Effects of Fe, W and Mo on Kinetics of Discontinuous Precipitation in the Ni–Cr System

Abstract

The effects of Fe, W and Mo on the kinetics of discontinuous precipitation in the Ni–Cr system were experimentally examined using a binary Ni–38Cr alloy and ternary Ni–38Cr–0.9Fe, Ni–38Cr–3.2W and Ni–38Cr–1.7Mo alloys. These alloys were homogenized at 1423 K for 3 h, solution treated at 1423 K for 1 h, and then isothermally annealed in the temperature range of 873–1023 K for various times up to 2300 h. Due to the solution heat treatment, all the alloys show the polycrystalline single-phase microstructure of the Ni-rich solid-solution (γ) phase with the face-centered cubic structure. During isothermal annealing, however, the cell of the lamellar microstructure consisting of the γ phase and the Cr-rich solid-solution (α) phase with the body-centered cubic structure is formed along the grain boundary of the γ matrix and then grows into the γ matrix. At each annealing temperature, the migration distance of the moving cell boundary is proportional to the annealing time. Hence, 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 873–1023 K. The cell growth is decelerated by W and Mo but not by Fe. The deceleration is more remarkable for Mo than for W. A kinetic model for the binary discontinuous precipitation controlled by boundary diffusion was used to analyze quantitatively the experimental result. According to the analysis, W and Mo retard the boundary diffusion of Cr along the moving cell boundary, but Fe slightly hastens that. Such influence of the alloying component on the boundary diffusion is ascribable to the thermodynamic interaction between Cr and the alloying component in the moving cell boundary. As a consequence, it is concluded that the retardation of the Cr boundary diffusion causes the deceleration of the cell growth.