Hirohisa Shiota

Yttria concentration dependence of tensile strength in yttria-stabilized zirconia

Abstract Tensile and bending tests were performed for ZrO2 polycrystals stabilized with 2.6–10 mol % Y2O3. There is no significant difference in the isothermal Young’s modulus of all the compositions. Both the tensile and bending strengths of PSZ are markedly higher than those of FSZ. This is mainly due to the fact that both strengths of PSZ reflect the increase in the resistance to crack propagation due to a stress-induced phase transformation. For both 3YSZ and 8YSZ, the tensile strengths are lower than the bending strength. The difference in the effective volume is responsible for those differences.

Influence of lamellar orientation on fatigue crack propagation behavior in titanium aluminide TiAl

Abstract Fatigue crack propagation (FCP) behavior of a titanium aluminide (TiAl) with a nearly fully lamellar microstructure has been studied on two different FCP directions relative to the lamellar orientation; i.e. parallel (type A specimen) and perpendicular (type B specimen) to the lamellar orientation, at ambient temperature in laboratory air. It was found that the FCP resistance of the former was considerably lower than that of the latter. Close examinations of crack morphology revealed significant differences between the two FCP directions. In type A specimens, several cracks along lamellae were seen on the surfaces and sections of the specimens, thus uncracked ligaments were formed in the wake of the crack tip. On the contrary, such ligaments were scarcely produced in type B specimens because only the main crack could propagate without remarkable deflections and branching. The FCP rates of type A specimens were decreased gradually with crack extension under constant stress intensity factor range, Δ K , tests, suggesting the role of crack bridging by uncracked ligaments. Finite element method (FEM) analysis indicated considerably reduced Δ K experienced at the crack tip, thus the difference in FCP resistance between two FCP directions based on the actual Δ K at the crack tip after allowing for crack bridging became much larger than that based on the nominal or applied Δ K .

Fatigue crack propagation in β Ti–15Mo–5Zr–3Al alloy

Abstract This paper describes the fatigue crack propagation (FCP) for five materials having different microstructures of β Ti–15Mo–5Zr–3Al alloy. With one exception, the microstructure was found to have very little or no influence on FCP behavior. The one exception was the material having the largest β grain size, which showed higher FCP resistance than all other materials due to higher crack closure levels. The effects of stress ratio on the FCP behavior could not be realized solely in terms of the crack closure mechanisms, because static fracture mechanisms operated during FCP depending on the magnitude of the maximum stress intensity factor, K max . Furthermore, it was found that the FCP resistance of all the materials was considerably lower than that of α / β Ti–6Al–4V alloy, which was primarily attributed to lower crack closure levels of the β alloy.

Microstructure dependence of fatigue strength and fatigue crack propagation in titanium aluminide

Abstract This paper describes the fatigue strength and fatigue crack propagation (FCP) at ambient temperature in a titanium aluminide, TiAl. In order to study the effects of microstructure on fatigue properties, two different microstructures were prepared: an as-cast material consisting predominantly of lamellar ( α 2 + γ ) grains, and a heat-treated (homogenized) material composed of equiaxed γ grains and lamellar grains. The as-cast material showed higher fatigue strength than the heat-treated material. As fatigue cracks were initiated at more than 90% of fatigue life, the observed difference in fatigue strength was attributed to the differences of the crack initiation resistances of the microstructures. The FCP rates were faster in the heat-treated material than in the as-cast material in the whole stress intensity factor range, ΔK , studied, and when the FCP data were plotted in terms of the effective stress intensity factor range, ΔK eff , the heat-treated material still exhibited faster FCP rates. The SEM observation revealed transgranular cleavage of γ grains in the heat-treated material, thus leading to lower FCP resistance.

Frequency dispersion of the internal friction in tetragonal and cubic zirconia polycrystals stabilized with yttria

A forced vibration method for measuring the frequency dispersion of internal friction under cyclic-tensile-compressive stress on the single axis was developed so that various mechanical tests can be conducted using a specimen having the same shape, machining, and sintering conditions. With this new method, frequency dispersions of the internal friction for tetragonal and cubic zirconia polycrystals stabilized with yttria were measured. The activation energy for the relaxation time was determined. As a result of comparison with the activation energy for the relaxation time measured by a conventional method, we conclude that the results of the internal friction measured in this study are valid. The activation energy for the ionic conductivity was determined, and compared with that for the relaxation time. The activation energies for the relaxation time in 2.6YSZ, 3YSZ, and the lower temperature peak of 10YSZ are very similar to those for the conductivity. As a result, we conclude that the internal friction peaks of 2.6YSZ and 3YSZ, and the lower temperature peak of 10YSZ are derived from rearrangement of point defects associated with oxygen ion vacancies.

