Rare-earth transition-metal intermetallic compounds produced via self-propagating, high-temperature synthesis
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Several binary intermetallic compounds—each containing a rare-earth (RE) element paired with a transition metal (TM)—were prepared by self-propagating, high-temperature synthesis (SHS). Thin multilayers, composed of alternating Sc or Y (RE element) and Ag, Cu, or Au (TM), were first deposited by direct current magnetron sputtering. Once the initially distinct layers were stimulated and caused to mix, exothermic reactions propagated to completion. X-ray diffraction revealed that Sc/Au, Sc/Cu, Y/Au, and Y/Cu multilayers react in vacuum to form single-phase, cubic B2 structures. Multilayers containing Ag and a RE metal formed cubic B2 (RE)Ag and a minority (RE)Ag2 phase. The influence of an oxygen-containing environment on the reaction dynamics and the formation of phase were investigated, providing evidence for the participation of secondary combustion reactions during metal-metal SHS. High-speed photography demonstrated reaction propagation speeds that ranged from 0.1–40.0 m/s (dependent on material system and foil design). Both steady and spin-like reaction modes were observed.
I. INTRODUCTION
A family of rare-earth (RE) transition-metal (TM) intermetallic compounds has recently been discovered, which exhibit large ductility in the B2 phase (CsCl structure).1–4 In contrast to other stoichiometric compounds that are often brittle5,6,7 or exhibit low ductility7 at room temperature, these line compounds exhibit surprising toughness (typically reaching more than 10% strain before fracture).8–10 As an example, the compound YAg was found to exhibit greater resiliency to fracture than commercial 3105 Al alloy under tensile loading at room temperature.1 Despite multiple observations of a B2 structure for many of these RE-TM compounds, there seems to be some uncertainty as to the conditions that lead to this phase. In general, the B2 structure is designated as the thermodynamically stable phase for all six equiatomic compounds examined in this report (i.e., YAg, YAu, YCu, ScAg, ScAu, and ScCu).11–16 However, the initial work by Chao et al.17 demonstrated that YAu and other binary intermetallic compounds were obtained only when cooled quickly. More recently, Gschneidner et al.18 reported that arc-melted, equiatomic YAu formed a B33 (CrB-type) structure—a uniquely different lattice type compared with the thermodynamically stable B2 phase of YAu.15 Phase formation of the RE-TM “fara)
Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/JMR.2010.0091
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from-equilibrium” is interesting in light of these recent observations and the promise of large ductility in a binary intermetallic compound. In the current work, RE-TM samples are prepared by self-propagating, high-temperature synthesis (SHS)—a far-from-equilibrium process characterized by rapid changes in temperature. SHS has been used in the past to synthesize a wide range of advanced materials, including some intermetallic compounds,19–21 ceramics,22,23 and ceramic-metal composites24–26 (for a review of this
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