Rafting in single crystal nickel-base superalloys - An overview
01 Feb 2003-Sadhana-academy Proceedings in Engineering Sciences (Springer India)-Vol. 28, Iss: 1, pp 115-128
TL;DR: In this article, the effect of stress state on γ/γ′ rafting, kinetics and morphological evolution are discussed with the recent experimental results, and the nature and influence of rafting on creep properties including pre-rafted conditions are discussed.
Abstract: Currently nickel-base single crystal (SX) superalloys are considered for the manufacture of critical components such as turbine blades, vanes etc., for aircraft engines as well as land-based power generation applications. Microstructure and high temperature mechanical properties are the major factors controlling the performance of SX superalloys. Rafting is an important phenomenon in these alloys which occurs during high temperature creep. It is essential to understand the rafting mechanism, and its characteristics on high temperature properties before considering the advanced applications. In this review article, the thermodynamic driving force for rafting with and without stress is explained. The nature and influence of rafting on creep properties including pre-rafted conditions are discussed. In addition, the effect of stress state on γ/γ′ rafting, kinetics and morphological evolution are discussed with the recent experimental results.
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15 Mar 2007-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, microstructural degradation in the CMSX-4 single crystal superalloy during creep deformation at 1150°C and 100MPa is studied. And it is shown that the high strain rates and shear stresses associated with the rupture process are sufficient to cause realignment of the rafted γ′ structure with respect to the γ matrix.
Abstract: Microstructural degradation in the CMSX-4 single crystal superalloy during creep deformation at 1150 °C and 100 MPa is studied. Tensile deformation in the 〈0 0 1〉 direction is considered due to its technological importance. Under these conditions, rafting of the γ′ structure is completed quickly and within the first 10 h. It is demonstrated that the creep rupture event is highly localised, the instability being associated with a critical and well-defined strain being reached with failure occurring within a further few tens of hours. It is shown that the high strain rates and shear stresses associated with the rupture process are sufficient to cause realignment of the rafted γ′ structure with respect to the γ matrix. Creep cavitation damage near to the rupture surface is prevalent, at microporosity inherited from the casting process but more significantly, at topologically close-packed (TCP) phases and associated pores and voids formed in their vicinity which have formed via vacancy condensation. Hot isostatic pressing (HIPing) prior to creep testing reduces the incidence of casting microporosity, but the creep rupture life is not improved significantly. It is suggested that it is the formation of TCP phases which limits creep rupture life.
144 citations
TL;DR: In this paper, a detailed characterization of interfacial dislocations in a DD6 superalloy after 12h at high temperature and low stress was carried out using transmission electron microscopy and high-angle annular dark field scanning transmission electron microscope techniques.
96 citations
TL;DR: In this article, a phase field model of an elastically inhomogeneous alloy was used to calculate elastic stress and strain fields for two-phase microstructures under a uniaxial applied stress.
84 citations
Cites background from "Rafting in single crystal nickel-ba..."
...The preferential coarsening of (dilatationally) misfitting precipitates in a direction parallel (P -type) or perpendicular (N -type) to an applied stress is known as rafting [5,9]: see [10] and [11] for reviews....
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TL;DR: A NASAIR-100 superalloy single crystal was tested in tension creep at 1000°C at a stress of 148 MPa, for a time period of 20h and to a strain of 1.1% as mentioned in this paper.
83 citations
TL;DR: In this paper, the authors used atomistic simulation samples generated from atom probe tomography data of a single crystal superalloy to study the interactions of matrix dislocations with a γ ′ precipitate in molecular dynamics simulations.
78 citations
References
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TL;DR: In this paper, the authors studied the effect of dislocation-free nickel base superalloy single crystals with high volume fractions of the γ′ phase on their deformation and found that the dislocation free precipitates are resistant to shearing by dislocations.
