Auger electron spectroscopy study of grain boundary segregation in alloy K-500: Part I. Behavior in as-processed state
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I.
INTRODUCTION
M O N E L K-500 is a precipitation-hardening Ni-Cu alloy widely used in sea water and other conditions requiring good corrosion resistance. However, there are relatively few studies on its mechanical properties, especially ductility and fracture, even though it has been in application for over 70 years. It has long been recognized that this alloy is prone to intergranular fracture under many conditions. Price and Henderson t~j reported that alloy K-500 shows a tendency toward intergranular separation during fatigue tests. Natishan et al. 12"31 found that severe intergranular fracture could occur in largesize bars or parts of alloy K-500, probably during hot working or subsequent heat treatment. Intergranular fracture has also been observed in in-house roomtemperature tensile tests leading to low ductility. While it is known that K-500 is rather sensitive to hydrogen embrittlement (HE), the works cited above and internal unpublished investigations have eliminated HE as a cause. From this, it is possible that intergranular embrittlement might be caused by grain boundary segregation of certain elements during processing, service, or testing, but very little detailed work, especially using a sophisticated surface analysis technique, has been performed to define WEI-DI CAO, Project Manager, and R.L. KENNEDY, VicePresident, are with the Research and Development Department, Teledyne Allvac, Monroe, NC 28110-0531. A. CHOUDHURY, Development Staff Member, is with the Metals and Ceramics Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. Manuscript submitted February 17, 1993. METALLURGICAL TRANSACTIONS A
the nature of this segregation. Natishan et al. in References 2 and 3 suggested that the origin of such intergranular embrittlement is a grain boundary graphite film which formed due to carbon precipitation at grain boundaries during processing. However, the existence of grain boundary graphite film has not been conclusively proven by the authors of References 2 and 3 due to some uncertainties in the experimental techniques used. For example, the grain boundary facets for Auger study were not analyzed after fracture under vacuum but directly taken from failed parts; hence, contamination must have occurred. Therefore, the true chemistry of grain boundaries cannot be determined even though heavy sputtering was applied. The Auger mapping technique applied in those studies could not give quantitative information on the nature and levels of carbon segregated to grain boundaries. While graphite films might occur in certain circumstances, other segregation phenomena could also be responsible for the low room-temperature tensile ductility observed in regular alloy K-500, but no study has been performed so far. Due to the importance of this subject, extensive research work has been undertaken to determine the grain boundary segregation behavior of various elements as a function of alloy chemistry and processing, using the Auger electron spectroscopy (AES) technique. The resuits of the grain boundary segr
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