Deformation behavior of Zr 3 Al-Nb alloys I: Room-temperature and high-temperature deformation study
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I.
INTRODUCTION
THE Zr3Al-based alloys were considered as candidate materials for structural components in pressurized heavy water reactor (PHWR). Zr3Al with its ordered fcc structure (Cu3Au type; L12 structure) shows isotropic behavior. In addition, it has a relatively low absorption cross section for thermal neutrons, a reasonably high melting point, and a moderate density. The ultimate tensile strength of the Zr3Al-based materials is 3 times that of the materials that are currently being used in PHWRs. The binary Zr3Al alloy containing fully ordered L12 structure exhibits a good combination of strength and ductility.[1] In spite of such good qualities, Zr3Al-based materials could not be used for structural applications in PHWRs due to some inherent problems that can be summarized as (1) inadequate control over the distribution of undesirable phases,[2] (2) high notch sensitivity,[3] and (3) irradiationinduced amorphization.[4] Ternary addition to other binary intermetallics, such as Ni3Al and Ti3Al, has improved the intermetallics’ room-temperature as well as high-temperature properties.[5–9] For example, upon microalloying with B, Ni3Al showed higher toughness compared to binary Ni3Al.[5] Addition of Cr improved hightemperature properties of binary Fe3Al intermetallic.[6] In Ti3Al,[7,8,9] macroalloying with niobium introduced new phases leading to improved ductility. This improvement in ductility is due to the refinement of the microstructure and retainment of the higher volume fraction of the -related phases. The R. TEWARI, Postdoctoral Fellow, is with the Department of Chemical and Materials Engineering, University of Cincinnati, Cincinnati, OH 45221. Contact e-mail: [email protected] G.K. DEY, Scientific Officer (H), Materials Science Division, and R.K. FOTEDAR, Scientific Officer (H), T.R.G. KUTTY, Scientific Officer (G), Radiometallurgy Division, are with Bhabha Atomic Research Centre, Mumbai 400 085, India. N. PRABHU, Associate Professor, is with the Indian Institute of Technology-Bombay, Mumbai 400 076, India. Manuscript submitted June 7, 2002. METALLURGICAL AND MATERIALS TRANSACTIONS A
presence of these phases improves the room-temperature and high-temperature properties of Ti3Al-based materials. However, in the case of Zr3Al, there has been no systematic study on the effects of alloying. Neither B nor Cr, due to very high absorption cross section for thermal neutrons, can be used as an alloying addition to Zr3Al. A systematic study to determine the effect of alloying Zr3Al with Nb is required to determine whether Nb can also improve these shortcomings of the binary Zr3Al-based alloys. There are many hardness-temperature relationships listed in the literature,[10,11] and the majority of these relations show the exponential temperature dependence of hardness, which can be expressed, in general, in the following form: H A exp (BT )
[1]
where the constants A and B can be related to deformation mechanisms at various temperatures. Westbrook[11] attempted to relate the constants A and B used in
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