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ScienceDirect Materials Today: Proceedings 3 (2016) 3086–3093

www.materialstoday.com/proceedings

ARRMA-2016

High resolution microstructural studies of the evolution of nanoscale, yttrium-rich oxides in ODS steels subjected to ball milling, selective laser melting or friction stir welding. G. J. Tatlocka,*, K. Dawsona, T. Boegeleina,b, K. Moustoukasa and A. R. Jonesa a

Centre for Materials and Structures, School of Engineering, University of Liverpool, Liverpool L69 3GH, UK b Institute for Materials Science, University of Erlangen-Nuremberg, 91058 Erlangen, Germany

Abstract Coarse-grained ferritic Oxide Dispersion Strengthened (ODS) FeCrAlY alloys were initially developed for use in high temperature environments due to their excellent creep strength and oxidation resistance. More recently, ODS materials capable of forming protective α-alumina scales have attracted increasing interest for nuclear applications, due to their potential corrosion resistance in super-critical water and liquid metal environments. However, there is still controversy about how and when the fine dispersoids form; and whether they are transformed during fabrication processes. In this paper high spatial resolution electron microscopy is used to monitor the evolution of the dispersoids under different conditions including: heat treatment of as-milled powder, selective laser melting and friction stir welding. © 2015 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Conference Committee Members of Advances in Refractory and Reactive Metals and Alloys (ARRMA-2016). Keywords: ODS alloys, PM2000, selective laser melting, friction stir welding

* Corresponding author. Tel.: +44-151-794-5367; fax: +44-151-794-4675. E-mail address: [email protected] 2214-7853 © 2015 Elsevier Ltd. All rights reserved. Selection and Peer-review under responsibility of Conference Committee Members of Advances in Refractory and Reactive Metals and Alloys (ARRMA-2016).

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1. Introduction Ferritic, Fe-Cr-Al Oxide Dispersion Strengthened (ODS) alloys containing, typically, 20wt% Cr and 5.5wt% Al with Y2O3 additions have been used for many years in applications which require good creep resistance and oxidation resistance at high temperatures [1-5]. Although Al is usually avoided in alloys used for nuclear applications, due to the potential formation of transmutation products, deleterious for reprocessing purposes, it has recently become apparent that alumina scales can give the necessary degree of protection to alloys used in liquid metal environments or super-critical water nuclear applications [6-10]. Hence aluminium containing ODS alloys may need to be considered alongside lower chromium ODS alloys which may contain tungsten and titanium, such as 14YWT [11,12]. The oxide dispersoids in all these alloys usually contain yttrium, and possibly other reactive elements such as hafnium or zirconium. In 14YWT type alloys, for example, the fine (2-10 nm) oxide dispersoids often adopt a pyrochlore (A2B2O7) type structure, in which A and B are typically Y and Ti respectively. In the aluminium containing alloys, pyrochlore structures are much less likely to be formed, with Y-Al-O dispersoids often adopting pervoskite (YAP, YAlO3), monoclinic (YAM, Y4Al2O9) or garnet (YAG, Y3Al5O12) structures [13,14]. In most cases the dispersoids are introduced by high-energy ball milling of powders of individual elements or pre-alloyed master alloys with Y2O3 powder and then consolidating the resulting material by hot isostatic pressing (HIPping) or extrusion, or both. An alternative approach is to take the milled powder and consolidate it by some form of additive manufacture, such as selective laser melting, into a near net shape component. One potential drawback of ODS alloys is the difficulty of joining them with conventional welding techniques. If the alloy is held in a molten zone for any appreciable length of time, the oxide particles will agglomerate and slag off, leaving a dispersion-free weld zone, with inferior mechanical properties when compared with those of the surrounding material. In order to avoid this, non-fusion joining techniques can be applied such as friction stir welding, pulse plasma processing or diffusion bonding [15-18], where the parent metal does not melt and the joint retains a useful volume fraction of dispersoids. During studies of these mechanical alloying and fabrication techniques, the question often arises: when are the oxide dispersoids formed and how are they affected by subsequent processing steps? Originally, it was assumed that the yttria additions just reacted with the surrounding matrix to form more stable, complex oxide structures, which remained unchanged during subsequent processing and consolidation. However high resolution (Scanning) Transmission Electron Microscopy (S)TEM and analysis and Atom Probe Tomography (APT) have shown that this is rarely the case. In this paper a series of high resolution STEM investigations are summarized which demonstrate that oxide particles may evolve, and even dissolve and reprecipitate into different crystallographic configurations, during subsequent processing. 2. Results 2.1. High temperature heat treatments of ball-milled powders Mechanically alloyed precursor powders of the ODS alloy PM2000, produced by Plansee GmbH were used in this study. The main constituents are Fe-19Cr-5.5Al-0.5Ti together with 0.5Y2O3 (wt%). TEM samples were prepared by Focused Ion Beam (FIB) sectioning or electropolishing of powders using specially designed preparation techniques [19]. Powders were in the as-received (milled) condition or heat treated over the temperature range 650°C to 1150°C to represent the range of temperatures experienced in various consolidation procedures such as degassing, extrusion, etc. TEM analysis of the as-milled PM2000 powder particles revealed a distorted ferritic microstructure consisting of fine lamellae often only 10 nm in width and 300 nm long. The large degree of deformation present in these specimens precluded the analysis of any very fine (10 nm) oxide precipitates are incoherent with the surrounding matrix and remain so after recrystallization. SLM and FSW both degrade the dispersoid distribution slightly, but post fabrication heat treatment appears to consolidate the multiphase particles into single phase oxide precipitates which remain stable for periods of prolonged high temperature exposure. Hence high temperature capabilities should be largely maintained. Work on the friction stir welding of dissimilar metals also confirms the feasibility of joining ODS materials to non-ODS alloys, while maintaining a graded structure between the two. There has been a resurgence in interest in ODS alloys, in the nuclear industry, once it was realized that the ODS particles can act as sinks for transmutation products such as He – thus delaying the swelling or grain boundary embrittlement of key components. Our recent work has dispelled some of the myths about the difficulty in

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consolidating and joining these materials, and we look forward to further advances in the use of ODS alloys in the future.

4. Acknowledgements We are grateful for the financial support of the work through the Advanced Research Materials (ARM) Program, US Department of Energy, Office of Fossil Energy, managed by U.T. Battelle, LLC, and the Engineering and Physical Sciences Research Council (EPSRC) grant EP/H018921/1 (Materials for Fusion and Fission Power).

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