Nanoscale characterisation and clustering mechanism in an Fe–Y2O3 model ODS alloy processed by reactive ball milling and annealing

doi:10.1016/j.actamat.2009.11.022

Acta Materialia

Volume 58, Issue 5, March 2010, Pages 1806-1814

https://doi.org/10.1016/j.actamat.2009.11.022 Get rights and content

Abstract

Reactive ball milling and annealing is proposed as a new production method for oxide dispersion strengthened (ODS) steels. A highly concentrated Fe–38 atm.% Y₂O₃ ODS model alloy was processed by reactive ball milling and annealing of YFe₃ and Fe₂O₃ powders so as to induce the chemical reaction 2YFe₃ + Fe₂O₃ → 8Fe + Y₂O₃. The model alloy was characterised after milling and annealing by complementary techniques, including atom probe tomography. Ball milling up to the stationary state results in the formation of two metastable nanometric interconnected phases: super-saturated α-iron and an yttrium and oxygen rich phase. Annealing leads the system towards equilibrium through: (i) a chemical evolution of each phase to nearly pure α-Fe and Y₂O₃ oxide slightly sub-stoichiometric in oxygen; and (ii) growth of the phases. A pure iron matrix reinforced by nanometric Y₂O₃ particles was successfully synthesised 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 ΔG_r(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.

References (43)

L.K. Mansur et al.
J Nucl Mater
(2004)
K.L. Murty et al.
Nucl Mater
(2008)
R.L. Klueh et al.
Nucl Mater
(2005)
R. Lindau et al.
Nucl Mater
(2002)
S. Ukai et al.
J Nucl Mater
(1993)
J. Saito et al.
J Nucl Mater
(1998)
M.B. Toloczko et al.
Nucl Mater
(2004)
T. Yoshitake et al.
J Nucl Mater
(2004)
A. Alamo et al.
J Nucl Mater
(2004)
T. Kuwabara et al.
J Nucl Mater
(1998)

S. Yamashita et al.

Nucl Mater

(2004)

P. Pareige et al.

Nucl Mater

(2007)

M.K. Miller et al.

J Nucl Mater

(2006)

M.K. Miller et al.

Intermetallics

(2005)

T.R. Allen et al.

J Nucl Mater

(2008)

M.J. Alinger et al.

J Nucl Mater

(2004)

S. Yamashita et al.

Philos Mag Lett

(2004)

N. Lorrain et al.

Mater Sci Eng A – Struct Mater Prop Microstruct Process

(2004)

P. Matteazzi et al.

Mater Sci Eng A – Struct Mater Prop Microstruct Process

(1992)

F. Legendre et al.

Alloy Compd

(2007)

P.R. Subramanian et al.

Calphad

(1984)

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