Hydride formation in a Ni-base superalloy
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INTRODUCTION S O L U T I O N - s t r e n g t h e n e d Ni-base superalloys have good resistance to Hydrogen Embrittlement (HE) and Stress-Corrosion Cracking (SCC). When the alloys undergo additional strengthening by cold-working and aging, they m a y fail 1-4 in extremely short times by H E in aqueous solutions containing C1 and S ions at temperatures near 300 ~ Two hypotheses have been advanced to account for the marked embrittlement of the alloys after coldworking or cold-working plus aging. Berkowitz and Kane, 3 following the ideas of Yoshino and McMahon, 5 suggest that thermomechanical processing promotes the segregation of certain impurities to grain boundaries. Once there, the impurites interact with H and cause intergranular failure at low applied stress. In contrast, AsphahanP postulates that slip mode, rather than grain boundary segregation, controls embrittlement. He suggests that because these alloys order during aging, slip mode changes from wavy to planar, so that embrittlement is accelerated in the manner described by Bernstein and T h o m p s o n . 6 A third possible mechanism of H E in these alloys is hydride formation. This mechanism, as reviewed by Birnbaum, 7 has been identified as being responsible for embrittlement in a variety of materials. Although hydrides are formed in Ni, 8-9 Ni-base alloys 1~ and high Ni austenitic stainless steels ~z-14 at high H fugacities, this mechanism has not been proposed to account for H E in superalloys, possibly because of the high fugacities required. Pickering 15 has shown however that, even in anodically polarized samples, local cathodic conditions and high H fugacities exist at the tips of microcracks and surface flaws. It is ELLINA LUNARSKA-BOROWIECKA,formerly a Senior Postdoctoral Fellow, Department of Metallurgical Engineering & Materials Science at Notre Dame, is now with the Institute for Physical Chemistry, Warsaw, Poland. NICHOLAS F. FIORE is Professor and Chairman, Department of Metallurgical Engineering & Materials Science, University of Notre Dame, Notre Dame, IN 46556. Manuscript submitted February 25, 1980.
possible therefore that hydrides might exist in the Nibase alloys at sufficiently high H fugacities and be responsible for embrittlement. It is the purpose of this study to investigate whether or not hydrides f o r m in a Ni-base superalloy ( H A S T E L L O Y Alloy C-276), which has been shown to undergo severe H E after cold-working and aging. Optical- and scanning-electron metallography (SEM), microhardness measurements, and X-ray diffraction measurements have been employed in this study of hydride-formation tendency during cathodic charging at high H fugacities. EXPERIMENTAL The nominal composition of Alloy C-276 is listed in Table I. Samples of the alloy were received in the form of 2 mm-thick strip which was 50 pct coldworked (cw). The cw strip was sheared into 1.5 cm squares. A number of the squares were aged at 500 ~ for 100 h (cw + a), and an additional n u m b e r were annealed at 1124 ~ in Argon for 10 min (ann). Hydride-formation
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