The effect of defects on the fatigue crack initiation process in two p/m superalloys: part i. fatigue origins

Hyzak, J. M.; Bernstein, I. M.

doi:10.1007/bf02642413

Cited by 87 publications

(48 citation statements)

References 9 publications

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“…Temperature [4,5], grain size [6,7], stress level [8], strain range [9], and size of non-metallic inclusions [10] have been found to affect fatigue performance. These microstructural features (grain boundaries, inclusions) introduce dislocation barriers and thus give rise to accumulation of damage, resulting in crack nucleation.…”

Section: Introductionmentioning

confidence: 99%

Crystal plasticity and high-resolution electron backscatter diffraction analysis of full-field polycrystal Ni superalloy strains and rotations under thermal loading

et al. 2014

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Electron back scattered diffraction (EBSD) has been employed to study a polycrystalline nickel superalloy containing a complex non-metallic agglomerate under thermal loading. Heterogeneous distributions of elastic strains are observed near the inclusion due to its complex geometry which have been quantified. Lattice rotations were also related to geometrically necessary dislocation (GND) density (~10 14 −2 ), indicating the development of localized plasticity arising from the mismatch in thermal expansivity between the Ni polycrystal and the inclusion. A crystal plasticity finite element (CPFE) model which explicitly represents the full detail of the complex microstructure was developed to interpret the experimental measurements, and good quantitative and qualitative agreement has been obtained. However, a limitation of the EBSD technique when investigating polycrystal systems is that full-field, trans-granular strain measurement remains difficult due to the necessity to reference a lattice spacing within a grain for strain calculation. An inverse reference shifting methodology has been developed using CPFE modelling to overcome this problem, thereby allowing like-for-like and grain-by-grain strain comparisons to be made. The method, in conjunction with high resolution-EBSD, shows promise for the determination of fullfield strains and rotations in polycrystalline materials, and provides key information for fatigue nucleation in these material systems.2

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Section: Introductionmentioning

confidence: 99%

Crystal plasticity and high-resolution electron backscatter diffraction analysis of full-field polycrystal Ni superalloy strains and rotations under thermal loading

et al. 2014

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“…Fractographic observations have revealed the crack initiation at surface and sub-surface non-metallic inclusions such as carbides, nitrides, and carbonitrides, when the alloy Inconel 718 is subjected to low-cycle fatigue with low-strain amplitude at intermediate temperatures [96]. Instead of inclusions, fatigue crack initiation at other metallurgical weak sites like porosities is also fairly common in cast alloys and powder-metallurgy alloys [97][98][99][100][101]. Such phenomena are most evident in VHCF.…”

Section: Crack Initiationmentioning

confidence: 99%

Surface Integrity and Fatigue Performance of Nickel-based Superalloys

Chen¹

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Gas turbines are widely used in e.g., power generation, and aero-industries. Due to global warming, the demand for more efficient gas turbines has increased. A way to achieve this is by increasing the operating temperature of gas turbines. Therefore, nickel-based superalloys have been developed to withstand these extreme temperatures and loads, especially in the hot sections.Today, the way of operating land-based gas turbines is changing. Instead of running for long periods of time, the operation is becoming more flexible, with everincreasing cyclic loads and number of start and stop cycles. To handle the increased stress and cycles, component resistance to fatigue failure needs to be improved.Surface integrity is critical to fatigue performance, since fatigue cracks are normally initiated at surfaces. Nickel-based superalloys are difficult-to-machine materials, primarily due to their high strength, high tendency for work-hardening, and low thermal conductivity. The machining process changes the surface integrity of the alloys which can result in worse fatigue resistance.The work presented in this Ph.D. thesis was conducted in collaboration with Siemens Industrial Turbomachinery AB in Finspång, Sweden. Surface integrity changes which are induced during machining and their effects on fatigue performance have been studied on alloy Inconel 718. Inconel 718 is a widely-used nickel-based superalloy for high temperature applications in modern gas turbines.Broaching, milling, and wire electrical discharge machining, related to component manufacturing in turbo machinery industries, were included in this study. Surface irregularity and defects induced by machining provide preferential sites for fatigue crack initiation which influence the fatigue performance of the alloy. If compressive residual stresses are induced during machining, they benefit the fatigue life by retarding fatigue crack initiation away from surface regions. Shot peening was performed on machined Inconel 718, by which high compressive residual stresses are deliberately induced. It results in increased fatigue performance. The high iv temperatures in gas turbines generally deteriorate the surface integrity. For instance, recrystallization often occurs in the highly plastically-deformed surface layer. Microstructural degradation, in a form of α-Cr precipitates, have also been observed in the deformed surface and sub-surface microstructure when subjected to thermal exposure. Oxidation at elevated temperatures was found to degrade the surface integrity and thereby also the fatigue performance. Fatigue cracks are preferably initiated at oxidized surface carbides, if thermal exposure has been made prior to the test, and a reduced fatigue life is often obtained. It is even worse when high temperatures relax the beneficial compressive residual stresses induced by shotpeening and thereby lowering the fatigue resistance. This increases the interest for research regarding how to optimize the shot peening process in order to enhance the thermal stability o...

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“…Nonmetallic inclusions are inevitably mixed into the Ni-based superalloy powders during the powder metallurgy (PM) forming process used to make components, such as turbine disks, 26,27 and are often found to be the fatigue crack initiation sites 28,29 and limit fatigue life of PM formed components. Inclusion sizes are carefully controlled by filtering out the large inclusions 30 so that the remaining inclusion sizes are comparable with nickel powder particles, which are approximately the size of several grains.…”

Section: Hr-ebsd Study Of Plasticity In Polycrystals Containing a Secmentioning

confidence: 99%

Toward Predictive Understanding of Fatigue Crack Nucleation in Ni-Based Superalloys

Jiang

Dunne

Britton

2017

JOM

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Predicting when and where materials fail is a holy grail for structural materials engineering. Development of a predictive capability in this domain will optimize the employment of existing materials, as well as rapidly enhance the uptake of new materials, especially in high-risk, high-value applications, such as aeroengines. In this article, we review and outline recent efforts within our research groups that focus on utilizing full-field measurement and calculation of micromechanical deformation in Ni-based superalloys. In paticular, we employ high spatial resolution digital image correlation (HR-DIC) to measure surface strains and a high-angular resolution electron backscatter diffraction technique (HR-EBSD) to measure elastic distortion, and we combine these with crystal plasticity finite element (CPFE) modeling. We target our studies within a system of samples that includes single, oligo, and polycrystals where the boundary conditions, microstructure, and loading configuration are precisely controlled. Coupling of experiment and simulation in this manner enables enhanced understanding of crystal plasticity, as demonstrated with case studies in deformation compatibility; spatial distributions of slip evolution; deformation patterning around microstructural defects; and ultimately development of predictive capability that probes the location of microstructurally sensitive fatigue cracks. We believe that these studies present a careful calibration and validation of our experimental and simulation-based approaches and pave the way toward new understanding of crack formation in engineering alloys.

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The effect of defects on the fatigue crack initiation process in two p/m superalloys: part i. fatigue origins

Cited by 87 publications

References 9 publications

Crystal plasticity and high-resolution electron backscatter diffraction analysis of full-field polycrystal Ni superalloy strains and rotations under thermal loading

Crystal plasticity and high-resolution electron backscatter diffraction analysis of full-field polycrystal Ni superalloy strains and rotations under thermal loading

Surface Integrity and Fatigue Performance of Nickel-based Superalloys

Toward Predictive Understanding of Fatigue Crack Nucleation in Ni-Based Superalloys

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