Creep crack growth behavior of several structural alloys
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I.
INTRODUCTION
THE realization that failure of high temperature components may occur by the propagation of a single crack that nucleates at some stress concentrated region, such as those due to preexisting defects or flaws produced during processing, has contributed to a growing interest in the study of crack propagation behavior in structural alloys in the creep range. Creep crack growth behavior of several superalloys, namely, Alloy 718,1-4 Discaloy, s Udimet 700, 6 Astroloy,2 Ren6 95, 2 Ren6 41, 7 Waspaloy,2 Nimonic 115,8 Nimonic 105, 9 IN 738, 9 IN 100, ~~ as well as advanced directionally solidified eutectic composites such as C73 carbide eutectic 13 and v/v' + 3 eutectic ~4has been studied so far. Results of these studies indicate that several of the above alloys are very susceptible to time-dependent crack growth. Furthermore, in some of these alloys, the contribution from the time-dependent process to crack growth even under cyclic load at high temperatures can be significant particularly at low frequencies or hold times. In spite of the fact that most of the alloys are 3" strengthened nickel base superalloys their crack growth rates are not the same but differ significantly among the alloys, and also even for the same alloy, the growth rates may differ by more than three orders of magnitude when heat treated differently. Thus, the published results indicate that high temperature timedependent crack growth is very sensitive to alloy chemistry and processing conditions, to microstructural variables such as grain size, precipitate morphology and distribution, and to grain boundary constituents in terms of the density and distribution of carbides or brittle phases. In addition, creep crack growth rates are also related to creep properties of the matrix as well as the grain boundaries in terms of creep strength, strain rate, and creep ductility. Superimposed on the creep damage is the environmental degradation which again differs among the alloys. Knowledge of creep crack K. SADANANDA and P. SHAHINIAN are Metallurgists with the Naval Research Laboratory, Washington, DC 20375. Manuscript submitted August 2, 1982.
METALLURGICAL TRANSACTIONS A
growth behavior of high temperature structural alloys is therefore important both to avoid any catastrophic failures in service as well as to develop a better understanding of the interrelation between microstructure and macroscopic growth behavior. In this paper the time-dependent crack growth behavior of the alloys, Inconel 600, Inconel 625, Inconel X-750, HasteUoy X, Incoloy 800, Haynes 25 (HS-25), and Nimonic PE-16 has been reported and the growth rates among the alloys have been compared in terms of the linear elastic stress intensity factor, K. In some of the specimens where it was convenient to measure loadline displacement during the test, crack growth rates were also analyzed in terms of nonlinear elastic-plastic fracture mechanics parameter, J*-integral. For a comprehensive review of the state of the art of the subject in terms of crack growth behavior, theore
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