Tomohide Haraguchi

Determination of density and vacancy concentration in rapidly solidified FeAl ribbons

Abstract In order to observe an anomalously large number of thermal vacancies retained in B2-type FeAl, an FeAl alloy is rapidly solidified into ribbons by a conventional melt-spinning technique, and the density and vacancy concentration of these ribbons are determined by an Archimedes water immersion technique and x-ray diffraction method. The lattice constant of as-spun ribbon, which has already B2-type ordered structure regardless of the rapid solidification by melt-spinning, is 2.8992×10 −10 m. This is smaller than that of water-quenched FeAl alloys previously reported, indicating lattice relaxation due to the existence of numerous thermal vacancies. The total vacancy concentration of as-spun ribbons is estimated to be approximately 1.38±0.15%, which is several hundred times as high as that of pure metals and disordered alloys near the melting point.

Surface mesostructure change of B2-type FeAl single crystals by condensation of supersaturated thermal vacancies

Mesoporous surfaces are formed in B2-type FeAl single crystals by quenching into iced water, followed by surface treatments and an ageing heat treatment. The pore shape changes depending on surface orientation, because the surfaces of the mesopores are surrounded by {100} planes in the same way as mesopores observed inside rapidly solidified FeAl ribbons. The pore size seems to increase with increasing temperature of the ageing heat treatment. Two relaxation peaks were observed in DSC thermograms, and the activation energy is estimated to be approximately 1.84 eV for the peak in a higher temperature range. TEM observations suggest that the observed DSC peaks correspond to vacancy condensation and/or absorption by ⟨100⟩-type dislocations. The experimental results also strongly suggest that the mesopore formation near surfaces is caused by the condensation of quenched-in, supersaturated thermal vacancies.

Nanostructure of Surface Formed by Vacancy Clustering in FeAl

In B2-type FeAl, supersaturated vacancies retained upon rapidly quenching are absorbed near surface during an aging treatment, being agglomerated into nano- to meso-clusters through the absorption process. Eventually, surface morphology is self-patterned in nano-order by the vacancy clustering. If FeAl was not quenched from high temperature or plastic strain remained near surface, the surface self-patterning never occurs, indicating that the change in surface morphology is caused by the clustering of supersaturated vacancies. The clusters have specific shape with cluster surfaces faceted toward {100} planes. Thus, the shape of the clusters formed near surface is controllable by changing surface orientation. Vacancy cluster size and its distribution density can be also controlled by varying the concentration of supersaturated vacancies and/or the clustering condition. These indicate that the vacancy clustering is a unique process to efficiently pattern the surfaces of metals, alloys and intermetallics in nano-scale.