Nickel-based superalloys demonstrate exceptional elevated-temperature strength and resistance to creep, oxidation, and corrosion, making them essential for aerospace and power-generation turbine applications. However, their resistance to fatigue-crack propagation at service temperatures remains a critical performance factor. These alloys are susceptible to crack formation during fabrication or service, with cracks propagating under stress in gas turbines and jet engines. Fatigue manifests as low-cycle fatigue from turbine start-stop cycles and high-cycle fatigue from vibrational loading. NASA-sponsored research revealed that fatigue crack propagation rates vary with stress frequency and hold-time, with lower frequencies unexpectedly increasing crack propagation rates. Grain-boundary engineering shows promise for enhancing fatigue resistance, particularly in polycrystalline nickel-based disk alloys like ME3.
Nickel-based superalloys represent a cornerstone technology in modern turbine applications, serving critical roles in both aerospace and land-based power-generation systems. These advanced materials owe their widespread adoption to their exceptional elevated-temperature strength, superior resistance to creep, oxidation, and corrosion, combined with excellent fracture toughness properties. However, among all performance characteristics, their resistance to fatigue-crack propagation at service temperatures stands as the most critical property determining operational safety and reliability.
Engine applications present engineers with two distinct fatigue challenges that must be addressed simultaneously. Low-cycle fatigue emerges from relatively large stress cycles associated with the stopping and starting of turbine operations, creating substantial thermal and mechanical stress variations. High-cycle fatigue (HCF) develops from vibrational loading during continuous service operations, presenting a different but equally dangerous failure mechanism.High-cycle fatigue results in rapid and often unpredictable failures through the propagation of fatigue cracks in blade and disk components under high-frequency loading conditions. These cracks typically initiate from small defects, frequently resulting from fretting wear or foreign-object damage during operation. The high vibrational frequencies involved create a particularly dangerous scenario where even cracks growing at relatively slow per-cycle velocities can propagate to complete failure within remarkably short time periods, potentially within a single flight segment.
Due to these severe consequences, HCF-critical turbine-engine components must operate below specific fatigue-crack initiation and growth thresholds. This operational requirement ensures that cracking cannot occur within approximately 109 cycles, providing adequate safety margins for extended service life.
Nickel-based superalloys have established themselves as indispensable materials in jet engines, land-based gas turbines, and other high-temperature machinery applications. These alloys must retain high strength and other desirable physical properties at elevated temperatures of 1000°F (540°C) or higher. Many of these advanced alloys contain γ' precipitates in varying volume percentages, which contribute significantly to their high-performance properties at elevated service temperatures.
The engineering community has increasingly recognized a significant problem with many nickel-based superalloys: their susceptibility to crack formation during both fabrication and service operations. These cracks can propagate and grow while components remain under stress during normal operation in structures such as gas turbines and jet engines. The propagation or enlargement of these cracks can lead to catastrophic part fracture or other critical failures, with particularly hazardous consequences when moving mechanical parts fail due to crack formation and propagation.
A principal finding from NASA-sponsored research revealed that fatigue crack propagation (FCP) rates are not uniform across all applied stresses or stress application methods. More importantly, the research demonstrated that fatigue crack propagation varies significantly with the frequency of stress application to components, particularly where stress application tends to enlarge existing cracks.The most surprising discovery from the NASA-sponsored study was the magnitude of the finding that lower-frequency stress application, rather than the higher frequencies previously employed in research studies, actually increased crack propagation rates. This groundbreaking research verified the existence of time dependence in fatigue crack propagation behavior. Furthermore, the time dependence of fatigue crack propagation was found to depend not solely on frequency but on the duration during which components remain under stress, known as hold-time.
The grain-boundary engineering approach has demonstrated particular success in promoting fracture resistance in specific applications, notably in addressing intergranular stress-corrosion cracking and creep resistance. However, its effects on fatigue resistance have remained largely unexplored until recent research initiatives.
According to recent studies, ambient temperature smooth-bar tension-tension fatigue lives for two γ/γ' superalloys showed significant improvements through grain-boundary engineering. An iron-based alloy demonstrated fatigue life increases by a factor of approximately 1.5 when the fraction of special boundaries increased from 20 to 65 percent. More dramatically, a nickel-based alloy showed fatigue life improvements by a factor of 3 when special boundary fractions increased from 9 to 49 percent, although researchers presented no mechanistic explanation for these improvements.
The effectiveness of grain-boundary engineering depends fundamentally on the nature of crack propagation paths, specifically the relative prevalence of intergranular versus transgranular cracking mechanisms. Understanding this relationship formed the foundation for recent research objectives investigating the feasibility of using grain-boundary engineering processing to promote resistance to fatigue-crack propagation, particularly at near-threshold stress levels.
Research focused on a new polycrystalline nickel-based disk alloy designated ME3 (Ni-Co-Cr Alloy), examining crack growth rates and threshold behavior of large through-thickness cracks ranging from 8 to 20 mm. Testing occurred across a range of temperatures including 25°C, 700°C, and 800°C to enhance the incidence of intergranular crack growth and better understand failure mechanisms.
Figure 1: The variation in fatigue-crack propagation behavior for small surface cracks in grain-coarsened ME3, along with EBSD characterization of crack propagation paths. Random boundaries are shown as black lines, twin boundaries in red, and other special boundaries in yellow
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