Phase Transformation Temperatures and Solute Redistribution in a Quaternary Zirconium Alloy
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TRODUCTION
EXCEL alloy is a quaternary zirconium alloy (Zr-3.5Sn-0.8Mo-0.8Nb) that was originally developed to replace the Zr-2.5Nb alloy used in CANDU pressure tubes,[1] and has more recently been proposed for use in Gen-IV nuclear reactors. Much of the recent study on the alloy has focused on its properties under irradiation.[2,3] Understanding phase equilibria and phase transformations in Excel alloy is complicated by the conflicting contributions of the alloying additions, as well as by the slow rate of diffusion of Sn, Mo, and Nb in Zr.[4] Sattari et al. report on the phase transformation temperatures of the alloy,[5] and variant selection and texture development during quenching was investigated by Sattari et al.[6] and Ahhmed et al.[7] In addition to the major alloying elements, Excel alloy contains controlled amounts of O and Fe, both of which are well established to modify the phase equilibria in zirconium alloys.[8–10] This study aims to extend the interpretations of these studies by investigating the phase transformation processes in situ. Yan et al. have used in situ neutron diffraction and Vegard’s law to interpret interphase diffusion during the ða þ bÞ ! b phase transformation in Zr-2.5Nb.[11] Vegard’s Law states that certain physical properties of solutions can be estimated by a linear combination of those properties in the constituents. For example, for a binary alloy of A and B, Vegard’s Law can be used to C. COCHRANE and M.R. DAYMOND are with the Department of Mechanical and Materials Engineering, Queen’s University, Kingston, ON K7L 3N6, Canada. e-mail: [email protected] Manuscript submitted September 01, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
predict the lattice constant of the alloy, aA1x Bx , assuming no phase change, from the lattice constants of its constituents, using aA1x Bx ¼ ð1 xÞaA þ xaB ;
½1
where aA is the lattice constant of pure A; aA is the lattice constant of pure B; and x is the concentration (in atomic pct) of B in solution in A. Although deviations from the linear behavior of Vegard’s law have been reported,[12] this approach has been used successfully for determining Nb concentration in b-Zr during heat treatments (e.g., References 11, 13, and 14). Using Vegard’s law, Yan et al. found that the transient behavior of Nb diffusion between b-Nb and b-Zr cannot be fully understood by the Zr-Nb phase diagram alone.[11] The Nb content of the b phase increased significantly on heating, as expected from the transition from b-Zr to b-Nb,[15] but decreased rapidly at temperatures above 550 °C. Above 550 °C, the Nb content of the b phase decreased toward the eutectoid composition of 23 wt pct Nb at 620 °C. This transient behavior can have a significant impact on microstructure and mechanical properties if material processing is performed at temperatures in the ða þ bÞ region. Kabra et al. report on the phase transformation in Zr-2.5 wt pct Nb alloy studied by in situ neutron diffraction.[16] They heated two samples of Zr-2.5Nb and monitored the diffraction pattern at temperatures
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