Nanoporous High-Entropy Alloy by Liquid Metal Dealloying

: High-entropy nanomaterials possessing high accessible surface areas have demonstrated outstanding catalytic performance, beating that found for noble metals. In this communication, we report about the synthesis of a new, nanoporous, high-entropy alloy (HEA) possessing open porosity. The nanoporous, high-entropy Ta 19.1 Mo 20.5 Nb 22.9 V 30 Ni 7.5 alloy (at%) was fabricated from a precursor (TaMoNbV) 25 Ni 75 alloy (at%) by liquid metal dealloying using liquid magnesium (Mg). Directly after dealloying, the bicontinuous nanocomposite consisting of a Mg-rich phase and a phase with a bulk-centered cubic (bcc) structure was formed. The Mg-rich phase was removed with a 3M aqueous solution of nitric acid to obtain the open, porous, high-entropy Ta 19.1 Mo 20.5 Nb 22.9 V 30 Ni 7.5 alloy (at%). The ligament size of this nanoporous HEA is about 69 ± 9 nm, indicating the high surface area in this material. length porous by chemical dealloying temperature correlated with inverse dealloying temperature” or T universal obtained liquid metal dealloying. a


Introduction
High-entropy alloys (HEAs) have attracted intensive research during the past decade due to their unique properties [1][2][3][4]. Unlike the conventional alloy systems based on one principal element, or occasionally two principal elements, HEAs contain at least five major elements at concentrations ranging between 5 and 35 at%. The implicit hypothesis in the term "high-entropy" is that the high mixing entropy of these modern types of alloys can exceed the enthalpies of compound formation. Therefore, single-phase solid solutions are formed. The outstanding chemical and mechanical properties of HEAs [1,[4][5][6][7][8][9][10][11] are directly related to the four "core effects", i.e., high entropy, sluggish diffusion, severe lattice distortion, and the cocktail effect [12]. Especially, the sluggish diffusion effect is important for the exceptional high-temperature strength [13,14] and high-temperature structural stability of HEAs [13]. Recently, a number of HEA nanomaterials possessing high intrinsic catalytic activity for the decomposition of ammonia, CO oxidation, and water splitting were discovered [15][16][17]. In this case, the excellent functional, e.g., catalytic, properties can only be revealed for HEAs possessing a very large accessible surface area.

