Initiation and growth of small fatigue cracks in a Ni-base superalloy
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I.
INTRODUCTION
A nickel-base superalloy (trademarked as INCOLOY* 908) has been developed recently tll for use *INCOLOY is a trademark of INCO Alloys International, Inc., Huntington, WV.
as the conduit material in "cable-in-conduit" superconductors for high-field superconducting magnets. 12,31 The conduit is a thin-walled tube that encloses superconducting cable. It serves both as a path for forced flow of the liquid helium coolant and as a distributed structural element. Because the conduit may be as thin as 3 to 12 mm and is often subject to cyclic Lorenz forces resulting from cyclic magnetic fields, its fatigue resistance in the small-crack regime at liquid helium temperature is an important issue in reliable design. Recent studies on the propagation of small fatigue cracks t4,5,61 show that the constitutive relations that govern large cracks become invalid when the crack length is very small. Small cracks often grow faster than large cracks at the same cyclic stress intensity and may propagate at cyclic stress intensities below the threshold value for large-crack growth. Several explanations for the anomalous growth of small fatigue cracks have been proposed in recent r e v i e w s . [7'8'91 These include: (1) reduction in the extent of crack closure when the crack is very short; (2) failure of the small-scale yield assumption when the crack size becomes comparable to the scale of local plasticity; (3) microstructural effects on the growth Z. MEI, formerly with Lawrence Berkeley Laboratory, is Development Engineer with the Hewlett-Packard Company, Palo Alto, CA 94304. C.R. KRENN, Graduate Student Research Assistant, and J.W. MORRIS, Jr., Professor of Metallurgy, are with the Center for Advanced Materials, Lawrence Berkeley Laboratory, Berkeley, CA 94304 and the Department of Materials Science and Mineral Engineering, the University of California, Berkeley, CA 94720. Manuscript submitted September 13, 1993. METALLURGICALAND MATERIALSTRANSACTIONSA
of cracks whose length is comparable to the size of relevant microstructural features; and (4) accelerated chemical attack due to the relative ease of access to the short crack tips. While mechanism (4) does not apply to high-Cr steels in liquid helium, the first three mechanisms may be important. To explore the rate and mechanisms of short-crack growth in INCOLOY 908, we used the approach taken in prior work on A286 steel, t'~ Three tests were used: (1) conventional large-crack growth rate tests at constant load ratio (R); (2) large-crack growth tests at constant maximum stress intensity (Kmax), which is the method proposed by Herman et al. 11~1 to simulate short crack growth by measuring crack growth rates under closurefree conditions; and (3) direct observations of the propagation of very small surface cracks. Because of the difficulty of testing at 4.2 K, the mechanisms of smallcrack propagation were determined at 298 and 77 K. The results establish the mechanisms that should be important at 4.2 K and suggest a method for estimating fatigue-safe design limits at
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