Absolute concentration of free volume-type defects in ultrafine-grained Fe prepared by high-pressure torsion

A maximum excess volume Delta V/V approximate to 1.9 x 10(-3) in ultrafine-grained Fe prepared by high-pressure torsion is determined by measurements of the irreversible length change upon annealing employing a high-resolution differential dilatometer. Since dislocations and equilibrium-type grain boundaries cannot fully account for the observed released excess volume, the present study yields evidence for a high concentration of free volume-type defects inherent to nanophase materials, which is considered to be the main source of their particular properties, such as strongly enhanced diffusivities. (C) 2010 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.