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

V.A. Vorontsov, J.S. Barnard, K.M. Rahman, H. -Y. Yan, P.A. Midgley, D. Dye

Research output: Contribution to journalArticle

17 Citations (Scopus)

Abstract

This work discusses the effects of alloying on the coarsening behaviour of the L12 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, Co-10Al-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. t1/3 coarsening rate coefficients and the structural gradient width has also been identified, whereby alloys with wider interfaces exhibit faster coarsening rates.

LanguageEnglish
Pages14-23
Number of pages10
JournalActa Materialia
Volume120
Early online date29 Aug 2016
DOIs
Publication statusPublished - 30 Nov 2016

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Coarsening
Superalloys
Precipitates
Nickel
Alloying
Tantalum
Ostwald ripening
Tungsten
Kinetics
Order disorder transitions
Aluminum
Stoichiometry
Temperature
Derivatives
Imaging techniques

Keywords

  • coarsening kinetics
  • cobalt-base superalloys
  • EELS
  • HRTEM
  • interface structure

Cite this

Vorontsov, V.A. ; Barnard, J.S. ; Rahman, K.M. ; Yan, H. -Y. ; Midgley, P.A. ; Dye, D. / Coarsening behaviour and interfacial structure of γ precipitates in Co-Al-W based superalloys. In: Acta Materialia. 2016 ; Vol. 120. pp. 14-23.
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Coarsening behaviour and interfacial structure of γ precipitates in Co-Al-W based superalloys. / Vorontsov, V.A.; Barnard, J.S.; Rahman, K.M.; Yan, H. -Y.; Midgley, P.A.; Dye, D.

In: Acta Materialia, Vol. 120, 30.11.2016, p. 14-23.

Research output: Contribution to journalArticle

TY - JOUR

T1 - Coarsening behaviour and interfacial structure of γ′ precipitates in Co-Al-W based superalloys

AU - Vorontsov, V.A.

AU - Barnard, J.S.

AU - Rahman, K.M.

AU - Yan, H. -Y.

AU - Midgley, P.A.

AU - Dye, D.

PY - 2016/11/30

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N2 - This work discusses the effects of alloying on the coarsening behaviour of the L12 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, Co-10Al-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. t1/3 coarsening rate coefficients and the structural gradient width has also been identified, whereby alloys with wider interfaces exhibit faster coarsening rates.

AB - This work discusses the effects of alloying on the coarsening behaviour of the L12 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, Co-10Al-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. t1/3 coarsening rate coefficients and the structural gradient width has also been identified, whereby alloys with wider interfaces exhibit faster coarsening rates.

KW - coarsening kinetics

KW - cobalt-base superalloys

KW - EELS

KW - HRTEM

KW - interface structure

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UR - https://www.sciencedirect.com/journal/acta-materialia

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DO - 10.1016/j.actamat.2016.08.023

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SP - 14

EP - 23

JO - Acta Materialia

T2 - Acta Materialia

JF - Acta Materialia

SN - 1359-6454

ER -