Aluminium segregation profiles in the (110), (100) and (111) surface regions of the Fe0.85Al0.15 random body-centered cubic alloy

Abstract

Thanks to a dedicated modelling of intensities, the depth sensitivity of X-ray photoemission is used to probe the segregation profile of aluminium at the (110), (100) and (111) low index surfaces of the body-centred Fe 0.85 Al 0.15 random alloy. Sputtered surface composition is close to the nominal bulk one, thus excluding preferential sputtering. Surface enrichment in aluminium upon annealing starts at around 700 K before reaching a stationary state above 1000 K. The average surface composition is close to Fe 0.5 Al 0.5 , corresponding to the B 2 CsCl structure on the phase diagram. The impacted depth, that is in the range of 2.5-3.5 nm, is quite significant. Although not evidenced previously in surface science conditions at FeAl single crystal surfaces, it is qualitatively in agreement with the segregation at grain boundaries and shear planes of Al-alloyed steels. This segregation tendency is rationalized through ab initio calculations.

Al-rich Fe0.85Al0.15(1 0 0), (1 1 0) and (1 1 1) surface structures

“…The novelty was the observation of a strong aluminum segregation which starts at around 700 K before reaching a steady state within the probed scale of photoemission. Whatever the orientation, the near surface composition obtained is close to the B 2 Fe 0.5 Al 0.5 with a typical affected depth of around 25Å, except on the (110) surface for which those figures are slightly different (Fe 0.6 Al 0.4 ; ∼ 35Å) [17]. Although suspected in the diffraction study of Kottcke et al [19] on FeAl(100) and in the Auger measurements of Hammer et al [20] on the open FeAl(111) surface, such a composition gradient in the Fe 1−x Al x system was never evidenced before, probably because experiments [21,22,23,19,24,20,25,14,26,23,27] have focused mostly on ordered FeAl alloys with higher Al contents.…”

“…Experiments were performed in an ultra-high vacuum (UHV) set-up previously described [28,17], that involves two connected vessels equipped with LEED (ErLEED, SPECS), STM (RT-Omicron) and XPS (Omicron EA125 under non-monochromatic excitation). STM images were acquired in constant current mode with chemically etched W tips and processed with the WsXM and Gwyddion software [29,30,31].…”

“…In the Fe-Al phase diagram [13,14,15,16], for increasing Al content, a body-centered cubic (bcc) random alloy phase A 2 in which sites are statistically occupied by Al and Fe appears first; it is followed by ordered phases of type B 2 (CsCl structure) and D0 3 (Heussler alloy), and by a series of complex Fe-rich phases, as shortly reviewed in Ref. [17]. In a first step toward the understanding of the alloy behavior, a previous study focused on the orientation dependence of aluminum segregation at the surface of Fe 0.85 Al 0.15 (100), (110) and (111) crystalsfor annealing temperatures between 300 K and 1100 K [18,17].…”

“…[17]. In a first step toward the understanding of the alloy behavior, a previous study focused on the orientation dependence of aluminum segregation at the surface of Fe 0.85 Al 0.15 (100), (110) and (111) crystalsfor annealing temperatures between 300 K and 1100 K [18,17]. In that temperature range, this composition corresponds to the ferritic A 2 solid solution that is also encountered in industrial steel grades, which makes the Fe 0.85 Al 0.15 alloy an appropriate model compound for Al-alloyed steels.…”

“…The present work aims at exploring the surface structures associated with the Al-enriched low-index orientations (Fe 0.85 Al 0.15 (100), (110) and (111)). If, according to a photoemission study [17], the average surface compositions are close to Fe 0.5 Al 0.5 , differences in bulk truncation surface atomic density (n S (100) < n S (111) < n S (110)) are anticipated to lead to specific surface structures. The surface reconstructions are explored by Low-Energy Electron Diffraction (LEED) and Grazing Incidence X-ray Diffraction (GIXD) while STM is used to unravel the associated modification of surface topography.…”