Simultaneous creep and oxidation of an Fe-1 wt pct Ai alloy at 973 to 1073 K
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INTRODUCTION
IN recent years it has been
adequately demonstrated that oxidation of pure metals and alloys can diminish or enhance high-temperature creep ductility through a combined influence on surface and substrate structures. 1~ Thus, certain oxidation products like an external, strongly adherent scale and internal oxides can exert a strengthening effect and, in contrast, oxidation mechanisms can lead to an increased vacancy concentration in the substrate which may promote either dislocation climb or cavitation resulting in a weakening effect. It is clearly desirable to establish the nature of the interactions between simultaneous creep and oxidation processes before attempting to predict the likely behavior of engineering components in aggressive atmospheres. However, some basic problems still remain. For example, if the creep/oxidation interactions arise primarily at the oxide/substrate interface and in the near-surface region of the substrate, do the thermally- and stress-activated events operate independently? It seems far more likely that the atom (or ion) and vacancy movements in a diffusioncontrolled parabolic oxidation process alongside selfdiffusion processes in diffusion or dislocation creep should be regarded as interdependent. If this were so, then it would be entirely justifiable to suppose a common apparent activation energy for the interdependent events and likewise a common material structure factor. Our earlier work has indicated that the values for the stress exponent n and for the apparent activation energy Qapp in the power-law expression for the steady state creep rate: ~ = Ao~ exp - - ~ both showed a dependence on oxygen partial pressure (Po2) of the test environment. 2'3 For Fe-lSi and Fe-4Si alloys, lower Qapp values were found for an intermediate Po2 = 10-3 mbar and higher Qapp values for the lowest R. ROLLS is Lecturer, Metallurgy Department, University of Manchester/UMIST, Metallurgy Building, Grosvenor Street, Manchester MI 7HS, United Kingdom. M. H. SHAHHOSSEINI, formerly a Research Student at the University of Manchester, is now with the University of Teheran, Iran. Manuscript submitted April 7, 1982. METALLURGICALTRANSACTIONSA
Po2 = 10-9 mbar for 973 to 1073 K. Strengthening arose from internal oxides, intergranular oxidation, and strongly adherent scales, whereas weakening was believed to be primarily from the flux of oxidation-induced vacancies which assisted dislocation climb. These events, of course, were confined to the near-surface or substrate zone of test specimens, although their influence on bulk creep behavior was clearly significant. This feature also raises questions as to the effects of specimen size, thickness, geometry, and grain size on creep behavior, which Davidson and Tien have already been examining. 4 The present work aimed to distinguish between the particular effects of oxidation products and mechanisms on dislocation creep in Fe-lwt pct A1 by varying oxygen partial pressure to form surface oxide films of different thickness and constitution. We expec
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