Microstructural evolution in Zr 3 Al-based alloys during various long-time annealing treatments
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I. INTRODUCTION
DUE to their high-strength (135 GPa at room temperature) and adequate toughness (KIC 50 MPa 1m at room temperature), Zr3Al-based alloys have been considered as candidate materials for structural components in pressurized heavy water reactors (PHWRs).[1,2] Unlike existing structural materials, which have highly anisotropic behavior due to hcp crystal structure, Zr3Al with its ordered fcc structure (Cu3Au type; L12 structure) shows no preferred orientation due to its higher symmetrical crystal structure.[2] In addition, Zr3Al has a relatively low absorption cross section for thermal neutrons, a reasonably high melting point (1723 K; operating temperature 0.34 Tm) and a moderate density (5970 kg/m3), and ultimate tensile strength 3 times that of the materials that are currently being used in PHWRs. However, Zr3Al-based materials have 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] Our recent studies on ternary addition of Nb to the binary Zr3Al have shown some promising results. It has been found that by the addition of niobium, the mechanical properties of these alloys can be tailored.[5,6] Addition of Nb to Zr3Al appears to alter the sequence of phase transformation, thus providing an opportunity to generate a variety of microstructures.[5,7,8] Some of the phases encountered in the binary and ternary systems of Zr, Al, and Nb elements are listed in Table I. To date, there have been no detailed studies on the microstructures of the Zr3Al-Nb alloys generated during various heatR. TEWARI, Postdoctoral Fellow, is with the Department of Chemical and Materials Engineering, University of Cincinnati, OH 45221. Contact e-mail: [email protected] G.K. DEY, Scientific Officer, Materials Science Division, and S. BANERJEE, Director, are with the Bhabha Atomic Research Centre, Mumbai 400 085, India. N. PRABHU, Professor, is with the Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology-Bombay, Mumbai 400 076, India. This article is based on a presentation made in the symposium entitled “Processing and Properties of Structural Materials,” which occurred during the Fall TMS meeting in Chicago, Illinois, November 9–12, 2003, under the auspices of the Structural Materials Committee. METALLURGICAL AND MATERIALS TRANSACTIONS A
treatment processes. This study is an attempt to fill this gap. In the present study, the temperature range of 1100 to 1723 K, where most of the phase reactions are found to occur in the binary Zr-Al system, has been selected and long-term annealing treatments were given to obtain the equilibrium microstructure. The goals of the present article are as follows: (1) To identify the various phases formed in the microstructures generated upon aging the Zr3Al-Nb alloys in the aforementioned temperature range, (2) To determine the chemical composition and volume fraction of these phases, and (3) To decipher the role
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