Diffusion Behaviour of Oxygen in Internal Oxidation in Ni-Al and Ni-Cr Alloys

Abstract

Dilute solid solution nickel alloys containing aluminium or chromium up to 4 wt% were internally oxidized at temperatures between 900° and 1300°C for various times, and the relation between the diffusion rate of oxygen and the size and distribution of oxide particles in the internally oxidized layer was investigated.The results obtained are as follows:(1) In each of the alloys, the diamater and interparticle spacing of dispersed oxide particles increased with increase in temperature, and the density of particles increased with solute content.(2) The effective diffusion coefficient of oxygen, D0, in the internally oxidized layer increased with increase in solute content, and the concentration dependency of D0 increased with temperature.(3) The value of D0 increased with increase in the net volume of the highly diffusive region around the oxide particles in the matrix, and decreased with the volume of particles, of which the diffusion rate was extremely low in comparison with that in the matrix. Therefore, the value increased with increasing size of particles up to a critical diamater, R*, after which it decreased and became that of the matrix at another critical diamater, Rmax. Both the values of R* and Rmax decreased with increase in temperature.(4) Analyzing the relation between D0 and the size and interparticle spacing of particles as a function of temperature and solute content, the diffusion coefficient in the interfacial layer around the particles, D0B, was found to be greater than that in the matrix D0L at 900°∼1300°C. Activation energies for D0L and D0B were analogous to the values for self-diffusion in the matrix and at large angle grain boundaries in nickel, respectively.(5) At higher temperatures the effect of the solute content on D0 was small. This seemed to be attributable to the growth of particles, decrease in particle density, and decrease in the particle effect on D0 with increase in temperature.