Surface oxide on fcc iron-nickel alloys
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K.B. REUTER, D.B. WILLIAMS, J.I. GOLDSTEIN, and E.P. BUTLER A transformation of interest in the Fe-Ni system is the disorder-to-order transformation whereby disordered Fe-Ni with the fcc structure transforms to ordered FeNi with an L10 superstructure. This transformation can be studied with the transmission electron microscope (TEM) because the superstructure gives rise to superlattice reflections in the selected area diffraction patterns (SADPs). A problem arises, however, because an oxide surface layer forms on thin-foil Fe-Ni specimens giving rise to extra reflections that may be confused with those arising from internal precipitation reactions.l'2'3 Through the work of Chen and Morris 4 this confusion has been alleviated for bcc Fe-Ni alloys. This study extends the work of Chen and Morris to include Fe-Ni alloys with the fcc structure with the aim of discriminating oxide and superlattice reflections in SADPs. Fe-35 wt pct Ni and Fe-50 wt pct Ni alloys were prepared from high purity (99.998 pct) Fe and Ni, homogenized, and rolled to 10 to 12 mils. Discs, 3 mm in diameter, were punched from the sheet and austenitized. The discs were thinned to 3 to 4 mils by grinding on fine SiC papers. Thin-foil specimens were then prepared in one of three ways: jet-polishing, ion-beam thinning, or jet-polishing followed by ion cleaning. The jet-polishing was carried out using a Fischione twin-jet electropolisher operated at 105 V using a 2 pct perchloric/98 pct ethanol solution at - 4 0 ~ Ion-beam thinning was performed on a Technics ion-beam milling machine with the specimen at a 15 deg angle to the 6 kV ion beam. Ion cleaning was also carried out on the Technics; the specimen was at a 12 deg angle to the ion beam and cleaned for 20 minutes. The Fe-35 wt pct Ni specimens were examined in a Philips EM300 operating at 100 kV and in a Philips EM400T operating at 120 kV. The Fe-50 wt pct Ni specimens were observed in an AEI-EM7 high voltage electron microscope (HVEM) operating at 1 MeV. TEM examination of all of the specimens revealed extra reflections in the SADPs. By assuming that a surface layer of NiFe204 or F e 3 0 4 had formed, all of the extra reflections could be accounted for. The lattice parameters of Fe304 (8.396 A) and NiFe204 (8.339 A) are too similar to determine which of the oxides was present using conventional electron diffraction. Chen and Morris found that the same oxides form on bcc Fe-Ni thin-foil specimens. K.B. REUTER, Research Assistant, and D.B. WILLIAMS and J. I. GOLDSTEIN, Professors, are with the Department of Metallurgy and Materials Engineering, Lehigh University, Bethlehem, PA 18015. E.P. BUTLER, formerly Lecturer, Department of Metallurgy and Materials Science, Imperial College, London SW7 2BP, England, is now with Alcan International Ltd., Banbury, Oxon OX16 7SP, England. Manuscript submitted November 28, 1984. METALLURGICALTRANSACTIONS A
Specimen preparation had an effect on the oxide layer formed. Specimens that were ion-beam thinned or jet-polished followed by ion cleaning had many oxide reflec
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