melting). Moreover, N and R were decreased with increasing melting temperature after two step heat-treatment (500 and 600°C). It was then shown that the difference of N between 1040 and 1300°C, ΔN≅500, was constant regardless of the duration of heat-treatment.
The essential condition for glasses and glass-ceramics to bond to living bone is the formation of a biologically active bone-like apatite layer on their surfaces in the body. In the present study, in order to investigate fundamentally the effect of B2O3 on the bioactivity of glasses and glass-ceramics, the compositional dependence of apatite formation on the surface of glasses in the system CaO-B2O3-SiO2 was examined in the simulated body fluid. Substitution of small amounts of B2O3 for SiO2 in CaO·SiO2 formula increased the rate of apatite formation on the surface of the glass, while substitution of more than 30 mol% B2O3 for SiO2 decreased it. The substitution of B2O3 for SiO2 in CaO-SiO2 binary glasses accelerated the dissolution of Ca(II) ion from the glasses, and hence enhanced the increasing rate of degree of supersaturation of the surrounding fluid with respect to apatite. The decrease in the apatite forming ability by substitution of large amounts of B2O3 for SiO2 is attributed to the suppression of formation of silica hydrogel layer which plays an important role in apatite nucleation.
The effects of Ag addition on the mechanical and superconducting properties of Bi2Sr2CaCu2Ox (BSCCO) superconductors were studied, and their microstructures were observed mainly by transmission electron microscopy (TEM). The addition of 10 wt% Ag resulted in an increase in average bending strength from 126 to 170MPa, in Weibul modulus from 3.8 to 9.8 and in the hardness normal to the c plane from 38.7 to 51.8kgf/mm2. The critical current density (Jc) of BSCCO bulk was also increased by addition of Ag. Ag existed as fine particles with the size of about 1μm inside BSCCO grains, and as the second phase at grain boundaries. The Ag phase was strengthened by coherent fine particles of Cu. At the interface between Ag and BSCCO, a Bi-O single layer existed. The mechanical properties of Ag/BSCOO composites were discussed from the view point of the microstructures.
The symmetric gradient materials of MoSi2/Al2O3/Ni/Al2O3/MoSi2 were fabricated by SHS/HIP compaction. Due to the thermal expansion mismatch between the outer MoSi2 and inner Al2O3-Ni layers, the compressive residual stress as much as 100MPa was induced in the MoSi2 surface, which enhanced the toughness to 5.8MPa·m1/2 comparing to 3.8MPa·m1/2 of the monolithic MoSi2. The flexural strength of the FGMs was 580MPa which is slightly higher than the monolithic MoSi2. It was found that Al2O3 in the FGMs acted as a diffusion barrier to Ni even under heat treatment at 900°C for 8hr.
High temperature strength, creep and creep-fatigue behavior were investigated in air for Al2O3, SiC and Si3N4 ceramics. The high temperature tensile strength was influenced by the existence of additives in ceramics. The tensile strength showed displacement rate dependence. In the creep-fatigue tests of Si3N4 at 1673K, not only hysteresis loops but also ratchetting behavior were observed in the stress-displacement curves. It seemed that the fracture occurred when the total plastic ratchetting deformation reached a constant value.
The tensile strength of yttria-stabilized tetragonal zirconia polycrystals (Y-TZPs) was measured and the fracture surfaces were analyzed by SEM-XMA. The fracture toughness, KIC, SEPB, and the crack extension resistance curve (R-curve) were evaluated by the single-edge-precracked-beam (SEPB) and the double cantilever beam (DCB) methods, respectively. The fracture toughness, KIC, slopel, evaluated from the tensile strength and the fracture surface analysis shows 3.4 to 3.7 MPa√m, which is much smaller than KIC, SEPB (5.0 to 5.2 MPa√m). The type of R-curve is a rising one, and KIC, slopel and KIC, SEPB are close to the initial and the saturated values of the R-curve, respectively. The fracture origins of the SEPB and the tensile test specimens are a relatively large precrack and a small natural flaw, respectivery. Therefore, the value of fracture toughness of the Y-TZPs with a rising R-curve depends on the size of fracture origin.
