Crack growth in a single crystal superalloy at elevated temperature
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I.
INTRODUCTION
AT elevated temperature the characteristics of fatigue crack propagation have been widely reported to be sensitive to the frequency of cycling and to the environment. In polycrystalline material transgranular crack propagation may occur at high frequencies, but decreases in the frequency may give rise to intergranular crack growth and an associated increase in the crack propagation rate. It has been considered that this behavior may arise from creep deformation giving localized grain boundary sliding and cavitation ~'2 or to the relaxation of crack tip stresses.3 Alternatively, environmental effects such as the dissolution of material 4 or the diffusion of embrittling species along grain boundaries s have been proposed. Superalloy materials may also exhibit a faceted mode of failure in which the facets show features characteristic of cleavage fracture. In smooth specimens Gell and Leverant6 have observed faceting over a wide range of temperatures and identified the plane of the facets as {111}. Facets formed during crack growth at elevated temperature have consequently been associated with failure on the slip plane. 7 More recently, Sadananda and Shahinian ~ investigated the crack growth behavior and faceting of Udimet 700 over a range of temperatures. The increased crack growth rates at elevated temperature were attributed to the decreased incidence of secondary cracking. Faceted crack growth was observed under all testing conditions and the plane of the facets identified as {100}. Crack growth in this manner has been considered to take place by the nucleation of microcracks on the cleavage plane ahead of the crack tip followed by subsequent linkage with the crack. 9 Table I. Co 10
Cr 8.8
A1 5.5
Ti 1.4
W 9.8
II.
EXPERIMENTAL T E C H N I Q U E S
The material used in the present work was single crystal Mar M 002, the composition of which is given in Table I. Single crystal blocks produced by unidirectional solidification were heat treated for 1 hour at 1100 ~ followed by 16 hours at 860 ~ This resulted in a microstructure of approximately 60 vol pct of cuboidal 3/' of 0.5 /zm edge and a small volume fraction of spherical 3" of approximately 15 nm radius. Approximately 6 vol pct of 3~-3/eutectic and a number of interdendritically located script carbides were also present (Figure 1). Center cracked tension specimens (Figure 2) were machined from single crystal blocks to have known orientations. The macroscopic crack propagation direction was principally [100], and several specimens with crack propagation directions of [110] and [210] were also investigated. The specimens were fatig___ueprecracked at room temperature with AK = 19.8 MPaVm, R = 0.1, and v = 10 Hz. Fatigue crack propagation tests were performed on a servo-hydraulic machine using triangular loading waveforms over a range of frequencies. Crack growth rates were measured in air at room temperature, 600 ~ and 850 ~ with R values of 0.1 and 0.25. Elevated temperatures were maintained by a furnace stable to • 1 K.
Alloy Composition (Wt
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