Changes in Aging Behavior and Defect Structure of Y 2 O 3 Fully Stabilized ZrO2 by In2 O 3 Doping

Based on the causes of decreased conductivity of yttria stabilized zirconia (YSZ) with aging, which were clarified by previous studies, methods of suppressing decreased conductivity of 8 mol % Y 2 O 3 stabilized 92 mol % ZrO 2 (8YSZ) with aging were investigated and a new stabilized zirconia was developed. For one method different amounts of In 2 O 3 were doped into 8YSZ. The electrochemical properties of these new zirconias were measured, and the changes in the micro and local structure were characterized using X-ray diffraction, high resolution electron microscopy, electron diffraction, and internal friction measurement. The present results indicate that the appropriate amount of In 2 O 3 doping, or 5 mol %, into 8YSZ effectively suppresses aging effect and dopant effects in 8YSZ.

Tensile and fatigue properties of long glass fibre-reinforced polypropylene immersed in hot water

Tensile tests and pulsating-tension fatigue tests have been carried out using smooth specimens of two materials of long glass fibre-reinforced polypropylene (LGF/PP), with and without acid modified polypropylene (APP). Specimens were immersed in distilled water at 80°C up to 6666 h. In both materials, the weight increased rapidly and reached the maximum, then decreased gradually with immersion time. The material without APP showed initially larger weight increase, but after immersion for over 2500 h more remarkable decrease was seen, indicating that the addition of APP was effective in improving water absorption characteristics of LGF/PP. Water entered along the fibre/resin interface and the weight loss of LGF/PP was attributed to leaching of surface treatment agents and dissolution of glass fibres, because the weight gain of the resin itself was negligible. Tensile and fatigue strengths were considerably decreased after hot water immersion because of the degradation of glass fibres, i.e. the reduction of fibre strength. The material with APP still exhibited higher strengths than the material without APP, also showing a beneficial effect of the addition of APP. Fracture mechanisms are discussed on the basis of close examination of fractured specimens.

Tensile Creep of Sintered Silicon Nitride Ceramics at Elevated Temperatures

A good understanding of high temperature creep properties of engineering ceramics is greatly necessitated to promote its application to high temperature-machine components. Our knowledge of high temperature creep of this material, however, is quite limited at present. One of main reasons for this is in difficulty in measuring displacement which is generated in ceramics sample loaded at elevated temperatures above 1000°C.

Fatigue crack propagation at elevated temperatures in titanium aluminide intermetallic

Abstract The objectives of the present study are to evaluate fatigue crack propagation (FCP) characteristics at elevated temperatures in TiA1 with a fully lamellar microstructure and to make better understanding for the effect of temperature on FCP behavior. FCP experiments have been performed at 600°C and 800°C and the obtained results were compared with the data at ambient temperature. In intermediate and high Δ K regions, FCP rates at 600°C were similar to those at 800°C and they were faster than those at ambient temperature. In low Δ K regions, FCP rates at 800°C decreased rapidly with decreasing Δ K compared with those at 600°C and higher threshold stress intensity factor range was attained at 800°C, which was almost the same as that at ambient temperature. This higher FCP resistance at 800°C in low Δ K region still existed in terms of Δ K normalized by elastic modulus. Based on crack path morphology, fracture surface examination and additional FCP experiments, pronounced crack closure induced by the formation of oxide film was found to be responsible for the higher FCP resistance at 800°C.

Optimum Design of Sintered Silicon Nitride Ceramics Specimen with Projections for High-Temperature Creep Test.

A tensile creep specimen with projections was designed to measure the creep displacement by means of the laser beam displacement meter at temperatures elevated beyond 1000°C. Stress/strain concentrations, however, occur around the root of projections in this specimen. The high-temperature creep strain was determined form the measurement of tensile displacement between the two projections by correcting influence of the projections. The procedure used to determine the steadystate creep constitutive equation of Si3N4 ceramics was proposed by combining a high-temperature creep test using the speciment with projections and the finite-element method (FEM) calculations for the specimen. Furthermore, the method for designing the optimum creep specimen with projections was proposed via the FEM calculations using the steady-state creep constitutive equation.