AUGER-ELECTRON SPECTROSCOPY STUDY OF GRAIN-BOUNDARY SEGREGATION IN ALLOY-K-500 .1. BEHAVIOR IN AS-PROCESSED STATE

Increased interest has been paid to grain boundary segregation in allo y K-500 due to severe intergranular cracking recently observed in forg ed bars. However, little systematic study of this segregation has been performed so far. A detailed auger electron spectroscopy (AES) study of grain boundary segregation in alloy K-500 has been carried out as a function of alloy chemistry. To determine C segregation, the C and O contamination rates in a vacuum chamber were measured and the necessar y condition for C grain boundary segregation determination was establi shed. It has been found that severe C, Al, and Cu segregation to grain boundaries occurred and depended on alloy chemistry. High bulk Ni and low bulk Al promoted C and Al grain boundary segregation, and low bul k Ni and high bulk Al significantly enhanced Cu segregation to grain b oundaries. The depth profiles of intergranularly segregated elements a lso showed different features for high and low Ni content Alloys. In h igh Ni alloys, C and Al levels dropped continuously as a function of d istance from the grain boundaries but the Cu level dropped only slight ly. In low Ni alloys, the Al and C levels rose from relatively low gra in boundary levels to a peak at a certain distance from the grain boun dary where the high grain boundary Cu level dramatically dropped. Tran smission electron microscope (TEM) observation revealed a grain bounda ry gamma'-depleted zone followed by a region with coarser and denser g amma' particles in low Ni and high Al alloys but quite uniformly distr ibuted gamma' particles with no depleted zone in high Ni and low Al al loys. These can be explained by the observed segregation behavior. The occurrence of Cu segregation is explained according to available theo ries about surface segregation in binary Ni-Cu alloys, and the segrega tion of C and Al to grain boundaries is suggested to be probably due t o their interaction with Ni and Cu.