Influences of Co, Cu and V on Kinetics of Discontinuous Precipitation in the Ni^|^ndash;Cr System

Abstract

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 (γ ) phase with the face-centered cubic structure. During isothermal annealing, however, the cell of the lamellar microstructure composed of the γ phase and the Cr-rich solid-solution (α) phase with the bodycentered cubic structure is produced 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 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.

Recent Developments on Discontinuous Precipitation

“…The influence of several elements on the behaviour of DP reaction in Ni-Cr system was investigated by Ueta, Hida and Kajihara [49,50]. They used Ni38Cr as the reference alloy, then added 0.9Fe, 3.2W, 1.7Mo, 1.0Co, 1.2Cu, 0.89V (wt.%) and annealed at temperatures of 873-1023 K for various time up to 2300 h. The cell growth was retarded by W and Mo but not by Fe.…”

Microstructural Evolution and Mechanical Properties of a Three-Phase Alloy in the Cr–Mo–Nb System

“…The TEM observations clearly indicate that the BCC 1 and BCC 2 phases are formed by a DP process, and that the precipitation products gradually replace the parent phase (original BCC) by migration of the reaction front. However, this is quite different from typical DP, 12,13) where precipitation typically occurs at GBs, and growth is accompanied by GB migration. 14) Nevertheless, in the present study, intragranular nucleation of the Cr-rich BCC 1 phase in this alloy dominates the early stage of the precipitation process, and becomes a trigger for precipitation of the neighboring Mo-rich BCC 2 phase, which in turn induces further precipitation of the Cr-rich BCC 1 phase, until an alternating BCC 1 /BCC 2 two-phase microstructure is finally formed.…”