The evaluation of bricks has been made based on perceptual engineering in order to study the possibility of material design which appeals to perception. The objective of this work was to try to obtain quantitative relationships between material properties and perception. The sense of heaviness was studied as an example and was examined whether it could be the material design standard using the method of constant stimuli and that of magnitude estimation. Both data were found to coincide with each other and it was thought that the sense of heaviness can be accepted as a material design standard. Using a semantic differential method the heaviness of the bricks with different density was evaluated before and after lift. It was found that the visual denseness and the density were important factors which controlled the heaviness of the material before and after lift, respectivery.
A novel test specimen geometry is proposed for quantitatively analyzing the creep deformation and the stress relaxation behavior of engineering ceramics at elevated temperatures. The specimen precisely yields a simple shear deformation under a compressive loading mode. The simple shear is the most essential deformation for time dependent rheological studies, because the conventional test specimen geometries such as flexural or tensile test geometries inevitably induce the volume change within the test specimen under load, leading to complicated nonlinear deformation. The present novel geometry is applied to the high-temperature deformation and flow of soda-lime glass to confirm the potential use for studies on the high-temperature rheology of engineering ceramics. A critical review is also made for some issues, problems and difficulties included in the conventional rheological approaches to the high-temperature deformation of ceramic materials.
In the thermal fracture testing technique proposed previously by the present authors, a one-side heated slab specimen is held by three points and the repulsive load appearing during heating is measured by the load cell placed at the upper point of the heated side. By this way, the thermal fracture stress of a ceramic specimen can be estimated from the repulsive load without knowing the heat transfer coefficient unlike the water quenching thermal shock test. However, it needs numerical calculation for every specimen at each experimental condition. In this paper, the numerical calculation of non-steady-state temperature distribution in an infinite plate, heated from one side with a constant heat flux were conducted in order to calculate the magnitude of thermal stress from the repulsive load data in the above-mentioned thermal fracture test. The temperature dependences of thermal conductivity, λ*=eAT and thermal diffusivity κ*=eBT were introduced in the calculation to realize a practical heat conductive condition. The time dependent temperature distributions were unified into a correlative equation that includes the coefficients of temperature dependence of material properties A, B, supplied heat flux Qi and Fourier's number ηi=κit/h2. By using this equation, the repulsive load and thermal stress at fracture were calculated for several ceramics, and the influences of material properties were discussed. The correlative equation is also useful to define the experimental conditions in thermal fracture tests.
The compressive large deformation of β-Ti alloy was investigated at temperatures from 77K to 673K and at strain rates upto 1.2×104S-1. At low temperatures, a work-softening was observed on the true stress-strain relation, while it was not so significant at high temperatures. The temperature change due to the heat conversion of plastic work and the strain rate change during deformation were taken to account the experimental results. It was found that the thermally activated process concept gave plausible understandings of the experimental results and could provide an informative prediction of dynamic behaviours of the material at large deformation. Metallographic observations revealed the frequent appearance of mechanical twinnings in the relatively early stage of high velocity deformation at low temperatures, and they played an effective barrier against the successive dislocation motion and caused significant work hardening at large strains.
Change in residual stress in the worked layer of type 304 autenitic stainless steel due to tensile deformation was measured by the X-ray diffraction residual stress measuring technique. The compressive residual stresses introduced by end-mill, side surface of end-mill and grinder were easily changed into tensile stresses when the plate specimens were subjected to tensile stress greater than the yield stress of the solid solution heat treated material. The residual stresses after the tensile deformation depend on the initial residual stresses and the degree of working. The residual stress change behavior can be interpreted when the surface worked material is regarded as a composite made of the solid solution heat treated material and the work-hardened material.
0.6C-Si-Mn steel has been studied to determine theeffects of bainitic transformation temperature in the range between 593 and 673K on the microstructure and tensile properties of Si-Mn steel.Carbide-free upper bainite whose individual ferrite was separated by the “thin-film” type of austenite appeared independent of temperature, while the ferrite width increased with increasing temperature. The “blocky” type of austenite was found with an increase in temperature. The carbon content decreased with increasing temperature and the thermal stability of the austenite decreased at and above 643K. Bainitic transformation at and above 643K followed by tempering at 473K significantly increased the percent elongation of the steel, as compared to bainitic transformation at and below 623K followed by tempering at 493K. From the true stress-true strain analysis, X-ray measurement and fractography, the improved ductility at and above 643K can be attributed to the transformation induced plasticity (TRIP) effect of austenite.
