Abstract
An improved understanding of high-temperature alloy oxidation is key to the design of structural materials for next-generation energy conversion technologies. An often overlooked, yet fundamental aspect of this oxidation process concerns the fate of the metal vacancies created when metal atoms are ionized and enter the growing oxide layer. In this work, we provide direct experimental evidence showing that these metal vacancies can be inseparably linked to the oxidation process beginning at the very early stages. The coalescence of metal vacancies at the oxide/alloy interface results initially in the formation of low-density metal and eventually in nm-sized voids. The simultaneous and subsequent oxidation of these regions fills the vacated space and promotes adhesion between the growing oxide and the alloy substrate. These structural transformations represent an important deviation from conventional metal oxidation theory, and this improved understanding will aid in the development of new structural alloys with enhanced oxidation resistance.
Highlights
The oxidation behavior of structural alloys at high temperatures is central to the function of power plants, aircrafts, and many other high-temperature applications.[1]
The behavior of these vacancies is not considered in conventional metal oxidation theory,[2] which effectively assumes that they are constantly annihilated at the oxide/alloy interface.[3]
In considering the STEM and atom probe tomography (APT) results collectively, we propose the process by which oxide growth has occurred
Summary
The oxidation behavior of structural alloys at high temperatures is central to the function of power plants, aircrafts, and many other high-temperature applications.[1]. For the case of voids formed at the oxide/alloy interface the process is analogous to void formation by the Kirkendall effect,[12] where different countercurrent diffusion rates result in a net flux of vacancies across an interface.[13] This phenomenon has been used, for example, to produce hollow metal oxide nanoparticles[14,15,16] and while these studies do confirm that void formation can occur by vacancy injection, it is difficult to relate results to the oxidation behavior of a structural alloy component in service. Difficulties in investigating the behavior of injected metal vacancies is partly related to the short time- and length-scales at which vacancy formation and migration occur This is especially true of the early stages of oxidation, which can be difficult to capture experimentally, and requires instrumentation capable of resolving and visualizing structural and compositional changes approaching the atomic scale.[19] it is during these early stages that oxide growth is fastest and the ensuing vacancy generation at a maximum. Network is dispersed throughout an otherwise voided zone, and together these features comprise the region of dark contrast
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