The influence of grain boundary precipitation on the measurement of chromium redistribution and phosphorus segregation in Ni-16Cr-9Fe

Abstract

The redistribution of chromium at the grain boundary and the segregation of phosphorus to the grain boundary in Ni-16Cr-9Fe is measured following thermal treatment at 700 °C for 1 to 100 hours. The addition of carbon to the base alloy results in the formation of Cr₇C₃ precipitates at the grain boundary and the formation of a chromium depleted zone in the adjacent matrix. Measurement of the Cr concentration is affected by the presence of Cr-rich carbides, and a technique of ratioing the Auger signal of the element of interest to a sum of the signals of elements present in the carbide and the matrix is required to minimize the scatter in the data. The presence of carbides does not affect the kinetics or extent of phosphorus segregation to the grain boundary, and there is no evidence of co-segregation of phosphorus with any major alloying element. The free energy of segregation of phosphorus is determined to be 46.2 KJ/mole at 1100 °C and 40.8 KJ/mole at 700 °C. Results show that the intergranular fracture path is along the carbide-matrix interface as opposed to through the carbides or some distance into the matrix. These results permit the calculation of the coverage of the grain boundary with carbides.