Materials and Methods
The design of a material system for liquid metal dealloying begins from the selection of elements. The elements for the liquid metal dealloying are selected based on the enthalpy of mixture (∆H mix (Mg−element) ) between a corrosive medium such as magnesium melt and the considered element ( Figure 1a). Elements exhibiting a positive value of ∆H mix (Mg−element) such as Ti, V, Cr, Fe, Mn, Co, Zr, Nb, Mo, Hf, and Ta ( Figure 1b) are immiscible in Mg and, therefore, self-organize into bicontinuous structures upon dealloying. At the same time, elements with a negative value of ∆H mix (Mg−element) such as B, Al, Si, P, Ca, Ni, Cu, Zn, Sr, Pd, Ag, In, Sn, Pt, and Au ( Figure 1b) dissolve in Mg upon the dealloying process. Thereby, a large number of element combinations can be selected for liquid/solid metal dealloying and, particularly, for obtaining multicomponent porous scaffolds, including porous high-entropy alloys. To demonstrate the effectiveness of the above-described approach, a precursor (TaMoNbV) 25 Ni 75 alloy (at%) was designed and dealloyed in magnesium at 1123 K for 20 min.
The precursor (TaMoNbV) 25 Ni 75 alloy (at%) in the shape of rods (1 mm in diameter) was fabricated and prepared from pure metals (99.99%) with an arc melting device coupled with a suction casting set-up under an argon (Ar) atmosphere (Mini Arc Melter MAM-1, Edmund Bühler, Germany). To carry out dealloying, rods 2 mm long together with a magnesium mesh were heated in a glassy carbon crucible under Ar flow using an infrared furnace (IRF 10, Behr, Switzerland). Upon heating (at a heating rate of about 40 K s −1 ), magnesium metal melts and diffuses into the precursor to selectively dissolve Ni out of the precursor alloy. The remaining elements diffuse along the metal/liquid interface [20,22] to form bicontinuous ligament structures. After dealloying, the crucible is cooled down to room temperature and the nanocomposite consisting of new phases, namely Mg-rich and high-entropy phases, is formed ( Figure 2). To obtain porous HEA material, the Mg-rich phase was selectively etched out in 3 M HNO 3 for 5 h (Figure 2). Structural investigation of the precursor alloys and porous samples was performed by X-ray diffraction in Bragg-Brentano geometry (D8 Advance, Bruker, Germany) with Cu-K α radiation. The device was equipped with a position-sensitive detector (LynxEye, Bruker, Germany), enabling us to achieve acceptable signal-to-noise ratios within a few hours of measuring time despite the smallness of the samples. Scanning electron microscopy (Nova Nanolab 200, FEI, Hillsboro, OR, USA) coupled with energy-dispersive X-ray analysis (EDAX, Weiterstadt, Germany) was used to explore the microstructure and composition.  The precursor (TaMoNbV)25Ni75 alloy (at%) in the shape of rods (1 mm in diameter) was fabricated and prepared from pure metals (99.99%) with an arc melting device coupled with a suction casting set-up under an argon (Ar) atmosphere (Mini Arc Melter MAM-1, Edmund Bühler, Germany). To carry out dealloying, rods 2 mm long together with a magnesium mesh were heated in a glassy carbon crucible under Ar flow using an infrared furnace (IRF 10, Behr, Switzerland). Upon heating (at a heating rate of about 40 K s −1 ), magnesium metal melts and diffuses into the precursor to selectively dissolve Ni out of the precursor alloy. The remaining elements diffuse along the metal/liquid interface [20,22] to form bicontinuous ligament structures. After dealloying, the crucible is cooled down to room temperature and the nanocomposite consisting of new phases, namely Mgrich and high-entropy phases, is formed ( Figure 2). To obtain porous HEA material, the Mg-rich phase was selectively etched out in 3 M HNO3 for 5 h (Figure 2). Structural investigation of the precursor alloys and porous samples was performed by X-ray diffraction in Bragg-Brentano geometry (D8 Advance, Bruker, Germany) with Cu-Kα radiation. The device was equipped with a position-sensitive detector (LynxEye, Bruker, Germany), enabling us to achieve acceptable signal-tonoise ratios within a few hours of measuring time despite the smallness of the samples. Scanning electron microscopy (Nova Nanolab 200, FEI, Hillsboro, OR, USA) coupled with energy-dispersive X-ray analysis (EDAX, Weiterstadt, Germany) was used to explore the microstructure and composition.  Figure 3 shows the X-ray diffraction pattern and microstructure of the final open nanoporous HEA. According to the X-ray analysis, the nanoporous HEA mainly consists of a phase with a bulkcentered cubic (bcc) structure and a minor amount of an unknown phase (Figure 3). According to the energy-dispersive X-ray analysis (EDX), the bcc phase is a solid solution of five elements with the following chemical composition: Ta19.1Mo20.5Nb22.9V30Ni7.5 (at%). The single-phase structure and multicomponent chemical composition (the concentration of each element is above 5 at%) of the current porous material indicate that this is a high-entropy alloy. The SEM analysis reveals the nanoporous structure of the designed HEA ( Figure 3). The average size of the ligaments is about 69 ± 9 nm, which is 1-2 orders of magnitude lower than is typically observed for the porous materials obtained by LMD [23,38]. The ligament coarsening during the liquid metal dealloying is primarily associated with surface diffusion [37]. Thus, the activation energy for the surface diffusion controls the coarsening. It seems that the sluggish diffusion effect observed in the high-entropy alloys [47] prevents coarsening in the current nanoporous HEA. However, the controversial results regarding the sluggish diffusion phenomenon require a deeper understanding of the basic mechanisms suppressing coarsening in the nanoporous HEAs.  Figure 3 shows the X-ray diffraction pattern and microstructure of the final open nanoporous HEA. According to the X-ray analysis, the nanoporous HEA mainly consists of a phase with a bulk-centered cubic (bcc) structure and a minor amount of an unknown phase (Figure 3). According to the energy-dispersive X-ray analysis (EDX), the bcc phase is a solid solution of five elements with the following chemical composition: Ta 19.1 Mo 20.5 Nb 22.9 V 30 Ni 7.5 (at%). The single-phase structure and multicomponent chemical composition (the concentration of each element is above 5 at%) of the current porous material indicate that this is a high-entropy alloy. The SEM analysis reveals the nanoporous structure of the designed HEA (Figure 3). The average size of the ligaments is about 69 ± 9 nm, which is 1-2 orders of magnitude lower than is typically observed for the porous materials obtained by LMD [23,38]. The ligament coarsening during the liquid metal dealloying is primarily associated with surface diffusion [37]. Thus, the activation energy for the surface diffusion controls the coarsening. It seems that the sluggish diffusion effect observed in the high-entropy alloys [47] prevents coarsening in the current nanoporous HEA. However, the controversial results regarding the sluggish diffusion phenomenon require a deeper understanding of the basic mechanisms suppressing coarsening in the nanoporous HEAs.

