Nagatoshi Okabe

Creep Rupture Map of Engineering Fine Ceramics

The main objective of this study is to clear the strength and strain behaviors of engineering fine ceramics in long term high temperature creep. For this purpose, a series of creep tests was carried out on a silicon nitride and a silicon carbide at high temperature levels up to 1723K and during long term up to 1000 hours by using a newly developed high temperature tensile creep testing machine. The results indicate that the relationship of applied stress vs. rupture time of both ceramics at each temperature level can be approximated by a straight line on full log paper with a little scatter. Then, discussion on the creep behaviors of the silicon nitride from the viewpoint of creep strain revealed that the minimum creep strain rate well governed the rupture time, and the creep rupture maps constructed from two different viewpoints indicated that the tertiary creep stage was a coalescence stage of multi-site microcracks.

EFFECT OF HIGH TEMPERATURE OXIDATION ON FLEXURAL STRENGTH OF SINTERED Si 3 N 4

The oxidation behavior of sintered Si 3 N 4 with AlN, Y 2 O 3 , HfO 2 as additives was studied over the range of 1573–1673K under static air. The oxidation kinetics of the ceramics followed a parabolic rate law. The 3-point flexural strengh of Si 3 N 4 at room temperature after oxidation decreased significantly but the strength at oxidation temperature was nearly equal to that before oxidation. Y 2 O 3 2SiO 2 was found to be precipitated in the oxide scale and an yttrium depleted zone was formed in the substrate beneath oxide scale. It was presumed that the strength degradation at room temperature of Si 3 N 4 after oxidation was caused by residual stress arising from different thermal expansions between the oxide scale and the ceramics substrate when cooling from oxidation temperature to room temperature.

Effects of Pre-Strain and Nb Content on Transformation Temperatures in Ti-Ni-Nb Shape Memory Alloy

Ti-Ni-Nb shape memory alloy has a wide transformation hysteresis, and has been used as coupling devices and so on. However systematic researches of the influence of the amount of Nb addition on transformation temperatures are few. The purpose of this work is to clarify the influences of pre-strain and Nb content on transformation temperatures in Ti-Ni-Nb shape memory alloys. The specimens were Ti-Ni-Nb alloys (Ni/Ti=1.0, Nb=0-15at%), annealed at 1223 K for 0.3 ks. The variation of transformation temperatures with pre-strain and Nb content were investigated experimentally. The variation of As, Ms and transformation temperature hysteresis with pre-strain and Nb content will be discussed in relation to the elastic strain energy and the volume fraction of slip-deformed martensite.

WS4b4 – FRACTURE STRENGTH OF SINTERED-Si3N4 UNDER HIGH TEMPERATURE AND UNIFIED ESTIMATION METHOD WITH CONSIDERING TEMPERATURE-DEPENDENCE

ABSTRACT The temperature dependence of sintered Si3N4 was investigated under temperature range above 1000°C up to 1300°C on the basis of unified estimation method, which are proposed for the evaluation of ceramic strength properties. The normalized fracture strength can be unificationally expressed by a formula with three parameters (▪, o0, n) and activation energy Q. The temperature dependence of fracture strength are expressed well by Arrheniuss formula. The strength shows the distribution of Weibull with two parameters. Consequentially if considering effective volume Veff and effective hold time teff, either fracture strength or fracture life can be probabilistically estimated under the temperature range above 1000°C up to 1300°C.

THE SOLID PARTICLE EROSION OF ENGINEERING CERAMICS

ABSTRACT The solid particle erosions of SiC, Si3N4 and PSZ were studied by using Fe3O4 and SiC powders as impingement particles. SiC powder impingement generated the heavy erosion, whereas Fe3O4 powder impingement did not give slight erosions on all target materials. Erosion volume of specimens depends on material properties such as hardness and fracture toughness, and test conditions such as velocity, mean radius and density of impingement particles. Their relationships were found to be good agreement with Evanss formula.

CRACK SIMULATION AND LIFE ASSESSMENT OF GAS TURBINE NOZZLES

ABSTRACT For the purpose of presuming damage and degradation of used gas turbine nozzles, hardness tests, low cycle fatigue tests and fatigue crack propagation tests are conducted on Co-base nozzle superalloy FSX414 thermal aged up to 10, 000 hours. Although hardening occurs due to thermal aging, there is no significant change in fatigue properties. The fatigue crack behavior is attributable to crack propagation morphology and is simulated by fractal geometry model of dendrite structure and random distribution of initial cracks. The multiple crack simulation is proved to be a promising technique for life assessment of nozzles which are used in the crack existing state after inspection and repairment.

STRENGTH AND STRAIN BEHAVIORS OF ENGINEERING FINE CERAMICS IN HIGH TEMPERATURE CREEP

ABSTRACT By using a newly developed high temperature tensile creep testing machine, a series of high temperature and long-term creep tests was conducted on a sintered silicon nitride and a sintered silicon carbide. The creep rupture curves of both ceramics showed straight line relationship at respective temperature levels, and the scatter of rupture lives could be negligible when compared with that of static and cyclic fatigue lives. The sintered silicon nitride showed 3-stage creep deformation process analogous to that of metallic materials, and the minimum creep rate well governed the rupture time of this ceramic.