The aqueous environment assisted cracking (EAC) resistance of a superalloy, Alloy 718 (Ni-19Fe-18Cr-5Nb-1Ti-0.6Al), was characterized by a rising displacement fracture mechanics method. This precipitation-strengthened alloy was susceptible to room-temperature EAC in acidified sodium chloride at cathodic and anodic potentials. The threshold for stable crack growth in chloride (KTH) was as low as 47 MPa√m, reduced from the laboratory air crack initiation toughness (KICi) of 81-85 MPa√m. The fracture morphology changed from ductile microvoids in air to a mixture of voids, transgranular facets, and intergranular facets in acidic chloride. Subcritical crack growth rates were on the order of 5x10-9 m/s for rising displacement at a stress intensity of 70 MPa√m and were an order of magnitude slower for constant displacement conditions. The degree of reduction in KTH from KICi, the amount and type of fracture surface features, and the crack growth rate depended on the applied electrode potential. Microstructure produced by sub- or super-δ solvus heat treatment affected these dependencies. Ion analysis indicated that alloy dissolution occurred at the crack tip even at cathodic polarizations.

Subject
Embrittlement, Acids, Environmental cracking, Crack growth, Materials, Microstructure, Fracture surfaces, Stress cracking, Intergranular cracking, Alloys, Cracks, Electrode potential, Hydrogen
Keywords:
crack chemistry, crack growth rate, electrode potential, fracture mechanics, hydrogen embrittlement, ion analysis, intergranular cracking, stress corrosion cracking, superalloys, transgranular cracking
© 1997 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).
1997
Association for Materials Protection and Performance (AMPP)
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