Triclinic Ni 2 AI phase in 63.1 atomic percent NiAl
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INTRODUCTION
I N nickel-rich NiA1 alloys, an ordered phase of Ni2A1 stoichiometry is known to form upon aging at intermediate temperatures. [~-9] The crystal structure of the Ni2AI phase existing in a matrix B2 phase was first determined by Reynaud m in 62.5 and 62.25 at pct NiA1 alloys annealed at 300 to 400 ~ for a week in the B2 phase. The structure of this phase was described, based on a hexagonal symmetry (space group: P 3 m l , no. 164), with the unit cell parameters aNi2Al = V~aB2 and CNi2AI --~ V/-3CB2. This hexagonal Ni2A1 phase was characterized by the presence of extra reflections at (n/3) (111) and (n/3) (211) B2 in the electron diffraction pattems. Aging of the L10 phase also results in formation of the Ni2A1 phase, the crystal structure of which was shown to be monoclinic, belonging to the space group C 2 / m . tSl From the electron-diffraction studies from 63.8 Ni 1 Co A1 alloys aged at 400 ~ Enami tS] derived the unit cell parameters of this monoclinic Ni2A1 phase as a [0T2k~0, b ~ [100]L10, C --~ [011]Lt0, and /3 = 80 deg. The presence of this phase was characterized by extra reflections at (n/3) (022) and (n/3) (021) in [100] L10, along with the main reflections from the L10 phase. In these studies, Enami t~j reported that no changes occurred in the microstructure, observed both optically and by transmission electron microscopy (TEM), upon aging the L 10 phase and the consequent formation of the monoclinic Ni2AI. Murthy and Goo, [91 however, were able to characterize the Ni2A1 precipitates existing in both the B2 and L10 phases in 63.1 at pct NiA1 on aging for different periods of time at 500 ~ By conventional TEM and high-resolution electron microscopy (HREM), the authors also showed in these studies that the precipitates of metastable hexagonal Ni2A1 existing in coherence with the B2 matrix phase transform martensitically and concurrently with the martensitic transformation of the B2 NiAI matrix to the L10 phase. Aging of the L10 phase in bulk at intermediate temperatures is also known to result in the formation of small localized regions which indicate the 7R phase on cooling, t6'~~ along with ordered Ni2A1 precipitates. Such E D W A R D GOO, Associate Professor, and A N A N D A S. MURTHY, Graduate Student, are with the Department of Materials Science and Engineering, University of Southern California, Los Angeles, CA 90089-0241. Manuscript submitted May 7, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
localized regions in the L10 phase are characterized by the presence of very high-density twins that are periodic or aperiodic. By extensive HREM combined with optical diffraction studies, the present authors recently characterized such microtwins in the L10 phase and evidenced the presence of stacked structures of varying stacking periodicities, t~~ In these studies a stacking periodicity of 7, which corresponds to the 7R structure, is observed to be the most predominant. The morphology and crystal structure of the Ni2A1 precipitates present in such regions of the L10 phase which are re
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