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Review

Properties and Applications of Supersaturated Metastable Alloys Obtained via Electrodeposition

by
Roberto Bernasconi
,
Luca Nobili
* and
Luca Magagnin
Politecnico di Milano, Dipartimento di Chimica, Materiali e Ingegneria Chimica Giulio Natta, Via Mancinelli 7, 20131 Milano, Italy
*
Author to whom correspondence should be addressed.
Crystals 2024, 14(9), 761; https://doi.org/10.3390/cryst14090761
Submission received: 26 July 2024 / Revised: 9 August 2024 / Accepted: 14 August 2024 / Published: 27 August 2024
(This article belongs to the Special Issue Dislocations and Twinning in Metals and Alloys)
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Abstract

:
Supersaturated alloys can exhibit superior properties and electrodeposition is a cost-effective and versatile technique to produce them. In this review, the chemical, mechanical and structural properties of supersaturated alloys are discussed, and connections with metallic glasses and high entropy alloys are also exposed. After discussing mechanisms causing supersaturation in electrodeposited alloys, an overview of the most important electrodeposited metastable alloys is provided, showing that they are mainly used as protective coatings able to improve corrosion resistance and tribological performance of a large variety of industrial components. Composition of the electrolytic baths and deposition parameters are also considered and discussed.

1. Introduction

Modern industrialized society makes extensive use of alloys obtained by mixing two or more metals and non-metals in specific proportions. The reason for this widespread practice is related to the vast range of mechanical, physical and corrosion-related properties that is possible to obtain by using alloys instead of pure metals. Different mechanisms are known in metallurgy to explain and predict the properties of alloys as a function of their composition [1]. As regards the mechanical properties, materials with high strength can be obtained by exploiting different processes, like grain refinement, precipitation hardening, cold working and solid solution strengthening [2]. Mechanical properties have to be considered in a wide sense, including capability of withstanding static stress or repeated loading (fatigue), resistance to wear and erosion or aptitude for saving a reflecting surface without scratches or blemishes. Because materials that are resistant to corrosive attack are required for many applications, a hardening mechanism resulting in improved or unimpaired corrosion resistance is highly desirable. Cold working causes the dislocation density to increase greatly and atoms along a dislocation line occupy defective sites, which are associated with higher free energy values compared to regular lattice positions. Thus, atoms inside a dislocation core are more reactive and typically are attacked more readily by an aggressive environment, where metal atoms are oxidized and transferred to the surrounding liquid medium (e.g., sea water) or to the liquid film covering the metal surface. Grain boundaries also are characterized by the presence of defective sites, in addition to chemical heterogeneity related to the segregation phenomenon. Although a significant grain refinement may reduce the intensity of segregation by a “dilution” effect, the risk of intergranular attack has to be considered, in connection with the specific environment. Precipitation of second-phase particles may produce a remarkable hardening effect, especially when a suitable distribution of fine particles is created by means of a specific heat treatment. Nevertheless, the difference in composition between the matrix and the second phase may induce a severe attack of the anodic constituent by galvanic-coupling corrosion. Solid solution strengthening derives from a variety of effects, which are related to the interaction between solute atoms and dislocations. Solute atoms may collect along dislocation lines and impede the dislocation movement through several locking mechanisms (elastic, electronic, etc.). In addition, solute atoms may act as stationary obstacles for a dislocation moving on a glide plane [3]. Solid solutions of metals may exhibit high corrosion resistance if the alloying elements are properly selected and the precipitation of second-phase particles with adverse effects is avoided. The most popular example is probably represented by stainless steels, where the addition of chromium is aimed at favoring the formation of a passivating surface layer.

