Nanostructure of Surface Formed by Vacancy Clustering in FeAl
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Nanostructure of Surface Formed by Vacancy Clustering in FeAl Kyosuke Yoshimi1, Tomohide Haraguchi1, Tomohisa Ogawa2, Takayuki Kobayashi3, Min-Seok Sung3 and Shuji Hanada3 1 Center for Interdisciplinary Research, Tohoku University, Sendai 980-8578, JAPAN 2 Department of Biomolecular Science, Tohoku University, Sendai 981-8555, JAPAN 3 Institute for Materials Research, Tohoku University, Sendai 980-8578, JAPAN ABSTRACT 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.
INTRODUCTION In our recent work on B2-type FeAl intermetallic compound (see Fig. 1), it was found that surfaces of rapidly solidified ribbons are made nanoporous during an aging heat treatment [1, 2]. Pore size observed is in the range between several tens and a few hundreds nm, and pore shape changes depending on surface orientation of each grain because of the surface orientation of pores faceting toward {100} planes [1]. This nanoporous phenomenon was interpreted in terms of excess vacancy clustering, since supersaturated vacancies of about 1 % or more retained by the rapid solidification process are restored during the aging heat treatment [2]. A question is whether the nanopore formation near surfaces is special phenomenon only under the rapid
Figure 1. Atomic structure of B2-type FeAl in the unit cell.
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solidification state. If it is possible to obtain such nanoporous surfaces in the bulk of FeAl intermetallic compound, we may be able to produce self-patterned surface morphology by controlling surface orientation using single crystals, concentration of supersaturated vacancies and so on. The purpose of the present work is to establish the way to prepare surfaces of bulk FeAl using single crystals for the self-patterned nanoporous surfaces, and to investigate nanoporous behavior with surface orientation, aging condition. How to
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