Mechanisms for the Precipitation of AIN on Nitriding of Fe-2 at% Al

Abstract

Mechanisms for the precipitation of AIN during gaseous nitriding of cold-rolled and recrystallized Fe-2at.% Al specimens were studied as a function of temperature and time using thermogravimetric analysis. A general method, involving measurements of transformation rates at two different temperatures for the same degree of transformation, was used to determine the activation energy for the precipitation process as a function of the stage of transformation. The activation energy determined for the cold-rolled material is constant and is determined by diffusional growth of the AIN particles only. The activation energy obtained for the recrystallized material varies with the degree of transformation and is determined both by nucleation and growth processes. The energy barrier for the formation of a nucleus of critical size in the recrystallized material changes during the precipitation process. The observations have been explained as consequences of pre-existing dislocations (in cold-rolled material) and of dislocations formed during precipitation (in recrystallized material).