1.1. Supersaturation and Metastability

Alloys are mixtures of different metal atoms. The percentage of each component can vary widely, but homogeneous single-phase systems are stable only in specific composition ranges. When a solubility limit is exceeded, the Gibbs free energy of the system decreases if a second phase (with different composition and structure) separates from the original mixture. If this precipitation transformation is impeded, the alloy can retain the single-phase structure with a percentage of the alloying elements larger than the solubility limit and this alloy is named “supersaturated”. Although the transformation into a stable two-phase system is expected to occur, kinetic obstacles may conserve the alloy in the supersaturated state for a long time, a condition that is called metastability. Then, supersaturated metastable alloys can be used as engineering materials with unusual properties.

1.2. Supersaturated Alloys

In many solid solutions, the solubility limit of the alloying elements is too low to achieve properties significantly better than those of the base element. In such cases, the production of supersaturated solutions is an attractive option. The transformation to the equilibrium state usually involves solid state diffusion, which typically occurs at an extremely low rate at the service temperature. Therefore, the “practical” stability of these systems is generally adequate. An example regarding stainless steels can be considered. Thermochemical treatments are performed to increase the content of interstitial elements, like nitrogen or carbon, below the surface of the treated part. Sensitization to intergranular corrosion can be induced in stainless steels by precipitation of chromium nitrides (or carbides) along grain boundaries, because of the chromium depletion occurring in the adjoining regions. Low-temperature thermochemical treatments (e.g., plasma nitriding) may inhibit chromium diffusion producing a nitrogen-supersaturated solid solution (named S phase or expanded phase) below the surface. This phase can provide a unique combination of superior hardness and improved corrosion resistance [4,5]. Due to its metastability, this phase tends to transform as temperature increases and the maximum service temperature may be near 200 °C or less, depending on the structure of the stainless steel [4,5].
Amorphous metallic alloys are even more interesting, because the lack of grain boundaries in principle removes sources of heterogeneity, which may induce localized corrosive attack. Moreover, amorphous alloys can attain very high yield strength because dislocations that are able to glide under low shear stress are not available. In metallic systems, the crystallization time below the liquidus temperature is usually very short and amorphous alloys can only be obtained with specific compositions and high cooling rates [6]. Since approximately the 1980s, bulk amorphous alloys have been developed, which can be solidified at relatively low cooling rates and make possible the production of pieces with thick sections (~101 mm) [7]. Bulk amorphous alloys can be divided in five groups, four of which are characterized by the presence of at least three main alloying elements having moderate negative enthalpy of mixing and large differences in atomic size [7]. The fifth group is based on the formation of stable clusters consisting of transition-metal atoms (Pd-Ni, Pt-Ni, etc.) located around a P atom [7]. Actually, amorphous Ni-P alloys are normally produced by electrochemical or electroless deposition, as discussed in the following sections.
Recently, high entropy alloys (HEAs) have attracted great interest because of their notable properties, like high yield strength, even at high temperatures, and high resistance to general corrosion [8]. These alloys typically consist of five or more components in equal or near equal atomic fraction and are characterized by high values of the configurational entropy of mixing. Although a single-phase structure is favored by the high entropy of mixing, some HEAs consist of two solid solutions with different crystalline structure or include nano-sized precipitates or amorphous phases [8]. Electrodeposition can be used to produce single-phase HEAs that cannot be fabricated by traditional metallurgical processes. Many electrodeposited quinary alloys consist of three common metals, Fe-Co-Ni or Fe-Cr-Ni, combined with other elements like Mn, Cr, Cu, Al, Zn, Bi, Gd [9]. Amorphous Fe-Co-Ni-Cr coatings exhibited low corrosion current densities (equivalent to corrosion rates approximately in the range of 0.01 to 0.1 mm year−1) in electrochemical tests performed in 3.5 wt.% NaCl solution and acidic (H2SO4) and alkaline (NaOH) solutions (1 mol L−1) [10]. Similarly, an Al-Cr-Cu-Fe-Ni alloy electrodeposited on a copper foil displayed a reduction of the corrosion current density in artificial seawater (3.5 wt.% NaCl solution) if compared to the uncoated substrate (4.0 μA cm−2 vs. 7.7 μA cm−2) [11]. Electrodeposited HEAs also include soft magnetic alloys that are characterized by high saturation magnetization and low coercivity. Examples are Co–Cu–Fe–Ni–Zn alloys [12] and amorphous-nanocrystalline Fe-Co-Ni-Mo-W alloys that combine low coercivity (11 Oe) with high hardness (682 HV) and high corrosion resistance (corrosion current density of 3.2 μA cm−2 in 3.5 wt.% NaCl solution) [13]. More recently, high-entropy materials have been developed for electrocatalytic applications [14]. For instance, Ni-Fe-Cu-Co-W alloys with controlled nano-porosity can achieve low oxygen-evolution overpotentials in water splitting technology [15].

