Influences of Co, Cu and V on Kinetics of Discontinuous Precipitation in the Ni-Cr System

The effects of Co, Cu and V on the kinetics of discontinuous precipitation in the Ni-Cr system were experimentally examined using ternary Ni-39Cr-1.0Co, Ni-39Cr-1.2Cu and Ni-39Cr-0.89V alloys. The alloys were homogenized at 1 423 K for 3 h, solution treated at 1 423 K for 1 h, and then isothermally annealed in the temperature range of 873-1 023 K for various times up to 2 300 h. Due to the solution heat treatment, all the alloys show the polycrystalline single-phase microstructure of the Ni-rich solid-solution (gamma) phase with the face-centered cubic structure. During isothermal annealing, however, the cell of the lamellar microstructure composed of the gamma phase and the Cr-rich solid-solution (alpha) phase with the body-centered cubic structure is produced along the grain boundary of the gamma matrix and then grows into the gamma matrix. At each annealing temperature, the migration distance of the moving cell boundary increases in proportion to the annealing time. Thus, the growth rate of the cell is constant independent of the annealing time. The growth rate and the interlamellar spacing of the cell almost monotonically increase with increasing annealing temperature at 873-1 023 K. The cell growth is slightly accelerated by Cu but not by Co and V. However, the acceleration is less remarkable at 1 023 K. A kinetic model for the binary discontinuous precipitation controlled by boundary diffusion was used to analyze quantitatively the experimental result. In the moving cell boundary, the thermodynamic interaction is attractive between Cr and V but repulsive between Cr and Cu. Therefore, it is anticipated that the boundary diffusion of Cr along the moving cell boundary may be expedited by V but retarded by Cu. According to the analysis, however, the Cr boundary diffusion is hardly affected by Cu and V as well as Co. On the other hand, the boundary diffusion is lightly faster for Cu than for Co but slower for V than for Co. Consequently, for Cu and V, the thermodynamic and diffusional contributions to the Cr boundary diffusion are compensated each other.