Microstructural Characterization and Damage Mechanism of the First Stage Blade GE MS9001 After Long-term Exposure

Document Type : Research Paper

Authors

• Hassan Ghorbani ¹
• Hassan Farhangi ²
• Mehdi Malekan ²

¹ PhD student of School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran

² Associate Professor , School of Metallurgy and Materials Engineering, College of Engineering, University of Tehran, Tehran, Iran

10.30495/jnm.2023.32570.2012

Abstract

Abstract
Introduction: Due to the outstanding high-temperature mechanical properties, precipitation-hardening nickel-based superalloys are capable of working in high-temperature conditions in the harsh environment. One of the applications of these alloys is the use in the hot part of gas turbines, which can be used as rotor and stator blades. The properties of these alloys after high-temperature (800-950°C) long-term operation have changed dramatically, including oxidation, severe local deformations, creep and fatigue, and microstructure. The mentioned damages can strongly affect the reliability of the operation of these components.  
Methods: In this research, damage mechanisms and microstructural properties of GTD-111 superalloy as a precipitation-hardening nickel base superalloy are investigated. Hence, the blade airfoil operated for 75,000 hours was used as the alloy under high-temperature conditions and the root section was used as the unexposed alloy. Characterization of the exposed alloy was carried out by optical microscope (OM), scanning electron microscope (SEM), transmission electron microscope (TEM), and X-ray energy dispersive spectroscope (EDS). In order to assess the damage mechanism, the radiography X-ray test (RT) and fluorescence penetration inspection (FPI) were used.   
Findings: Based on the finite element analysis results, the blade airfoil experienced a high temperature condition about 800-925°C. The non-destructive tests results showed that some damages such as spallation of thermal barrier coating, local deformation, local oxidation, tip rubbing and discoloration. The microstructure observation results demonstrated that there are considerable changes in the microstructure features such as the decomposition of MC carbide, coarsening and spheroidization of primary γ′, dissolution of secondary γ′, and formation of huge deleterious topological closed packed (TCP) phases located in interdendritic regions which profoundly impact its mechanical properties. Based on the results obtained from the transmission electron microscope (TEM), dislocations have been observed in the γ channels and the γ-γ' interface. It is worth mentioning that due to the low stress level, the density of dislocation in the γ′ phase, which can be in the form of SFE and APB, is limited. Microstructural observations indicated severe structural deterioration after 75,000 hours of turbine operation. The main structural degradation that can lead to a sharp decrease in mechanical properties at high temperature and consequently a sharp decrease in reliability is the change in the size and morphology of precipitation hardening phases γ' and the continuity of brittle carbide phases in the grain boundaries of GTD-111 alloy.

Keywords

• Nickel based superalloys
• microstructural degradation
• Dislocation
• long-term exposure

20.1001.1.22285946.1402.14.51.4.9

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