1.3. Electrodeposition and Electroless Deposition

In the electrodeposition process, metal atoms are deposited on a substrate surface by chemically reducing metal ions contained in the surrounding liquid electrolyte. Current circulation in the power supply circuit and the electrolyte is needed in this process. Similarly, a metal layer is grown on the substrate by electroless deposition, but the reduction reaction is now sustained by a reducing species dissolved in the electrolyte. Then, current circulation is not required, and the deposited layer is typically characterized by high thickness uniformity.
The thickness of electrodeposited products is typically in the order of 10−1 to 101 μm, but thickness values near 1 mm can be attained in electroformed parts by properly selecting the bath formulation and deposition parameters. Because corrosion resistance and tribological behavior are mainly related to surface properties, electrodeposited coatings are widely exploited to improve the durability of industrial components made of low-cost, easily processed and tough materials.
In electrodeposition, the growth kinetics of the metallic layer at the interface with the liquid electrolyte in most cases are fast enough to avoid the rearrangement of the elements according to the thermodynamic equilibrium conditions [16]. The result is the formation of a metastable solid solution characterized by a solute content exceeding the equilibrium limit. Electrodeposition is thus a viable method to produce layers of supersaturated alloys having unique properties, in some cases almost impossible to obtain employing other methods. Some of these materials, like nickel–phosphorus alloys, have a wide range of industrial applications while some others have niche applications. The main aim of the present review is to discuss in detail the mechanisms that make possible the formation of electrodeposited alloys, and supersaturated solid solutions in particular. As a complement to this discussion, some properties and applications of the most common supersaturated alloys obtained via electrodeposition are presented. Due to the high complexity of multicomponent alloys, the discussion is limited to binary alloys (whose formation mechanisms are understood at acceptable levels).

