High temperature deformation of Mn-Zn ferrite single crystals

Abstract

Compressive creep tests in a temperature region higher than 900° C were conducted on Mn-Zn ferrite single crystals of various compositions. The strain rates in the stationary creep vary with stress and temperature according to the well-known relation $$\mathop \varepsilon \limits^\cdot = A\sigma ^m \exp ( - H/kT)$$ The stress index,m, is nearly 3, independent of the composition, but the activation energy,H, depends on it. From TEM and etch pit observations the distribution of mobile dislocations were almost uniform and the effective stress index,m*, of the dislocation velocity was nearly 1. In the compression test with constant deformation velocity, the internal stress during the deformation was about 17% lower than the applied stress. From these experimental results the high temperature deformation of Mn-Zn ferrite crystals is concluded to be governed by the viscous movements of dislocations which drag the atmosphere of pinning atoms or ions. The chemical composition dependency of the activation energy is considered to be caused by the structural defects due to the deviation from stoichiometry which act as the obstacles for the diffusion of pinning atoms or ions.