Mitsutoshi Okada

Derivation of Temperature-Estimation Equation Based on Microstructural Changes in Coatings of In-Service Blades of Gas Turbines

A CoNiCrAlY-coated blade of an in-service gas turbine is analyzed, and a diffusion layer is formed along the boundary between the coating and the substrate due to the interdiffusion in the middle and tip of the blade. Such a layer is not observed in the vicinity of the blade root because of a comparatively low temperature during the operation. Coated specimens are prepared from the portions of the blade devoid of the diffusion layers, and the specimens are exposed to a high temperature in air. On the basis of the increase in the diffusion layer thickness, an equation for estimating the temperature of the blade is derived. An analysis of another in-service blade with a thermal barrier coating is carried out. The aluminum content decreases below the bond coat surface due to Al diffusion caused by the Al-oxide formation. This results in the formation of an Al-decreased layer (ADL) along the leading and trailing edges. The ADL is not observed at the center of the blade chord. The specimens are extracted from the portions of the blade that are devoid of ADL, and they are subjected to a high temperature in air. On the basis of the increase in the ADL thickness, a temperature-estimation equation is derived.

Microstructural Change in SiCf/SiC Composite in High-Temperature Atmosphere

In order to clarify the microstructural change of ceramic matrix composite (SiCf/SiC) which can influence the strength, chemical composition and crystal structure of ceramic fiber and matrix are examined by means of electric probe micro-analyzer (EPMA) and Raman spectrometer. Oxidation tests are carried out at the temperature of 1473K-1673K for 1000h in air. Little change in chemical composition is observed in ceramic fiber. Excess amount of carbon exists as disordered graphite in ceramic fiber, and it is found that the disorder proceeds as the test time increases according to R value, which is defined as the ratio of two carbon peaks in Raman Spectra. The crystallization of SiC in fiber is rarely observed in extent of the test. As for matrix, besides silicon and carbon, oxygen is contained even before the test in most of the matrix, while there is the other kind of matrix which contains relatively low amount of oxygen and large amount of carbon. It implies that the microstructure of matrix is not uniform. Inside the specimen, the chemical composition of matrix does not change due to the oxidation test. However, after the oxidation test, the content of oxygen increases and that of carbon decreases due to the oxidation at the vicinity of surface. Raman spectra show the crystallization of SiC in matrix after the oxidation test. And even in matrix, disordered graphite exists, and, according to R value, the disorder proceeds as the test temperature and time increase.

Development of Assessment Method for Oxidation Resistance of MCrAlY Coating on Gas Turbine Bucket: Part 1

Analysis is performed to investigate the degradation of CoCrAlY coating on a stage 1 bucket used in a 1100°C-class gas turbine running on LNG fuel for about 38000 hours. It is confirmed that the diffusion or movement of aluminum in the coating is important to understand the degradation of the oxidation resistance of the coating. In order to clarify the degradation behavior, high temperature oxidation tests are carried out in electric furnaces by means of CoCrAlY coating specimens. The density of aluminum below Al-oxide of the surface is decreased and such area forms a layer. This layer grows as the oxidation proceeds, and its behavior is observed. Moreover, the influence of temperature, partial pressures of steam and stress on the oxidation and the microstructural degradation are examined. They depend only on temperature and time not on partial pressure of steam or stress.Copyright

Fatigue Fracture of Directionally Solidified Superalloy Mar-M247 at Room Temperature and Dependence on the Solidification Direction.

Low cycle fatigue tests using smooth specimens of Mar-M247 are conducted at room temperature to simulate the fracture under out-of-phase thermal fatigue and the behavior of initiation and growth of small cracks is identified. Three kinds of specimen are cut from a cast plate such that their axes possess angles of 0°, 45° and 90° with respect to the ‹001› orientation that is parallel to the solidification direction; these specimens being denoted the specimen 0°, the specimen 45° and the specimen 90°, respectively. The results are discussed in connection with the anisotropic and composite microstructures. They are summarized as follows. (1) Transgranular cracks are initiated in all specimens. The first crack in Specimen 0° nucleates at 8000 cycles and those in other two specimens at about 1000 cycles. Cracks of Specimen 0° are initiated from casting defects at surface of the specimen, whereas cracks of Specimens 45° and 90° originate mostly in slip bands and the number of cracks per unit area is 200 times as large as that of Specimen 0°. (2) Cracks in Specimen 0° grow each other independently. On the other hand, cracks in Specimens 45° or 90° coalesce frequently. Grain boundaries, dendrite arms and γ-phase precipitates do not work as barriers to the crack growth unlike the creep fatigue crack. However, the direction of crack growth has a strong dependence on the crystallographic orientation of the γ-matrix. (3) The crack growth rate tends to be the lowest in Specimen 0° for the same half crack length. This is caused by the smallest Youngs modulus, the largest resistance for slip band formation, and the lowest crack density and hence the crack growth without coalescence.