metals Article Composition Stability and Cr-Rich Phase Formation in W-Cr-Y and W-Cr-Ti Smart Alloys Damian Sobieraj 1,† , Jan S. Wróbel 1, * ,† , Mark R. Gilbert 2,† , Andrey Litnovsky 3,4,† , Felix Klein 3,† , Krzysztof J. Kurzydlowski 5,† and Duc Nguyen-Manh 2,6, * ,†   Citation: Sobieraj, D.; Wróbel, J.S.; Gilbert, M.R.; Litnovsky, A.; Klein, F.; Kurzydlowski, K.J.; Nguyen-Manh, D. Composition Stability and Cr-Rich Phase Formation in W-Cr-Y and W-Cr-Ti Smart Alloys. Metals 2021, 11, 743. https://doi.org/10.3390/ met11050743 Academic Editor: Carlos Garcia-Mateo Received: 26 March 2021 Accepted: 28 April 2021 Published: 30 April 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 Faculty of Materials Science and Engineering, Warsaw University of Technology, ul. Woloska 141, 02-507 Warsaw, Poland;damian.sobieraj.dokt@pw.edu.pl 2 United Kingdom Atomic Energy Authority, Culham Science Centre, Abingdon, Oxon OX14 3DB, UK; mark.gilbert@ukaea.uk 3 Forschungszentrum Jülich GmbH, Institut für Energie- und Klimaforschung, 52425 Jülich, Germany; a.litnovsky@fz-juelich.de (A.L.); fe.klein@fz-juelich.de (F.K.) 4 National Research Nuclear University MEPhI, Kashirskoe Sh. 31, 115409 Moscow, Russia 5 Faculty of Mechanical Engineering, Bialystok University of Technology, ul. Wiejska 45C, 15-351 Bialystok, Poland; krzysztof.kurzydlowski@pw.edu.pl 6 Department of Materials, University of Oxford, Parks Road, Oxford OX1 3PH, UK * Correspondence: jan.wrobel@pw.edu.pl; (J.S.W.); duc.nguyen@ukaea.uk; (D.N.-M.) † These authors contributed equally to this work. Abstract: W-Cr-Y smart alloys are potential material candidates for plasma facing components due to their protective behaviour during the loss-of-coolant accident (LOCA), while maintaining beneficial properties of W during the normal operation of the fusion power plant. During plasma exposure, the lighter alloying elements are preferentially sputtered at the surface, but in case of a LOCA, the plasma quenches and sputtering stops and diffusion of the alloying elements to the surface becomes intensive. The diffusion of Cr to the surface due to alloying elements (Y, Ti) yields a protective oxide layer stopping the sublimation of WO 3 . The phase stability and short-range ordering of ternary alloys in W-Cr-Y(Ti) systems has been investigated, using combination of Density Functional Theory (DFT) and Cluster Expansion (CE) methods with Monte-Carlo (MC) simulations. It has been found out from the DFT calculations, that all pairs in the W-Cr-Y system have positive values of the enthalpy of mixing, while most of the Cr-Ti and Ti-W binary structures have negative enthalpies of mixing. The shift in the short-range order as a function of temperature between Cr and W has been predicted as a result of Y addition in W-Cr-Y alloys compared to W 70 Cr 30 , by around 400 K towards lower temperatures. A strong tendency towards clustering of Y has been observed even at elevated temperatures (1800 K). The decrease of the order–disorder transition temperature (ODTT) as a result of the Y addition has been observed, while the addition of Ti has not shown any significant changes in the ordering of W-Cr-Ti alloys compared to W-Cr alloy. Our MC simulations showed that for the W 70 Cr 29 Y 1 alloy the enthalpy of mixing (H mix ) value is positive in the whole analysed temperature range. Free energy of mixing above 1000 K has been calculated from the first nearest neighbours approximation for W 70 Cr 29 Y 1 and W 70 Cr 29 Ti 1 alloys. The results of the present investigations provide an insight enabling for optimizing chemical composition of materials for future plasma facing components. Keywords: smart alloys; short-range order; Plasma-Facing-Materials; W alloys; Cluster Expansion 1. Introduction Plasma-Facing-Materials (PFM) for the first wall of future fusion power plants are required to possess extraordinary properties while being able to withstand very high temperatures and radiation damage. High melting point, low erosion yield from plasma particles, low tritium retention, resistance towards neutron irradiation, and stability during failure/abnormal event are examples of required properties for potential candidates for Metals 2021, 11, 743. https://doi.org/10.3390/met11050743 https://www.mdpi.com/journal/metals