Dynamic aging of 18 wt-%Ni maraging 250 steel, based on studies of serrated flow

Abstract

The dynamic aging of an 18 wt-%Ni 250 maraging steel was investigated. The first stage of dynamic aging is suggested to be the formation of Mo atmospheres on dislocations, which occurs at approximately 204°C. The activation energy for Mo-atmosphere formation was found to be 1·01–1·38 eV. It is also suggested that Mo clustering in the matrix independent of the atmosphere–dislocation interaction occurs in the first stage of dynamic aging. In the later stage of dynamic aging, the Mo atmospheres are depleted from the dislocation lines by a precipitation reaction which occurs during the arrest period of the dislocations at the matrix Mo clusters. The second stage of dynamic aging (depletion of Mo atmospheres and growth of matrix Mo clusters), occurs at and above 316°C. As the growth of the Mo clusters progresses, a driving force for Ni diffusion to the clusters occurs in order to form matrix Ni₃Mo precipitates. Ni diffusion to the Mo clusters occurs with an activation energy of 2·7-3·2 eV, and is considered to be the rate-controlling process sustaining the later stage of dynamic aging. As the reaction in the matrix begins to saturate, the dislocations retain the remaining Mo as atmospheres. At this point, Ni begins to diffuse to the dislocation lines, where a precipitation reaction to form Ni₃Mo directly on the dislocation lines occurs.

MST/109