Results and Discussion
Metals 2020, 10, x; doi: FOR PEER REVIEW www.mdpi.com/journal/metals centered cubic (bcc) structure and a minor amount of an unknown phase (Figure 3). According to the energy-dispersive X-ray analysis (EDX), the bcc phase is a solid solution of five elements with the following chemical composition: Ta19.1Mo20.5Nb22.9V30Ni7.5 (at%). The single-phase structure and multicomponent chemical composition (the concentration of each element is above 5 at%) of the current porous material indicate that this is a high-entropy alloy. The SEM analysis reveals the nanoporous structure of the designed HEA (Figure 3). The average size of the ligaments is about 69 ± 9 nm, which is 1-2 orders of magnitude lower than is typically observed for the porous materials obtained by LMD [23,38]. The ligament coarsening during the liquid metal dealloying is primarily associated with surface diffusion [37]. Thus, the activation energy for the surface diffusion controls the coarsening. It seems that the sluggish diffusion effect observed in the high-entropy alloys [47] prevents coarsening in the current nanoporous HEA. However, the controversial results regarding the sluggish diffusion phenomenon require a deeper understanding of the basic mechanisms suppressing coarsening in the nanoporous HEAs. As was proposed by Chen and Sieradzki [48], the microstructural length scale (ligament size) of porous materials fabricated by chemical dealloying at room temperature is correlated with the inverse "homologous dealloying temperature" or 1/TH = Tmelting point/T298K. This universal correlation was expanded by McCue et al. [49] to the porous materials obtained by liquid metal dealloying. It was shown that the porous materials obtained by LMD follow a similar trend. The homologous dealloying temperature, in this case, was modified to TH = Tmelting point/Tdealloying temperature. Later, Joo and As was proposed by Chen and Sieradzki [48], the microstructural length scale (ligament size) of porous materials fabricated by chemical dealloying at room temperature is correlated with the inverse "homologous dealloying temperature" or 1/T H = T melting point /T 298K . This universal correlation was expanded by McCue et al. [49] to the porous materials obtained by liquid metal dealloying. It was shown that the porous materials obtained by LMD follow a similar trend. The homologous dealloying temperature, in this case, was modified to T H = T melting point /T dealloying temperature . Later, Joo and Okulov et al. demonstrated that porous HEAs synthesized by LMD possess a different correlation of ligament size with homologous temperature [19]. Specifically, the universal relationship for the nanoporous HEAs shifts down by one order of magnitude towards the smaller-size regime. Similar to the reported nanoporous HEAs [19], the currently designed nanoporous HEA also deviates from the universal relationship proposed by Chen and Sieradzki [48] and validates the recent high-entropy design strategy [19] for suppressing thermal coarsening in nanoporous materials obtained by dealloying (Figure 4).  [19]. Specifically, the universal relationship for the nanoporous HEAs shifts down by one order of magnitude towards the smaller-size regime. Similar to the reported nanoporous HEAs [19], the currently designed nanoporous HEA also deviates from the universal relationship proposed by Chen and Sieradzki [48] and validates the recent high-entropy design strategy [19] for suppressing thermal coarsening in nanoporous materials obtained by dealloying ( Figure 4). In summary, we have successfully designed a new, open, nanoporous, high-entropy Ta-Mo-Nb-V-Ni alloy by liquid metal dealloying based on the design strategy proposed in our recent study [19]. The currently developed open, porous HEA predominantly consists of a body-centered cubic (bcc) solid solution phase. The ligament size of the nanoporous Ta-Mo-Nb-V-Ni alloy after 20 min of dealloying at 1123 K is 69 ± 9 nm, which indicates its high stability against thermal coarsening. The nanoporous, high-entropy materials are promising candidates for functional applications such as  In summary, we have successfully designed a new, open, nanoporous, high-entropy Ta-Mo-Nb-V-Ni alloy by liquid metal dealloying based on the design strategy proposed in our recent study [19]. The currently developed open, porous HEA predominantly consists of a body-centered cubic (bcc) solid solution phase. The ligament size of the nanoporous Ta-Mo-Nb-V-Ni alloy after 20 min of dealloying at 1123 K is 69 ± 9 nm, which indicates its high stability against thermal coarsening. The nanoporous, high-entropy materials are promising candidates for functional applications such as catalysis.

Conflicts of Interest:
The authors declare no conflict of interest.