Microstructural analysis of impurity segregation around β-Nb precipitates in Zr–Nb alloy using positron annihilation spectroscopy and atom probe tomography

The effect of crystallographic mismatch on the obstacle strength of second phase precipitate particles in dispersion strengthening: bcc Nb particles and nanometric Nb clusters embedded in hcp Zr

“…In this study, we investigate the irradiation-induced precipitation in a prototype J-Alloy™, called J3, recently developed as the fuel cladding tubes of light water reactors (LWRs), optimized for extended burn-up operations [10,11]. The alloy composition is Zre2.5Nb in atomic percent, with minor amounts of impurity Fe and Cr, less than the impurity level of those contained in the highpurity nuclear-grade sponge zirconium (ASTM B349, Grade R60001): up to~0.25 at.% (~1500 wt.…”

“…The microstructure of the JAlloy™ (J3) consists of a fully recrystallized Zr matrix having a hexagonal close packed (hcp) structure and Nb precipitates having a body centered cubic (bcc) structure: the diameter of bcc Nb precipitates is~57 nm (Fig. 1aec) [10,11]. APT analysis showed that the Zr matrix contains Nb,~0.5 at.%, and the Nb precipitate particles contain Zr,~10.7 at.% [10,11]: these compositions correspond to their solubility at~973 K (Fig.…”

“…1aec) [10,11]. APT analysis showed that the Zr matrix contains Nb,~0.5 at.%, and the Nb precipitate particles contain Zr,~10.7 at.% [10,11]: these compositions correspond to their solubility at~973 K (Fig. 1d) [12].…”

“…The bcc Nb particles are softer than the hcp Zr matrix with respect to shear modulus (28 GPa [13,14] and 33 GPa [15], respectively); however, they may be the Orowan-type strong obstacles due to the following crystallographic constraint. Unlike the well-known crystal orientation relationship proposed by Burgers for taking into account the structural transition from bcc Zr to hcp Zr, {0002} hcp //{110} bcc [16,17] [10,11]; the discrepancy arises presumably because the lattice constant of bcc Nb is significantly different from that of bcc Zr: the difference is~8.5% [10,11]. The slip plane of bcc Nb particles, {110} bcc , is not parallel to that of the matrix, {0001} hcp .…”

“…1. Microstructure of J-Alloy™ (J3) before irradiation [10,11]: (a) STEM bright-field image, (b) EDS Nb map, and (c) APT Nb 3þ map; and (d) the ZreNb binary phase diagram [12]. modulus of hcp Zr (33 GPa), b is the magnitude of the Burgers vector of dislocations in hcp Zr (0.323 nm).…”