2. Electrodeposition of Metastable Alloys

In the electrodeposition of alloys, metal ions are reduced and adsorbed onto the cathodic surface as adatoms, which can diffuse on the surface itself until the growth of the atomic layer is completed. Reliable simulations of the process would be useful for defining the operating conditions necessary for achieving specific properties of the deposited alloys. Many phenomena are involved in the overall process, then a variety of modeling methods may be needed. For instance, finite-difference methods for calculating ion diffusion in the electrolyte can be combined with computational models that simulate the events occurring on the deposition surface [17]. In the literature, Kinetic Monte Carlo (KMC) simulations of metals electrodeposition (Cu, Zn, Ag) are reported [18,19,20,21]. In a KMC simulation, particles move stochastically according to specific events and the mean values of a set of properties are calculated [22]. Phenomena occurring on the nanometer scale can be modeled, and the surface morphology of the deposited layer can be simulated [21]. Instead, Density Functional Theory (DFT) is used to calculate adsorption energies and diffusion activation energies of adatoms [21,23], even in combination with the KMC approach [21]. Probably, the application of KMC models to the electrodeposition process will be expanded in the future, including the simulation of alloys deposition. The phase-field approach can also be mentioned, which is used for simulating phase transformation and microstructural evolution in solids [24,25,26] and might be applied to some electrodeposited alloys.
Focusing attention on the electrodeposition of alloys, the process is depicted in Figure 1, where schematic pictures of the top surface layer during the deposition of a binary alloy are shown. Kinetic Monte Carlo simulations of metals electrodeposition [18,19,20,21] consider a variety of adatom diffusion mechanisms including surface diffusion, where adatoms hop to vacant nearest-neighbor sites, and interlayer diffusion, where adatoms move downward from the top atomic layer to unoccupied nearest sites in the sub-surface layer [18,19,20,21]. These simulations demonstrate that surface diffusion is the dominant mechanism, with a greater number of diffusion events [18,19,20], and is characterized by low activation energy when hopping occurs on dense crystalline planes [21].
For the sake of simplicity, only adatom displacements within the top layer are considered in Figure 1. Atoms adsorbed on the alloy surface can move to unoccupied nearest-neighbor sites at the same time as new atoms are deposited from the electrolyte. This process repeats itself regularly, with a period equal to the time required for depositing a single atomic layer. The atomic rearrangement that takes place during this time will produce the final structure of the deposited alloy. In systems characterized by the existence of a miscibility gap, atomic rearrangement would lead to demixing through the formation of clusters of the second phase, while excess solute atoms would aggregate in precipitate clusters in alloys with restricted mutual solubility. If atomic rearrangement is inhibited or does not proceed until the equilibrium state is attained, a supersaturated alloy will grow. For the most common electrodeposited metals, under a wide range of deposition current densities (10 to 1000 A m−2), the time required for monolayer completion results to vary between 1 ms and 0.9 s. It is not surprising that demixing or precipitation phenomena occurring over this short time may be largely incomplete.

3. Properties and Applications of Some Electrodeposited Supersaturated Alloys

Supersaturated solid solutions obtained via electrolytic deposition constitute an important part of the modern plating industry, with established processes like Ni-P deposition. In this section, some notable examples of electrodeposited supersaturated solid solutions are mentioned, including the aforesaid widely used materials, and also more unusual alloys like Ag-Ni or Co-Cu. These less-known materials present many interesting properties that make possible their future application in the galvanic industry. The attention in this section is focused on the properties of the alloys presented and their possible uses in current industrial practice. The most common electrolytic baths used for depositing these alloys are presented as well.

