High Temperature Nickels: Part Two

Nickel-based superalloys typically constitute 40–50% of the total weight of an aircraft engine and are used most extensively in the combustor and turbine sections of the engine where elevated temperatures are maintained during operation. Creep-resistant turbine blades and vanes are typically fabricated by complex investment casting procedures that are essential for introduction of elaborate cooling schemes and for control of grain structure. Such components may contain equiaxed grains or columnar grains, or may be cast as single crystals, completely eliminating all high-angle grain boundaries.

Because grain boundaries are sites for damage accumulation at high temperatures, the blades in the early stages of the turbine are typically single crystals, whereas the blades in the later (cooler) stages of the turbine are fabricated from equiaxed alloys. Structural components such as engine cases are also fabricated by investment casting processes. Turbine disks are fabricated via wrought processing approaches that either use cast ingots or consolidated superalloy powder performs. Exceptional combinations of strength, toughness, and crack-growth resistance can be achieved in these materials by close control of microstructure through the multiple stages of wrought processing.

As turbine-disc requirements have evolved, materials have been designed to optimize certain key features. The initial cast-and-wrought nickel-based superalloys were developed for increased temperature and strength capability. In the early 1970s, the emphasis turned toward powder-metallurgy (P/M) alloys for even higher temperature and strength applications. More recently, nickel-based superalloy developments have been driven toward improved damage tolerance through highly optimized fatigue-crack-growth properties. Additionally, issues such as component residual stresses have become critical to component design and lifing.

Optimized fatigue-crack-growth rates and reduced component residual stress levels have led to reductions in other properties, such as strength and temperature capabilities. The mechanical properties of nickel-based superalloys can be optimized by tailoring alloy chemistry and microstructural features through refined special practices and process control. Nickel based superalloys are complex alloys with various microstructural features that contribute to the control of the mechanical properties. These features include grain size, γ size and distribution, carbide- and boride-phase content, and grain-boundary morphology.

Grain size is one of the most important features, as it greatly influences strength, creep, fatigue crack initiation and growth rate. Grain-size optimization and control is one of the primary goals during the forging of turbine-disc components.

Nickel-based superalloys have a relatively high yield and ultimate tensile strengths, with yield strengths often in the range of 900–1300 MPa and ultimate tensile strengths of 1200–1600MPa at room temperature. Turbine-disk alloys are typically developed to have higher strengths for flexibility at temperatures below 800°C in the design to protect against burst of the turbine disk in the event of an engine over speed. Note that the tensile properties do not substantially decay until temperatures are greater than approximately 850°C.

The slight rise in the yield strength of the alloys at intermediate temperatures is due to the unusual flow behavior of the Ni₃Al γ׳ phase. Deformation of the precipitates gives a corresponding, but weaker, rise in the flow stress of superalloys at intermediate temperatures. Note also that the two-phase superalloys are much stronger than either the matrix or precipitate materials in their bulk form. Strengthening in two-phase superalloys arises from multiple microstructural sources, including solid-solution strengthening, grain size strengthening, and the interaction of dislocations with precipitates (Orowan bowing between precipitates or shearing through precipitates in strong- or weak-coupled modes).

The primary precipitates are not solutioned in the late stages of processing and therefore inhibit grain growth and indirectly provide grain size strengthening. Disk alloys with these large (≈1μm) precipitates are referred to as subsolvus processed. If only one or two populations of submicron-size precipitates are present, because of complete solutioning of the primary γ׳, then the grains are much larger (30–50 μm); these microstructural conditions result from supersolvus processing. Modeling that considers the various possible strengthening mechanisms for each microstructural constituent in PM IN100 also demonstrates that there is no unique combination of microstructural parameters that results in a high-strength alloy.

Because superalloys spend extended periods of time under stress at high temperature, a high resistance to time-dependent creep deformation is essential. This is very important for cast lade alloys, because they will experience temperatures up to 1100°C, whereas disk alloys are typically limited to less than 700°C. For a fixed stress and temperature, two-phase superalloys have a much higher creep resistance compared to their single-phase counterparts. As with all properties that are governed by plastic deformation processes, creep properties are sensitive to microstructure.

The metallugical technology applied to high temperature gas turbine discs has always been at the cutting edge. A unique combination of severe stress/temperature cycles, long life requirements, minimum weight and the safety critical nature of the components has driven alloy and process design to the limit. Recent developments have seen a move away from cast and wrought nickel alloys to higher strength powder alloys that combine excellent strength with creep, fatigue and environmental resistance.

The complexity of the alloy systems and the high sensitivity to process conditions have necessitated the development of materials models that provide strong support for the design at all stages, from the initial alloy design through to the prediction of final machining stresses and estimation of in-service performance. The operating temperature of turbine discs in advanced engines exceeds 700°C so that creep and environment interactions have a large influence on the fatigue life.

Modelling tools have enabled a far better understanding of the the interactions between the alloy, the process and the properties such that it is now possible to design components that have their microstructure modified locally to resist the dominant local damage mechanisms.