High temperature mechanical properties of Cr 2 Nb-based intermetallics
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The results of high-temperature compressive deformation tests on a two-phase alloy consisting of Cr 2 Nb, a C15 Laves phase, and a soft Nb-based solid solution, are presented along with measurements of oxidation kinetics at 1273 K in air. These alloys are deformable only at temperatures above 1273 K. The measured 0.2% yield stress decreases steadily with increasing temperature and is only slightly sensitive to alloy composition. The steady state flow stress decreases steadily with increasing temperature and depends on alloy composition. A constitutive equation was fitted to the experimental data with a composition-independent stress exponent of about 2.7 and an apparent activation energy which ranges between 477 and 391 kJ/mol, also depending on alloy composition. Microstructural examination shows that cracking (cavitation) and interfacial sliding between the two constituent phases, in addition to bulk deformation of the constituent phases, are responsible for the deformation. The oxidation resistance of these alloys is very good.
a high melting temperature (-2003 K), low density (—7.7 g/cm 3 ), and potential resistance to corrosion and
I. INTRODUCTION Cr2Nb has C15 (Laves phase) structure which is a relatively complicated structure, compared with more widely studied B2, L l 2 , and D0 19 structures. The unit cell is cubic with 24 atoms, a lattice parameter of about 0.698 nm, and a three-layer stacking sequence, each layer of which is composed of four interpenetrating atomic planes. Cr2Nb-based intermetallics are attractive for high temperature structural applications because of
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1 mm FIG. 1. Phase diagram of the C r - N b binary alloy system.4 The three alloy compositions used in this study are shown by arrows. J. Mater. Res., Vol. 8, No. 12, Dec 1993 http://journals.cambridge.org
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FIG. 2. Cross-sectional view in as-cast Cr 6 iNb 39 button. Note that the lower is the bottom of the button. © 1993 Materials Research Society IP address: 203.64.11.45
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T. Takasugi, S. Hanada, and K. Miyamoto: High temperature mechanical properties of C^IMb-based intermetaiiics
oxidation. In addition, the C15 phase has a relatively wide solid solution range, can accommodate relatively large amounts of substitutional additions such as Ti, Zr, Ta, Hf, and Co, leading to pseudo-binary Cr 2 Nb-Cr 2 X (X = Ti, Zr, and Hf) and Cr 2 Nb-X 2 Nb (X = Co) alloys, and also can be equilibrated with ductile bcc Nb and Cr phases. These features suggest the possibility of improving its mechanical properties by alloying and
microstructural modification. In spite of these promising features, relatively few studies have been performed on these alloys. Work on solidification processing,1 mechanical alloying,2-3 phase equilibrium,4 and the deformation behavior of Cr2Nb-based alloys5"8 have recently been reported. However, the fundamental mechanisms of high temperature deformation in these all
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