The high efficiency of coal-fired power plants has been achieved by increasing the steam temperature and pressure. Due to the strong demand for higher efficiency of the plants, recently, an advanced plant called “Ultra Super Critical (USC)” thermal power plant was developed. The operating conditions of the plants have become harsher, such as the main steam temperature of 600℃, reheated steam temperature of 610°C and the steam pressure of 25 MPa, and thus, the transmission end efficiency was improved to 42%. It is used for about half of domestic coal-fired power plants in Japan because of its high performance. In USC, ferritic steel and austenitic stainless steel are generally used as heat-resistant alloy used for turbines and pipes. In order to further reduce CO~2~ emission as the countermeasure of global warming, thermal power plants are expected to continuously improve their thermal efficiency. A-USC (Advanced-Ultra Super Critical) thermal power plant, which is 700^o^C-class in steam temperature, has been developed in various countries. The typical steam temperature is 750^o^C and the maximum steam pressure is 35 MPa. Since the allowable working temperature of ferritic materials is around 630°C, and austenitic steels are considered insufficient in strength for steam conditions of 700°C class, the main heat-resistant alloys should be replaced by higher strength alloys. Ni-based superalloys are strong candidates for A-USC because their 100,000-h fracture strength is about 100 MPa at temperatures from 650°C to 750°C. Unfortunately, however, the lifetime of Ni-base superalloys was found to decrease drastically under the application of creep-fatigue load at elevated temperatures. The random change of the output of the advanced thermal power plants is indispensable for compensating the unstable output of renewable energies for assuring the safe and reliable power supply. The main reason for the drastic decrease was attributed to the accumulation of dislocations around certain grain boundaries and it degraded the strength of the grain boundaries drastically. Intergranular cracking accelerated the crack growth rate and thus, decreased the fracture time.

In order to explicate the mechanism of the drastic decrease of the lifetime under creep-fatigue loading, in this study, molecular dynamics (MD) method was applied to the analysis on the degradation of the crystallinity of a grain boundary and its effect on the change of the effective strength of the grain boundary.  Various bicrystal structures were modelled by considering the combinations of the crystallographic orientation of the two grains in the bicrystal structure and the crystallinity, in other words, the total density of defects in those grains. In particular, the effect of the accumulation of vacancies and dislocations on the strength of the grain boundary between them was analyzed in detail. Both Uniaxial static or dynamic strains perpendicular to the grain boundary was applied at the ends of the bicrystal structure. It was found that the fracture mode of the bicrystal structure varied from ductile transgranular fracture to brittle intergranular one as a strong function of the combination of Schmid factor of the two grains. When the difference in the Schmid factor of the grains was large, brittle intergranular cracking tended to occur. In addition, the accumulation of vacancies and dislocations around the grain boundary degraded the strength of the grain boundary drastically, and thus, intergranular cracking was accelerated clearly. Both the fracture strain and fracture strength of the bicrystal structure decreased as a strong function of the total density of dislocations and vacancies. The accumulation of these defects was accelerated when the loading mode was change from simple creep loading to creep-fatigue loading. The local heavy plastic deformation occurred around the grain boundary with large difference in Schmid factor between nearby grains and the diffusion of the newly grown dislocations and vacancies was suppressed by the large strain field due to the large mismatch of the crystallographic orientation between the grains. Therefore, the mechanism of the acceleration of intergranular cracking under creep-fatigue loading was successfully clarified by MD analysis.


Molecular Dynamics Analysis of the Acceleration of Intergranular Cracking of Ni-Base Superalloy Caused by Accumulation of Vacancies and Dislocations Around Grain Boundaries

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Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

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