Date of Award
Master of Science
Metallurgical and Materials Engineering
Stephen W. Stafford
Superalloys are metallic materials that exhibit excellent mechanical strength and creep resistance at high temperatures. They have good surface stability and are corrosion resistant. Superalloys are mostly used in the aerospace industry, gas turbine engines and blades (hot zones of gas turbines), and where extreme heat is encountered. The focus of this research was on the GTD-111 Ni-base superalloy, which is a General Electric (GE) proprietary superalloy mostly used in gas turbine blades with the form of high pressure or first stage buckets. This alloys features better mechanical properties, creep resistance, and a higher stress rupture temperature than the commonly used Inconel 738LC Ni-base superalloy.
The purpose of this research was to characterize the microstructural differences between two different sections (airfoil and shank) of a GTD-111 General Electric Frame 3/2 Model "J" Dry Low NOx Unit Stage 1 bucket. This research was divided into several stages: a) microstructure, b) elemental Composition determination using energy dispersive x-ray spectroscopy (EDS), c) microhardness testing, d) gamma-prime (γ΄) diameter using the scanning electron microscope (SEM), and e) transmission electron microscopy (TEM). The buckets had been in service for 48, 064 hours at an approximate service temperature of 1700 - 1800 ÂºF.
Microstructural changes were examined by the use of metallography, scanning electron microscopy, transmission electron microscopy, and microhardness testing in the airfoil and shank sections of the turbine buckets. These techniques provided the means to establish specific microstructural alteration which included:
1.) γ΄ coarsening and particle coalescence;
2.) the essence of solute re-distribution between the γ and γ΄ phases, and MC carbide decomposition;
3.) the apparent thermal modification of γ΄ eutectic structures;
4.) and the strengthening of the trailing edge regions of the airfoils.
The overall γ΄ coarsening and coalescence occurs mainly by the Ostwald ripening effect and follows the Lifshitz, Sloyozov, and Wagner theory. All of the elemental changes in the microstructure suggest that vacancy diffusion took place and could have contributed to the strengthening (higher hardness and tensile strength) of the trailing edge of the airfoil. It is important to mention that the material's performance depends on the ability to retain its original microstructural features during service, since preserving this microstructural state represents an optimum design condition.
Metallographic inspection by replication can be used to analyze the microstructural changes the buckets have undergone. Such technique is non-destructive and can be used in preventive maintenance routines to assess the microstructural degradation and determine if component can continue in service. Future work includes a more in depth microstructural investigation for different service conditions to help determine a more approximate rate of degradation of the γ΄ phase. Also, any future work would have to entail creep rupture testing to confirm residual elevated temperature properties.
Received from ProQuest
Quintero, Oscar, "Microstructural Characterization of Overaged GTD-111 HP Turbine Buckets" (2009). Open Access Theses & Dissertations. 2758.