Abstract
ABSTRACT The crystal structure of η″-Fe3Al7+x , the low-temperature phase of η-Fe2Al5 with a composition on the Fe-rich side of the solid solubility range, has been determined by synchrotron X-ray single-crystal diffraction combined with scanning transmission electron microscopy. The η″ phase possesses commensurate long-period-ordered superlattice structures (space group Pmcn) based on the parent orthorhombic unit cell of η-Fe2Al5, consisting of twin domains (orientation variants) alternately stacked along the long-periodicity axis. Each of the twin domains possesses a motif structure belonging to the base-centered monoclinic space group C2/m, with a cell volume twice that of the parent orthorhombic unit cell (space group Cmcm). One-fourth of the c-axis chain sites corresponding to Al2- and Al3-sites in the η phase are respectively occupied by both Fe and Al atoms and exclusively by Al atoms in a regular manner. This regularity is disturbed in the twin-boundary region, giving rise to structural/compositional modulation. Because of the different chemical compositions between the motif structure and twin-boundary region, the η″ phase with various compositions can be constructed only by changing the number of the parent orthorhombic unit cells to be stacked along the orthorhombic c-axis, without changing the atomic arrangements for the motif structure or the twin boundary to account for the observed solid solubility range. The chemical formula of the η″ phase can thus be expressed as Fe3Al7+x under a simple assumption on the occupancies for Al/Fe atoms in the c-axis chain sites.
Highlights
Aluminized steels have received increasing attention in recent years, because of their excellent resistance to corrosion and oxidation at high temperatures as well as high ARTICLE HISTORYReceived 7 March 2019Revised 26 April 2019Accepted 26 April 2019 KEYWORDSIntermetallic compound; superlattice structure; orientation variants; nanoscale twins; icosahedron CLASSIFICATION10 Engineering and Structural materials; 106Metallic materials; 212Surface and interfaces; 302
Becker et al [22,23] have reported, on the other hand, that when the alloy composition is on the Fe-rich side of the solubility range (~Al-29.4 at.%Fe), Al and Fe atoms in the c-axis chain are arranged in an ordered manner to form another low-temperature phase (η′′) [23]
We investigate the crystal structure of the η′′ phase with two different compositions, by using single-crystal synchrotron X-ray diffraction (SXRD) and electron diffraction combined with atomic-resolution scanning transmission electron microscopy (STEM), as we have successfully applied to the crystal structure refinement of a series of intermetallics of the Fe-Zn system formed on galvanized/galvannealed steels [25,26,27,28,29,30,31]
Summary
Aluminized steels have received increasing attention in recent years, because of their excellent resistance to corrosion and oxidation at high temperatures as well as high. Becker et al [22,23] have reported, on the other hand, that when the alloy composition is on the Fe-rich side of the solubility range (~Al-29.4 at.%Fe), Al and Fe atoms in the c-axis chain are arranged in an ordered manner to form another low-temperature phase (η′′) [23]. Those authors believed that the η′′ phase has an incommensurately modulated crystal structure of long periodicity along the orthorhombic c-axis [23]. We investigate the crystal structure of the η′′ phase with two different compositions, by using single-crystal synchrotron X-ray diffraction (SXRD) and electron diffraction combined with atomic-resolution scanning transmission electron microscopy (STEM), as we have successfully applied to the crystal structure refinement of a series of intermetallics of the Fe-Zn system formed on galvanized/galvannealed steels [25,26,27,28,29,30,31]
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