Creep of a niobium beryllide, Nb 2 Be 17
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T. C. Chou Lockheed, O/93-10, B/204, 3251 Hanover Street, Palo Alto, California 94304-1191
D. Owen and A. H. Chokshi Department of Applied Mechanics and Engineering Sciences, University of California, San Diego, La Jolla, California 92093-0411 (Received 23 July 1992; accepted 7 December 1992)
A niobium beryllide, Nb 2 Bei 7 , has been prepared by powder-metallurgy techniques and the mechanical properties characterized both at room and elevated temperatures. Microhardness and fracture toughness were measured at room temperature. Hardness and hot-hardness test results indicated that, although the material was brittle at low temperatures, it became plastic at elevated temperatures (>1000 °C). Creep properties of Nb 2 Be 17 were studied at temperatures from 1250 to 1350 °C and applied stresses from 10 to 90 MPa. The stress exponent, determined from stress-change tests, was about 3, and the activation energy, determined from temperature-change tests, was about 575 kJ/mol. The creep of Nb 2 Be 17 at high temperature is apparently controlled by dislocation glide; this proposal was supported by transient creep experiments. Comparisons have been made between the creep properties of Nb 2 Bei 7 and other intermetallics.
I. INTRODUCTION Amongst all of the ordered alloy groups, the beryllides possess the greatest potential for high-temperature structural applications, particularly at temperatures above 1000 °C. This is because the beryllides have low density, good high-temperature strength, and good oxidation resistance.1"3 Extensive studies on beryllides were undertaken in the early 1960s,4"7 but the research was terminated as a result of the difficulties of reducing impurity levels and room temperature brittleness problems. Recently, the interest in using beryllides has been revived because conventional materials cannot meet the sophisticated structural requirements of advanced flight vehicles. Beryllides, and in particular refractory metal beryllides, are considered to offer the best potential of meeting the high temperature properties required for these applications. Despite the fact that almost all refractory metal beryllides are brittle at room temperature, as an intrinsic result of their extremely complex crystal structures, they can, nonetheless, be plastic at elevated temperatures. For example, Nieh and Wadsworth demonstrated that a ductile-to-brittle transition occurs at a temperature that is about 0.50-0.55 of the homologous temperature for refractory metal beryllides.8 Also, recently, Bruemmer et al.9 observed the operation of a large number of dislocation slip systems in NbBe12 deformed at 1200 °C. For example, there exist 1/2(101] {121), 1/2(101] {101), and 1/2(100] {011) partial dislocations, and 1/2(111] {101) and 1/2(111] {121) perfect dislocations. J. Mater. Res., Vol. 8, No. 4, Apr 1993 http://journals.cambridge.org
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The creep properties of many intermetallics, and in particular the aluminides (e.g., TiAl,10 Ti 3 Al, n NiAl,12'13 CoAl,13-14 and Ni 3 Al 15 ) have been widely studied
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