Bending Ductility of Heavily Cold-Rolled Ni 3 Al Thin Foils
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BENDING DUCTILITY OF HEAVILY COLD-ROLLED Ni3Al THIN FOILS Satoru Kobayashi*, Masahiko Demura, Kyosuke Kishida and Toshiyuki Hirano, National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki, 305-0047, JAPAN. TEL: +81-298-59-2415, FAX: +81-298-59-2501. E-mail: [email protected] ABSTRACT Our recent studies revealed that heavily cold-rolled Ni3Al foils have a good bending ductility in spite of almost no elongation in tensile test. In this paper, bending characteristics of 95% coldrolled foils around transverse and rolling directions (TD and RD, respectively) were examined to understand the bending mechanism. Fracture elongation on the tension surface shows a large bending anisotropy: 5 % for the TD bending, while less than 1% for the RD bending. The bending ductility is due to {111} slip deformation. In the TD bending, slip occurs on the slip systems operated during cold rolling, and cracks initiate along the shear bands. In the RD bending, slip occurs on the other {111} planes besides the slip planes operated during cold rolling, and fracture occurs as a result of the dislocation interaction in the both planes. INTRODUCTION Recently we have succeeded in fabricating thin foils of binary Ni3Al by cold rolling of the single or near single crystals without intermediate annealing [1-3]. The thinnest foils obtained so far are 23 mm in thickness with 99% reduction [3]. By using these foils, we have tried to fabricate the honeycomb structure [4], which is known as lightweight structure with high strength and stiffness. The honeycomb structure of Ni3Al foil is expected for use in high-temperature structural/functional applications. Our previous studies revealed that the foils have very high fracture strength (1.3-2.0 GPa) at room temperature, depending on the reduction, deformation texture and tensile direction, i.e. parallel or perpendicular to the rolling direction (RD) [2, 3]. And these extremely work-hardened foils fracture with almost no elongation in tensile tests [2, 3]. However, the foils possess good ductility in bending tests. The 95% cold-rolled foils could be plastically bent about 90° around the transverse direction (TD) of cold rolling. This corresponded to about 12% elongation on the tension surface [2]. It is interesting to understand the two seemingly inconsistent results on tensile ductility. In order to know this point, it is important to investigate the bending mechanism. It is also important from a viewpoint of application because the good bending ductility is favorable for forming corrugation structure, which leads to honeycomb structure fabrication [3]. In this paper, we performed bending tests of the cold-rolled Ni3Al foils around the TD and RD and investigated their ductility, deformation markings and cracks initiation to understand the bending mechanism. EXPERIMENTAL PROCEDURE The samples used in this study are 95% cold-rolled foil (87 mm in thickness) with a nominal composition of binary _Ni-24 by cold rolling a single crystal with ___ at% _Al. ___The foil was prep
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