Due to their promising mechanical properties and superior irradiation resistance, over the last decades, oxide dispersion strengthened ferritic steels (ODS FS) have been developed in order to increase the security and thermal efficiency of Generation IV advanced nuclear reactors [1
]. Their properties are connected to the high precipitates density in the ferritic matrix that not only obstructs the deformation mechanisms (dislocation movement or grain boundary sliding) but also reduces irradiation effects by being sinks for vacancies and other defects [3
]. These caused their excellent mechanical behavior under irradiation and high temperatures, leading ODS steels to be suitable for cladding and tubes in the nuclear industry.
The composition in these alloys is a crucial parameter. Every alloying element has a purpose: 14 wt% chromium ensures the ferritic (BCC) nature of the steel and improves its corrosion behavior; Aluminum likewise can enhance corrosion resistance by developing an alumina layer [7
] and acts as an oxide former during the consolidation step [8
]; Tungsten is an effective element for solid-solution strengthening and promotes high temperature strength [9
]; yttrium and titanium are usually included because of their highly reactive nature and their ability to combine with other elements like oxygen to form complex nano-precipitates [10
]. A typical composition of ferritic ODS steel is the one used in this research: Fe-14Cr-5Al-3W-0.4Ti-0.25Y2
However, existing ODS steels need a good balance between their ultimate tensile strength (UTS) and toughness, especially Al containing ODS alloys. Therefore, new strategies must be investigated. Kimura et al. found a small addition (0.6 wt%) of Zr significantly enhanced the creep strength of Al and Ti added ODS steel [7
]. Since the ODS steel properties are closely connected to microstructural characteristics, such as the distribution of precipitates and their composition, the method of adding oxide formers to obtain thermally stable nano-oxides is critical. The main preparation route followed in this work is to mechanically alloy (MA) the ferritic steel powder with Y2
and other oxides formers like Ti. For that reason, this work proposes to synthesize a complex oxide nanopowder by using co-precipitation. This nanopowder would act as a carrier containing all the oxide formers (in this case Y, Ti, and Zr). This method has unparalleled precision to control the precipitate composition compared to traditional MA of pure elements. The new compound is introduced in the milling stage to ensure the formation of an enriched environment regarding these elements. During the subsequent consolidation, nano-oxides form through precipitation [12
]. Zr is a remarkable addition in this compound, as other authors have reported positive effects such as: refining the oxide particles [14
], decreasing the formation of coarse Y-Al-O type oxides, and ensuring there is an elevated dispersion and segregation of the oxides [15
], thus diminishing grain growth during the consolidation stage [17
Another goal of the present work is to investigate the influence of boron (B) addition, since B is known to segregate at grain boundaries, inclusions, or precipitates. Boron is a small, reactive element often used to increase the material hardness [18
] or to improve the creep behavior by inhibiting the coarsening of M23
carbides and stabilizing the carbides fine distributions at grain boundaries [19
]. Another reported feature of B is its ability to activate the sintering process as it promotes the appearance of a liquid phase wetting the surfaces of the powder particles, and thus contributes to an increase in the sintered compact density [25
Therefore, a significant objective of this work is to develop an alternative processing route with fewer stages, such as posterior heat treatments or thermomechanical treatments, while focusing on strategies that are able to provide an optimum strengthening by using precipitates that limit the formation of coarse Y-Al-O oxides.
The main goal is to study the effect of four different compositions of ODS ferritic steels with different alloying elements additions on the microstructure and, hence, on mechanical behavior, and to compare them with a base material extensively studied previously in the literature [26
] also processed in this work.
2. Materials and Methods
Four ODS steels were developed following a (powder metallurgy) PM route where powders were mechanically alloyed and further consolidated by spark plasma sintering (SPS) following the compositions listed in Table 1
. The initial raw materials were a spherical prealloyed grade Fe-14Cr-5Al-3W (Sandvik Osprey Powder Group, d50
= 30 µm), spherical highly pure Ti powder (GfE mbH, Nuremberg, Germany, d50
= 50 µm), spherical Y2
powder (TJ Technologies & Materials Inc., Shanghai, China, d50
= 7 µm), spherical pure B powder (Good Fellow Cambridge, d50
= 0.9 µm), and synthesized nanoparticles of a Y-Ti-Zr-O composition.
