Nickel-based superalloys play an invaluable role in gas turbine engines, offering creep resistance across a wide temperature range, high tensile strength and yield strength, good fatigue properties, and corrosion protection.
Ni-base alloys contain several GCP phases, the primary one being g, an ordered L12 phase with Ti or Al on its faces and Ni atoms at its corners. Unfortunately, g is nonstoichiometric and requires additional iron addition to stabilize itself.
High-Temperature Strength
Nickel-based superalloys can withstand temperatures close to their melting points at up to 70% of their melting points, enabling jet engines to operate at higher temperatures for greater thrust and efficiency. Advanced nickel superalloys superior temperature strength can be attributed to the coherence and stability of intermetallic phase g' (cubic L12 structure), with Ni on its faces and Ti or Al in its corners; this provides structural hardening and ductility while other carbides offer dispersion strengthening and grain boundary stabilization properties.
Single-crystal PM superalloys NR3 and RR1000 have been engineered with fine grain microstructure (15 um), which provides exceptional mechanical properties at higher temperatures while being exposed to long-term exposures up to 750 deg C.
Furthermore, their fine grain microstructure makes these alloys suitable for swaging without losing mechanical properties; an important consideration when creating wind turbine blades; however swaging requires precise control in order to preserve microstructure integrity during this process.
Ductility
Nickel superalloys attain high-temperature strength through the formation of an FCC phase known as Gamma Prime (g': Ni3Al), which works to interlock with surrounding matrix crystal structures to provide additional strength at higher temperatures. Compositions typically include nickel on its faces with aluminum or titanium added at its corners, increasing gamma prime formation while simultaneously improving the ductility of Ni-base superalloys.
Although these materials possess excellent mechanical properties, fabrication and repair operations often result in weld cracking in the heat-affected zone (HAZ). This issue can lead to costly outages of gas turbines or other critical equipment.
Recent research has demonstrated the efficacy of Fusion AM for processing and rewelding superalloys that have been swaged, using cold work on cylindrical bars of Nickel Superalloy RR1000. Grain size analysis and electron back-scattered diffraction revealed significant modifications in microstructure as a result of cold work; specifically a strong 111 texture paralleling the longitudinal bar axis was noticed as evidence of modification due to cold work.
Creep Strength
Advanced nickel alloys are widely utilized in gas turbine engines that operate at elevated temperatures under sustained mechanical loads, featuring high strength, good ductility, and remarkable hot corrosion resistance properties. Their combination of strength, good ductility and impressive hot corrosion resistance makes these alloys the material of choice.
Creep occurs when materials are exposed to prolonged stress over time, causing them to deform permanently. The rate at which materials creep is determined by several factors: stress magnitude, activation energy, and grain size - especially grain size's impact on dislocation motion in materials as it determines dislocation motion can be inhibited when particles such as oxide or carbide form at grain boundaries preventing dislocation motion from happening as easily.
The precipitate phase, known as g', which is covalent with the matrix phase is one of the primary contributors to creep strength. Their quantity, morphology, and stability at creep temperature determine their strength; closer-spaced precipitates provide higher creep resistance than more widely-scattered ones.
Corrosion Resistance
Nickel-based superalloys have gained immense acclaim due to their superior properties at higher temperatures. As such, researchers have conducted in-depth analyses of all aspects of their manufacturing, performance, and development.
One such aspect is the influence of various intermetallic phases on the mechanical behavior of cast and wrought Ni-based superalloys, such as IN 718 alloy. Intermetallic phases may either enhance or diminish mechanical properties depending on their morphology, orientation, and quantity; for example, the g-phase in IN 718 improves creep resistance by slowing dislocation motion in its matrix.
Another aspect under investigation is the relationship between Ni-based PM superalloys' microstructures and high-temperature mechanical properties. To this end, cylindrical bars of RR1000 alloy were subjected to swaging before their microstructure was characterized with scanning electron microscopy (SEM) and EBSD analysis. Furthermore, cold work's effect on their microstructure and mechanical properties was assessed at various test temperatures.
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