Abstract: Creep deformation in 〈001〉 oriented nickel base superalloy single crystals has been studied in an effort to assess the factors which contribute to the overall creep resistance of superalloys with high volume fractions of γ′ phase. Detailed observations of three dimensional dislocation arrangements produced by creep have been made with the use of stereo electron microscopy. In the temperature range of 800–900°C at stresses of 552 MPa or lower, the dislocation-free γ′ precipitates are resistant to shearing by dislocations. As a result, creep deformation occurs by forced bowing of dislocations through the narrow γ matrix channels on {111} planes. At moderate levels of temperature and stress there are incubation periods in virgin crystals prior to the onset of primary creep. The incubations arise because of the difficult process of filling the initially dislocation starved material with creep dislocations from widely spaced sources. When the newly generated dislocations percolate through the cross section, incubation comes to an end and primary creep begins. In primary creep neither work hardening nor any type of recovery plays an important role. The creep rate decelerates because the favorable initial thermal misfit stresses between γ and γ′ phases are relieved by creep flow. Continued creep leads to a build-up of a three-dimensional nodal network of dislocations. This three-dimensional network fills the γ matrix channels during steady state creep and achieves a quasi-stationary structure in time. In situ annealing experiments show that static recovery is ineffective at causing rearrangements in the three-dimensional network at temperatures of 850°C or lower. The kinematical dislocation replacement processes which maintain the quasi-stationary dislocation network structures during apparent steady state creep are not understood and require further study. Because of the impenetrability of the γ′ precipitates, dislocations move through the γ matrix by forced Orowan bowing, and this accounts for a major component of the creep resistance. In addition, the frictional constraint of the coherent or semi-coherent precipitates leads to the build-up of pressure gradients in the microstructure, and this provides load carrying capacity. There is also a smaller component of solid solution strengthening. Work hardening is comparatively unimportant. Finite element analysis shows that the non-deforming precipitates are increasingly stressed as creep deformation accumulates in the matrix. In the later stages of steady state creep and during tertiary creep the stresses in the precipitates rise to high enough levels to cause shearing of the γ′ particles by dislocations entering from the γ matrix. The recovery resistance of the material is in part due to a very low effective diffusion constant and in another part due to the fact that the three-dimensional dislocation networks formed in the γ matrix serve to neutralize the misfit between the γ and γ′ phases.
782 citations
"Rafting in single crystal nickel-ba..." refers background in this paper
...…(Swalin 1967; Tien & Copley 1971; Pineau 1976; Nathalet al1982; Shewmon 1989; Henderson & Mclean 1983; MacKay & Ebert 1983; Chang & Allen 1991; Pollock & Argon 1992; Svetlovet al 1992; Labergeet al1997; Ohashiet al1997 ; Pariset al1997; Nabarro 1997; Chen & Immarigeon 1998; Kamarajet al1998;…...
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...Some recent experimental studies have shown opposite effects and demonstrated that the directional coarsening is a softening process which deteriorates high temperature mechanical properties (Henderson & Mclean1983; MacKay & Ebert 1983; Pollock & Argon 1992; Mughrabiet al 1994)....
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TL;DR: In this article, the diffusional processes responsible for rafting were investigated on the local scale of the precipitates, with preferential local dissolution of the γ′ and diffusional flow of alloying elements around the periphery of precipitates.
Abstract: Directional coarsening has been investigated experimentally in two nickel-base single crystal alloys with high volume fractions of precipitates. The initial elastic stresses due to the positive or negative misfit of the coherent precipitates biased the orientation of the precipitates developed during coarsening. However, directional coarsening of the precipitates did not occur until some limited amount of creep deformation was initiated. With the application of an external stress, dislocations penetrated preferentially into the most highly stressed matrix channels and directional coarsening occurred by coalescence of the γ′ precipitates in the plane of the initially less highly stressed channels, where the misfit stresses were not altered by the presence of misfit relieving dislocations. The diffusional processes responsible for rafting apparently operate on the local scale of the precipitates, with preferential local dissolution of the γ′ and diffusional flow of alloying elements around the periphery of the precipitates.
447 citations
"Rafting in single crystal nickel-ba..." refers background in this paper
...…misfits (d) and elastic constants betweenγ /γ ′ phases (Pineau 1976; Chang & Allen 1991; Svetlovet al 1992; Ignatet al 1993; Socrate & Parks 1993; Pollock & Argon 1994; Nabarro 1996, 1997; Nabarroet al 1996; Svoboda & Lukǎs 1996; V́eronet al1996; F̈ahrmannet al1996; Laberge t al1997;…...
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