Influence of Al on Kinetics of Discontinuous Precipitation in Ni–38Cr Alloy

Abstract

The influence of Al on the kinetics of discontinuous precipitation in the Ni–Cr system was experimentally examined using ternary Ni–38Cr–Al alloys with Al concentrations of 3.8 mass% and 4.2 mass%. The ternary alloys were homogenized at 1473 K for 16 h, solution treated at 1473–1493 K for 30 min, and then isothermally annealed in the temperature range of 773–1073 K for various times up to 50 h. Due to the solution heat treatment, both ternary alloys show the polycrystalline single-phase microstructure of the Ni-rich solid-solution (γ) phase with the face-centered cubic structure. In the early stages of isothermal annealing, however, fine particles of the Ni3Al (γ′) phase with the L12 structure are formed in the γ matrix by continuous precipitation. On the other hand, 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. The fine particles of the γ′ phase are dispersed also in the γ phase of the cell. The migration distance of the moving cell boundary is proportional 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 monotonically increase with increasing annealing temperature. A kinetic model was used to analyze quantitatively the relationship between the growth rate and the interlamellar spacing. Although the discontinuous precipitation in the ternary Ni–38Cr–Al alloys occurs in a complicated manner, the analysis indicates that the growth of the lamellar cell is controlled by the boundary diffusion of Cr along the moving cell boundary.