Oxidation of Metals

, Volume 12, Issue 3, pp 257–272 | Cite as

High-temperature oxidation of directionally solidified Ni-Cr-Nb-AI (γ/γ′-δ) eutectic alloys

  • J. Stringer
  • D. M. Johnson
  • D. P. Whittle
Article

Abstract

The oxidation of Ni-23.1Nb-4.4Al and Ni-19.7Nb-6 Cr-2.5Al alloys in air at temperatures in the range 870–1100°C has been studied for times up to 168 hr, in the as-cast, slowly cooled, and directionally solidified forms. The oxidation rate decreases with increasing temperature for the ternary alloy, and this appears to be due to the increasing tendency to establish a continuous Al2O3 layer at the metal surface, although at no temperature in this range is a complete layer established. At the lowest temperature the δ-Ni3Nb lamellae are preferentially oxidized, with fingers of oxide extending into the metal, but at 900°C and above a continuous single-phase 8-free layer is established at the metal surface very early in the oxidation. The oxidation rate of the quaternary alloy increases with increasing temperature. At the lower temperatures a continuous Al2O3 layer is established at the metal surface, but at the highest temperature the aluminum oxidizes internally and a continuous layer is not established, internal oxidation penetrating down the lamellae. It appears that niobium, like chromium, is able to promote the formation of external Al2O3 layers; if this fact is accepted, the beneficial role of chromium in these alloys is difficult to explain.

Key words

eutectic alloy oxidation oxidation mechanisms alumina-formers 

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Copyright information

© Plenum Publishing Corporation 1978

Authors and Affiliations

  • J. Stringer
    • 1
  • D. M. Johnson
    • 2
  • D. P. Whittle
    • 2
  1. 1.Electric Power Research InstitutePalo Alto
  2. 2.Department of Metallurgy and Materials ScienceUniversity of LiverpoolLiverpoolEngland

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