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Friday, 3 January 2014

Fatigue of Nickel-Based Superalloys: Part Two

Fatigue of Nickel-Based Superalloys: Part One

A critical property of nickel-based superalloys is their resistance to fatigue-crack propagation, particularly at service temperatures.
Many nickel-based superalloys are subject to formation of cracks or incipient cracks, either in fabrication or in use, and that the cracks can actually propagate or grow while under stress during the use in structures such as gas turbines and jet engines.
Nickel-based superalloys are widely used in turbines for both aerospace and land-based power-generation applications, due to their exceptional elevated-temperature strength, high resistance to creep, oxidation, and corrosion, and good fracture toughness. However, a critical property of these alloys is their resistance to fatigue-crack propagation, particularly at service temperatures.
In engine applications, there are often two components to this problem:
low-cycle fatigue, which results from relatively large cycles associated with the stopping and starting of the turbine, and
high-cycle fatigue (HCF), associated with vibrational loading during service.
Fatigue results in rapid, and often unpredictable, failures due to the propagation of fatigue cracks in blade and disk components under high-frequency loading, where the cracking initiates from small defects, in many instances resulting from fretting or foreign-object damage. Due to the high vibrational frequencies involved, even cracks growing at slow per-cycle velocities can propagate to failure in short time periods, possibly within a single flight segment. Consequently, HCF-critical turbine-engine components must be operated below the fatigue-crack initiation or growth thresholds, such that cracking cannot occur within ~109 cycles.
Nickel-based superalloys have been used extensively in jet engines, in land based gas turbines and other machinery where they must retain high strength and other desirable physical properties at elevated temperatures of 1000°F (540°C) or more. Many of these alloys contain a γ’ precipitate in varying volume percentages. The γ’ precipitate contributes to the high performance properties of such alloys at their elevated use temperatures.
A problem which has been recognized to a greater and greater degree with many nickel-based superalloys is that they are subject to formation of cracks or incipient cracks, either in fabrication or in use, and that the cracks can actually propagate or grow while under stress as during use of the alloys in such structures as gas turbines and jet engines. The propagation or enlargement of cracks can lead to part fracture or other failure and the consequences of failures of the moving mechanical part due to crack formation and propagation can be particularly hazardous.
A principal finding of a NASA sponsored study was that the rate of propagation based on fatigue phenomena or in other words, the rate of fatigue crack propagation (FCP), was not uniform for all stresses applied nor to all manners of applications of stress. More importantly, the finding was that fatigue crack propagation actually varied with the frequency of the application of stress to the part where the stress was applied in a manner to enlarge the crack.
More surprising still was the magnitude of the finding from the NASA sponsored study that the application of stress of lower frequencies rather than at the higher frequencies previously employed in studies, actually increased the rate of crack propagation. In other words the NASA study verified that there was a time dependence in fatigue crack propagation. Further, the time dependence of fatigue crack propagation was found to depend not on frequency alone but on the time during which the member was held under stress, so-called hold-time.
To date, the grain-boundary engineering approach has been shown to be particularly successful in promoting fracture resistance in specific cases, notably in the context of intergranular stress-corrosion cracking and creep. However, its effect on the fatigue resistance has largely been unexplored.
According to one study, ambient temperature, smooth-bar, tension-tension fatigue lives for two γ / γ’ superalloys were reported to be increased by a factor of ~1.5 in an Fe-based alloy by increasing the fraction of special boundaries from 20 to 65 pct and by a factor of 3 in a Ni-based alloy by increasing this fraction from 9 to 49 pct, although no mechanistic explanation was presented.
Clearly, the effectiveness of grain-boundary engineering will depend upon the nature of the crack path, specifically, the preponderance of intergranular versus transgranular cracking. In light of this, the objectives of the studies was to investigate, the feasibility of using grain boundary engineering processing to promote resistance to fatigue-crack propagation, particularly at near-threshold levels, in a new polycrystalline nickel-based disk alloy, ME3 (Ni-Co-Cr Alloy). Specifically, the crack growth rates and threshold behavior of large (8 to 20 mm) through-thickness cracks were examined over a range of temperatures (25°C, 700°C, and 800°C) in order to enhance the incidence of intergranular crack growth, Figure 1.
Figure 1: Variation in fatigue-crack propagation behavior for small surface cracks in the grain-coarsened ME3, together with EBSD characterization of the path for small crack propagation. Random boundaries are shown as black lines twin boundaries are in red, other special boundaries are in yellow.

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)nN 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.

Fatigue of Nickel-Based Superalloys: Part One

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