Grain boundary segregation of boron and sulfur and its effect on ductility in rapidly solidified Ni-base L1 2 compounds

Abstract

This paper reports a study of boron segregation in Ni 3Al, Ni 3Si, Ni 3Ge, and Ni 3Ga. Sulfur segregation in Ni 3Al is also considered. The results show that boron segregates in Ni 3Al and that the amount of segregation tends to increase as the bulk concentration of boron in the alloy increases. However, because the samples had not reached equilibrium during the heat treatment, there was some scatter in this correlation. Boron also segregated in Ni 3Si and Ni 3Ge, but there appeared to be no dependence of the amount of segregation on the bulk concentration. This result is not surprising because in these alloys the boron concentration was above the solubility limit. Boron segregation also occurred in Ni 3Ga and it appeared to increase with increasing bulk concentration. However, only a small amount of data was obtained for this system. Sulfur segregated in Ni 3Al and its concentration on the grain boundaries increased with increasing bulk concentration. It did not appear to compete with boron for grain boundary sites. Aluminum also segregated in Ni 3Al, but there was a large amount of scatter in the data. The plastic strain to failure measured for the samples of Ni 3Al did not correlate with the amount of boron segregation. In particular, we could not explain the fact that boron additions enhance grain boundary cohesion more effectively in Ni-rich alloys by an increase in boron segregation in these alloys. Stoichiometric alloys and Ni-poor alloys that were very brittle had boron segregation in equivalent amounts to that found in ductile Ni-rich alloys.