Influence of Stoichiometry on the Strength of Nickel Beryllide
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INFLUENCE OF STOICHIOMETRY ON THE STRENGTH OF NICKEL BERYLLIDE T.G. Nieh*, J. Wadsworth* and C.T. Liu+ * Lockheed Missiles and Space Co., Inc., Research and Development Division, 0/93-10, B/204, 3251 Hanover Street, Palo Alto, CA, 94304 + Metals and Ceramics Division, Oak Ridge National Laboratory, P.O. Box 2008, Oak Ridge, TN 37831-6115 ABSTRACT It is demonstrated that the hardness of NiBe is dependant upon its stoichiometry; a minimum hardness is observed at the equiatomic composition. This behavior is similar to intermetallics that have the same crystallographic structure, e.g., NiAI and CoAl. The hardness increase for the off-stoichiometric compositions is a result of defect and anti-site defect structures, but may also, in part, be caused by interstitial oxygen. Nickel beryllides appear to have some intrinsic room temperature ductility, as evidenced by the absence of cracking near hardness indentations. INTRODUCTION Advanced aerospace systems of the future, e.g., hypersonic vehicles, will depend strongly on the availability of high temperature structural materials which are both strong and lightweight. Unfortunately, the mechanical strength for almost all structural materials decreases as the temperature increases; an exception is the group of long-range-order (LRO) alloys [1-3]. The major disadvantages of LRO alloys is that they are brittle in nature, because of their complex crystallographic structure, and they are often too dense for aerospace applications. Amongst all of the LRO alloys, the beryllide group offers the greatest promise of low density. In the early 1960's, researchers concluded that some beryllides do exhibit good high temperature properties. A direct comparison of the mechanical properties of some of these beryllides with current state-of-the-art materials is presented in Fig. 1 [4]. It is obvious from this figure that beryllides possess a great potential for high temperature structural applications, particularly at temperatures above 1000 0 C. Research in the area of beryllides was, however, terminated because of the difficulties of reducing impurity levels and the room temperature brittleness problem associated with the compounds. The generic brittleness problem in intermetallics has been studied heavily in more recent years and some breakthroughs have been made. For example, polycrystalline Ni 3 Al exhibits almost no ductility, but if it contains a small amount (0.2 to 0.5 wt.%) of B, then a room temperature tensile ductility of up to 40% is observed [5]. The high density of Ni 3 Al remains an intrinsic problem. Nickel beryllide, i.e., NiBe, is of particular interest for fundamental studies because it has a relatively simple, ordered, B2 crystal structure and exhibits a wide range of stoichiometry. These features are similar to those found in NiAl and CoAl. In the cases of NiAl and CoAl, it was found that they exhibit a hardness minimum at the equiatomic composition. The hardness increases observed in off-stoichiometric compositions were attributed to defect and anti-site defect structure
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