Tailoring the Ti-C nanoprecipitate population and microstructure of titanium stabilized austenitic steels N. Cautaerts a, b, * , R. Delville a , E. Stergar a , D. Schryvers b , M. Verwerft a a Fuel Materials Group, Institute for Nuclear Materials Science, SCK-CEN, Boeretang 200, BE-2400, Mol, Belgium b EMAT, Department of Physics, University of Antwerp, Groenenborgerlaan 171, BE-2020, Antwerp, Belgium highlights  A new heat of DIN 1.4970 cladding steel was aged and characterized.  Ti-C nanoprecipitate number and size distribution evolution were studied by TEM.  They appeared at lower temperatures (600  C) than previously reported.  Results were explained referring to a solubility product and a diffusion model.  Nanoprecipitate formation coincided with dislocation dissociation. article info Article history: Received 12 January 2018 Received in revised form 26 April 2018 Accepted 28 April 2018 Available online 30 April 2018 abstract The present work reports on the microstructural evolution of a new heat of 24% cold worked austenitic DIN 1.4970 (15-15Ti) nuclear cladding steel subjected to ageing heat treatments of varying duration between 500 and 800  C (by steps of 100  C). The primary aim was studying the finely dispersed Ti-C nanoprecipitate population, which are thought to be beneficial for creep and swelling resistance dur- ing service. Their size distribution and number density were estimated through dark field imaging and bright field Moir e imaging techniques in the transmission electron microscope. Nanoprecipitates formed at and above 600  C, which is a lower temperature than previously reported. The observed nucleation, growth and coarsening behavior of the nanoprecipitates were consistent with simple diffusion argu- ments. The formation of nanoprecipitates coincided with significant dissociation of dislocations as evi- denced by weak beam dark field imaging. Possible mechanisms, including Silcock's stacking fault growth model and Suzuki segregation, are discussed. Recrystallization observed after extended ageing at 800  C caused the redissolution of nanoprecipitates. Large primary Ti(C,N) and (Ti,Mo)C precipitates that occur in the as-received material, and M 23 C 6 precipitates that nucleate on grain boundaries at low tempera- tures were also characterized by a selective dissolution procedure involving filtration, X-ray diffraction and quantitative Rietveld refinement. The partitioning of key elements between the different phases was derived by combining these findings and was consistent with thermodynamic considerations and the processing history of the steel. © 2018 Elsevier B.V. All rights reserved. 1. Introduction Titanium stabilized austenitic stainless steels have been quali- fied as nuclear fuel cladding for the sodium-cooled fast reactors more than four decades ago. The ’15-15Ti’ class austenitic steels with grades such as D9 (US [1], India [2]), AIM1 (France [3e5]), JPCA (Japan [6,7]), DIN 1.4970 (Germany/Belgium, developed for SNR- 300 [8,9]) remain the prime candidate materials for the cladding of the first cores of several planned lead or sodium cooled fast re- actors. This is because these materials possess excellent mechanical properties, an established fabrication technology and an extensive database of properties under irradiation. The historical develop- ment of titanium stabilized steels for use as fuel cladding in fast reactors is well documented by other authors [8, 10]. A significant drawback of austenitic steels is their tendency to swell excessively under irradiation, which in some reactor concepts * Corresponding author. Fuel Materials Group, Institute for Nuclear Materials Science, SCK-CEN, Boeretang 200, BE-2400, Mol, Belgium. E-mail address: niels.cautaerts@sckcen.be (N. Cautaerts). Contents lists available at ScienceDirect Journal of Nuclear Materials journal homepage: www.elsevier.com/locate/jnucmat https://doi.org/10.1016/j.jnucmat.2018.04.041 0022-3115/© 2018 Elsevier B.V. All rights reserved. Journal of Nuclear Materials 507 (2018) 177e187