Hydrogen effects in [001] oriented nickel-base superalloy single crystals
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I.
INTRODUCTION
NICKEL-based superalloy single crystal materials appear to offer improved ambient and high temperature mechanical properties compared to polycrystalline and directionally solidified structures.~ However, it has been suggested that specific single crystal materials can be susceptible to varying degrees of hydrogen embrittlement under a variety of testing conditions in gaseous high pressure hydrogen, 2 an important environment, for example, for space shuttle main engine turbo pumps. A particular alloy, CMSX-2, described below, has shown a sensitivity to hydrogen, an effect which depended on heat treatment; 3 a treatment reported to improve the high temperature creep resistance of this alloy4 also was found to reduce the susceptibility to hydrogen embrittlement for both smooth and notched tensile configurations. 2'3 While such results suggest a significant role of both internally dissolved hydrogen and microstructure on embrittlement sensitivity, evidence of the associated requirement that hydrogen can be dissolved and then transported in significant quantities in nickel-base superalloys, in particular, single crystals is very limited. Latanision and Kurkela found that the room temperature diffusivity dropped below measurable values for highly alloyed materials such as INCOLOY 800, 5 although Robertson6 in a study on INCONEL 718" and INCOLOY 703 was able to measure *INCOLOY and INCONEL are trademarks of the INCO family of companies.
a diffusivity at 150 ~ of near 10-8 c m 2 S -1. In the latter study no microstructural details were given. In an attempt to clarify the role of hydrogen in high y ' alloys, we have investigated some aspects of hydrogen interaction in the single crystal material CMSX-2, which showed microstructure-dependent properties, ,using as the main technique cathodic charging in hydrogenated molten salts at 150 ~ in order to provide a high input concentration of hydrogen. Results were obtained and herein described on the transport kinetics of hydrogen and on its trapping and redistribution in the structure, as well as information on how C.L. BAKER is with Coming Glass Works, Coming, NY; J. CHENE is with the Laboratoire de Metallurgie Structurale, University of ParisSud, Orsay, France; I.M. BERNSTEIN is Provost, Illinois Institute of Technology, Chicago, IL 60616; and J.C. WILLIAMS is Dean of Carnegie Institute of Technology, Carnegie Mellon University, Pittsburgh, PA 15213. Manuscript submitted November 8, 1985. METALLURGICALTRANSACTIONS A
such interactions lead to the degradation of room temperature tensile properties, an effect found to scale with the depth of a hydrogen-rich layer.
II.
EXPERIMENTAL PROCEDURE
Material was obtained from a single melt of the nickelbase superalloy CMSX-2, provided in the form of 1.27 cm diameter x 10 cm length single crystal bars, with orientations within 10 deg of [001], and having the chemical analysis reported in Table I. The material was supplied from the vendor in the as-solution heat treated state of 3 hours at 1315 ~ Tensile samples were t
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