Understanding the high-temperature deformation
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I.
INTRODUCTION
THE primary goal of intermetallics research thus far has been to find a ductile intermetallic system with a high melting point, a high modulus, and good creep resistance. Using the knowledge gained through extensive work on Ni3AI as a basis, t~-4j researchers began examining other aluminum-based systems: FeA1, NiA1, TiA1, Ti3A1, and the niobium aluminides. Thermodynamicists began experimenting with ternary additions which would transform promising B2 phases into the more desirable ductile L12 phase. Despite the variety of systems examined, all of this research has primarily focused on one goal: finding an L12 intermetallic system that would behave like Ni3A1 but with a high heat-resistant strength shifted to a higher temperature corresponding to its higher melting point. If Ni3A1, with a melting temperature of 1395 ~ peaks at 760 ~ then a similarly behaving L12 intermetallic with a melting temperature of 2000 ~ could be expected to exhibit an anomalous yield maximum near 1100 ~ Such an L12 system would have all of the welldocumented benefits of Ni3A1,15-8] but its beneficial properties would be expected to exist over a much greater temperature range. Based upon this premise, this study examines the behavior of one of the highest melting L 12 intermetallic systems, Ir3Zr (Tin = 2280 ~ Ir3Zr belongs to a class of intermetallics known as Engel-Brewer intermetallics, which are extremely stable compounds of high melting point transition metals. In the 1940s, Engel examined the high heat of atomization of several transition metals and proposed that the more stable metals resulted from a large number of bonding d orbitals, t91 Brewer applied this theory to transition metal solid solutions in an effort to predict stable phases in transition metal phase diagrams. The application of ANGELA M. GYURKO, Graduate Research Assistant, GREGORY E. VIGNOUL, Postdoctoral Fellow, JOHN K. TIEN, Rashid Engineering Regents Chair, and JUAN M. SANCHEZ, Professor, are with the Department of Materials Science and Engineering, University of Texas at Austin, Austin, TX 78712. Manuscript submitted February 11, 1992. METALLURGICAL TRANSACTIONS A
Engel's theory to Brewer's phase diagrams yielded the prediction that intermetallic compounds of metals from the second and third transition series would be unusually stable. The configuration of the electrons in the s and p orbitals would determine the crystal structure of the compound, and the number of bonding d orbitals would determine its relative stability. A mixture of the two metals would allow electron transfer from the metal with a surplus of electrons to the metal with low vacant orbitals, maximizing the use of all electrons and orbitals and giving highly directional bonding. For example, Zr and Ir have 25 bonding electrons in their pure form, while the Ir3Zr compound has as many as 31 bonding electrons per molecule, with Ir giving up electrons and Zr accepting them. u~ On this basis, Ir3Zr, with its highly negative free energy of formation, was expected to exhibit very high
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