The effect of tungsten on microstructure and mechanical performance of an ultrafine Fe-Cr steel
Introduction
Ferritic/martensitic steels with high Cr content have been considered as promising materials for nuclear energy applications since the last half of the XXth century [1]. These steels demonstrate high resistance to thermal and irradiation effects and they are less sensitive to irradiation swelling [2] compared to austenitic stainless steels. Thus, they are promising candidates for internal structures and out-core components of current reactors but also for future fast neutron IVth generation reactors and fusion reactors [1], [3]. The steels designed up to date are mostly based on the Fe-Cr binary system with a chromium content in a range of 7 to 14 wt% Cr. It has been shown that micro-alloying with Mo or Nb for example helps stabilizing the ferrite phase. However, their amount should be minimized to improve the steel ability to quickly release post-irradiation radioactivity. Here, the addition of W or V seems to be promising for the fast decay of radioactivity induced by fast neutrons [4], [5].
Besides, it has been recently demonstrated that the resistance of austenitic and ferritic steels to neutron and ion irradiation could be significantly improved if the grain size is reduced down to the submicrometer scale [6], [7], [8], [9], [10]. It is however not clear how alloying elements may influence the mechanical behavior of Fe-Cr steels in the nanostructured state. Indeed, atoms in solid solution could lead to the accumulation of a higher density of defects or they could segregate along grain boundaries (GBs) which might eventually promote the thermal stability and the strength but at the expense of ductility [11].
The aim of the present study was to investigate the influence of W on grain refinement by severe plastic deformation of a Fe-14Cr model alloy and the related mechanical behavior.
Section snippets
Experimental
Two alloys have been investigated: Fe-Cr (nominal composition wt.%: 14.0Cr, 0.08Ni, 0.3Si, 0.03P) and Fe-Cr-W (nominal composition wt.%: 14.0Cr, 1.0 W, 0.3 Mn, 0.2Ni, 0.3Si). The Fe-Cr alloy has been casted and then hot rolled at 930 °C. The Fe-Cr-W alloy was ball-milled under H atmosphere during 176 h followed by hot extrusion at 1100 °C. The mean grain size in the alloys was about 200 μm and 5 μm, respectively. The alloys were then nanostructured by high pressure torsion (HPT). As a result
Results and discussion
The TEM data clearly show that the HPT process successfully led to an ultrafine grained (UFG) structure in both alloys (Fig. 1). The inverse pole figures maps are displayed in Fig. 1(a) and (b) for Fe-Cr and Fe-Cr-W, respectively. Images testify that microstructures of both steels are formed by grains elongated in shear direction and delineated by high angle GBs. The fraction of low angle GBs was estimated to be about 20%.
Quantitative data computed from the ACOM datasets (Fig. 1c–f) show that
Summary
The influence of W on the microstructure and mechanical properties of HPT-processed Fe-Cr alloy was studied. It was found that addition of W leads to formation of finer but more elongated grains after nanostructuring. Since W does not segregate at GBs, this effect is attributed to a reduced defect recovery during HPT due to the solid solution. The final UFG structure with W exhibits better mechanical properties, both in terms of strength and plasticity. This is mainly attributed to the enhanced
Acknowledgements
This work was supported by EraNet.RUS program through the satellite grants by Russian Foundation for Basic Research #16-53-76020-era-a; by Deutscher Zentrum für Luft und Raumfahrt #01DJ16001 and by #163472 Nanodes grant for the French team. Experiments in Rouen were performed on GENESIS platform instruments supported by the Région Haute-Normandie, the Métropole Rouen Normandie, CNRS via LABEX EMC3 and French National Research Agency via “Investissements d’avenir” program (ANR-11-EQPX-0020).
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