Creep and Creep-Fatigue Crack Growth in the Directionally-Solidified Ni-base Superalloy 247LC-DS
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Towner, Zachary
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Abstract
Hot-section components of industrial gas turbine engines are exposed to extreme environments that consist of high temperatures, high sustained and cyclic loads, and corrosive gases. Creep, fatigue, damage, crack growth, and fracture are thus critical design considerations for these components. Ni-base superalloys, such as alloy 247LC-DS, that are known for their high strength and creep resistance, are commonly used to fabricate hot-section turbine blades. Turbine blade designers face the question of determining the inspection criterion and how frequently blades should be inspected during service and when they should be repaired or removed from service. Fracture mechanics-based approaches are useful for characterizing crack growth rates in these materials and for estimating the service life of components.
This work aimed to experimentally determine the creep and creep-fatigue crack growth behavior of alloy 247LC-DS in the temperature range on 750°C to 950°C using nominally 50.80mm wide compact type C(T) specimens with thicknesses of nominally 3.00mm and 12.70mm. The crack plane was oriented in the direction normal to the direction of primary loading (also called transverse direction) that coincides with the direction of solidification during casting of the blades. Hold times of 2min and 20min were used during creep-fatigue crack growth testing. Microstructural characterization was performed to document the crack tip interaction with the microstructure during creep and creep-fatigue crack growth to gain insights into the cycle and time-dependent crack growth mechanisms. Some specific observations and conclusions from the work are as follows:
• The creep and creep-fatigue crack growth experiments revealed that, at 750°C, crack growth was dominated by fatigue loading and time-dependent crack growth was negligible. The fatigue crack growth rates were characterized by ΔK.
• At 850°C, there was competition between transgranular fatigue crack growth and time-dependent crack growth due to creep deformation and damage during the hold times. The fatigue crack growth rates were characterized by ΔK and the time-dependent crack growth rates both during sustained creep loading and during hold times of 2min and 20min were characterized by Ct and (Ct)avg, respectively.
• At 950°C, the fatigue crack growth behavior correlated with ΔK but the creep and creep-fatigue crack growth rates showed considerable scatter. Crack tip metallography showed that the time-dependent damage at this temperature occurred by diffuse cavitation and microcracking in the interdendritic regions in a direction normal to the main crack plane and by microcrack formation ahead of the main crack tip. Thus, traditional fracture mechanics parameters that are meant to characterize crack growth that proceeds in a self-similar manner, were no longer suitable for characterizing the crack growth rates.
• The mechanisms of time-dependent crack growth were identified as the dissolution of γ′ strengthening precipitates to form brittle oxide layers and microcrack formation near carbides in the interdendritic regions. This resulted in an interdendritic crack path.
• Alloy 247LC-DS exhibited creep-ductile crack growth behavior at temperatures ≥850°C. This is contrary to the common belief that creep-resistant materials behave in a creep-brittle fashion. This was especially the case in thin specimens where the crack front encountered only a few grains, and the crack tip is dominantly under plane stress conditions. This required the use of time-dependent fracture parameters such as Ct and (Ct)avg to characterize the crack growth rates. However, the effectiveness of Ct or (Ct)avg was also called into question when the crack tip damage was extensive due to interdendritic crack growth and microcrack formation. This damage pattern resulted in scattered data that cannot be correlated with any fracture mechanics parameters.
• The lack of self-similar damage ahead of the crack tip was also observed under some conditions in nominally 12.70mm thick specimens tested at 850°C that led to more scatter in the data when time rates of crack growth were correlated with Ct or (Ct)avg. It was more prevalent at the lower values of applied K. Thus, both K levels and specimen thickness effects through constraint influence the onset of interdendritic cracking and microcracking.
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2023-04-27
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Dissertation