Hexagonal close-packed high-entropy alloy formation under extreme processing conditions
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FOCUS ISSUE
INTRINSIC AND EXTRINSIC SIZE EFFECTS IN MATERIALS
Hexagonal close-packed high-entropy alloy formation under extreme processing conditions Ram Devanathan1,a) , Weilin Jiang1, Karen Kruska1, Michele A. Conroy1,b), Timothy C. Droubay2, Jon M. Schwantes3 1
Energy and Environment Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, USA Physical and Computational Sciences Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, USA 3 National Security Directorate, Pacific Northwest National Laboratory, Richland, Washington 99352, USA a) Address all correspondence to this author. e-mail: [email protected] b) This work was performed while M.A. Conroy was at Pacific Northwest National Laboratory. 2
Received: 7 September 2018; accepted: 29 October 2018
We assess the validity of criteria based on size mismatch and thermodynamics in predicting the stability of the rare class of high-entropy alloys (HEAs) that form in the hexagonal close-packed crystal structure. We focus on nanocrystalline HEA particles composed predominantly of Mo, Tc, Ru, Rh, and Pd along with Ag, Cd, and Te, which are produced in uranium dioxide fuel under the extreme conditions of nuclear reactor operation. The constituent elements are fission products that aggregate under the combined effects of irradiation and elevated temperature as high as 1200 °C. We present the recent results on alloy nanoparticle formation in irradiated ceria, which was selected as a surrogate for uranium dioxide, to show that radiation-enhanced diffusion plays an important role in the process. This work sheds light on the initial stages of alloy nanoparticle formation from a uniform dispersion of individual metals. The remarkable chemical durability of such multiple principal element alloys presents a solution, namely, an alloy waste form, to the challenge of immobilizing Tc.
Introduction High-entropy alloys (HEAs), which are composed of five or more metals in near-equiatomic proportions [1, 2], have attracted growing attention due to fundamental scientific interest in phase stability and promising structural applications. While initial work focused on entropic stabilization of disordered solid solutions, it is now recognized that contributions other than configurational entropy must be considered to understand and predict the occurrence of single-phase solutions [3, 4, 5, 6, 7, 8, 9, 10, 11]. Configurational entropy is sensitive to shortrange order that varies with temperature. Vibrational entropy also contributes to total entropy. This contribution can be predicted from density functional theory (DFT) [12] and hybrid Monte Carlo/molecular dynamics simulations, which were reviewed in a recent paper [13]. This finding has given rise to terms such as multi-principal element alloys (MPEAs) [14] or complex concentrated alloys [15] to describe alloys that are located away from the edges and vertices of multicomponent phase diagrams (ternary and higher). The common feature of these definitions is the alloying strategy using
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