A structural study of oxidation in a zirconia-toughened alumina fiber-reinforced NiAl composite
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I.
INTRODUCTION
THE intermetallic compound NiAI exhibits a range of attractive properties, such as high melting temperature (1640 ~ low density (5.86 g/cm3), high modulus (297 GPa), 11'21and good resistance to oxidation,131which make NiA1 a suitable candidate material for use at elevated temperatures. However, inadequate roomtemperature toughness14] and poor tensilet5'6'7] and creep strengthI8,91 have prevented the use of NiA1 in aeroturbine engines. These deficiencies may possibly be circumvented by reinforcing NiAI with a suitable ceramic fiber. Among the various ceramic fibers that are currently available, the Al203-based fibers (PRD-166* and FIBER FP*) appear to be a suitable choice for reinforce*PRD-166 and Fiber FP are trademarks of E.I. Du Pont de Nemours, Inc., Wilmington, DE.
ment. This is due to their chemical compatibility with N i A I , a coefficient of thermal expansion (9 • 10 -6 ~ which is closer to that of NiAI (12.5 • 10 -6 ~ t h a n S i C fiber (4.86 • 10 -6 ~ high elastic modulus, l~~ high room-temperature strength, t~~ and high creep resistance, l~31 Since NiAI is highly resistant to oxidation, the composites of NiAI reinforced with continuous A1203 fiber are also expected to be resistant to oxidation. The aim S. NOURBAKHSH, Professor and Department Head, O. SAHIN, Postdoctoral Research Associate, and H. MARGOLIN, Distinguished Research Professor, are with the Department of Materials Science and Engineering, Polytechnic University, Brooklyn, NY 11201. Manuscript submitted June 23, 1993. METALLURGICALAND MATERIALS TRANSACTIONSA
of the present study is to report on the effect of heat treatment, in vacuum and air, on the stability of microstructure in a NiA1 intermetallic composite reinforced with an A1203-based fiber, PRD-166. PRD-166 is a 20-/zmdiameter polycrystalline a-A1203 fiber which contains 20 wt pct partially stabilized ZrO2. Pressure casting has been employed to fabricate the composite. The outcome of this investigation demonstrated that, although NiAI and A1203 fiber are both resistant to oxidation, a composite made of these materials offers negligible resistance to oxidation.
II.
EXPERIMENTAL PROCEDURE
The intermetallic base ahoy used in the investigation was produced by arc melting of its pure constituents under Ar. The NiA1 alloy had a nominal chemical composition of Ni-45Al-lTi (all compositions in atomic percent). Titanium was added to facilitate the infiltration of the fibers by the molten NiA1 during pressure casting. The experimental procedure used to fabricate the PRD-166 fiber-reinforced NiA1 composite is described in Reference 14. The composite fabricated was in the form of a disk 50-mm in diameter and 15-mm thick, with fibers unidirectionally aligned parallel to the flat face of the disk. The cast composite disk was cut into 6 • 6 x 6 mm pieces. These pieces were heat-treated either in a vacuum of 10-2 Pa or in air at 1100 ~ for times ranging from 2 to 48 days, followed by air cooling to room temperature. For transmission electron microscopy examination, 0.5mm-thick s
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