Lattice modulation and formation of lamellar duplex ferrite/cubic carbide microstructure in rapidly solidified Fe-Ni-Al-
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I.
INTRODUCTION
MODULATED structure of many substitutional alloys, such as Au-Pt, ~Au-Ni, 2 Cu-Ti, 3 Ni-Ti, 4 Cu-Ni-Fe, 5 Cu-NiSn, 6 and F e - M o 7 alloys have been adequately investigated. On the other hand, occurrence of interstitially induced modulated structures is rare because structural stability of interstitial alloys in the phase decomposition temperature range more or less favors the formation of the precipitate which is polymorphic to the parent phase. For this reason, modulated structures of interstitial alloys have been reported sporadically. In the meantime, it has been recently shown that the carbide precipitate of ferrous alloys high in A1 and C, such as Fe-7 wt pct AI-2.0 wt pct C 8 and Fe-31 wt pct Mn-8.8 wt pct Al-l.2 wt pct C 9 is found to be isostructural to the parent T-phase, and therefore, these austenitic alloys have been reported to form modulated structures during the early heat-treatment stage9 or even during waterquenching. 8 No distinction was made, however, as to the structure of the bulk precipitate phase from that of the discontinuous precipitate phase, of which distinction will be made later in this paper. The T-phase stability of Fe-7 wt pct A1-2 wt pct C is very tenuous at elevated temperatures at best. A (100) darkfield transmission electron (TE) micrograph of a waterquenched specimen was given, 8 but it showed a dotted contrast indicative of fine particle precipitation that could have occurred during quenching. The apparent elongated (200) diffraction spot was taken to correspond to two spots. Thus the matrix of the water-quenched specimen was deduced to be composed of two isomorphous phases. Because of high temperature instability, no heat treatment effect of this alloy was investigated. The structural change on heat treatment of a single crystalline Fe-31 wt pct Mn-8.8 wt pct AI-1.2 wt pct C has been investigated by the X-ray diffraction method. 9 Since the austenite phase of the latter alloy is
WOONG K. CHOO, Professor, and DONG G. KIM, Graduate Student, are with the Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, P.O. Box 131, Cheongryang Seoul, Korea. Manuscript submitted April 24, 1986. METALLURGICALTRANSACTIONS A
relatively stable in the 873 K range, the background diffraction change of the (200) and (220) spots on heat treatment was investigated. The (220) diffraction spot became progressively diffuse, but this spot was not resolved to indicate clearly the directionality of the sidebands. As for 3,-phase Fe-Ni-A1-C alloys, TEM investigation of rapidly solidified (RS) samples was previously conducted by Inoue et al. io They reported a fine modulated domain structure and the presence of superlattice reflections. Based on the observation, they suggested formation of L12 (Cu3Au type) structure during the rapid solidification process (RSP). Recently, Han and Choo It showed how X-ray (200) sidebands of a series of RS Fe-Mn-A1-C and Fe-Ni-A1-C could be controlled. In the work, they were able to completely suppress the si
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