Sulfur segregation to growing Al2O3/alloy interfaces

It is well known that impurities and/or solutes in an alloy can segregate to the free surface and to the alloy grain boundaries at elevated temperatures [1, 2]. When a metal or alloy undergoes oxidation, an external oxide scale is formed from the transport of metal outward and of oxygen inward. It has been proposed that sulfur, which is a common impurity, segregates to the growing scale/alloy interface and weakens the interfacial bonds [3] as it does when present at alloy grain boundaries [4, 5]. However, it is not well established if sulfur should segregate to an intact interface [6], where the oxide remains adherent to the underlying alloy. Rather, the presence of sulfur that have been observed [7] is often attributed to segregation after scale detachment due to the thermal stresses present in cooling, or from accumulation as a result of oxide growth, where the interface advances inward [8]. The purpose of this paper is to report the build-up of sulfur and other impurities at growing scale/alloy interfaces to better understand the segregation phenomena associated with oxidation. Two Al2O3-forming alloys were used and their composition and sulfur content are listed in Table I. The Fe3Al-based iron aluminide was prepared by arc melting and casting, followed by hot rolling. The FeCrAl was induction melted and cast, then homogenized at 1200◦C for 24 h. The amount of sulfur in each alloy was determined using glow discharge mass spectrometry (GDMS). Specimens, typically 10 mm× 10 mm× 1 mm, were cut from the alloys with one of the main faces polished to a 1 μm finish using diamond paste. Oxidation was performed in flowing, dry oxygen at 1000◦C under 1 atm. Most specimens were air cooled after the desired oxidation time; a few were water quenched. The chemistry of the scale/alloy interface after oxidation was determined using Scanning Auger Microscopy (SAM) after the surface oxide was scratched to spallation in the ultra high vacuum (2–3× 10−10 torr) using a Vickers micro-indenter [9]; an example is given in Fig. 1. A 0.5–1 μm diameter electron beam was then placed on the exposed metal surface to study its composition. Ten to fifteen areas were surveyed to assess the compositional uniformity. The underside of the spalled oxide was also analyzed when it was exposed. Very different segregation behavior was found on the two alloys, even though both form Al2O3 scales. sulfur was the major impurity, but with significantly different concentrations. Fig. 2 shows the Auger spectra of the two interfaces at steady state. Apart from sulfur, the FeCrAl interface was also enriched with Cr and C. The concentration of interfacial sulfur with oxidation time for both alloys is plotted in Fig. 3. For the FeCrAl alloy, saturation was reached after less than 10 min. The rate is comparable in relation to sulfur diffusion in Fe [10]. For the Fe3Al alloy, on the other hand, the sulfur concentration was initially high, went through a minimum of almost no sulfur at the interface, then increased again slowly to a steady state. The same kind of trend was observed with oxidation at 900◦C, except that the rate is almost an order of magnitude slower. Furthermore, the spatial variation, as seen by the error bars, decreased considerably with time, indicating large local differences under thinner scales. Whenever the underside of the spalled scale was examined, it was found to contain only Al and O. To the free surface of the Fe3Al alloy, sulfur was found to readily segregate and reach a saturation level after about 4 min at 1000◦C [9]. Compared to this behavior, the segregation to growing Al2O3/alloy interfaces differs in a few major ways. For the FeCrAl alloy, the saturation concentration is significantly higher than the usual 0.25–0.5 monolayer coverage when S segregates to free surfaces of Fe or Ni single crystals [1]. This high concentration may be related to a cosegregation effect of S with Cr [11]. There may be some sulfide formation at the interface, whether as very small particles or as a two-dimensional compound. When