Nanoporous Behavior Induced by Excess Vacancy Clustering in Rapidly-Solidified B2 FeAl Ribbons
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Nanoporous Behavior Induced by Excess Vacancy Clustering in Rapidly-Solidified B2 FeAl Ribbons Tomohide Haraguchi1,2, Kyosuke Yoshimi1,2, Man H. Yoo1, Hidemi Kato1,2, Shuji Hanada2 and Akihisa Inoue1,2 1 Center for Interdisciplinary Research, Tohoku University, Sendai 980-8578, JAPAN 2 Institute for Materials Research, Tohoku University, Sendai 980-8577, JAPAN ABSTRACT Rapidly solidified ribbons of B2-ordered Fe-40, 45 and 50mol%Al were produced by a conventional single-roll melt-spinning method. The lattice parameters of as-spun ribbons are fully restored by annealing at 723 K for 24 h. This suggests that large numbers of supersaturated thermal vacancies are removed by the heat treatment. After the heat treatment, it is found that clustering of the supersaturated vacancies leads to a large number of pores that have a few hundreds nm or less in diameter near the surfaces, thus creating nanoporous surfaces. DSC measurements show irreversible exothermic peaks due to vacancy clustering. Vacancy complexes such as dislocations and pores are also observed inside the ribbons by TEM. The volume fraction of the overall vacancy complexes shows Al concentration dependence, demonstrating that defect structure formed by clustering of the excess vacancies is controllable by changing Al concentration in rapidly solidified FeAl ribbons.
INTRODUCTION It is well known that thermal vacancy concentration in B2-ordered FeAl alloys is very high at high temperatures and most of the thermal vacancies can be readily frozen-in by rapid cooling from the high temperatures [1–6]. The supersaturation of thermal vacancies can lead to their binding into forming divacancies as predicted by the theoretical calculation [7] and further clustering of them into higher-order vacancy complexes and extended defects as experimentally observed [8–11]. Quite recently, it was revealed in our previous work [12] that pores of tens to hundreds nm size in diameter are formed, particularly near the surfaces, by a heat treatment in melt-spun Fe-45Al ribbons. (Chemical compositions are given in molecular percent throughout this paper.) The pore formation is interpreted in terms of excess vacancy clustering [12]. In addition, the concentration of frozen-in vacancies in B2 FeAl strongly depends on Al concentration [6, 13, 14]. This suggests that the concentration of supersaturated vacancies in rapidly solidified FeAl alloys can be controllable by changing Al concentration. The purpose of this paper is to investigate and characterize excess vacancy clustering in rapidly solidified FeAl ribbons having different Al concentration, particularly focusing on the nanoporous behavior.
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EXPERIMENTAL DETAILS Three kinds of Fe-Al alloys, Fe-40, 45 and 50Al, were produced from 99.99 mass% electrolytic iron and 99.99 mass% aluminum by an arc-melting technique. The alloy ingots were re-melted in a quartz tube, and rapidly solidified into ribbons of about 2 mm in width and 20 µma in thickness by a conventional single-roll melt-spinning apparatus with a copper rol
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