In order to clarify the temperature dependences of fracture strength and fracture toughness of sintered silicon carbide, the fracture strength and fracture toughness tests were carried out under 3-point bending conditions at the temperature range up to 1500°C in air and in vacuum. The single edge precracked rectangular bar specimens were used in the fracture toughness tests. The fracture strength in air exhibited a maximum value at about 1200°C. On the other hand, the fracture strength in vacuum and the fracture toughness in air and in vacuum increased monotonically with an increase in temperature. The increase in fracture toughness at elevated temperature in air is attributed to healing due to the oxide, SiO2, formed on the precracked surface. To evaluate the effect of healing on fracture toughness, the precracked specimens were post heat-treated at several elevated temperature levels, and then the fracture toughness tests were conducted on these specimens at room temperature. The results revealed that the higher was the annealing temperature, the higher was the fracture toughness depending on the healing of the precracked surfaces.
Crack shielding by fiber bridging under high temperature (473K) fatigue conditions was investigated using the Double Cantilever Beam (DCB) specimens of unidirectional AS4/PEEK laminates. Under a constant applied stress-intensity-factor-range, ΔKapply, the crack propagation rate decreases due to the shielding by fiber bridges between the crack surfaces as the crack extends. After the crack propagates to a few mm long, the propagation rate is saturated at the lower bound. The highest propagation rate can be obtained by extrapolating the propagation rate at the initial slit. The rate is evaluated as the intrinsic value of crack propagation rate having no effect of fiber bridging, and it is correlated well with ΔKapply. The fact implies that the crack propagation is governed by the intrinsic stress-intensity-factor range at the crack tip, ΔKtip, which is equal to ΔKapply-ΔKbridge where ΔKbridge is the stress-intensity-factor range carried by fiber bridging. In constant load amplitude tests, the crack propagation rate shows a transition from the upper bound represented by the da/dN-ΔKtip relation without bridging to the lower one obtained from the constant ΔKapply tests.
The effects of total strain range and test temperature on the fatigue life of a cast Intermetallic compound, Ti-34wt.%Al, which possesses a lamellar structure, γ+α2, were investigated. The behaviors of initiation and propagation of small cracks were observed in the smooth bar specimens subjected to push-pull cyclic loading. The results observed are summarized as follows. (1) A small amount of inelastic strain is observed even in fatigue under high strain range up to 1%. The fatigue life is sensitive to the total strain range. (2) The fatigue life shows a positive temperature dependency. (3) Small cracks observed on/near the surface of specimens are classified into three types, Type A cracks which initiate from defects introduced during casting process and form a main crack in fatigue below 1073K. Type B cracks which initiate along an interface of lamellae being parallel to the stress axis and are blocked by grain boundaries never to grow as a long crack. Type C cracks which initiate from the cracking of the surface oxide-layer and bring numerous fracture sites in the matrix over 1173K. (4) Although the initiation and early propagation of Type A cracks show an eminent fluctuation caused by the distribution of casting defects and the orientation of lamellar structure, they follow by a continuous propagation with an increase in crack length. Therefore, Type A cracks are typical of this material where by the fatigue fracture is brought about at the early stage of life.
Rotating bending fatigue tests of smooth specimens were carried out using an austempered ductile cast iron (ADI) in the temperature range between room temperature and 400°C. In order to clarify the fracture origin, the fracture surfaces were investigated by SEM. The amount of retained austenite was measured by X-ray diffraction method. The fatigue limit reached maximum at about 300°C due to not only the transformation of retained austenite but also the cyclic strain ageing. The retained austenite transformed at 350-400°C. Due to the surface hardening by the transformation and strain ageing, most of the cracks caused at elevated temperatures initiated at the location of internal defects.
