Atomistic Study of Surfaces and Interfaces of Fe-Cr and Fe-Cr-Al Alloys

Surface and interface properties of Fe-Cr, Fe-Al, and Fe-Cr-Al are studied using Exact Muffin-Tin Orbitals and Monte Carlo methods and with x-ray photoelectron and Auger electron techniques. Surface composition is investigated as a function of oxidation (heating) time. Hard x ray photoelectron spectroscopy (HAXPES) is used to scan non destructively the compositions below the surface. It is found that Cr boosts the Al segregation to the surface. Introduction The fundamental physical difference between Fe and Cr plays a decisive role in the thermodynamics of Fe-Cr based alloys: Cr favours an antiferromagnetic coupling to its neighbours whereas Fe prefers a ferromagnetic one. Small amount of Cr, coupled antiferromagnetically to the host matrix, can be dissolved in Fe. However, with increasing Cr content the average distance between, in this case, ferromagnetically coupled Cr atoms decreases inducing increasing of the CrCr pair energy; the energy of a Cr-Cr dimer in Fe increases 240 meV when Cr atoms are brought from a third-neighbour position to a first-neighbour position [1]. Therefore, with increasing Cr content the energy of the homogeneous Fe-Cr alloy is pushed up inducing: peculiar Fe-Cr mixing enthalpy [2], bulk Cr threshold (≈10 at.% Cr in bulk) for a Cr containing surface [3], and α-α’ phase separation [4]. Chromium in bulk affects also surface properties: within 8–23 at.% Cr in bulk the Cr containing surface has lower surface energy than the pure Fe surface [1] and adding 10 at.% Cr to Fe-Al, the Al concentration can be dropped from ≈15 to ≈3 at.% without weakening the protective alumina surface scale [5,6]. In the present work some effects of substitutional Cr in Fe and Fe-Al are investigated using photoelectron and Auger electron measurements and computer simulations. Materials Science Forum Online: 2013-07-01 ISSN: 1662-9752, Vol. 762, pp 728-733 doi:10.4028/www.scientific.net/MSF.762.728 © 2013 Trans Tech Publications, Switzerland All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of Trans Tech Publications, www.ttp.net. (#69805089, Pennsylvania State University, University Park, USA-17/09/16,22:38:06)