Investigation of spark plasma sintered oxide-dispersion strengthened steels by means of small-angle neutron scattering
Introduction
Oxide dispersion strengthened (ODS) steels are considered as candidate materials for components of generation IV (Gen IV) fission and fusion nuclear reactors [1], [2]. Ferritic Fe-14Cr alloys exhibit good resistance to corrosion and swelling compared to austenitic steels [3]. They are creep resistant up to 550–600 °C [4], but in order to apply these materials at higher operation temperatures (e.g. 650 °C in Gen IV fission reactors [5]), the creep properties have to be improved. The addition of ODS particles (e.g. Y2O3) was found to be a good option [2], [6], [7]. Besides refining the grain size of the matrix, the particles themselves have an influence on the mechanical behavior at high temperatures. Moreover, the interface between small particles (few nm) and the matrix is assumed to act as a sink for radiation-induced primary defects, such as vacancies and interstitials, and as a nucleation site for small He-agglomerations which are uncritical as opposed to large pressurized He bubbles [8], [9]. Along with the grain size, the oxide particle size distribution is an important factor with regard to the radiation resistance. Here, two factors play a role: the mean free diffusion path towards sinks and the sink capacity. The finer the dispersion of ODS particles (at constant volume fraction) is, the larger is their specific area and, simultaneously, the smaller is the mean free diffusion path towards sinks/nucleation sites.
Usually, ODS steels are produced applying a powder-metallurgy route which consists of mechanical alloying (MA) with subsequent consolidation via hot isostatic pressing (HIP) [10], [11], [12] or hot extrusion [12], [13], [14], [15] and thermal/thermomechanical treatments. Recently, MA followed by spark plasma sintering (SPS) was shown to be suitable to achieve oxide particle sizes in the lower nm range in ODS Fe-Cr alloys [12], [16], [17], [18], [19].
It has been observed that changes in the structural properties of the Y2O3 take place during the MA process. After milling for several hours, the intensities of the Y2O3 peaks in powder X-ray diffraction (XRD) patterns decrease and finally disappear [20], [21], [22], [23], [24], [25], [26]. This may be due to either complete dissolution of Y and O [23], [24] or, alternatively, to fragmentation of Y2O3 down to the sub-nm scale [21], [22]. The details are still under debate. Finally, the milling parameters govern the size distribution, spatial distribution and structure of oxide particles after consolidation. During sintering of the milled powders, Y2O3 reappears [17] or, if the matrix contains Ti, Y–Ti complex oxides form [27]. The variation of the Y2O3 content allows the Y/Ti ratio to be adjusted.
Consolidation by means of SPS is based on heat generation directly at the powder particle contacts. The main advantage of SPS is the shorter holding time and a limitation of grain growth [28] compared to conventional techniques such as HIP.
The nature of nanoclusters in ODS steels is still a matter of debate [18], [29], [30], [31]. Yttrium-containing oxide particles have been extensively studied by means of TEM, atom probe tomography (APT) and XRD [32], [33], [34], [35]. Zhang et al. applied TEM and APT to identify the particles in a spark plasma sintered material to be Y2Ti2O7, which is known to form stable particles [36], [37]. Nagini et al. investigated the influence of milling time on the microstructure, including the particle size and composition, of a 9Cr ODS steel by means of TEM. However, the volume analyzed using these techniques is very small, which can cause problems in samples with an inhomogeneous spatial distribution of particles. In contrast, small-angle neutron scattering (SANS) provides information about the size distribution averaged over a statistically representative number of oxide particles randomly sampled from a macroscopic volume (∼50 mm3 in the present case) [38]. In existing SANS studies, several aspects of the ODS particle distribution and its role in the fabrication process are considered [39], [40].
This work is focused on the influence of milling parameters on the size distribution and composition of oxide particles in ODS Fe-14Cr alloys consolidated by means of SPS. For this purpose, SANS is applied to ODS and non-ODS samples. The SANS analysis allows bimodal particle size distributions to be explored and both particle populations to be analyzed individually. The main aim is to identify a set of milling parameters suitable to obtain a high number density of nm-size stable strengthening particles in the steel matrix. The suitability of the non-ODS samples as reference samples is examined in order to distinguish the desired strengthening particles from contamination oxides. Moreover, the ratio between the magnetic and nuclear scattering derived from SANS is critically compared to values calculated on the basis of the structure and composition of oxide particles reported in the literature.
Section snippets
Materials
The investigated samples were fabricated by MA of a gas atomized Fe-14Cr based pre-alloyed powder provided by Nanoval GmbH & Co. KG and Y2O3 powder from PCT Ltd. (particle size ≈ 30 nm) with subsequent consolidation via SPS. The composition of the steel powder is given in Table 1. MA was carried out in a Pulverisette P5 planetary ball mill under purified argon atmosphere. The ball-to-powder weight ratio was 10:1 and the milling tools (bowls and balls) were made of stainless steel. In order to
Results
The measured and fitted magnetic difference scattering curves and the reconstructed size distributions of scatterers are shown in Fig. 2, Fig. 3, Fig. 4. The fits covered the Q-range from 0.04 nm−1 to 2.5 nm−1. The fit curves are the Fourier transforms of the given size distributions. The goodness of fit therefore confirms the validity of the reconstructed size distributions. In Table 3, the integral characteristics of the calculated particle-size distributions, i.e. number density N, mean
Non-ODS samples
Multimodal particle size distributions are usually caused by the presence of different particle types with respect to their chemical composition and/or structure. It is therefore reasonable to interpret the two components of the observed bimodal size distributions of scatterers as manifestation of (at least) two different populations of particles. Bimodal particle size distributions were also reported by Mathon et al., Olier et al. and Sakasegawa et al. [14], [50], [62].
The shift of the
Conclusions
Oxide dispersion strengthened (ODS) Fe-14Cr alloys were produced using mechanical alloying and spark plasma sintering, varying the Y2O3 content and milling parameters. The compacts were analyzed using small-angle neutron scattering. The scattering curves, particle size distributions and A-ratios were critically compared among the investigated alloys and with results from the literature. The following conclusions were drawn:
- (1)
Non-ODS samples exhibit bimodal particle size distributions with a
Acknowledgements
This work received funding by the European Commission within the MATTER project (Grant Agreement No. 269706) and MatISSE project (Grant Agreement No. 604862). It also contributes to the Joint Programme on Nuclear Materials (JPNM) of the European Energy Research Alliance (EERA).
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