Ni 3 Al Thin Foil by Cold Rolling
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Ni3Al Thin Foil by Cold Rolling Toshiyuki Hirano, Masahiko Demura, Kyosuke Kishida and Yozo Suga1 National Research Institute for Metals, 1-2-1 Sengen, Tsukuba, Ibaraki 305-0047, Japan 1 Nippon Cross Rolling Co. 697 Mobara, Chiba 297-0026, Japan ABSTRACT Thin foils of stoichiometric Ni3Al below 100 µm in thickness were successfully fabricated by cold rolling of the sheets which were sectioned from directionally solidified ingots. Maximum rolling reduction in thickness amounted to 96%, irrespective of the initial orientation or the existence of columnar grains in the starting sheets. The as-rolled foils were characterized in terms of microstructures, textures and dislocation structures. The deformation microstructures were of a dual banded structure composed of two different {110} textures in the case of rolling direction, while a rather homogeneous structure with a single {110} texture resulted in the case of rolling direction. TEM observation revealed homogenous dislocation structures in either case without cell formation, accompanied by very fine grained-regions at higher reduction. INTRODUCTION Considerable efforts have been made in recent decades to use Ni3Al as high-temperature structural materials in a bulk form [1]. In contrast, we are focusing on a plan to use it in a foil form, e.g. honeycomb structure known as a lightweight and high-rigidity structure. However, at present it is impossible to cold-roll to thin foil below 100 µm in thickness, because of severe intergranular brittleness [2]. Even with the beneficial effect of boron addition which Aoki and Izumi discovered [3], the ductility is not enough to reduce the thickness below 800 µm [4,5]. Alternatively, we found that directional solidification (DS) using a floating zone (FZ) method provides us a significant ductility improvement for Ni3Al without any boron additions [6,7]. The DS materials with columnar-grained structure show very high tensile elongation, more than 70% in ambient air [7,8]. The high ductility is ascribed to the large fraction of low angle and low-Σ value boundaries [9]. Using the same technique we have succeeded in growing single crystals of binary stoichiometric Ni3Al [10,11]. As is well known, single-crystalline Ni3Al has substantial ductility, more than 100% elongation [2]. Taking advantage of the high ductility of the DS materials, we fabricated thin foil by cold rolling in this study. We present the details of the results. EXPERIMENTAL Four rods, designated as Nos. 31-2, 41-1, 42-2, and 47-1, of boron-free binary stoichiometric Ni3Al were grown by a FZ method in the same way as previously described [6]. Optical microscopic observation showed that Nos. 31-2 and 47-1 were mostly single crystal but contained columnar grains with low angle boundaries in places, while Nos. 41-1 and 42-2 were fully single crystals. Table 1 summarizes the Al contents of the grown rods. The grown rods were sectioned into sheets along the growth direction by electric discharge machining. The initial rolling direction (RD) or the growth direction and norm
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