The synthesis of Y-Ti-Zr-O powders, a compound containing the nano oxides formers responsible for a better precipitation in the ODS steels, was carried out by using co-precipitation involving yttrium nitrate (Y(NO3
O), titanium isopropoxide (Ti(OCH(CH3
) and zirconium n-butoxides (Zr(OCH(CH3
) as precursors (all of them were purchased at Sigma Aldrich). Each precursor was mixed by using dissolution individually in different proportions to achieve the desired stoichiometry in 5 ml of isopropanol to avoid unexpected segregations in the next step. After that, the three dissolutions were mixed in an aqueous solution, which was kept at 10 pH through the addition of concentrated NH4
OH at room temperature [28
]. The co-precipitation continued for one hour. To remove all the residues produced during the co-precipitation, the resultant precipitated powders were filtered out and washed up several times using distilled water and a mixture of methanol and ethanol in equal proportion (50% in vol.). Subsequently, the powders were dried up in a hot plate and pyrolyzed at 700 °C for one hour in air atmosphere. To crystallize the powders, another heat treatment was performed at 850 °C for 30 min using a heating rate of 10 °C/min in air.
The amount of complex oxide (Y-Ti-Zr-O) was calculated and added to the ODS steels in order to keep the level of Zr at 0.6 wt% equivalent to previous results reported in [29
MA was performed in a horizontal ZOZ Attritor mill Simoloyer CM01 type under a high purity argon atmosphere (99.9995 vol%) after being purged in the vacuum system. Steel balls with a diameter of 5 mm were used, and the balls to powders ratio was 20:1. The rotation speed was set up to be 800 rpm, and the final effective milling time was 40 h for all the compositions. The objective was to involve the same milling energy through the massive collisions, on all 4 ODS FS.
To study the crystallographic parameters of the milled powders, X-ray characterization was carried out in an Xpert Phillips diffractometer (Amsterdam, Netherlands) using Cu-Kα radiation; 2Θ-scans were conducted with a step size of 0.02 degree and a step time of 2.4 s. The study of crystallite size and microstrain was performed with the Scherrer Method using Xpert Highscore software (version 2.2.5 Malvern Panalytical, Amsterdam, Netherlands).
A laser particle size analyzer was used (Malvern Mastersizer 2000, Malvern, Worcestershire, UK) to measure the particle size. The powder morphology and microstructures of the alloys were studied by employing scanning electron microscopy (Philips XL-30 and FEI Teneo FEG-SEM, Hilsboro, OR, USA) in combination with TEM (FEI Talos F2000X, Hilsboro, OR, USA) and FEG STEM (FEI Talos F2000X, Hilsboro, OR, USA) for the nano-scale investigations. For TEM examinations, disc-samples of 3 mm in diameter were polished and electropolished at −15 °C and 25 V in a solution of 5% perchloric acid in methanol.
A low diffusion sintering technique was selected to consolidate the milled powders while limiting the grain growth in the steels. Samples were sintered by using SPS (FCT System GMBH, HPD25, Frankenblick, Germany). Furthermore, to impede C diffusion from the graphite die, a high-purity (>99.97 wt.%) tungsten foil of 25 μm thickness was used during the sintering process. Subsequently, the milled ODS powders were put in into a 20 mm cylindrical graphite die and heated in vacuum (10−2
mbar). A sintering temperature of 1100 °C was achieved and maintained for 5 min using a heating rate of 400 °C/min from room temperature (R.T.), a pressure of 80 MPa was applied in order to obtain samples without pores [29
The grain microstructure analysis was performed by using different techniques such as SEM (FEI Teneo, Hilsboro, OR, USA) secondary electron (SE FEI Teneo, Hilsboro, OR, USA) imaging and EBSD (Zeiss NVision 40 FEG-SEM equipped with a Bruker EBSD acquisition system, Berlin, Germany). EBSD mappings were done on the transversal section of the consolidated samples by applying a step size between 60 nm and 150 nm. Evaluation was done using an in-house written software; a misorientation threshold of 5 degrees was used for grain determination.
Concerning the study of the nano-sized precipitates, TEM examinations were effectuated using weak beam dark field (WBDF), bright field (BF), STEM, and energy dispersive X-ray spectroscopy (EDX) modes. The precipitates size distributions were determined by measuring the precipitates using the STEM images (3 images per ODS steels) with the image analysis software JMicrovision (version 1.3.3). At least 1000 precipitates were included for each steel specimen.
The mechanical properties were analyzed by measuring Vickers’ microhardness (Zwick Roell Microhardness, Indentec Hardness Testing Machines Limited, Southern Avenue, UK), ex-situ micro-tensile tests (Micro Tensile Module, Kammrath and Weiss, Dortmund, Germany) at room temperatures, and small punch tests at both room and high temperatures. For the microhardness measurements, a load of 1.96 N was applied. The HV0.2 data corresponds to the average of 20 individual measurements, 10 in the center of the sample and 10 more in the outer part of the considered section. The tensile tests were carried out with miniature T-bone samples, under a strain rate of 2 × 10−3 mm/s, performing 4 tests for every ODS steel. In order to study the fracture mode and ductility, small punch tests with a displacement rate of 0.3 mm/min were performed at room temperature, 300 °C, and 500 °C. For the small punch tests, disc specimens of 3 mm in diameter and with a thickness of 0.250 mm were prepared and finely polished (last step: silica 0.3 µm).