Properties and Potential of High-Temperature Niobium Beryllides
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Properties and Potential of High-Temperature Niobium Beryllides S. M. Bruemmer, J. L. Brimhall, C. H. Henager, Jr. and J. P. Hirth* Pacific Northwest Laboratory, Richland, WA 99352 "Washington State University, Pullman, WA 99164 Abstract Recent research on the low- and high-temperature properties of two berylliumniobium intermetallic compounds, Be12Nb and Be17Nb2, is reviewed and discussed. Strength (bend and compression), hardness and fracture toughness has been mapped as a function of test temperature up to 1200 0 C. Results for hotisostatically-pressed Be12Nb and Be17Nb2 are highlighted illustrating the potential for reasonable strength at both low and high temperatures. Chemical compatibility between Be12Nb and several high-temperature materials such as SiC, A120 3, MoSi2 and various refractory metals is evaluated. Limitations for the structural use of the beryllides are identified and discussed including low-temperature toughness, intermediate-temperature embrittlement, high-temperature creep strength and composite compatibility. Introduction Beryllium-refractory metal intermetallic compounds, i.e. Be12X, Be1 3X and Be, 7X2, exhibit excellent strength and oxidation resistance at high temperatures. Because of these properties and their very low densities, beryllides are promising candidate materials for aerospace applications. The combination of properties exceeds other intermetallics such as refractory metal silicides and aluminides in most cases. Unfortunately, similar to many other intermetallics, beryllides have complex crystal structures which lead to poor low-temperature toughness. The Be12X compound is the simplest with a body-centered-tetragonal (bct) structure (14/mmm) and 26 atoms per unit cell. Elements such as Nb, Mo, Ta, Ti, Cr, V and W all form isostructural Be12 X compounds with little change in the lattice dimensions. A number of metallic elements also form isostructural Be, 3X (cubic, Fm3c) and Be17 X2 (hexagonal-rhombahedral, R3m) compounds as summarized in Figure 1. Phase stability in beryllides (e.g., formation of Be, 3 X versus Be12 X) depends on atom size of the metal atom (XMreacting with Be and, to a lesser extent, its bonding character.1 Face-centered-cubic (fcc) Be13 X is favored when the diameter of metal atom (d,) is greater than - 0.3 nm, while Be12X is stable for smaller atom sizes, d× = 0.25 to 0.29 nm. Among the Be12X compounds, larger (>0.28 nm) metal atoms (e.g., Nb, Ta and Ti) also form Be17 X2, but smaller ones (e.g., Cr, Mn, V, Mo and W) do not. Recent work by the authors on Be12Nb and Be17Nb2 intermetallics is reviewed with emphasis on: (1) phase stability, (2) mechanical properties, (3) deformation mechanisms and (4) composite compatibility. Specific properties of these compounds are discussed in relation to their potential for high-temperature structural applications. Mat. Res. Soc. Symp. Proc. Vol. 288. @1993 Materials Research Society
800
l Be13 X Only IM Be13X + Be 7X2
11A
I1B
IVB
VB
BeX
•
Be12 X +
M
Be1 2X Only
VIB
2
VIIB
Figure 1. Refractory and trans
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