A nickel-based single crystal superalloy with or without a Re/Al coating was Al-diffusion treated and then oxidized in air at 1373K for up to 2.54Ms under thermal cycling and isothermal conditions. The Al-diffusion treatment, carned out at 1273K for 57.6ks in a mixture of FeAl and Al2O3 powder, resulted in a triplex layer structure in the sequence β -NiAl / Re-rich layer / Al-diffusion zone on the alloy. The Re-rich layer contained 25Ni, 19Al, and 16Cr (at%). When oxidized at 1373K for 460ks in air under thermal cycling, a very protective α -Al2O3 scale formed, while the β -NiAl layer changed to Ni-aluminide (presumably γ’-Ni3Al) containing 21at%Al and the Re-rich layer to a Re-based alloy containing 31Cr, 21Ni, 12Co, 6W, and 3Al (at%) plus Re, which corresponded to σ -phase in the Ni-Cr-Re phase diagram. The protective α-Al2O3 scale and the resulting triplex coating structure of γ’-Ni3Al/σ-phase (Cr-Re-Ni system) / alloy substrate were maintained with little change for up to 2.54Ms under isothermal oxidation. The formation of the triplex structure was discussed by using the diffusion path on the ternary Ni-Cr-Re and binary Ni-Al phase diagrams. In the Al-diffusion and subsequent initial stage of oxidation the Al, Ni, Co and Cr diffused rapidly through the Re/Al coating and the Re-Ni-Al-Cr alloy layer. With further oxidation at 1373K the Re-Cr-Ni (Co) (σ -phase) alloy layer formed between the alloy substrate and γ’-Ni3Al layer and it was found to act as a very effective diffusion barrier to the alloying elements and Al.

Subject
Nickel coatings, Coatings, Composition, Substrates, Diffusion, Nickel based alloys, Structures, Superalloys, Scale, Elements, Aluminum alloys, Barrier coatings, Oxidation
Keywords:
Re-coating, Al-diffusion, diffusion barrier, nickel-based superalloy, high temperature oxidation
© 2001 Association for Materials Protection and Performance (AMPP). All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means (electronic, mechanical, photocopying, recording, or otherwise) without the prior written permission of AMPP. Positions and opinions advanced in this work are those of the author(s) and not necessarily those of AMPP. Responsibility for the content of the work lies solely with the author(s).
2001
Association for Materials Protection and Performance (AMPP)
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