Microstructure and mechanical properties of NiAl/Al 2 O 3 composites
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Microstructure and mechanical properties of NiAlyAl2 O3 composites Chii-Shyang Hwang and Tien-Jui Liu Department of Materials Science and Engineering, National Cheng-Kung University, Tainan, Taiwan, Republic of China (Received 15 August 1997; accepted 9 February 1998)
To improve mechanical properties of NiAl, a method for making NiAl matrix composites containing oxide ceramics is introduced. The method involves oxidation of NiAl powder in air to form a thin and uniform oxide scale, mainly Al2 O3 , on the NiAl particles. The Al2 O3 contents increase with increasing oxidation temperature. The NiAlyAl2 O3 composites are then formed by hot-pressing the oxidized NiAl powder under vacuum atmosphere. Al2 O3 inhibits the grain growth of NiAl during the hot-pressing. The residual stress and the Ni-rich NiAl composition exist in the hot-pressed NiAlyAl2 O3 composites. Strength and toughness data on NiAlyAl2 O3 composites indicate that the use of oxidation of NiAl powder is a viable technique for improving these properties over that of monolithic NiAl.
I. INTRODUCTION
Nickel aluminide (NiAl) is a kind of potentially high temperature material because of its high melting point (1638 ±C), large elastic modulus (294 GPa), and good oxidation resistance. The thermal conductivity of NiAl is 3 to 8 times that of Ni-base superalloys. The density of NiAl is lower than that of superalloys. The potential of using NiAl in aeronautical engineering and nuclear energy industries is thus high.1–4 However, NiAl exhibits the disadvantages of general intermetallic compounds, i.e., the brittleness at low temperature and the inadequate strength at high temperature. To overcome these properties much work has been done. Except doping with Nb, Mo, Ti, etc. elements, ceramics such as TiB2 ,5,6 HfC,7 ZrO2 ,8,9 and Al2 O3 10 have been added to nickel aluminide matrix to form composites. For the composites containing Al2 O3 , there are several processing techniques, including cryomilling NiAl and then following by HIP and extrusion,11 pressure casting,12,13 and hot-pressing of NiAl powder and Al2 O3 whisker.14 A method to prepare NiAlyAl2 O3 composites is introduced in the present study. The method contains two steps: (a) oxidizing the NiAl powder to form a thin and uniform oxide scale on the surface of NiAl powder, and (b) hot-pressing the oxidized NiAl powder to form NiAlyoxide composites. Effects of oxidizing conditions on the microstructure and mechanical properties of NiAlyoxides composites were investigated. II. EXPERIMENTAL PROCEDURES
A NiAl powder with particle sizes about 100 mesh and a composition Ni–68.3, Al –31.3 (49.97 at. %), O–0.28, Fe –0.042, Si–0.006 wt. % (Kyouritsu Ceramic J. Mater. Res., Vol. 14, No. 1, Jan 1999
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Materials Co. Ltd.) was ball-milled in an alumina pot mill for 24 h. After milling, the average particle size was 12.8 mm. The NiAl powder was oxidized at a temperature range from 700 to 1000 ±C for 2–4 h in air. The oxidi
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