Auger electron spectroscopy study of grain boundary segregation in alloy K-500: Part II. Behavior in slow strain rate te
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I.
INTRODUCTION
IT was established in Part I of this series of articles 1~1 that bulk chemistry-related grain boundary segregation exists in Ni-Cu alloy K-500 after regular thermomechanical processing. Severe Cu segregation was found in low Ni and high A1 alloys, and C, together with Al segregation, was more pronounced in high Ni and low A1 alloys. The A1 distribution was closely associated with 7' distribution in grain boundary regions; there was a 7'depleted zone and a nearby 7'-discontinuous precipitation region which existed in low Ni and high A1 alloys. As mentioned in Part I of this series, an extreme intergranular brittleness has been observed in various sizes of forged and rolled bars of alloy K-500. It was suggested I2j that this intergranular cracking occurred sometime during forging and heat-treatment processes and was caused by the formation of grain boundary graphite. However, the graphite formation was not observed in any of the alloys in the as-processed state examined in this study, including some very brittle alloys. One possibility for this discrepancy could be the difference in melting and thermomechanical history between the alloys examined. As postulated in Reference 2, the severe cracking occurred mostly in air-melted alloy K-500 in which the carbide forming elements, such as Ti, were tied up as oxides or nitrides, leaving carbon free to precipitate on the grain boundaries during forging or heat treatment. To reveal the existence of free graphites, a slow strain rate tensile test (SSRTr) was included in QQN 286 Revision F I31 which requires less than 5 pct intergranular fracture area along the entire circumference of a threaded stud specimen after a SSRTT at 209 ~ (400 ~ 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
Thus, further Auger electron spectroscopy (AES) study was indicated to confirm or refute the presence of free graphite films and to determine if graphite or another cause was responsible for the observed intergranular separation. In addition, it has not been clearly shown that free graphite or other precipitates formed during thermomechanical working or heat-treatment processes, or during the SSRTT itself. As a continuation of Part I of the work, a detailed AES study was performed on alloy K500 with various chemistries after SSRTTs and the results were correlated with their performance in SSRTTs. The emphasis of this work was on examining the change in grain boundary segregation and the occurrence of precipitation during SSRTT.
II.
EXPERIMENTAL PROCEDURE
The chemistries of most of the test alloy K-500 are the same as those used in the previous study (Table I). Only a few alloys were added in this study to fully examine the effect of alloy chemistry on segregation behavior.
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