Elsevier

Acta Materialia

Volume 60, Issues 6–7, April 2012, Pages 2840-2854
Acta Materialia

Microstructural extremes and the transition from fatigue crack initiation to small crack growth in a polycrystalline nickel-base superalloy

https://doi.org/10.1016/j.actamat.2012.01.049Get rights and content

Abstract

The fatigue behavior of the nickel-base superalloy René 88 DT has been investigated at room temperature with fully reversed loading in an ultrasonic fatigue apparatus operating at a frequency close to 20 kHz. A characterization protocol based on the electron backscatter diffraction technique has been developed to identify the combination of microstructural features within crack initiation sites and surrounding neighborhoods that leads to the transition from initiation to early small crack growth. Surface grains that were more than three times the average grain size, that were favorably oriented for cyclic slip localization and that also contained Σ3 twin boundaries inclined to the loading axis were most favorable for fatigue crack initiation. Fatigue cracks subsequently grew in grain clusters within which grains are misoriented by less than 20° relative to the initiation grains. More highly misoriented neighboring grains resulted in crack arrest. The material characteristics that promote crack initiation and small crack growth exist only at the extreme tails of the microstructural distributions. The implications for modeling of fatigue life and fatigue life variability are discussed.

Introduction

In metallic materials with a limited content of metallurgical defects, cyclic damage and fatigue crack initiation occur primarily due to the accumulation of irreversibly slipping dislocations [1]. The morphology of the dislocation substructure that develops during cyclic strain localization may take on different forms, depending on the microstructure of fatigued material and testing conditions [2]. In pure metals, e.g. copper, cyclic deformation normally leads to the formation of well-known persistent slip bands (PSBs) [3]. Cyclic strain localization within PSBs roughens the specimen surface and may act as fatigue crack initiation sites [3], [4]. PSBs can also impinge on grain boundaries (GBs), and the PSB–GB interaction may also result in fatigue crack initiation [5], [6].

Most engineering alloys have a complex microstructure, with microstructural heterogeneities such as inhomogeneously distributed precipitates, a distribution of grain size and microtextures, and varying grain boundary character. Under cyclic loading, microstructure inhomogeneities may act as either local stress concentrators or sources of dislocations, contributing to cyclic strain localization. In the high cycle or very high cycle regime, the loading stresses are far below the nominal yield stress. Under this circumstance, only a small fraction of grains within a typical test volume have favorable conditions for cyclic plastic deformation [7]. It can thus be expected that microstructural extremes play a significant role in fatigue crack initiation in these high cycle or very high cycle regimes. In this paper, we define fatigue crack initiation as the formation of a crack in a single grain by intense cyclic strain localization, where grain size is defined using a grain tolerance angle of 5°. This means that some cracks will not propagate beyond the initiating grains, while other may propagate through several grains until they encounter a grain boundary with a high misorientation angle (>20°). This definition allows us to differentiate the microstructural contributions to crack initiation and to small crack growth.

Polycrystalline nickel-base superalloys are a group of engineering alloys with complex microstructures that possess excellent elevated temperature properties [8]. Cyclic deformation and the accumulation of fatigue damage in these alloys depend both on deformation modes operating under testing conditions and on microstructure [9], [10]. At low temperature, precipitate shearing is the dominant deformation mode in nickel-base superalloys since thermally activated dislocation processes are inhibited at low temperature. When planar slip dominates at low temperature, dislocations are restricted to {1 1 1} slip planes, leading to the formation of slip bands with a high density of dislocations [11], [12], [13], [14], [15]. Besides precipitate structure, a variety of other microstructural features contribute to fatigue damage in nickel-base superalloys [16], [17], [18], [19], [20], [21], [22], [23], [24]. A better understanding of the role of microstructure in the development of fatigue damage in nickel-base superalloys is therefore crucial for the prediction of fatigue life and fatigue life variability [25].

In our previous study [24], the critical microstructure features associated with subsurface crystallographic fatigue crack initiation in the polycrystalline nickel-base superalloy René 88 DT at elevated temperature were quantitatively examined. Grain sizes at the high end of the size distribution, grain orientation and the presence of twins were associated with the crack initiation process. However, at elevated temperature, surface oxidation prevents direct observation of cyclic strain localization processes and fatigue crack initiation at specimen surfaces. As a result, examination of subsurface fatigue crack initiation processes at elevated temperature requires extensive metallographic serial sectioning. The present study investigates the effects of microstructure, especially the role of “microstructural neighborhoods”, on cyclic plastic deformation, fatigue crack initiation and early small crack propagation in René 88 DT at room temperature. At room temperature, without the effect of oxidation, cyclic deformation processes and crack initiation at the specimen surface can be examined directly. Electron backscatter diffraction (EBSD) can also be employed to collect microstructural information, including crystallography, grain size, grain boundary character, and size and orientation of neighboring grains, for large collections of grains in the vicinity of surface crack initiation sites. In this paper, the probability of encountering critical combinations of microstructural features that result in crack initiation and early small crack growth is analyzed and the implications for fatigue life prediction are discussed.

Section snippets

Microstructure characterization

The material used in this study is the polycrystalline nickel-base superalloy René 88 DT, prepared by advanced powder metallurgy techniques [26]. The nominal composition of René 88 DT is: Ni–16Cr–13Co–4Mo–4W–3.7Ti–2.1Nb–2.1Al–0.7Nb–0.03Zr–0.03C–0.015B (wt.%). The microstructure consists of two major phases: γ grains with a face-centered cubic crystal structure and γ′ strengthening precipitates with an L12 crystal structure. Fig. 1a shows a large inverse pole figure map of γ grains in René 88

Fatigue properties

The dependence of fatigue life on alternating stress amplitude for the specimens tested in this study is shown in Fig. 4. In the present study fatigue life is determined by the initiation and propagation of a dominant crack to a length sufficient to cause loss of resonance in the ultrasonic fatigue test. Cyclic stress amplitudes were in the range of 460–720 MPa, which are well below the 1200 MPa yield strength of this alloy at room temperature [21]. All fatigue specimens failed at life times

The role of grain clusters in fatigue cracking of René 88 DT

The process of fatigue crack initiation and early propagation in René 88 DT is illustrated in Fig. 13. Fatigue cracks are formed by cyclic strain location in the region close to favorably oriented Σ3 twin boundaries in surface large grains. Previous studies indicate that elastic incompatibility stresses in the vicinity of twin boundaries play an important role in cyclic strain localization and fatigue crack initiation [24], [31], [32], [33], [34]. After fatigue crack initiation, fatigue cracks

Conclusions

The fatigue behavior of a polycrystalline nickel-base superalloy René 88 DT was investigated under fully reversed loading at room temperature using ultrasonic fatigue techniques in high cycle regimes. Under current testing conditions, all fatigue failures initiated from specimen surfaces.

Regions close to favorably oriented Σ3 twin boundaries in surface grains with size larger than three times the average grain size are dominant sites for cyclic strain localization and fatigue crack initiation.

Acknowledgements

This work was supported by the Air Force Office of Scientific Research (F49620-03-1-0069, Dr. J. Tiley, Program Manager), DARPA (Dr. L. Christoudolou, Program Manager) and Rackham Predoctoral Fellowship Program.

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