Nanoscale characterisation and clustering mechanism in an Fe–Y2O3 model ODS alloy processed by reactive ball milling and annealing
Introduction
Both Generation IV fission reactors and future fusion reactors are challenging in terms of structural materials due to their high operating temperatures (∼500–1000 °C) and high level of neutron displacement damage [up to ∼200 displacements per atom (d.p.a.)] [1], [2]. Oxide dispersion strengthened (ODS) steels are promising structural materials for such reactors because they exhibit good mechanical properties, such as tensile and creep strength, at high temperature [3], [4], [5] and good radiation resistance to hardening, swelling and embrittlement [6], [7], [8], [9], [10].
These properties are partly due to the nanometric and dense dispersion of complex oxides in the matrix [11], [12], [13], [14], [15]. To obtain this specific microstructure a pre-alloyed metallic powder and Y2O3 powder are usually ball milled and then thermo-mechanical treatments are applied. The widely proposed mechanism of formation of nano-oxides is dissolution of Y2O3 during ball milling followed by precipitation of oxides during annealing [13], [16], [17]. Since the properties of ODS steels depend on the dispersion of nano-oxides, controlling their nucleation and growth during the process is essential, but is difficult in the standard production method. This is why other routes need to be considered. Ball milling can induce a solid-state chemical reaction when the appropriate reactants and conditions are chosen. This is termed reactive ball milling. Being based on nucleation of the chemical products, reactive ball milling usually promotes the formation of nano-structures [18], [19], [20]. Thus, reactive ball milling appears an interesting method for creating nanometre sized precipitates. As a first step the relevancy of this method was evaluated on a model alloy.
The objective of this work was to study the formation of yttrium oxide in an iron-based material during reactive ball milling and subsequent annealing. For this an ODS model alloy, highly concentrated in yttrium oxide in an iron matrix, was synthesised by reactive ball milling of YFe3 and Fe2O3 powders, followed by annealing. It is worth noting that YFe3 and Fe2O3, being less thermodynamically stable than Y2O3, should be easily dissolved during ball milling according to the reaction 2YFe3 + Fe2O3 → 8Fe + Y2O3. Indeed, the change in Gibbs free energy due to this reaction is ΔGr(T = 30 °C) = −1.01 × 106 J mol−1 [21], [22], meaning that this reaction is thermodynamically favourable at room temperature. The kinetics of the reaction is detailed in Couvrat et al. [23]. In this study the microstructure of the alloy was characterised in the ball milling stationary state [24], [25] and after annealing by complementary techniques.
Atom probe tomography (APT) was chosen for its already demonstrated capability of mapping the chemistry of a material three-dimensionally at the atomic scale [26], [27], [28]. APT use has traditionally been limited to conductive materials. However, the recent development of laser-assisted APT allows the analysis of insulating materials, among them oxides [29], [30]. In addition to APT, techniques giving structural and chemical information on a larger scale and/or on larger volume were also used, including X-ray diffraction (XRD), electron probe X-ray microanalysis (EPMA) and Mössbauer spectrometry.
Section snippets
Experimental
A Fe–38 atm.% Y2O3 ODS model alloy was produced by reactive ball milling of 1.19 g of Fe2O3 and 3.81 g of YFe3. Powders were milled in a Fritsch P0 mill for 280 h, under secondary vacuum (5 × 10−5 Pa), with a WC ball and vial. The milling intensity as defined by Chen et al. [25] was 2000 m s−2. XRD was used to check that beyond 280 h of ball milling under these conditions the system reached a stationary state, i.e. there was no more evolution of the microstructure. Annealing was performed under an argon
XRD results
From the as milled powder XRD spectrum (Fig. 1) it was deduced that the reactants YFe3 and Fe2O3 had disappeared after 280 h ball milling and that a cubic iron phase had been formed. The crystallite size of this phase was estimated to be less than 10 nm and the cell parameter was 0.2865 ± 0.0001 nm, which can be considered equal to that of pure body centred cubic (α) iron (a = 0.28663 nm), given the uncertainty. A second phase diffracts in the angular range of monoclinic Y2O3, but the peak was too
Discussion
All techniques used here show that the initial reactants YFe3 and Fe2O3 disappeared and new products, α-Fe nano-crystallites and a Y and O enriched phase, were formed during 280 h ball milling. The Fe crystallites contained a significant amount of Y and O. As was shown by Fu et al. [38], using first principles calculations O in Fe has a strong affinity for vacancies. As a consequence the formation energy of O-vacancy pairs becomes extremely small if these vacancies pre-exist. Moreover, in the
Conclusion
A Fe–38 atm.% Y2O3 ODS model alloy was processed by reactive ball milling of Fe2O3 and YFe3 powders. It was shown that reactive ball milling and annealing were efficient in synthesising a nano-scaled ODS material. The microstructure was characterised after milling and after annealing at 800 °C for 1 s by a set of complementary techniques, including atom probe tomography, which successfully provided a nanometre insight into the powder.
After ball milling YFe3 and Fe2O3 for 280 h the obtained powder
Acknowledgement
The authors gratefully acknowledge G. Le Caër of the University of Rennes for his interest in this work and for his helpful comments and suggestions.
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