High-resolution electron microscopy of dislocation ribbons in a CMSX-4 superalloy single crystal

Vorontsov, V.A.; Kovařík, Libor; Mills, Michael J.; Rae, C.M.F.

doi:10.1016/j.actamat.2012.05.014

Cited by 116 publications

(60 citation statements)

References 29 publications

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“…This not only concerns rafting regimes, but other deformation modes as well. Recent studies have shown the interface interacting with matrix dislocations, wrapping around them to form characteristic ridges and thus reducing the local elastic strain fields in the crystal [12][13][14][15].…”

Section: Introductionmentioning

confidence: 99%

Coarsening behaviour and interfacial structure of γ′ precipitates in Co-Al-W based superalloys

Vorontsov¹,

Barnard

Rahman³

et al. 2016

Acta Materialia

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This work discusses the effects of alloying on the coarsening behaviour of the L1 2 ordered γ phase and the structure of the γ/γ interfaces in three Co-Al-W base superalloys aged at ∼90 • C below the respective solvus temperatures: Co-7Al-7W, Co10Al-5W-2Ta and Co-7Al-7W-20Ni (at.%). The coarsening kinetics are adequately characterised by the classical Lifshitz-Slyozov-Wagner model for Ostwald ripening. Co-7Al-7W exhibited much slower coarsening than its quaternary derivatives. Alloying can be exploited to modify the coarsening kinetics either by increasing the solvus temperature by adding tantalum, or by adding nickel to shift the rate controlling mechanism towards dependence on the diffusion of aluminium rather than tungsten. Lattice resolution STEM imaging was used to measure the widths of the order-disorder (structural) and Z-contrast (compositional) gradients across the γ/γ interfaces. Similarly to nickel base superalloys, the compositional gradient was found to be wider than the structural. Co-7Al-7W-20Ni had much wider interface gradients than Co-7Al-7W and Co-10Al-5W-2Ta, which suggests that its γ phase stoichiometry is less constrained. A possible correlation between temperature and misfit normalised r vs. t 1 3 coarsening rate coefficients and the structural gradient width has also been identified, whereby alloys with wider interfaces exhibit faster coarsening rates.

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Section: Introductionmentioning

confidence: 99%

Coarsening behaviour and interfacial structure of γ′ precipitates in Co-Al-W based superalloys

Vorontsov¹,

Barnard

Rahman³

et al. 2016

Acta Materialia

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“…At the high strain rate in a tensile test the activated movement of partial dislocations is not possible, and the dislocations cut through the tertiary precipitates as single dislocations producing APB faults which are rectified by subsequent dislocations, i.e. "weak coupling" [11]. In the creep tests above yield the rapid multiplication of dislocations, triggered by the freedom of the dislocations to glide through the precipitate structure, leads to a greatly increased dislocation density and hence the increase in flow strength associated with forest hardening adding to the precipitate hardening of the virgin material.…”

Section: Discussionmentioning

confidence: 99%

Relating fundamental creep mechanisms in Waspaloy to the Wilshire equations

Deen

Whittaker

Harrison

et al. 2014

MATEC Web of Conferences

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Abstract. Creep tests of the polycrystalline nickel alloy Waspaloy have been conducted at Swansea University, for varying stress conditions at 700 • C. Investigation through use of Transmission Electron Microscopy at Cambridge University has examined the dislocation networks formed under these conditions, notably those with stresses above and below the yield stress. This paper highlights how the dislocation structures vary throughout creep and proposes a dislocation mechanism theory for creep in Waspaloy. In particular, the roles of recovery, tertiary gamma prime particles and dislocation foresting are examined, and related back to observations from the Wilshire fits. The virgin (untested) material has been forged and heat treated, containing some recrystallised material together with areas of more heavily deformed and recovered material clustered around the grain boundaries. Observations from tests below the 0.2% proof stress show relatively low dislocation densities away from grain boundaries and dislocation movement can be seen to be governed by interactions with the γ precipitates. In contrast, above the 0.2% proof stress, TEM observations show a substantially greater density of dislocations. The increased density provides an increment of strength through forest hardening. At stresses above the original yield point, determined by the precipitates, the creep rate is controlled by inter-action with the dislocation forest and results in an apparent activation energy change. It is proposed that the activation energy change is related to the stress increment provided by work hardening, as can be observed from Ti, Ni and steel results.

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“…The primary slip plane was identified using Laue X-ray imaging and the test-piece sectioned vertically, parallel to the tensile axis. For high resolution imaging the primary slip plane (-111) was perpendicular to the foil normal [110] and the Burgers vector of the highest Schmid Factor [1][2][3][4][5][6][7][8][9][10][11][12](-111) slip system lies in the plane of the foil. Hence an edge dislocation in this system a/3 [1][2][3][4][5][6][7][8][9][10][11][12] has its line vector parallel to the foil normal and would be in the ideal configuration to image the atom alignment at high resolution.…”

Section: Methodsmentioning

confidence: 99%

“…For high resolution imaging the primary slip plane (-111) was perpendicular to the foil normal [110] and the Burgers vector of the highest Schmid Factor [1][2][3][4][5][6][7][8][9][10][11][12](-111) slip system lies in the plane of the foil. Hence an edge dislocation in this system a/3 [1][2][3][4][5][6][7][8][9][10][11][12] has its line vector parallel to the foil normal and would be in the ideal configuration to image the atom alignment at high resolution. The sample was also cut on the other vertical section normal to [1][2][3][4][5][6][7][8][9][10] and horizontally normal to [001] for imaging the dislocations and stacking faults at lower magnification.…”

Section: Methodsmentioning

confidence: 99%

“…In this paper we examine the microstructure of the alloy CMSX-4 R crept to 8% strain just at the onset of secondary creep. In a previous paper we established the structure of the superlattice partial dislocations, a/3 112 and also the way in which the lattice dislocations are embedded in the γ γ interface [7]. In this paper we look at how dislocations combine and contribute to the creep and the implications this has for creep under these conditions.…”

Section: Introductionmentioning

confidence: 99%

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Dislocations in a Ni-based superalloy during low temperature creep

Rae

Vorontsov

Kovařík

et al. 2014

MATEC Web of Conferences

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Abstract.The nature and variety of the dislocations passing through the two-phase γ γ microstructure of Ni-based superalloys is key to the properties of these materials. The chemistry, size and arrangement of the precipitates greatly affects the nature of these dislocations. We present High Angle Annular Dark Field (HAADF) TEM observations of the structure of dislocations entering, passing through the γ precipitates in the singlecrystal superalloy CMSX-4 R . The creep deformation of the sample was interrupted after 8 hours at 750 • C and 750 MPa, a critical stage just as secondary creep was being established, and shows a range of defects in both phases, not always those predicted by the Schmid factor for the deformation geometry. We show that dislocations lodged in the γ γ interfaces have a significant effect on the structure of the interface and that they combine to produce stacking faults which cut through the γ . The implications of these observations for secondary creep deformation are discussed.CMSX-4 R is a registered Trade Mark of the Cannon Muskegon Company.

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High-resolution electron microscopy of dislocation ribbons in a CMSX-4 superalloy single crystal

Cited by 116 publications

References 29 publications

Coarsening behaviour and interfacial structure of γ′ precipitates in Co-Al-W based superalloys

Coarsening behaviour and interfacial structure of γ′ precipitates in Co-Al-W based superalloys

Relating fundamental creep mechanisms in Waspaloy to the Wilshire equations

Dislocations in a Ni-based superalloy during low temperature creep

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