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Detailed analysis was carried out on proton and a neutron irradiated Nb-containing Zr-alloy to study the evolution of dislocation loop size and densities as well as the formation and evolution of irradiation-induced precipitation/clustering. The results obtained here have been contrasted against previously published work on a Nb-free Zr-alloy [1, 2] to investigate the mechanistic reason for the improved resistance to irradiation-induced growth of Nb-containing Zr alloys. The combined use of bright field scanning transmission electron microscopy, ultra-high-resolution energy dispersive spectroscopy and atom probe tomography analysis provides evidence of evenly distributed radiation-induced Nb clusters that have formed during the early stage of proton irradiation and Fe-rich nano-rods near Fe-containing second phase particles. The former seems to have a profound effect on loop and subsequent loop formation, keeping loop size small but number density high while loops seem to initially form at similar dose levels compared to a Nb-free alloy but loop line density does not increase during further irradiation. It is hypothesized that the formation of the Nb nano-precipitates/clusters significantly hinders mobility and growth of loops, resulting in a small size, high number density and limited ability of loops to arrange along basal traces compared to Nb-free Zr-alloys. It is suggested that it is the limited loop arrangement that slows down loop formation and the root cause for the high resistance of Nb-containing Zr-alloys to irradiation-induced growth.

Transmission electron microscopy (TEM) studies provide mechanistic understanding of nanoscale processes, whereas advanced synchrotron XRD (SXRD) enables precise measurements on volumes that are more representative of bulk materials. Therefore, the combined strengths of these techniques can provide new insight into irradiation-induced mechanistic processes. In the present study, their application to Zircaloy-2, proton-irradiated to 2.3, 4.7, and 7.0 dpa at 2 MeV and 350 °C and neutron-irradiated to 9.5 and 13.1 × 1025 n m−2 are exemplified. The application of correlative spectral imaging and structural TEM investigations to the phase transformation of Zr(Fe,Nb)2 precipitates in Low-Sn ZIRLO™, neutron-irradiated to 8.9–9 × 1025 n m−2, demonstrates the possibility of a Cr core nucleation site. Anomalous broadening is observed in SXRD profiles, which is believed to be caused by defect clusters and precursors to dislocation loop nucleation. The challenges to quantitative analysis of dislocations by SXRD are highlighted with reference to the segregation of Fe and Ni to basal planes and dislocation cores, observed by spectral imaging in the TEM.

We report here the microstructural changes occurring in the zirconium alloy Excel (Zr–3.5 wt% Sn–0.8Nb–0.8Mo–0.2Fe) during heavy ion irradiation. In situ irradiation experiments were conducted at reactor operating temperatures on two Zr Excel alloy microstructures with different states of alloying elements, with the states achieved by different solution heat treatments. In the first case, the alloying elements were mostly concentrated in the beta (β) phase, whereas, in the second case, large Zr3(Mo,Nb,Fe)4 secondary phase precipitates (SPPs) were grown in the alpha (α) phase by long term aging. The heavy ion induced damage and resultant compositional changes were examined using transmission electron microscopy (TEM) in combination with scanning transmission electron microscope (STEM)-energy dispersive x-ray spectroscopy (EDS) mapping. Significant differences were seen in microstructural evolution between the two different microstructures that were irradiated under similar conditions. Nucleation and growth of -component loops and their dependence on the alloying elements are a major focus of the current investigation. It was observed that the -component loops nucleate readily at 100, 300, and 400 °C after a threshold incubation dose (TID), which varies with irradiation temperature and the state of alloying elements. It was found that the TID for the formation of -component loops increases with decrease in irradiation temperature. Alloying elements that are present in the form of SPPs increase the TID compared to when they are in the β phase solid solution. Dose and temperature dependence of loop size and density are presented. Radiation induced redistribution and clustering of alloying elements (Sn, Mo, and Fe) have been observed and related to the formation of -component loops. It has been shown that at the higher temperature tests, irradiation induced dissolution of precipitates occurs whereas irradiation induced amorphization occurs at 100 °C. Furthermore, dose and temperature seem to be the main factors governing the dissolution of SPPs and redistribution of alloying elements, which in turn controls the nucleation and growth of -component loops. The correlation between the microstructural evolution and microchemistry has been found by EDS and is discussed in detail.

