Nanoporous Surfaces of FeAl Formed by Vacancy Clustering

Abstract

Numerous thermal vacancies are frozen into FeAl B2-ordered alloy ribbons by a conventional rapid-solidification technique. Through heat treatment at 723 K, clustering of the supersaturated vacancies generates a large number of nanopores, particulary near surfaces, thus creating nanoporous surfaces. The nanoporous surface structure was confirmed by scanning electron microscopy and atomic force microscopy. The behavior of this vacancy clustering was examined by differential scanning calorimetry. An exothermic, irreversible peak, probably due to vacancy clustering, was observed around 800 K, giving an activation evergy of about 1.17 eV. The nanopore formation was also observed by in-situ heating experiments in a transmission electron microscope. The pores have specific morphology and crystallography with pore surfaces faceting toward {100} planes. These results suggest that the vacancy clustering is a unique process which enables us to efficiently make nanoporous surfaces.

Differential scanning calorimetry study on annihilation behavior of supersaturated thermal vacancies in B2-type FeAl

“…2. In the previous studies [17,29], similar changes in the peak position with heating rate were also observed. Generally, the position shift of an exothermic peak toward high temperature as a heating rate increases corresponds to that the reaction is controlled by a thermal activation process.…”

“…Fig. 1 shows DSC curves of rapidly solidified Fe-44.9 at.% Al ribbons [17] and a water-quenched Fe-48.5 at.% Al single crystal [22]. The two exothermic peaks appear only for the first heating step.…”

“…That is, the apparent activation energy is not the material constant but the variable depending on the concentration of the supersaturated vacancies. [17]. (b) A Fe-48.5 at.% Al single crystal disk water-quenched from 1273 K [22].…”

“…Yang and Baker [30] also concluded from the results obtained in quenched Fe-40 at.% Al single crystals that the low-temperature peak is associated with the rearrangement of quenched-in point defects and the high temperature peak is due to the annihilation of vacancies. In our previous studies [17,19,22], the high temperature peak was linked to microstructural evolution such as the increase in the density of h100i-type dislocations or the formation of cuboidal voids inside the bulks.…”

Surface Oxidation of Fe-48 mol%Al Single Crystal under a High Vacuum

“…Following this, the surface morphology was self-patterned on a nano-to meso-scale. 7) In our subsequent paper, the nanopore formation near the surface was further confirmed for a waterquenched Fe-48 mol%Al single crystal. 8) Based on these results, we are convinced that the nanopore formation in FeAl is due to supersaturated thermal vacancy condensation.…”