Phase separation tendency in the as-solidified Zr 3 Al-Nb alloys studied by microstructural observations and thermodynam
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I. INTRODUCTION Zr3Al is the first intermetallic in the Zr-rich side of the Zr-Al binary phase diagram. The equilibrium Zr3Al phase has an ordered fcc-like structure (Cu3Au type; L12 structure) due to which it exhibits isotropic physical and mechanical properties;[1] the bravais lattice is primitive cubic. In addition, it has a low absorption cross section for thermal neutrons, a reasonably high melting point, and a moderate density. It is also known to be a ductile material under appropriate conditions.[2] It can be formed into shapes such as strip, rod, and tubing. Hot working at temperatures above 1273 K, followed by aging at lower temperatures, produces a fully ordered L12 structure with a good combination of strength and ductility.[1,2] Due to some inherent problems such as inadequate control over the distribution of undesirable phases,[1] high notch sensitivity,[3] and irradiation-induced amorphization,[4] Zr3Al-based materials have not been used for structural applications in nuclear reactors.[4] Schulson[1] has reviewed a major part of the work on Zr3Al and has shown that in the presence of a notch the otherwise ductile Zr3Al fails in a brittle manner. He has also shown that some of these problems could be solved by altering the initial microstructure, by controlling the grain size and shapes, and by introducing new phases. In this connection, a ternary addition in Zr3Al is thought to be beneficial. Some of the problems associated with binary intermetallics such as Ni3Al and Ti3Al have been addressed by the addition of ternary alloying additions.[5–8] In Ni3Al, mechanical properties have been shown to improve upon microalloying with B.[5] Addition of B to Zr3Al is not acceptable from
R. TEWARI, G.K. DEY, and P. MUKHOPADHYAY, Scientific Officers, Materials Science Division, and S. BANERJEE, Director, Materials Group, are with the Bhabha Atomic Research Centre, Mumbai 400 085, India. Contact e-mail: [email protected] N. PRABHU, Associate Professor, is with the Indian Institute of Technology-Bombay, Mumbai 400 076, India. Manuscript submitted July 31, 2001. METALLURGICAL AND MATERIALS TRANSACTIONS A
the nuclear application point of view, as it is a “poison” for thermal neutrons. This problem would not arise if Nb were used as a ternary addition to Zr3Al. Therefore, attempts could be made to develop Zr3Al-Nb alloys. This study constitutes a preliminary step toward the development of Zr3Al-Nb alloys. The Zr-rich side of the Zr-Al phase diagram is shown in Figure 1. As can be seen from this diagram, the formation of Zr3Al involves one eutectic (L →  ⫹ Zr5Al3; at temperature (Te) ⫽ 1623 K) and two peritectoid ( ⫹ Zr5Al3 → Z2Al; at temperature (Tp1) ⫽ 1523 K and  ⫹ Zr2Al → Zr3Al; (Tp2) ⫽ 1248 K) reactions.[9] Microstructural studies on the as-solidified binary Zr3Al alloy have shown that the second peritectoid reaction is suppressed during chill casting. Studies on isothermal aging treatments of as-solidified microstructure have also shown that the kinetics of the second peritectoid reaction is very s
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