To study the effect of γ′ precipitate size on the deformation behaviour of a polycrystalline nickel-based superalloy, model microstructures with a unimodal γ′ size distribution were developed and subjected to loading experiments at 750 °C. Neutron diffraction measurements were carried out during loading to record the elastic lattice strain response of the γ and γ′ phase. A two-site elasto-plastic self-consistent model (EPSC) assisted in the interpretation of the elastic lattice strain response. In addition, the microstructures of the deformed specimens were analysed by (scanning) transmission electron microscopy (STEM). Excellent agreement was found between the EPSC and STEM results regarding a joint deformation of the γ and γ′ phase in the fine γ′ microstructures and for low plastic strains in the medium γ′ microstructures. With increasing γ′ size and increasing degree of plastic deformation, both experimental methodologies revealed a tendency of the two phases to deform independently. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

In-situ stair case loading experiments at 500 °C using neutron diffraction have been carried out on two γ′ strengthened polycrystalline nickel base superalloys, RR1000 and Udimet 720LI. In order to study the effect of γ′ particle size, uni-modal γ′ size distributions were generated in both alloys. Both alloys showed signs of dynamic strain aging (DSA) although the strain bursts were far more significant in RR1000 than Udimet 720LI. The neutron diffraction experiment revealed load partitioning between γ and γ′ in the case of the coarse γ′ microstructure but not in the fine γ′ microstructure. When load partitioning occurred, γ′ picked up significantly larger elastic strains during a DSA related strain burst than the γ matrix, indicating that mainly the matrix phase deforms plastically during such an event. The peak broadening data confirmed this. Subsequent to the strain burst, a reduction in the elastic strain partitioning was observed indicating preferred yielding of the γ′ phase during this stage. The differences in DSA detected between RR1000 and Udimet 720LI are discussed in terms of possible contribution of the various alloying elements.

Deformation mechanisms under tensile loading at room temperature have been studied in a polycrystalline nickel-based superalloy containing close to 50 vol.% γ′. In order to identify the effect of γ′ particle size on deformation mechanisms, model microstructures with unimodal γ′ size distributions were developed. The investigations were carried out by combining in situ loading experiments using neutron diffraction and two-site elasto-plastic self-consistent plasticity modelling with detailed post-mortem electron microscopy. The microscopy work also includes results for samples strained at 500 °C. During early plastic deformation, the diffraction data demonstrate that γ and γ′ display the same elastic strain response, indicating that at this stage γ′ is cut by dislocations regardless of the γ′ particle size. Scanning electron microscopy studies showed an abundance of shearing processes in all three microstructures, hence supporting the conclusions drawn from the diffraction experiment. As the material is further deformed, elastic load transfer from γ to γ′ was observed in the medium (130 nm) and coarse (230 nm) γ′ microstructures but not in the fine (90 nm) γ′ microstructure. The load transfer can be explained by assuming that Orowan looping becomes an additional operative deformation mode. Transmission electron microscopy confirmed that in the fine γ′ microstructure deformation takes place by strongly coupled dislocations cutting the γ′, while the medium and coarse γ′ microstructures showed additional signs of Orowan looping.

The desire to improve the corrosion resistance of Zr cladding material to allow high burnup has resulted in a general trend among fuel manufacturers to develop alloys with reduced levels of Sn. While the detrimental effect of Sn on high temperature aqueous corrosion performance is widely accepted, the reason for it remains unclear. High-Energy synchrotron X-ray diffraction was used to characterise the oxides formed by autoclave exposure on Zr-Sn-Nb alloys with tin concentrations ranging from 0.01 to 0.92 wt.%. The alloys studied included the commercial alloy ZIRLO® and two variants of ZIRLO with significantly lower tin levels, referred to here as A-0.6Sn and A-0.0Sn. The nature of the oxide grown on tube samples from each alloy during autoclave testing at 360°C was investigated by cross-sectional Scanning and Transmission Electron Microscopy (SEM & TEM). Non-destructive synchrotron X-ray diffraction analysis on the oxides revealed that the monoclinic and tetragonal oxide phases display highly compressive in-plane residual stresses with the magnitudes dependent on both phase and alloy. Additional in-situ Synchrotron X-ray diffraction experiments during oxidation at 550°C provided further confirmation of the trends seen for autoclave tested samples and demonstrated the presence of elevated levels of tetragonal phase in the initial stages of oxidation. In-situ and ex-situ measurements demonstrate unambiguously that the amount of tetragonal phase present and, more importantly, the degree of transformation from tetragonal to monoclinic oxide both decrease with decreasing tin levels, suggesting that tin stabilises the tetragonal phase. It is proposed that in Zr-Nb-Sn alloys with low Sn, the tetragonal phase is mainly stabilised by very small grain size and therefore remains stable throughout the corrosion process. By contrast, in alloys with higher tin levels larger, stress stabilised, tetragonal grains can form initially, but then become unstable as the corrosion front progresses inwards and stresses in the existing oxide relax.