Ion nitriding has been used to improve the wear resistance of machine elements such as gears, rollers, etc.. However, it is scarcely known that ion nitriding causes an appreciable reduction in bending impact fatigue strength. Together with the conventional tension and Charpy impact tests, the cyclic impact tests were carried out to clarify the effect of case hardening by ion nitriding on the bending impact fatigue strength of a high tension steel (HT80), where a low carbon alloy steel (SM50) was used as a reference material. In the case of the low carbon alloy steel with a tensile strength of about 500MPa, ion nitriding reduced the impact fatigue life to about one tenth of the same steel without nitriding. The reduction ratio became smaller (about 1/3) in the case of the high tension steel with a tensile strength of about 800MPa. The impact fatigue strength of nitrided steel is closely related to the impact resistance determined by the Charpy impact test.
Sound transmission loss (TL) of double glazed panels with different gases in the cavity was measured using the two room transmission suite method. Five kinds of gases were used in the experiment: freon, helium, the mixed gas of air and helium, etc.. In addition, the calculation of TL was carried out based on the distributed constant circuit theory, which showed similar frequency characteristics to the experimental result. Valuable informations about the refraction and the propagation of sound through the intervening gas space were obtained from the experimental and the calculated results. The double panels filled with freon in the cavity, in which sound velocity was lower than that in air, provided a sharp reduction in TL generated by the double-wall resonance (wall-gas-wall). In the frequency range higher than the resonance frequency, TL was much greater than that of the double panel filled with air in the cavity. This characteristic was due to the intensification of the resonance caused by the sound refraction. For the case in which the cavity was filled with helium, the sound velocity was higher than that in air. This case provided greater TL in a wide frequency range. This was probably due to the mitigation of double-wall resonance and the total reflection of sound on the incident-side wall.
Woodceramics are the porous carbon materials obtained from wood or woody materials impregnated with phenol formaldehyde resin and sintered in a vacuum furnace. The purpose of this paper is to analyze fundamental friction properties of woodceramics in sliding with several materials. A reciprocating friction apparatus was used in the experiments. An alumina ball (R=1.5mm, 4.0mm), a silicon nitride ball (R=4.0mm), a bearing steel ball (R=4.0mm) and a diamond pin (R=0.075mm) were used for the upper specimens, and several woodceramics made of medium density fibreboard (MDF) and beech wood, impregnated with phenol formaldehyde resin and sintered in a vacuum furnace, were used for the lower specimens. The experiments were carried out under an unlubricated condition in air, under a base-oil impregnated condition and in water, with several normal loads at different sliding velocities. The following principal results were obtained in this investigation. (1) The friction coefficient is around 0.15 under the unlubricated condition in air, under the base-oil impregnated condition and in water. (2) The friction coefficient slightly decreases and then keeps a constant value with increasing normal load. (3) The friction coefficient is not affected by sliding velocity. (4) Woodceramics have a good self-lubricity.
The purpose of this investigation is to analyze fundamental wear properties of woodceramics. For this purpose, wear tests were carried out by using an alumina ball (R=1.5, 4.0mm) or a semispherical diamond pin (R=0.075mm) sliding against a woodceramics plate (MDF-800), under an unlubricated condition in air, under a base-oil impregnated condition and in water. The specific wear rate was calculated from the profile of worn surface in order to evaluate wear characteristics. By introducing the contact pressure parameter (W/R2)1/3, the relationship between the specific wear rate and the contact pressure parameter was obtained. The worn surfaces were observed microscopically by using a scanning electron microscope (SEM) in order to clarify the microscopic wear mechanisms. The following principal results were obtained. (1) The specific wear rate of woodceramics increases rapidly with increasing contact pressure parameter (W/R2)1/3, under the unlubicated condition in air, under the base-oil impregnated condition and in water. (2) When the contact pressure parameter (W/R2)1/3 is less than a certain critical value, the specific wear rate of woodceramics is less than 10-8[mm2/N] which is low enough for practical use. (3) The wear mode of woodceramics can be classified into the following three modes; the large scale brittle fracture induced wear (flake formation), the small scale brittle fracture induced wear (powder formation), and the ultra mild wear (ploughing).