Evolution of Dislocation and Precipitate Structure in Zr Alloys Under Long-Term Irradiation

Abstract

Tubes from zirconium-base alloys are used widely in the pressure tube reactor core. The lifetime of the zirconium component in the reactor core will be determined by structure changes and alloy properties under long-term neutron irradiation. The studies were carried out using Zr-1Sn-1Nb-0.4Fe (E635) and Zr-2.5Nb (E125) alloy samples cut out of a pressure tube (PT) in the initial condition and after 7 and 15.5 years operated (42 000 and 95 000 effective hours) under irradiation to the neutron fluxes of 3 × 1017 and 2 × 1017 n/m2 s (E > 1 MeV) at 304°C in RBMK-1000 and 314°C in RBMK-1500, respectively. The E125 alloy PTs were in two conditions, as cold worked and annealed (A) and as thermomechanically treated (TMT-1) (B). The E635 alloy PTs were cold worked and annealed (A) (Tablel). The examinations were implemented using analytical transmission electron microscopy (TEM), energy dispersive X-ray (EDX), and X-ray diffraction (XRD) analyses. New data showing the microstructure changes are presented. Both the alloys have a partially recrystallized grain structure with a high density of intragranular dislocations in the initial state. The main part of dislocations belong to ⟨a⟩ type. Density of secondary phase precipitates is high. They are β-Nb (bec) in Zr-2.5Nb. In Zr-1.3Sn-1Nb-0.4Fe, precipitates consist of Zr, Nb, and Fe, and the constituent ratio is close to 1:1:1 Zr(Nb,Fe)2 (hcp). Linear dislocations (Type a) are annealed under irradiation, while the density of ⟨c⟩-component dislocations is not practically changed. Grain structure of the Zr-2.5Nb alloy is retained, and it is practically completely recrystallized in Zr-1.3Sn-1Nb-0.4Fe. The phase structure and microchemical composition are modified by irradiation. Nb concentration changes in β-Nb are observed in Zr-2.5Nb. A substantial decrease of Fe concentration and irradiation defect accumulation are observed in the intermetallic precipitates Zr(Nb,Fe)2 in the E635 alloy. This leads to crystal lattice disordering and new precipitates Nb-enriched are formed. Dislocation loops are formed under irradiation. Loop dimensions vary widely in Zr-2.5Nb. They show a tendency to ordering under high-fluence irradiation. Uniform structure of loops with a high tendency to ordering is formed in the alloy Zr-l.3Sn-lNb-0.4Fe; 70% of them are interstitial loops of the ⟨a⟩ type. Irradiation-induced Fe depletion of intermetallic particles and a Fe content increase in saturated α-Zr matrix may be a cause of the microstructure and performance changes in E635 alloy pressure tubes. The correlation between irradiation-induced dislocation structure and hardening of the E125 alloy is discussed.

The characterisation of second phases in the Zr-Nb and Zr-Nb-Sn-Fe alloys: A critical review

“…Interestingly, the authors observed no β-Nb phases within α-Zr when the alloy Nb content is 0.29 wt.% and did observe β-Nb when the Nb content was 0.49 wt.%. This leads to the conclusion that Nb TSS in α-Zr is in the range 0.29-0.49 wt.% at temperatures below the Zr-Nb monotectoid temperature (<600 °C), a value much greater than that suggested by Canay et al Figure 1 Ternary Zr-Nb-Fe diagram generated from data after [3,[20][21][22][23][24][25][26][27]29,[41][42][43][44]46,[62][63][64]. All compositions are given in at.%.…”

“…The Fe and Nb is enriched in the metastable phase states, and forms discrete ZrNbFe precipitates in advance of the complete decomposition of the β-Zr phase to β-Nb [10]. The compositional range for β-Nb is given from several studies in the ternary diagram of Figure 1 using data from [20][21][22][23][24][25][26], an analysis of which is shown in Figure 2. The diagram in Figure 1 represents the phases identified at room temperature, some of which may not be equilibrium phases.…”

“…mostly above the Zr-Nb monotectoid temperature. Under commercial alloy heat treatments and compositions, the vast majority of studies report that there is no Fe detected in β-Nb [3,13,20,22,24,26,27,[29][30][31][32]. Some early work did report Fe in β-Nb in commercial alloys and we will now take the opportunity to assess the validity of those claims in order to alleviate any confusion on the topic.…”

Redistribution of alloying elements in Zircaloy-2 after in-reactor exposure

“…This is approximately a factor of 10 higher than the expected number densities of <a>-type dislocation loops after a similar neutron irradiation dose. The build-up of layers of particles resembles the arrangement of <a>-loops seen with TEM [2,[21][22][23][24] and it is therefore possible -but not certain -that there is some correlation between the clustering and such defects. In particular there is agreement between the size and number densities of clusters and the small prismatic loops that were observed in Zircaloy-2 by Idrees [24].…”

Oxidation behaviour of zirconium alloys and their precipitates – A mechanistic study

“…The Zr(Fe,Cr) 2 precipitates can already exhibit the hexagonal and the face centred cubic structure, being both close packed structures. The ZrCr 2 , Zr(Fe,V) 2 [31] and Zr(Fe,Nb) 2 [33,34] exhibiting delayed oxidation all have either the hexagonal or the face centred cubic structure.…”