Full length articleHydrogen assisted crack initiation and propagation in a nickel-based superalloy
Graphical abstract
Introduction
Structural materials used in the oil and gas deep wells face extremely aggressive environments, including low pH (H2S), high temperature (>250 °C) and high pressure (>100 MPa) [1], [2]. High strength nickel-based superalloys are promising structural materials for application in these challenging conditions, because of their capability to operate at the high temperature and high pressure conditions providing outstanding corrosion resistance. One longstanding challenge, however, is the hydrogen-induced embrittlement when these alloys are exposed to hydrogenating conditions such as the sour environments of deep wells: adsorption of hydrogen leads to changes in the fracture mode of nickel-based superalloys from ductile to brittle [3], [4], [5], [6].
A number of mechanisms have been proposed to account for the hydrogen embrittlement in various materials under different hydrogen charging conditions [7], [8], [9], [10], [11], [12], [13], [14]. For the metallic materials that do not form hydrides, such as Fe and Ni, the most commonly invoked embrittlement mechanisms are hydrogen-enhanced decohesion (HEDE) [12], [13] and hydrogen-enhanced localised plasticity (HELP) [8], [9]. According to the HEDE mechanism, the hydrogen can reduce the cohesive strength of the atomic bonding. Consequently, grain boundaries and precipitate/matrix interfaces that accumulate hydrogen beyond a critical concentration will fracture when the material is stressed. The HEDE mechanism was raised to interpret the load relaxation and intergranular failure caused by hydrogen. Although atomic scale simulations [15], [16], [17] support it, the lack of direct experimental evidence places some doubt on the relevance of this mechanism. The HELP mechanism was first suggested by Beachem [8], and was then underpinned by in-situ transmission electron microscopy (TEM) analysis, where the mobility of dislocations was evidently enhanced by the presence of hydrogen [9]. The theoretical basis for the HELP mechanism was rationalized by the hydrogen shielding effect on dislocations [18], [19]. In this framework, the dislocation slip mode and dislocation-dislocation interaction are modified and specifically the equilibrium distance between dislocations is decreased [20], [21]. Furthermore, dislocation slip planarity is promoted due to the dragging of hydrogen [19]. Accordingly, extensive dislocation slip is expected, and dislocation motion and dislocation-dislocation interaction are largely constrained to the initial slip plane due to limited cross slip in the presence of hydrogen.
Phenomenologically, hydrogen embrittlement in metallic materials is generally characterised by intergranular cracking with quasi-cleavage fracture surface. Experimental studies have demonstrated that extensive dislocation activities and plasticity are associated with hydrogen embrittlement [11], [22], [23], [24], [25], [26], [27]. Martin et al. studied the microstructure beneath the quasi-cleavage fracture surface of a ferritic steel [22], [23] and pure Ni [24], and found a good correlation between the features on the fracture surface and the underneath dislocation substructure, which indicates the importance of dislocation activities on the hydrogen embrittlement. More importantly, they flagged up that the dislocation substructure in the hydrogen containing material is finer than in the one without hydrogen [24], [27], [28], which provides further support for the HELP mechanism. Besides the HEDE and HELP mechanisms, the hydrogen enhanced vacancy stabilization mechanism has also been proposed [10], [11], [29]. In this mechanism, hydrogen is supposed to stabilize vacancies generated by the dislocation activities and promote the formation of voids by the vacancy agglomeration [29]. Although the central aspect of this mechanism is different from that of the HELP mechanism, conceptually, in both cases hydrogen induced fracture originates from, and is promoted by, dislocation activities and therefore the crack nucleation is associated with the regions of extensive dislocation slip activities.
To date, most studies dedicated to understand the mechanism of hydrogen embrittlement of non-hydride forming metallic materials have been conducted on pure nickel [9], [24], [30] and some steels [22], [25], [26], [27], [31], [32], [33]. For nickel-based superalloys, investigations have mainly focused on the fracture mode and the effect of different heat treatments on mechanical properties in regard to hydrogen embrittlement [3], [4], [5], [34], [35], [36], [37]. Since in precipitation-hardened nickel-based superalloys the interfaces between the precipitates (γʹ, γʺ, δ and carbides) and γ matrix are often assumed to be the trapping sites for hydrogen, studies have focused on modifying the size and volume fraction of the various precipitates by specific heat treatments and compare hydrogen embrittlement susceptibility of the resulting microstructures [4], [5], [36], [37]. Even though the activity of dislocations is demonstrated to be of crucial importance for hydrogen embrittlement in several materials, to the authors’ knowledge, there is no work carried out to clarify the mechanism on this aspect in nickel-based superalloys.
In the present study, hydrogen-induced embrittlement was studied for the precipitation-hardened nickel-based superalloy UNS N07718 (Alloy 718), which is a widely used structural material for deep well application in the oil and gas industry. The correlation between the microstructural features, especially the dislocation substructure developed during straining, and the crack initiation and propagation was explored in detail by extensive Scanning Electron Microscopy (SEM), high resolution Electron Channeling Contrast Imaging (ECCI), Electron Backscattered Diffraction (EBSD) and TEM characterisations. The observations are discussed in context of previous studies related to hydrogen-induced embrittlement and a mechanistic framework is proposed for precipitation-hardened nickel-based superalloys involving dislocation slip localisation in conjunction with hydrogen promoting vacancy production.
Section snippets
Experimental
Cylindrical blanks with a diameter of 8 mm and a length of 60 mm were machined from the Alloy 718 ingot (commercially heat-treated to aerospace specifications) and solution treated at 1040 °C for 1 h followed by the furnace cooling (cooling rate is approximately 15 °C/min). The material was subsequently aged at 774 °C for 6 h followed by cooling in air. To minimise the surface oxidation, all heat treatments were carried out in an argon atmosphere. The purpose of this heat treatment was to
Results
The initial microstructure of Alloy 718 after the heat treatment described above is shown in Fig. 1 . The grain morphology, which was mapped out by EBSD, indicates an average grain size of about 62 μm with a ∑3 twin boundary (TB) fraction of 44% (see Fig. 1a). A dark field TEM image with the incident electron beam along the 〈100〉γ zone axis shows the needle-shaped γ˝ with a mean length of 23.5 nm (see Fig. 1b).
The stress-strain curves of the material, following the SSR tests with and without
Localised dislocation slip and crack initiation
The evidence presented here clearly indicates that crack initiation and propagation are closely associated with planar dislocation slip bands formed during tensioning of Alloy 718 in the presence of hydrogen. These dislocation boundaries are well aligned with the specific dislocation slip planes, i.e. the {111} crystallographic planes of γ. It is well established that in γ′/γ′′ strengthened nickel-based superalloys, the shearing of these precipitates by weakly or strongly coupled dislocation
Conclusions
Hydrogen induced embrittlement in hydrogen charged Alloy 718 slowly strained to failure was investigated by means of detailed electron microscopy in order to develop a better mechanistic understanding of crack initiation and propagation. Observations within the hydrogen charged region were compared with the non-hydrogen charged region. The main findings can be summarized as follows:
- 1)
Hydrogen charging results in significant reduction in the tensile ductility of Alloy 718, with quasi-cleavage
Acknowledgement
The authors would like to acknowledge the funding and technical support from BP through the BP International Centre for Advanced Materials (BP-ICAM), which made this research possible. We would like to thank Dr Viviane Smith and Dr John Martin for extensive discussions and Dr Octav Ciuca for the initial training and help on the FEI Magellan microscope.
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