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Nickel-based superalloys have proved themselves over the past 50 years in demanding applications such as hot sections of gas turbine engines due to their excellent creep strength, oxidation resistance and corrosion protection; however, new challenges require materials with greater endurance at higher temperatures.
Commercial cast Ni-superalloys typically exhibit gamma prime precipitates that coexist within grain boundaries. Their size and location in the microstructure can be controlled with appropriate heat treatment:
Thermo-Mechanical Properties
Nickel-base superalloys consist of austenitic matrix structures enriched with elements like cobalt, iron, molybdenum and 3d printed aluminum that act as solid-solution strengtheners. When these are heated-treated, precipitation of an FCC phase known as g' is promoted - increasing in strength with temperature. Therefore these alloys are often utilized for high temperature applications.
Al, Ti and TiNi add elements which significantly improve flow stress and ultimate tensile strength of alloys. Furthermore, dispersions strengthened with Mo, Re and W can further strengthen grain boundary strength and grain boundary integrity - in turn providing enhanced grain boundary strength. Moreover, these alloys also show superior time-dependent creep performance.
For optimal properties in alloys, it is crucial to comprehend their liquid-phase behavior. Onera has developed an extensive multi-scale framework which combines two length scales: size-dependent dislocation density models at sub-grain level and homogenized crystal plasticity models with account for size dependence and tension-compression asymmetry in homogenized crystal plasticity models at macro-grain level.
Corrosion and Oxidation Resistance
Nickel-based superalloys have been driven by the need to develop materials capable of meeting stringent strength, creep resistance and corrosion/oxidation resistance requirements. Their development results from precipitating an ordered cubic g' phase within a disordered cubic L12 phase with nearly identical lattice constant.
Physical metallurgy of these alloys is complex and requires extensive understanding. Yet some general principles can provide insights into their microstructure/property relationships; one strong correlation has been identified between solid solution hardening of nickel and the increase in stacking fault energy associated with various solute elements being added as solute elements are added.
One such atomic structure is that of the complex g' phase found in many advanced nickel superalloys. This phase exhibits an extraordinary property: increasing strength with temperature due to planar defect structures (antiphase boundaries and stacking faults) found within it.
Microstructure and Mechanical Properties
Nickel-based superalloys are known for being highly ductile under stress, making them suitable for components that must stretch repeatedly. Furthermore, they possess high strength as well as resistance against oxidation and wear.
High-temperature mechanical properties in alloys are dependent on multiple factors, including microstructure, precipitate phase composition and deformation mechanisms. Recent advances in characterization techniques (such as electron microscopy and atom probe tomography ) and advanced modeling tools have enabled more accurate and predictive models for these alloys.
Nickel-based superalloys designed to improve damage tolerance have seen reductions in Al, Ti, and Nb content and an increase in solid solution strengthening elements such as W and Mo, with more W and Mo being added as strengthening agents. Furthermore, their gamma prime phase plays an integral part in their mechanical properties by interacting with dislocations during plastic deformation - thus being one of the main drivers behind strength and creep performance - so careful composition design must be undertaken when considering their design.
Fabrication
Nickel-base superalloys are ideal materials for high temperature mechanical applications such as gas turbine engines and other demanding uses due to their excellent creep and fatigue resistance, long term stability at elevated temperatures, as well as superior creep resistance of gamma prime precipitates precipitates.
Gamma prime (g') precipitates are at the core of high-temperature performance in superalloys and superelastics, providing crucial strength, creep resistance, strength distribution, volume fraction and distribution characteristics critical for strength and creep behavior. Furthermore, superalloys' production of precipitates allows dislocations to interact more freely when plastic deformation takes place.
Aluminum 3d printing Nickel-base superalloys can significantly enhance their high temperature mechanical properties through the addition of elements like Ti, Nb, Al and Mo; combined with careful solidification processing to remove grain boundaries which become weak points when exposed to elevated temperatures. Furthermore, single crystal (SX) forms can provide additional creep and fatigue resistance.
