MICROSCOPIC ANALYSIS OF THE INITIATION OF HIGH-TEMPERATURE DAMAGE OF NI-BASED HEAT-RESISTANT ALLOY

It is imperative to reduce greenhouse-effect gas such as CO2. Since the emission of CO2 from fossil fuel combustion to generate electricity is a critical quantity, accounting for 42% of global CO2 emissions in 2013 [1], high efficiency of thermal power plants is indispensable for reducing the CO2 emissions. In order to further improve thermal efficiency of thermal power plants, various R&D projects have been conducted (such as Thermie 700 in the EU and DOE-Vision 21 in the US) to develop A-USC (advanced ultra-supercritical) power plants of the 700 degrees C class. Alloy 617 is a candidate alloy for boiler tubes and pipes. Since it has higher coefficient of thermal expansion than conventional ferritic steels, however, the increase in the thermal stress is of concern in the power plant components. In addition, it is important to consider the effect of creep-fatigue loading on the lifetime of the components in the design and maintenance of the components. This is because frequent output change is inevitable for assuring the stable and reliable supply of electricity under the combination with renewable energies. Conventionally, the creep-fatigue damage has been evaluated by linear cumulative damage rule. However, it has been found that there are a lot of loading conditions and materials to which the rule can't be applied [2-3]. Therefore, it is indispensable to establish the method for evaluating the total damage of materials under creep-fatigue loading conditions. Thus, the authors conducted fatigue and creep-fatigue tests and observed the change of the micro texture to elucidate the damage evolution of the alloy from the viewpoint of the change of the order of atom arrangement using EBSD (Electron Back-Scatter Diffraction) analysis. As a result, it was found that the difference of damage accumulation under fatigue and creep-fatigue loadings appeared in the change of the GROD (Grain Reference Orientation Deviation) value in the inelastic strain range. Therefore, the difference in the damage mode between fatigue and creep loads can be analyzed by using these KAM and GROD values.