Transmission Electron Microscopy of Fe 2 Nb Laves Phase with C14 structure in Fe-Nb-Ni Alloys
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1128-U08-06
Transmission Electron Microscopy of Fe2Nb Laves Phase with C14 structure in Fe-Nb-Ni Alloys Naoki Takata1, 2, Shigehiro Ishikawa1, Takashi Matsuo1, 2 and Masao Takeyama1, 2 1 Department of Metallurgy and Ceramics Science, Tokyo Institute of Technology 2-12-1, Ookayama, Meguro-ku, Tokyo 152-8552, JAPAN 2 "Fundamental Studies on Technologies for Steel Materials with Enhanced Strength and Functions" Consortium of JRCM (The Japan Research and Development Center for Metals)
ABSTRACT The lattice structure of the C14 Fe2Nb Laves phase with Ni in solution in Fe-Nb-Ni ternary alloys was examined by transmission electron microscopy. Binary stoichiometric Fe2Nb (Fe-33.3 at.% Nb) exhibits a featureless morphology with a low dislocation density. A similar morphology was observed in stoichiometric Fe2Nb containing 20 at.% Ni and in binary Fe-rich Fe2Nb (Fe-27.5 at.% Nb). In contrast, many planar faults parallel to the basal plane of the C14 structure were observed in Fe-rich Fe2Nb with Ni in solution, and the fault density increases with increasing Ni content up to 33.1 at.%. The high resolution transmission electron microscope (HRTEM) analysis revealed that the planar faults are related to the local change in the stacking sequence of the three 36-nets (triple layer) of the C14 structure. These results suggest that the presence of both, the point defects (Fe sublattice sites occupied by Ni atoms) and the anti-site defects (Nb sublattice sites occupied by excess Fe atoms), facilitate the formation of the planar faults. INTRODUCTION We have systematically investigated the phase equilibria between γ-Fe and Fe2M Laves phases in Fe-M-Ni ternary systems (M: Nb, Ti, Mo) at elevated temperatures in order to seek the possibility for the development of a new type of austenitic heat resistant steel strengthened by intermetallic compounds [1-5]. In those studies, we revealed that the homogeneity region of the Fe2Nb Laves phase with C14 structure extends towards the equi-Nb concentration direction up to 44 at.% Ni [1,2]. This indicates that more than two thirds of the Fe sublattice sites in Fe2Nb can be replaced by Ni atoms. In addition, the hardness of Fe-rich Fe2Nb decreases significantly from 9 GPa to 6 GPa with increasing Ni content [1]. This significant change should be somehow related to the defect structures created by Ni in solution. In the present study, we have examined the lattice structure of Fe2Nb with and without Ni in solution using transmission electron microscopy. EXPERIMENTAL PROCEDURE Two series of alloys previously prepared for the phase diagram studies [1,2] were used; one is stoichiometric Fe-33.3Nb alloys containing Ni up to 20 at.% (Fe-33.3Nb-(0-20)Ni) and the other is Fe-15Nb alloys containing Ni up to 40 at.% (Fe-15Nb-(0-40)Ni). These alloys were prepared by arc-melting and equilibrated at 1473 K/240 h. The experimental methods were described in detail elsewhere [1,2]. The stoichiometric alloys exhibited the single phase Fe2Nb,
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