Grain Boundary Self-Diffusion in Nickel

Abstract

High-purity nickel bicrystals with controlled grain boundary misorientations have been grown from the melt using an electron beam floating-zone technique. These bicrystals have symmetrical tilt grain boundaries with 10° misorientation between {111} planes tilted about a 〈112〉 axis, and 10° twist about a {111} axis. The diffusion of nickel-63 in these boundaries and similar sintered grain boundaries has been studied over the temperature range 600°–970°C using high-resolution contact autoradiography. Using the dislocation pipe model for low-angle grain boundaries, edge and screw dislocations were found to have similar activation energies of 40.7 and 44.9 kcal/mole, respectively. The value for the tilt boundaries is shown to agree well with recent work on isolated edge dislocations in nickel. It is concluded that the low-angle tilt boundaries are composed of 〈112〉 {111} edge dislocation arrays which exhibit the same diffusion behavior as isolated edge dislocations formed by plastic deformation. Also, the higher value of the activation energy for 〈112〉 tilt boundaries as compared to previously reported results on 〈100〉 tilt grain boundaries in nickel points out a basic difference in the energy required for the mass transport process in the two types of dislocation structures found in 〈100〉 and 〈112〉 tilt boundaries. A theoretical explanation of this activation energy difference is consistent with an assumed vacancy mechanism for dislocation pipe self-diffusion.