Room-temperature mechanical properties of cold-rolled thin foils of binary, stoichiometric Ni 3 Al
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I. INTRODUCTION
SINCE the intermetallic compound Ni3Al has excellent high-temperature properties,[1] Ni3Al thin foils could be used as lightweight, high-temperature structural materials in the form of a honeycomb structure. Unfortunately, use of the foils has been unrealistic so far, because of the grain-boundary brittleness.[2] As is well known, the brittleness can be overcome by microalloying with boron,[3,4] but even with all the beneficial effects, the ductility is not sufficient to fabricate the thin foils on an engineering scale, e.g., by cold rolling.[5] We previously found that the directional solidification by the floating-zone (FZ) method is very effective in improving the ductility of Ni3Al without additions of any ternary elements.[6,7] By using the ductile, directionally solidified ingots, we, for the first time, have succeeded in fabricating thin foils less than 100 m in thickness by cold rolling without any intermediate annealing steps.[8] Figure 1 shows an example of the foils: a 1-m-long, 10-mm-wide, and 90m-thick foil cold-rolled to a 96 pct reduction in thickness. The surface is crack-free and smooth, with a shiny metallic luster. It was found that the recrystallized foils possessed some room-temperature tensile ductility (3.0 to 14.6 pct) in air.[8] Thus, the grain-boundary brittleness is not a serious problem, which means that the foils are applicable to hightemperature use. Since the Ni3Al foils have just been fabricated, their fundamental properties are not well studied at present. The purpose of this article is to present the room-temperature mechanical properties of the cold-rolled foils, the knowledge of which MASAHIKO DEMURA and KYOSUKE KISHIDA, Researchers, and TOSHIYUKI HIRANO, Senior Researcher, are with the National Institute for Materials Science, Tsukuba, Ibaraki 305-0047, Japan. Contact e-mail: [email protected] YOZO SUGA, Vice-President, is with Nippon Cross Rolling Corporation, Mobara, Chiba 297-0026, Japan. Manuscript submitted September 4, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
is important in assembling the honeycomb structure. In our previous study,[9] it was found that there were two types of deformation microstructures in the cold-rolled foils, depending on the initial rolling direction. In this article, the mechanical properties such as Vickers hardness, tensile properties, and bending ductility were examined for both the microstructures. II. EXPERIMENTAL Three boron-free single-crystalline ingots with different growth orientations, designated as samples 47-1, 42-2, and 41-1, were grown by the FZ method. The feed-rod preparation and the crystal growth procedure were previously described.[6] The Al contents of the grown ingots were determined by inductively coupled plasma spectroscopy. As summarized in Table I, the Al contents are close to the stoichiometric composition (25 at pct Al). The grown ingots were sectioned into sheets (2 mm in thickness) along the growth direction by electrodischarge machining (EDM), and the sheet surfaces were mechanically p
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