Particle stability in model ODS steel irradiated up to 100 dpa at 600 °C: TEM and nano-indentation investigation

doi:10.1016/j.jnucmat.2012.04.001

Journal of Nuclear Materials

Volume 426, Issues 1–3, July 2012, Pages 240-246

https://doi.org/10.1016/j.jnucmat.2012.04.001 Get rights and content

Abstract

This paper is an experimental investigation of high temperature-dose stability of yttria particle dispersions in pure Fe matrix. Irradiation experiments were performed using single Fe and dual Fe and He ion beams, up to 100 dpa/360 appm He at 600 °C. Irradiation-induced evolutions are investigated by means of TEM observations, in combination with nano-indentation measurements. Particle stability at 600 °C is directly confirmed, up to 25 dpa/40 ppm He, while pronounced evolution of particle size distribution is evidenced at 80 dpa/360 ppm He. Diminution of particle density and particle coarsening is reflected in a significant evolution of the nano-indentation response. The change in the micro-mechanical evolution is ascribed to enhanced strain localization, associated with irradiation-induced particle size changes. Radiation-induced defect cluster and void formation are not detectable using TEM observations, in the whole investigated temperature/dose domain. Specific effect of implanted He is the augmentation of the micro-mechanical yields stress.

Introduction

Elevated temperature strength in typical oxide dispersion strengthening (ODS) alloys is obtained through microstructures containing a high density of nano-sized yttria particles, dispersed in a ferrite matrix [1], [2]. Quite naturally, long-term mechanical strength of these steels depends on micro-structural stability of the particle dispersions. Particle stability after irradiation [3], [4], [5], [6], [7], [8] and thermal ageing [9] is under investigation by many different groups worldwide. This paper presents an experimental investigation of stability of yttria particles included in a high purity Fe matrix. High temperature/dose experimental conditions are obtained through Fe ion irradiation with simultaneous injection of He. The simplest possible specimen chemical composition is adopted here, with a view to use different fine-scale simulation techniques for subsequent, detailed study of irradiation-induced evolutions, including molecular dynamics and ab initio calculations [10], [11].

This paper is divided into four main sections. Fabrication route of the selected model ODS alloy is given in Section 2. In Section 3, details regarding experimental techniques for investigation of particle stability under ion irradiations are presented (Section 3.1: ion irradiation conditions, Section 3.2 pre and post-irradiation experiments). Section 4.1 presents pre-irradiation particle distributions, whereas Section 4.2 describes irradiation-induced evolutions, in terms of microstructure and micro mechanical response. Section 5 is a discussion on the irradiation stability of the nano-sized yttria precipitates, based on the present results.

Section snippets

Material fabrication method

High purity Fe and yttria powders are first blended at 100 rpm for 15 min. Ball milling is then performed with a ball to powder ratio of 1:10, at 350 rpm, in Ar atmosphere at room temperature for 20 h. These conditions were selected to limit particle size growth and carbon intake, while preserving an adequate mixing of the metal with the yttria. Processed ODS powder is then compacted in the form of cylinders. Hot iso-static pressing (HIP) duration is 2 h at 195 MPa and 1150 °C. The heating and

Ion irradiations

Specimens were irradiated in the form of 3 mm discs (disc thickness = 80 μm), using single and dual ion beams (5 MeV Fe and 1.5 MeV He) at 500 °C and 600 °C. Fe ions, which are the self-ions of the steel matrix, were selected to minimize the effects of heavy-ion implantation. For He implantation, 3 and 3.8 micron thick aluminium degrader foils are used, to improve He and Fe range compatibility. The maximum Fe and He doses were 2.6 × 10¹⁶ and 7.6 × 10¹⁶ ions/cm², respectively. In these conditions, SRIM

Particle distributions in as-received material

In Fig. 2, particle distribution obtained through SANS data is compared along with the particle distribution obtained from TEM observations. The particles (particle volume fraction f_v = 0.85%) have a mean radius of 1.6 ± 0.2 nm. Only 0.15% of the particles have a radius of 4.8 nm and above. Consistency between those results confirms, firstly, the long-range uniformity of particle dispersions with HIP fabrications; secondly, validate the adopted particle counting method based on TEM imaging (see also

Discussion

Comparison between Fig. 3, Fig. 4 show: (i) elimination of the finer particles (<3 nm), (ii) marked diminution of overall particle density, and (iii) particle size coarsening. Presented evolutions suggest that dissolution of fine particles is followed by diffusion of dissolved atoms in the matrix and then by re-precipitation of these atoms, either homogeneously³ or at surviving (and initially) larger particles [24]. This mechanism implies

Conclusions

This paper is an experimental investigation of high temperature/dose stability of ODS particle dispersions in pure Fe grains, up to 100 dpa/360 ppm He at 600 °C. Irradiation experiments were performed using single Fe and dual Fe and He ion beams (5 MeV Fe and 1.5 MeV He), at 500 °C and 600 °C. Irradiation-induced micro-structural evolutions are investigated by means of TEM, in combination with nano-indentation measurements, interpreted using a specific reverse analysis method (identification of

Acknowledgments

The authors acknowledge Dr. Ramar Amuthan for preparing and providing the specimens used in this investigation. Subsequent work was performed under the aegis of Indo-French collaboration: Agreement on basic research and modelling of physical phenomena, as per the Implementing Agreement on Investigation of irradiation-induced microstructure in ODS materials using JANNUS facilities and multi-scale modelling. The authors are endowed to Dr. A. Alamo and Dr. B. Raj, who led the aforesaid Agreement

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¹: Present Address: CVD/PVD Laboratory, Surface Engineering Division, National Aerospace Laboratories, Post Bag No. 1779, 560 017 Bangalore, India.

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