Microstructural Evolution in Zr-1Nb and Zr-1Nb-1Sn-0.1Fe Alloys
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INTRODUCTION
THE b fi a allotropic transformation in pure zirconium and its dilute alloys, the possibility of the displacive b fi x transformation in alloys containing b-stabilizing elements to a suitable level, and the clustering tendency in the b phase leading to a phase separation are responsible for the variety of transformations in the Zr-Nb system. The various possible sequences of phase transformations in alloys of different compositions are (1) martensitic transformation from the b (bcc) to the a (hcp) phase in alloys containing 0 to 7 wt pct Nb; (2) the b to x (hexagonal) transformation in alloys containing 7 to 17 wt pct Nb during b quenching (athermal x) or isothermal holding (x precipitation); (3) the precipitation of a from the supersaturated b phase; (4) phase separation, b fi bI (Zr rich) + bII (Nb rich), which is initiated by a spinodal decomposition in a suitable temperature and composition regime; and (5) a monotectoid reaction, bI fi a + bII (at 883 K and 20 wt pct Nb).[1] Menon et al.[2] have shown that due to a large miscibility gap in the b phase, the rate of monotectoid decomposition of the bI phase to bII phase depends strongly on the heat-treatment S. NEOGY, Scientific Officer D, D. SRIVASTAVA, Scientific Officer G, J.K. CHAKRAVARTTY, Scientific Officer H, G.K. DEY, Scientific Officer H, Materials Science Division, and S. BANERJEE, Director, Materials Group, are with the Bhabha Atomic Research Centre, Trombay, Mumbai 400 085, India. Contact e-mail: [email protected] Manuscript submitted April 7, 2006. Article published online March 27, 2007. METALLURGICAL AND MATERIALS TRANSACTIONS A
temperature. In general, the bI phase is retained to a large fraction in the samples annealed at temperatures higher than the monotectoid temperature (Tm). Annealing at temperatures below Tm, on the other hand, results in b precipitates with composition varying between bI and bII. However, at T«Tm, a long duration heat treatment results only in formation of the equilibrium bII precipitates. Recently, Zr-1Nb and Zr-1Nb-1Sn-0.1Fe alloys have drawn considerable interest because of their application in nuclear reactors as cladding material.[3] The microstructure of the Zr-1Nb alloy is different from those observed in the other commonly used zirconium-based alloys. Like Zr-2.5Nb, the Zr-1Nb alloy also has a twophase microstructure consisting of the a and the b phases. However, in these alloys, the shape, size, and size distributions of the b phase are significantly different. In Zr-1Nb alloy, the b phase is distributed as fine spherical particles within the a matrix. Zircaloys also have a microstructure similar to the Zr-1Nb alloy, but they essentially have a single-phase microstructure in which intermetallic precipitates are distributed within the a matrix. In Zr-1Nb alloy, therefore, the size, size distribution, and composition of the b phase are expected to vary with aging time and temperature. In quaternary Zr-Nb-Sn-Fe systems, where Sn and Fe are present in small quantities, the phase transformation behavior is not expected
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