Size dependent compositional variations for the ordered L1 2 -structure gamma prime (γ′) precipitates in the commercial Ni-based superalloy RR1000 have been investigated using scanning transmission electron microscope (STEM) imaging combined with energy-dispersive X-ray (EDX) spectroscopy. To address the problem of quantitative compositional determination for nanoscale particles within a metal matrix we have applied a novel electrochemical method to extract individual precipitates. The use of a high-efficiency EDX detector enabled compositional measurements to be obtained for particles with diameters as small as 20 nm with acquisition times of the order of a few minutes. We have studied compositional variations across the different size families of γ′ precipitates within a microstructure generated by slow cooling. Our results demonstrate the importance of kinetic factors for determining the precipitates compositions. In particular, we provide new evidence for the role of aluminium antisite atoms on the low-temperature growth kinetics of fine scale γ′ precipitates. Our findings provide valuable structural data towards improving the accuracy of predicting the microstructural evolution in Ni-based superalloys.

Zirconium alloys used as cladding materials in nuclear reactors can exhibit accelerated irradiation induced growth, often termed linear growth, after sustained neutron irradiation. This phenomenon has been linked to the formation of -component dislocation loops and to the concentration of interstitial solute atoms. It is well documented for the Zircaloys that Fe dissolves from second phase particles (SPPs) during irradiation thus increasing the interstitial solute concentration in the matrix. However, no progress has yet been made into understanding whether a similar process occurs for the newer ZIRLO™ alloys. We aim to overcome this shortcoming here by studying compositional changes in second phase particles in Low Tin ZIRLO™ after neutron and proton irradiation using energy dispersive X-ray (EDX) spectroscopy. Material irradiated to 18 dpa (displacements per atom) using neutrons and to 2.3 and 7 dpa by protons was investigated. The results show that Fe is lost from Zr–Nb–Fe-SPPs during both neutron and proton irradiation. Prior to irradiation, Fe was detected at the interface of β-Nb-SPPs. This Fe enrichment is also dispersed during irradiation. Qualitatively, excellent agreement was found regarding the elemental redistribution processes observed after proton and neutron irradiation.

Energy-dispersive X-ray (EDX) spectroscopy in the scanning transmission electron microscope (STEM) has been used to demonstrate the presence of size-dependent compositional variation for L1 2 -structured Ni 3 Al-type gamma-prime (γ′) precipitates within a commercial RR1000 Ni-based superalloy. This semi-quantitative elemental analysis has been achieved using electrochemical extraction of the γ′ precipitates from the γ matrix. The applicability of this approach to size-dependent compositional analysis of precipitates was confirmed by a comparison of the size distribution for the extracted precipitates with those present in traditional electropolished foil specimens in the size range 20–250 nm. By applying suitable thickness-dependent absorption-corrections we have demonstrated that the composition of γ′ precipitates in our material depends on the size of the precipitate in the range of 5 nm to 3 μm. In particular, the Al content was observed to increase in smaller γ′ precipitates while Ti and Ta contents are constant for all sizes of precipitate. Hf was observed to be present only in the largest precipitates. This type of local compositional information provides invaluable input to assess the accuracy of microstructural modelling for these complex alloys and provides new evidence supporting the importance of anti-site diffusion.

Extended abstract of a paper presented at Microscopy and Microanalysis 2013 in Indianapolis, Indiana, USA, August 4 – August 8, 2013.