Mechanical properties of nickel beryllides
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C. T. Liu Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, Tennessee 37831-6115 (Received 16 May 1989; accepted 1 August 1989)
The elastic properties of nickel beryllide have been evaluated from room temperature to 1000 °C. The room temperature modulus is measured to be 186 GPa, which is relatively low by comparison with other B2 aluminides such as NiAl and CoAl. Hardness measurements were carried out on specimens that had compositions over the range from 49 to 54 at. % Be, using both a Vickers microhardness tester and a nanoindentor. It was found that the hardness of NiBe exhibits a minimum at the equiatomic composition. This behavior is similar to that of aluminides of the same crystal structure, e.g., NiAl and CoAl. The effect of interstitial oxygen on the hardness of NiBe has also been studied and the results show that the presence of oxygen in NiBe can cause a significant increase in hardness. It is demonstrated that the hardness increase for the off-stoichiometric compositions is primarily caused by interstitial oxygen and can only be attributed partially to anti-site defects generated in off-stoichiometric compositions. Nickel beryllides appear to have some intrinsic room temperature ductility, as evidenced by the absence of cracking near hardness indentations.
I. INTRODUCTION Advanced aerospace and aircraft systems, e.g., hypersonic vehicles, will depend strongly on the availability of high temperature structural materials which are both strong and lightweight. Most conventional metal alloys lose their mechanical strength at temperature above 0.5 Tm, where Tm is the absolute melting point of the alloys; an exception is the group of ordered intermetallic alloys.1"3 This is because diffusion processes which lead to softening in an ordered lattice are generally sluggish compared to those in conventional metal alloys. The major disadvantages of intermetallic alloys are that, because of their complex crystallographic structure, they are brittle and exhibit a relatively high ductile-to-brittle transition temperature4'5; also they are often too dense for aerospace applications. Amongst all of the ordered intermetallic alloys, the beryllide group offers the greatest promise of low density. In the early 1960s, researchers concluded that some refractory beryllides do exhibit good mechanical and oxidation properties at high temperatures. 6 Despite the fact that beryllides possess a great potential for high temperature structural applications, particularly at temperatures above 1000 °C, research in the area of beryllides was terminated because of the difficulties of reducing impurity levels and the room temperature brittleness problems associated with the compounds. The generic brittleness problem in ordered intermetallics has been studied heavily in recent years and some breakthroughs have been made.7"9 For example, by proper control of chemical stoichiometry, NiAl can exhibit some J. Mater. Res., Vol. 4, No. 6, Nov/Dec 1989
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