Elastic anisotropy of γ'-Fe4N and elastic grain interaction in γ'-Fe4N1-γ layers on α-Fe:: First-principles calculations and diffraction stress measurements

The three independent single-crystal elastic-stiffness constants C-ij of cubic gamma'-Fe4N (face-centred cubic (fcc)-type iron substructure) have been calculated by first-principles methods using the density functional theory: C-11 = 307.2 GPa, C-12 = 134.1 GPa and C-44 = 46.0 GPa. The Zener elastic- anisotropy ratio, A = 2C(44)/(C-11 - C-12) = 0.53, is strikingly less than 1, implying < 1 0 0 > as stiffest directions, whereas all fcc metals show A > 1. This elastic anisotropy is ascribed to the ordered distribution of N on the octahedral interstitial sites. X-ray diffraction lattice-strain measurements for a set of different hk1 reflections recorded from gamma'-Fe4N1-y layers on top of alpha-Fe confirmed the "abnormal" elastic anisotropy of gamma'-Fe4N1-y. Stress evaluation, yielding a compressive stress of about -670 MPa parallel to the surface, was performed on the basis of effective X-ray elastic constants determined from the calculated single-crystal elastic constants C-ij and allowing a grain interaction intermediate between the Reuss and the Voigt models. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.