Hydrogen permeation behavior in IN718 and GH761 superalloys
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INTRODUCTION
DuE to good ductility and excellent strength within a wide range of temperatures, the precipitationstrengthened nickel-iron-base superalloys are widely used in many fields such as jet engines, rocket motors, possibly the first wall of fusion nuclear reactors, petrochemical plants, e t c . For these applications, when the materials used are selected, satisfactory mechanical properties are necessary, and the influences of hydrogen on properties and hydrogen transport must be considered because of the presence of hydrogen in service environments. It is well known that one of the reasons why hydrogen embrittlement occurs in some alloys is the high diffusivity of hydrogen in metals. Clearly, the susceptibility of alloy-to-hydrogen embrittlement correlates with the ingress ability and transport process for hydrogen in alloys. Therefore, when clarifying the factors that would affect the hydrogen permeation and diffusion, it is important to understand the mechanism of hydrogen embritttement of materials. Although the hydrogen permeation data for some superalloys have been reported in previous work, 1~-6l few studies on the dependencies of hydrogen permeation behavior on microstructural features in superalloys have been carried out to date. INCONEL718* (IN718), a typical nickel-iron-base *INCONEL is a trademark of Inco Alloys International, Inc., Huntington, WV.
alloy, is one of the most used superalloys. Some hydrogen embrittlement problems for this alloy have been extensively studied. 17'8j However, it appears that the relationship between hydrogen permeation behavior and microstructure in this alloy has not been examined in detail. Recently, Turnbull e t al. 19l studied the permeation of hydrogen in several Ni-Cr-Fe-base alloys, including IN718, as a function of heat treatment using electrochemical permeation techniques and revealed that the J. XU, Assistant Professor, and X.K. SUN, Associate Professor, are with the State Key Laboratory of RSA. Q.Q. LIU, Research Assistant, and W.X. CHEN, Professor, are with the Institute of Metal Research, Academia Sinica, Shenyang 110015, China. Manuscript submitted April 8, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
precipitation phases 3'" and 6 showed hydrogen trapping effects, but the measurement in their study was conducted only at 80 ~ GH761 is another nickel-ironchromium-base superalloy developed by Chinese researchers, t~~ It has been found that in this alloy, the addition of a small amount of boron and misch metal improved its tensile ductility as a result of grainboundary strengthening. However, the understanding of its hydrogen performance is absent presently. Moreover, the strengthening phase in these two superalloys is somewhat different. In the former, the strengthening contribution mainly comes from the precipitation of the 3'" phase (tetragonal D022 structure), while in the latter, the primary strengthening phase is only 3" precipitates (cubic L12 structure). Thus, comparison of the hydrogen permeation behavior between these two alloys is also
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