Acta Materialia 51 (2003) 1393–1407 www.actamat-journals.com The lateral growth strain accompanying the formation of a thermally grown oxide D.R. Clarke * Materials Department, College of Engineering, University of California, Santa Barbara, CA 93160-5050, USA Received 14 August 2002; received in revised form 5 November 2002; accepted 7 November 2002 Abstract The high-temperature oxidation of alloys produces an oxide that grows laterally as well as thickening with time. Constrained by the underlying alloy being oxidized, the lateral strain produces a compressive stress in the growing oxide. The lateral growth strain is modeled using a dislocation climb process in which “unlucky” counter-diffusing cations and anions are trapped at cores of dislocations. The model predicts that at a fixed oxidation temperature the lateral growth strain rate increases linearly with the oxide thickening rate in accord with observations.  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. Keywords: Oxidation; Modeling; Mechanical properties; Grain boundary diffusion 1. Introduction When a metallic element or alloy is oxidized at high temperatures in air, a protective oxide gener- ally forms [1,2]. The commonest alloys designed for high temperature applications typically form either single-phase aluminum oxide or chromium oxide, although on some alloys a layered sequence of different oxides are formed. This latter is exem- plified by the formation of an outer layer of NiO, an intermediate layer of Ni–Al spinel and an inner layer of alpha-aluminum oxide when Ni 3 Al and nickel-based superalloys, such as N5 or PWA 1484, are oxidized [3,4]. Irrespective of the parti- * Tel.: +1-805-893-8275; fax: +1-805-893-8983. E-mail address: clarke@engineering.ucsb.edu (D.R. Clarke). 1359-6454/03/$30.00  2002 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved. doi:10.1016/S1359-6454(02)00532-3 cular alloy being oxidized or the type of oxide for- med, the growing oxide is usually under compress- ive stress at temperature. The existence of a stress accompanying the growth of the oxide has been known for many years and is manifest in several ways [5–10]. For instance, thin sheets and rods of an alloy elongate during oxidation [5,9,10]. Simi- larly, the pitch of alloy wire helices increase on oxidation [8]. In these cases, extension and shape changes are attributed to a tensile stress in the alloy, generated in response to the compressive oxide stress in the growing oxide, exceeding its creep or yield stress (Fig. 1). Although growth stresses have been recognized as being due to the constraint of a lateral growth strain, its origin has intrigued investigators for many years. However, the lack of quantitative experimental measurements hindered detailed consideration until very recently. This is now changing in large part as a result of