Effects of Carbon, Boron, and Nitrogen on the Grain Boundary Chemistry and Microstructure of Nickel - Chromium-16 - Iron-9.

Abstract

The purpose of this study was to determine how the grain boundary chemistry and microstructure of high purity Ni-16Cr-9Fe-.03C alloys are modified by the additions of boron or nitrogen. The alloys were (a) undoped, (b) doped with 300 wppm C, (c) doped with 50 wppm B and 300 wppm C, and (d) doped with 50 wppm N and 360 wppm C. Heat treatment at 600 to 700(DEGREES)C of the C doped alloy results in the intergranular precipitation of Cr rich M(,7)C(,3) (M > 90 at% Cr) and in Cr depletion of the surrounding matrix. Neither the M(,7)C(,3) composition nor the details of Cr depletion are sensitive to the addition of 50 wppm B or N. Adding B results in B segregation and in the precipitation of Ni + B rich M(,23)X(,6) (M > 80 at% Ni, X (DBLTURN) 100 at% B). The free energy of segregation for B is 42.6 kJ/g-mol. Adding N has no measureable effects on the microstructure. The measured compositions of M(,7)C(,3) and M(,23)X(,6) can be explained by a thermodynamic model. The compositions are calculated by minimizing the Gibbs free energy change for the transfer of atoms from the matrix to the precipitates. The Gibbs free energy of a precipitate is modelled as the sum of (a) a weighted average of the free energies of formation of the constituent binary phases and (b) a free energy of mixing term assuming ideal solution behavior on the metallic and non-metallic sublattices. The calculation of stable Ni + B rich M(,23)X(,6) requires the use of matrix activities in equilibrium with intergranular Cr(,7)C(,3). This means that Ni + B rich M(,23)X(,6) depends on Cr(,7)C(,3) for its existence, and that precipitate composition can depend on local departures from the bulk chemistry. In calculating the matrix activities at the grain boundary as required by the precipitate composition model, a theoretical grain boundary composition is also obtained. This is done using the Kohler equation formalism and a data base that includes binary interaction coefficients derived from the Fe-Cr, Fe-Ni, Cr-Ni, C-Fe-Ni, and C-Cr-Ni systems. The experimental grain boundary Cr concentrations, which are as low as 4.9 at%, are accurately modelled at 600 and 700(DEGREES)C. In addition, these calculations correctly explain variations in the experimental Fe to Ni concentration ratios, which range from 0.127 in the bulk to 0.16 at the grain boundary. This demonstrates that a Ni rich quaternary alloy can be modelled without quaternary system data. In the B + C doped alloy, M(,7)X(,3) and M(,23)X(,6) precipitates containing B form at 1100 to 1200(DEGREES)C. This is in contrast to the B free alloys, in which annealing at 1100(DEGREES)C for 20 min results in precipitate free grain boundaries. Precipitation at high temperature causes a 50% reduction in grain growth at 1100(DEGREES)C, and a 25% reduction at 1200(DEGREES)C. This high temperature precipitation is not successfully explained by the precipitate composition model.