Short crack initiation and growth at 600°C in notched specimens of Inconel718

Abstract

The natural initiation and growth of short cracks in Inconel ® 718 U-notch specimens has been studied at 600 o C in air. U notches were introduced through broaching, and hardness traces and optical microscopy on cross sections through the U notch broaching showed that the broaching process had introduced a deformed, work hardened layer. Fatigue tests were conducted under load control using a 1-1-1-1 trapezoidal waveform, on specimens with as-broached and polished U-notches. Multi-site crack initiation occurred in the notch root. Many of the cracks initiated at bulge-like features formed by volume expansion of oxidising (Nb,Ti)C particles. In unstressed samples, oxidation of (Nb,Ti)C particles occurred readily, producing characteristic surface eruptions. Scanning electron microscopy on metallographic sections revealed some sub-surface (Nb,Ti)C oxidation and localised matrix deformation around oxidised particles. A mechanism for crack initiation by carbide expansion during oxidation is discussed. Surface short crack growth rates in the notch root of polished specimens were measured using an acetate replica technique. Observed short crack growth rates were approximately constant across a wide range of crack lengths. However, there was a transition to rapid, accelerating crack growth once cracks reached several hundred microns in length. This rapid propagation in the latter stages of the fatigue life was assisted by crack coalescence. Polishing the U-notch to remove broaching marks resulted in a pronounced increase in fatigue life. IntroductionNickel-base superalloys like Inconel ® 718 (IN718) are used extensively for turbine discs and other components in industrial and aerospace gas turbines. Since turbine discs are safety-critical components, considerable effort is expended in determining their safe operating life. The lifing procedure adopted must enable the reliable prediction of the safe life of the disc, without being overly conservative. Once engines are in service, regular inspection of the engines and refinements of lifing models may enable life-extension programs to be considered. In the traditional predicted safe life philosophy, the life is declared based on laboratory specimen and component testing. In recent years a damage-tolerant approach to lifing has been adopted, in which it is accepted that components contain defects or inhomogeneities from which cracks can initiate. Assuming an initial flaw size c o , the fatigue life is determined as the number of cycles required to propagate the crack to some critical size c c . The initial size c o can be based on the maximum defect size present in the material, or on the minimum defect size detectable by nondestructive testing (NDT) [1]. Selection of c c is based on knowledge of the fracture toughness of the material, the limit load, the maximum allowable strain or maximum permitted compliance change for a particular component. Damage-tolerant lifing requires accurate information on fatigue crack initiation and growth rates. Fatigue lives are determi...

Effect of broaching on high-temperature fatigue behavior in notched specimens of INCONEL 718

“…Previous work by the authors on Batch A specimens [25] demonstrated that primary carbides at or close to the surface of IN718 oxidised at elevated temperatures, and there was a substantial volume expansion associated with oxidation. This was observed in unstressed specimens and U-notch fatigue specimens exposed at 550 o C and 600 o C in air.…”

“…Volume expansion of oxidising primary carbides was identified as a mechanism of environmentally assisted crack propagation in notch-rupture tests on IN718 [21] . The role of oxidised primary carbides in fatigue crack initiation has also been studied [22,23,24,25] .…”

“…The natural initiation and growth of short cracks in U-notch IN718 specimens has been studied at 600 o C in air and reported in detail elsewhere by the authors [25] . Fatigue tests were conducted under load control using a 1-1-1-1 trapezoidal waveform, on specimens with as-broached and polished U-notches.…”

Effect of Phosphorus on Microstructure and Mechanical Properties of IN718 Alloy after Hot Corrosion and Oxidation

“…It has been reported that the oxidation behavior at intermediate temperature of IN718 alloy was controlled by the oxidation of NbC carbide and the formation of Cr 2 O 3 and Fe 2 O 3. [7,8] The Cr 2 O 3 would preferentially form at grain boundaries, which inhibits the progression of oxygen along grain boundaries. However, the Cr contents were not high enough to form continuous protective oxide layer at the initial oxidation stage, therefore other elements can be oxidized within the grain.…”

Probabilistic Life of DA718 for Aircraft Engine Disks

“…[1,3,[8][9][10]). The effect of grain size reduction ,assumed to be beneficial on fatigue lives, was more particularly studied for conventional and DA 718 alloys (see e.g.…”