nanomaterials Article Stable, Ductile and Strong Ultrafine HT-9 Steels via Large Strain Machining Osman El-Atwani 1, * , Hyosim Kim 1 , Jonathan G. Gigax 2 , Cayla Harvey 1,3 , Berk Aytuna 4 , Mert Efe 4,5 and Stuart A. Maloy 1   Citation: El-Atwani, O.; Kim, H.; Gigax, J.G.; Harvey, C.; Aytuna, B.; Efe, M.; Maloy, S.A. Stable, Ductile and Strong Ultrafine HT-9 Steels via Large Strain Machining. Nanomaterials 2021, 11, 2538. https://doi.org/10.3390/ nano11102538 Academic Editor: Csaba Balázsi Received: 13 September 2021 Accepted: 26 September 2021 Published: 28 September 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Materials Science and Technology, Los Alamos National Lab, Los Alamos, NM 87545, USA; hkim@lanl.gov (H.K.); cayla@lanl.gov (C.H.); maloy@lanl.gov (S.A.M.) 2 Center for Integrated Nanotechnologies, Los Alamos, NM 87545, USA; jgigax@lanl.gov 3 Chemical and Materials Engineering, University of Nevada, Reno, NV 89557, USA 4 Department of Metallurgical and Materials Engineering, Middle East Technical University, Ankara 06800, Turkey; aytuna.berk@metu.edu.tr (B.A.); mert.efe@pnnl.gov (M.E.) 5 Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, WA 99354, USA * Correspondence: osman@lanl.gov Abstract: Beyond the current commercial materials, refining the grain size is among the proposed strategies to manufacture resilient materials for industrial applications demanding high resistance to severe environments. Here, large strain machining (LSM) was used to manufacture nanostructured HT-9 steel with enhanced thermal stability, mechanical properties, and ductility. Nanocrystalline HT-9 steels with different aspect rations are achieved. In-situ transmission electron microscopy annealing experiments demonstrated that the nanocrystalline grains have excellent thermal stability up to 700 ◦ C with no additional elemental segregation on the grain boundaries other than the initial carbides, attributing the thermal stability of the LSM materials to the low dislocation densities and strains in the final microstructure. Nano-indentation and micro-tensile testing performed on the LSM material pre- and post-annealing demonstrated the possibility of tuning the material’s strength and ductility. The results expound on the possibility of manufacturing controlled nanocrystalline materials via a scalable and cost-effective method, albeit with additional fundamental understanding of the resultant morphology dependence on the LSM conditions. Keywords: nanocrystalline; large strain machining; microtensile; nanoindentation; HT-9 steel 1. Introduction Fourth generation nuclear (Gen IV) fission reactor designs are currently explored and developed for attaining ultimate goals of sustainability, efficiency, and safety. These new designs require novel structural materials that can withstand higher radiation doses, temperatures, and mechanical stresses when compared to current light water reactors [1,2]. Therefore, the search for advanced nuclear materials is paramount and a priority to achieve success in Gen IV reactors. Ferritic/Martensitic (F/M) steels are known to be primary can- didates as structural and cladding materials for Gen IV reactors given their documentation over a long period of time in research [3]. Some of these steels are the first generation F/M steels (HT-9 with 12% Cr, 1% MoVW) and the second generation modified steels (e.g., Grade 91 with 9% Cr and 1% Mo) [4]. These steels exhibit advantages over austenitic steels in terms of void swelling and physical properties (reduced thermal expansion coefficient and improved thermal conductivity) [4–7]. While these steels provide excellent resistance to atmospheric corrosion and many organic media, their utilization is however limited to around ~560 ◦ C due to thermal creep and associated loss of strength at higher tempera- tures [1]. This is a key concern, given that the use temperature of fuel cladding materials in the future fleet of reactors is expected to approach 650–700 ◦ C[8]. Another challenge, for example with HT9, is embrittlement (loss of fracture toughness) that occurs due to defect Nanomaterials 2021, 11, 2538. https://doi.org/10.3390/nano11102538 https://www.mdpi.com/journal/nanomaterials