Grain boundary segregation in Ni and binary Ni alloys doped with sulfur
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I.
INTRODUCTION
IT has been known for nearly 60 years that sulfur is deleterious to the mechanical properties of Ni,1 and in recent years S has been shown to be segregated to grain boundaries in Ni and Ni-base alloys. 2'3'4 Nevertheless, very little is known about the details of S segregation in Ni alloys, and in particular there is no information about interactions between S and the common alloying elements in Ni which may influence segregation. Interactions between impurity and alloy elements (cosegregation) have been shown to be among the most significant factors in temper embrittlement of low alloy steels, 56 ' and a major goal of the present work was to determine if these effects occur in Ni alloys. In order to facilitate this and allow comparison with theoretical models, a careful characterization of the kinetics and temperature dependence of S grain boundary segregation in Ni and binary Ni alloys doped with 70 to 100 atomic ppm S was undertaken.
II.
EXPERIMENTAL
Chemical compositions of the alloys utilized in this study are shown in Table I. All alloys were vacuum cast and poured into rectangular molds using standard foundry practice and contained approximately 75 atomic ppm sulfur. Because of the danger of hot shortness and hot tearing due to the high sulfur content, no attempt was made to hot work the ingots, and they were given a homogenization heat treatment of 24 hours at 1200 ~ instead. All heat treatments were done in evacuated quartz capsules. Auger samples, approximately 3 mm • 3 mm • 15 mm were cut from the ingots, solution treated at 1200 ~ for one hour, water quenched, and then isothermally aged at temperatures between 500 ~ and 1000 ~ followed by a final water quench. Auger spectroscopy was done on a Physical Electronics model 590 Scanning Auger Spectrometer. All samples were fractured in situ in the vacuum chamber, generally at 1 x 10-1~torr. Intergranular failure was easily obtained in most samples. However, a few samples with low amounts R.A. MULFORD, formerly with General Electric Company Corporate Research and Development Center, Schenectady, NY 12301, is now with General Electric Company, Knolls Atomic Power Laboratory, Schenectady, NY 12301. Manuscript submitted September 8, 1981. METALLURGICALTRANSACTIONS A
of grain boundary sulfur segregation (including all the Ni-20 Cr and 0.5 Hf samples and also the Ni, Ni-10 Cu, and Ni-5 A1 samples aged at 900 ~ or above) were cathodically charged with hydrogen to" insure grain boundary fracture. The procedure used was to Pd-plate the samples and then immerse in a 0.5 N H2SO 4 solution for 24 hours at 99 ~ with a current density of 800 mA/cm 2. Samples were Cdplated upon completion of charging and immediately loaded into the vacuum chamber of the Auger spectrometer. The model 590 spectrometer allows individual grain boundary facets to be observed and analyzed. Generally, large differences are observed in the amount of grain boundary segregation from facet to facet in any given sample. However, these variations are not seen in the grain boundary
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