TEM Observation of Vacancy Clustering Process in FeAl Single Crystal
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0980-II05-11
TEM Observation of Vacancy Clustering Process in FeAl Single Crystal Masafumi Tsunekane, Kyosuke Yoshimi, and Kouichi Maruyama Graduate School of Environmental Studies, Tohoku University, Sendai 980-8579, Japan
ABSTRACT Nanoporous surfaces are formed in B2-type FeAl single crystals by quenching into iced water, followed by surface treatment and aging heat treatment. From our previous work, it was indicated that the nanoporous phenomenon is caused by the clustering of supersaturated thermal vacancies. In this study, the clustering process of supersaturated thermal vacancies was systematically investigated by transmission electron microscopy (TEM). An irreversible exothermic peak was detected for as-quenched specimens by differential scanning calorimetry (DSC). The isochronal changes of surface morphology and substructure around the exothermic peak temperature were clarified by TEM observation. The average size of the surface pores monotonously increased with increasing the temperature. Dislocations whose Burgers vectors are parallel to existed in the isochronally-heated single crystals, and their density was changed with heat treatment temperature. On the other hand, there was a dislocation-free zone from the surface to the depth of several tens nanometers. A growth model of the surface pores is discussed based on the results.
INTRODUCTION B2-type FeAl is well known as an intermetallic compound that has a large number of thermal vacancies are generated at high temperature and these vacancies are easily frozen in by conventional quenching processes. The vacancy concentration should reach several per cent at its melting point [1-7], which is higher than that of pure metals and disordered alloys by two orders of magnitude. These unique phenomena are caused mainly from the properties of the low vacancy formation enthalpy and the high vacancy formation entropy in the B2-type FeAl [8-11]. Recently, Yoshimi et al. [12] reported that surface pores and voids of several to several hundreds nm were formed in rapidly solidified Fe-45mol%Al ribbons during aging heat treatments. Since the lattice constant of the rapidly solidified ribbons evidently increased due to the heat treatment, it appeared that the formation of the mesopores and mesovoids was caused by the clustering of supersaturated thermal vacancies introduced by the rapid solidification. The surfaces of the mesopores near surfaces and mesovoids inside the ribbons were faceted to {001} planes, so their shape was changed with the surface orientation of each grain. The formation of surface mesopores was also confirmed in quenched Fe-49mol%Al [13] and Fe-48.5mol%Al [14] single crystals during aging heat treatment. From this previous work, the large potential for nanoscale surface self-patterning by vacancy clustering process was indicated in B2-type FeAl single crystals. However, the mechanism on the clustering process of supersaturated thermal vacancies is still unclear. In the present work, the clustering process of supersaturated thermal vacancies in FeAl
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