Microstructures of Ni 3 A1 Rapidly Solidified by Hammer-Anvil Technique
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MICROSTRUCTURES OF Ni3 Al RAPIDLY SOLIDIFIED BY HAMMER-ANVIL TECHNIQUE YOSHINAO MISHIMA, SHIROU SASAKI AND TOMOO SUZUKI Research Laboratory of Precision Machinery and Electronics, Tokyo Institute of Technology, Nagatsuta, Midori-ku, Yokohama 227, Japan *Graduate Student, Department of Materials Science and Engineering, Tokyo Institute of Technology, Nagatsuta. Now with Showa Denko K.K., Shimokagemori, Chichibu, Saitama 369-18, Japan ** Department of Metallurgical Engineering, Tokyo Institute of Technology, O-okayama, Meguro-ku, Tokyo 152, Japan.
ABSTRACT The Hammer-Anvil technique is employed for the rapid solidification of Ni3 Al in the present work. The effect of off-stoichiometry on the change in the microstructure in the quenched alloy is examined for compositions within the L1 2 phase field at equilibrium. It is shown that non-equilibrium phases of various morphologies appear with fine grains of Ll2 phase, which have never been reported for the compound rapidly solidified by the roll techniques. With increasing aluminum concentration and with increase in cooling rate, the latter being judged by the difference in the thickness of the quenched foil, the fraction of the non-equilibrium phases appearing is found to increase giving more complex microstructure.
INTRODUCTION The Ll2 Ni3 Al is a Berthollide-type compound having compositional range within the stable phase field and yet maintaining its ordered structure up to the melting point. It is the reason why there has been no report on observing the thermal antiphase domain boundary(APB) in this compound fabricated by conventional castings. The rapid solidification technique has originally been applied to the compound with the expectation of being able to provide an improved ductility despite the inherent brittleness. There have been several reports showing fine equi-axed grains of L1 2 phase with well defined APBs within them in the compound formed using single or twin roll technique[l-5]. In the present work, the Hammer-Anvil technique is employed which is thought to provide an order of magnitude faster cooling rate than the conventional roll methods. The microstructures of the foils are examined for alloys with different Al concentration ranging from 24 to 26 at%. It is also possible to examine the effect of cooling rate on the microstructure of the alloy. This comes from the fact that the rapidly quenched foil has a thickness gradient from the center to circumference and hence it is assumed that the cooling rate is faster for the thinner part of the foil. The results obtained provide useful information on the stability of the Ll 2 phase with respect to the other intermetallic phases appearing on the binary Ni-Al phase diagram.
EXPERIMENTAL PROCEDURES Binary Ni3 Al alloys with varying aluminum concentration within the stable L1 2 phase field, being 24 to 26 at%Al, were prepared by arc melting under an argon atmosphere using the highest purity raw materials readily Mat. Res. Soc. Symp. Proc. Vol. 186. @1991 Materials Research Society
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