Irradiation-Induced Growth and Microstructure of Recrystallized, Cold Worked and Quenched Zircaloy-2, NSF, and E635 Alloys

Abstract

This paper is devoted to the study of the effect of the texture, phase composition, and microstructure on the irradiation-induced growth strain (GS) of zirconium-based alloys. GS measurements and TEM microstructural examinations were performed on Zry-2, NSF, and E635 samples in the recrystallized, beta quenched and cold-worked (CW) conditions. The samples were irradiated in the BOR-60 reactor in the temperature range of 315–325°C up to a neutron fluence level of 1.1 ×1026 n/m2 (E>1 MeV), i.e., up to a damage dose of 23 dpa. Growth strains of NSF and E635 alloys in all states and in the longitudinal and transverse directions are lower as compared to those of Zry-2, and do not exceed 0.2 % even at the maximum fluence level. As for recrystallized Zry-2, the GS kinetics are characterized by the appearance of the accelerated growth stage. A combination of a certain amount of Nb, Fe, and Sn in the matrix content plays a key role in GS kinetics. The higher the degree of CW, the higher the irradiation growth but its rate of increase with increasing fluence is different for alloys of different compositions. The maximum GS, reaching 0.72 %, is observed in the 20 % CW Zry-2 samples. Texture, along with the alloy composition, is one of the main GS-determining factors. Irradiation growth of the transversal samples is lower as compared to the longitudinal ones because of texture. As for quenched alloys, the texture is practically isotropic and GS values are low, independent of the alloy composition. In CW materials, the density of <c>- dislocations greatly affects the irradiation growth strain. Particles of Zr(Fe,Cr)2 and Zr2(Fe,Ni) phases in Zry-2 as well as Zr(Nb,Fe)2 in NSF and E635 are depleted in iron under irradiation. The Fe goes into the matrix and modifies its properties. The HCP lattice structure in the Laves phases in NSF and E635 changes into BCC (β-Nb-type). FCC (Zr,Nb)2Fe precipitates preserve on the whole their composition and structure; no amorphization of the Nb-containing precipitates is observed. The Zr2(Fe,Ni) precipitates with a BCT lattice remain crystalline, and HCP Zr(Cr,Fe)2 precipitates undergo amorphization. The average particle size in the irradiated alloys is larger and the concentration is a little lower as compared to the unirradiated ones. Irradiation-induced fine dispersed precipitates about 3 nm in size, probably enriched in niobium, appear in NSF and E635. The observed changes of microhardness are discussed from the viewpoint of generation of radiation defects (clusters, dislocation loops), evolution of the initial dislocation structure, and matrix composition (enrichment in Fe, Cr, and, probably, Nb).

Radiation Effects in Zirconium Alloys

“…For the extreme case of SRA zirconium alloys, which could undergo up to 80% cold working followed by a stress-relieving annealing, the growth rate is so high that the stationary growth rate is not observed, and from the beginning of the irradiation, the growth rate is comparable to the growth rate measured for RXA zirconium alloys after the breakaway growth. The dependence of the amount of cold-working on the growth rate has been also observed for Zy-2 and Zr-Nb-Sn-Fe alloys by Kobylyansky et al [115]. Several authors, as reviewed by Fidleris et al [297] and Holt [104], have clearly correlated the increase of the growth rate with the increase of the dislocation density [106] due to the cold working.…”

“…A review by Harte et al [157] summarizes the behavior of these precipitates at higher doses, under neutron irradiation: the cubic phase (Zr,Nb)2Fe retains its crystalline core and shows a polycrystalline microstructure at the particle periphery that is likely Nb-rich platelets. This behavior was described in several publications [113] [163] [117] [115].…”

“…These nano-precipitates are also present after neutron irradiation of some quaternary alloys (Zr-Fe, Nb, Sn) such as 3R [113], Q12 [15], E635 [117] or O2-O8 [166]. Simultaneously to the formation of these precipitates, a noticeable decrease of Nb content occurs in the matrix [115]. Another interesting observation is the fact that these nano-precipitates are not more numerous at the vicinity of native βNb.…”

Contribution on the phase equilibria in Zr–Nb–Fe system

“…For example Zr(NbFe) 2 was observed to precipitate together with bNb in ZIRLO™ [1]. In E635, mostly Zr(NbFe) 2 and lesser amounts of (ZrNb) 2 Fe were observed [2,3]. Even in HANA 6, where Fe was not an alloying element but is introduced as an impurity from the nuclear grade sponge Zr, hcp Zr(NbFe) 2 was still discovered [4].…”

Corrosion of E-635 alloy in the reactor core of a nuclear icebreaker

“…The cladding made of this alloy is not subject to hot-spot corrosion and no effects due to boiling were found. The changes in the microstructure of E-635 alloy cladding after operation in the reactor core of an icebreaker correspond to the irradiation temperature and fast-neutron fluence attained [2,3].…”