Fatigue of Nickel-Based Superalloys: Part Two

It has been discovered that it is feasible to construct parts of nickel based superalloys for use at high stress in turbines and aircraft engines with greatly reduced crack propagation rates and with good high temperature strength. The properties needed for moving parts of the engine are usually greater than those needed for static parts, although the sets of needed properties are different for the different components of an engine.

The development of the superalloy compositions and methods of their processing of this invention focuses on the fatigue property and addresses in particular the time dependence of crack growth. Crack growth, i.e., the crack propagation rate, in high-strength alloy bodies is known to depend upon the applied stress (ς) as well as the crack length (a). These two factors are combined by fracture mechanics to form one single crack growth driving force; namely, stress intensity factor K, which is proportional to ς√a.

Under fatigue conditions, the stress intensity in a fatigue cycle may consist of two components, cyclic and static. The former represents the maximum variation of cyclic stress intensity (ΔK), i.e., the difference between Kmax and Kmin. At moderate temperatures, crack growth is determined primarily by the cyclic stress intensity (ΔK) until the static fracture toughness KIC is reached.

Crack growth rate is expressed mathematically as da/dN α(ΔK)n. N represents the number of cycles and n is material dependent. The cyclic frequency and the shape of the waveform are the important parameters determining the crack growth rate.

For a given cyclic stress intensity, a slower cyclic frequency can result in a faster crack growth rate. This undesirable time-dependent behavior of fatigue crack propagation can occur in most existing high strength superalloys. To add to the complexity of this time-dependence phenomenon, when the temperature is increased above some point, the crack can grow under static stress of some intensity K without any cyclic component being applied (i.e. ΔK=0).

The design objective is to make the value of da/dN as small and as free of time-dependency as possible. Components of stress intensity can interact with each other in some temperature range such that crack growth becomes a function of both cyclic and static stress intensities, i.e., both ΔK and K.

Following the documentation of this unusual degree of increased fatigue crack propagation at lower stress frequencies there was some belief in the industry that this phenomena represented an ultimate limitation on the ability of the nickel based superalloys to be employed in the stress bearing parts of the turbines and aircraft engines and that all design effort had to be made to design around this problem.

However, it has been discovered that it is feasible to construct parts of nickel based superalloys for use at high stress in turbines and aircraft engines with greatly reduced crack propagation rates and with good high temperature strength. It is known that the most demanding sets of properties for superalloys are those which are needed in connection with jet engine construction. The properties needed for moving parts of the engine are usually greater than those needed for static parts, although the sets of needed properties are different for the different components of an engine.

Nickel-base superalloys, strengthened by a high volume fraction of Ni3Al precipitates, have been the undisputed choice for turbine discs in gas turbines as they exhibit the best available combination of elevated temperature tensile strength and resistance to low cycle fatigue (LCF), which is essential for a disc alloy. Alloy 720LI is a wrought nickel-base superalloy developed for disc application and exhibit superior elevated temperature tensile strength and LCF properties. It is distinct because of its chemistry, especially Ti, Al and interstitial C and B contents, its processing and heat treatment. However, literature available in open domain to develop an understanding of these properties in alloy 720LI is rather limited.

The effect of temperature and strain rate on monotonic tensile properties were assessed at different temperature in the range of 25–750°C (0.67 Tm) at a strain rate of 10-4 s-1 and strain rate effects were explored in detail at 25, 400, 650 and 750°C at different strain rates between 10-5 s-1 and 10-1 s-1. Yield and ultimate tensile strength of the alloy remains unaffected by temperature till about 600°C (0.58Tm) and 500°C (0.51Tm), respectively, beyond which both decreased drastically. Negligible strain rate sensitivity exhibited by the alloy at 25 and 400°C indicated that flow stress is a strong function of strain hardening rather than strain rate hardening. However at 650 and 750°C, especially at low strain rates, strain rate sensitivity is relatively high.

The cast nickel-based superalloy Inconel 792-5A is used for the gas-turbine integral wheels of auxiliary power units in the aircraft industry. As well known, turbine wheels are subjected to repeated elastic-plastic straining as a result of heating and cooling during the start-up and shut-down periods. Consequently, low cycle fatigue is an important consideration in the design of the components, and the cyclic stress-strain and fatigue-life data are needed up to the working temperature of 900°C.

The fatigue behaviour of Inconel 792-5A (12.28 Cr; 8.87 Co; 3.98 Ti; 3.36 Al; 4.12 Ta; 4.1 W; 1.81 Mo; 0.1 Nb; 0.16 Fe; 0.031 Zr; 0.078 C; 0.015 B, all in wt %) has been reported only scarcely. Strain localization is one of the most important stages during the fatigue damage of crystalline materials. It is closely connected to crack nucleation and manifests itself in specific changes to the internal structure and in the formation of a characteristic surface relief.

Slip bands parallel to the active slip plane are formed and slip markings originate in the vicinity of the intersection of slip bands with the free surface. Slip bands and slip markings have been reported for many materials, including nickel-based superalloy single crystals, and polycrystals.

The investigation of the dislocation structure in superalloy polycrystals at room and at high temperatures indicated planar slip bands parallel to {111} slip planes and cutting of the strengthening particles. Surface slip markings were observed in Inconel 713 LC at room and at high temperature. The effect of slip bands on the cyclic stress-strain response in polycrystalline superalloys has not been studied systematically. The fragmentation and shearing of γ’ particles were considered to be the reason for the observed cyclic softening at room temperature.

Because some sets of properties are not attainable in cast alloy materials, a solution is sometimes to apply powder metallurgy techniques. However, one of the limitations for the use of powder metallurgy techniques in preparing moving parts for jet engines is the issue of the powder purity.

If the powder contains impurities such as a speck of ceramic or oxide the place where that speck occurs in the moving part becomes a latent weak spot where a crack may initiate. Such a weak spot is in essence a latent crack. The possible presence of such latent cracks makes the problems of reducing and inhibiting the crack propagation rate all the more important. It is possible to inhibit crack propagation both by the control of the composition of alloys and by the methods of preparation of such metal alloys.

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