Keun-Bong Yoo

Quantitative analysis of carbides and the sigma phase in thermally exposed GTD-111

The effects of thermal exposure on γ′, carbides and TCP-type phases, such as the sigma and eta phases, in the nickel-based superalloy GTD-111 were investigated in this study. The thermal exposure was performed at 871 °C and 982 °C for 1,000, 5,000 and 10,000 h, after which the thermally exposed specimens were compared in terms of their microstructure, weight fraction and growth rate of each precipitate. Electrolytic extraction and Rietveld refinement were successfully applied for a quantitative analysis of the carbides and the sigma phase in the thermally exposed GTD-111. The weight fractions of MC, Cr-rich M23C6 and the sigma phase for various thermal exposure times and temperatures were obtained to understand the formation and decomposition rate of each phase.

Low-Cycle Fatigue in Ni-Base Superalloy IN738LC at Elevated Temperature

For many years, high-strength nickel-base superalloys have been used to manufacture turbine blades because of their excellent performance at high temperatures. The prediction of fatigue life of superalloys is important for improving the efficiency of the turbine blades. In this study, low cycle fatigue tests are performed for different values of total strain and temperature. The relations between strain energy density and number of cycles before failure occurs are examined in order to predict the low cycle fatigue life of IN738LC super alloy. The results of low cycle fatigue lives predicted by strain energy methods are found to coincide with experimental data and with the results obtained by the Coffin-Manson method.

Transient oxidation of a Nickel-base superalloy with varying oxygen partial pressure

Abstract The transient oxidation behaviour of a nickel-base superalloy in different oxygen partial pressures studied using electron and ion microscopy showed that at high pO2, a triple-layered oxide developed: outer nickel oxide, then a complex aluminium-chromium oxide, and inner alumina layer. Lower pO2 produced only alumina, as reduced oxygen activity suppressed nickel oxide formation. An aluminium-depleted region in the alloy with width proportional to pO2 was observed.

Low Cycle Fatigue Behavior of Cobalt-Base Superalloy ECY768 at Elevated Temperature

Abstract : The Co-base super heat resisting alloy ECY768 is employed in gas turbine because of its high temperature strength and oxidation resistance. The prediction of fatigue life for superalloy is important for improving the efficiency. In this paper, low cycle fatigue tests are performed as variables of total strain range and temperature. The relations between strain energy density and number of cycle to failure are examined in order to predict the low cycle fatigue life of ECY768 super alloy. The lives predicted by strain energy methods are found to coincide with experimental data and results obtained from the Coffin-Manson method. The fatigue lives is evaluated using predicted by Coffin-Manson method and strain energy methods is compared with the measured fatigue lives at different temperatures. The microstructure observing was performed for how affect able to low-cycle fatigue life by increasing the temperature. Key Words : low cycle fatigue, co-based superalloy, strain energy method, elevated temperature

LOW CYCLE FATIGUE BEHAVIOR AND LIFE PREDICTION OF A CAST COBALT-BASED SUPERALLOY

Co-base superalloys have been applied in the stationary components of gas turbine owing to their excellent high temperature properties. Low cycle fatigue data on ECY-768 reported in a companion paper were used to evaluate fatigue life prediction models. In this study, low cycle fatigue tests are performed as the variables of total strain range and temperatures. The relations between plastic and total strain energy densities and number of cycles to failure are examined in order to predict the low cycle fatigue life of Cobalt-based super alloy at different temperatures. The fatigue lives is evaluated using predicted by Coffin-Manson method and strain energy methods is compared with the measured fatigue lives at different temperatures. The microstructure observing was performed for how affect able to low-cycle fatigue life by increasing the temperature.

Mechanical Properties and Changes in Microstructure for IN738LC with Thermal Exposure

High-strength nickel-based super alloys have been widely used in aircraft engines, vessel engines, and turbine blades because of their high strength and excellent fatigue and oxidation resistance. In this study, tests were carried out to determine the total strain range and temperature for high-strength nickel-based super alloys. Prepared specimens of IN738LC were exposed to temperatures of and for 1,000.10,000 h. These specimens were subjected to tests of mechanical properties and microstructure observations. The changes in mechanical properties were related to changes in according to the thermal exposure time.