3.1. Ni-P and Co-P

The two phosphorus-based alloys Ni-P and Co-P are by far the most important electrodeposited supersaturated solid solutions from the industrial point of view. The two materials can be discussed together due to the notable similarities in their thermodynamic behavior. Nickel–phosphorus alloys are in general distinguished by the P content: low P alloys contain less than 5 wt.% of P, medium P alloys 5–9 wt.% and high P alloys more than 9 wt.%. However, there is not a single classification, and the compositional ranges can differ according to the technological application branch. Cobalt–phosphorus is usually not classified, even if a classification similar to that used for Ni-P can be applied. Ni-P and Co-P alloys are characterized by high hardness (400–650 HV), good corrosion behavior and good surface finishing. All these properties depend on the P content and higher concentrations generally cause corrosion resistance to increase and hardness or ductility to decrease.
By considering the Ni-P phase diagram [27], it is evident that the portion of the diagram in the range of 0 to 25 at.% P is characterized by complete immiscibility of the two elements in the solid state. Only an almost pure Ni phase (with negligible P content) and the Ni3P intermetallic are present in thermodynamic equilibrium conditions. If a non-equilibrium production process is applied, however, it is possible to deposit a solid solution in the cited composition range [28]. The material obtained is characterized by a degree of amorphism that depends on the P content: an increasing percentage of this element gradually hinders crystallization and leads to the formation of an amorphous alloy. This behavior is clearly evidenced by X-ray diffraction (XRD) analysis [29]. In the case of Co-P, the phase diagram is similar [30]. Even in this case, the addition of increasing quantities of P changes the structure of the alloy from crystalline to amorphous [31].
Electroless deposition of a Ni80P20 (11.7 wt.% P) alloy has been modeled by molecular dynamics simulations based on the embedded atom model (Figure 2) [32]. This study is aimed at comparing the atomic structures of the alloys obtained by electroless deposition and melt quenching and shows that short-range order is similar in both alloys. The three-dimensional local topology for this alloy displays a strong five-fold symmetry component around both Ni and P atoms, with minor fcc ordering. Five-fold symmetry is incompatible with the translational symmetry of crystals and may favor formation of amorphous phases. In addition, pair-correlation functions demonstrate a strong Ni–P affinity, with the absence of P–P first neighbors [32].
Upon exposure to high temperatures, the metastable alloy decomposes into pure Ni and NixPy intermetallics. In the case of low P content, the intermetallic formed is mainly Ni3P [33] for all annealing temperatures, while in the case of medium and high P content other intermetallics can also form at the lower annealing temperatures (Ni2P and Ni12P5 below 400 °C) [33]. If the temperature is increased over 400 °C, however, only Ni3P is in general observed also in the case of medium and high P content. The explicative examples reported in Figure 3 and Figure 4 represent the different microstructures obtained without annealing (Figure 3) and with exposure to 400 °C (Figure 4) for samples electrodeposited at increasing P content [34]. Co-P coatings present a similar behavior, but only Co2P is obtained upon annealing [28].
Ni-P coatings are typically electrodeposited from modified Watts nickel baths (containing Ni sulfate, Ni chloride and boric acid) at pH lower than the usual range used for pure Ni deposition [31,35,36,37,38,39] or, more rarely, from electrolytes based on Ni sulfamate [40,41,42,43] or Ni chloride [44]. Phosphorous acid or sodium hypophosphite are in general added as P sources. Co-P is deposited from sulfate-based baths where, again, phosphorous acid or sodium hypophosphite are added [16,45]. By tuning the composition of the solution and the deposition parameters, it is possible to deposit alloys having a P content as high as 24 at.% and even more in the case of Co-P.
From the application point of view, Ni-P and Co-P are alloys of major importance in current industrial practice. Due to their notable mechanical and anticorrosive properties [16], they can be used as functional coatings. Target applications include mainly wear protection, corrosion prevention and decorative plating. Ni-P is in particular a notable candidate for the replacement of chromium in some specific applications [46]. Ni-P can be deposited with cathodic efficiencies considerably higher than Cr, better throwing power and improved surface morphology [16]. Upon annealing and Ni3P precipitation, a hardness exceeding that of hard chromium can be achieved (~1050 HV). Another advantage of Ni-P with respect to Cr is that the alloy, once the intermetallic is precipitated, retains its high hardness even after short-term exposure to high temperatures (700 °C and more). In addition, ceramic particles (e.g., carbides) can be codeposited with Ni-P to further increase mechanical properties [46]. Co-P is another suitable candidate for replacing Cr, and some industrial processes are already available.

3.2. Fe-P

Fe-P alloys present interesting mechanical, anticorrosive and magnetic properties. They are characterized by high hardness, good corrosion resistance and a soft magnetic behavior.
From the thermodynamic point of view, they behave in a similar way with respect to Ni-P. The solubility of P in Fe is low and the Fe-P phase diagram [47] exhibits a two-phase field consisting of almost pure Fe and Fe3P intermetallic, at low temperature (below 400 °C) and P content smaller than 25 at.%. Low P percentages in supersaturated Fe-P alloys induce the formation of a crystalline phase, while an increasing degree of amorphism is gradually introduced by increasing the P content [48]. Annealing treatments promote the transformation into the α-Fe phase and the Fe3P and Fe2P intermetallics. The latter is found only in the case of very high P concentrations over 22 at.% [47].
Electrolytic baths used for Fe-P deposition include chloride [48] and sulfate based [49,50] formulations. These baths are standard Fe deposition electrolytes modified by adding sodium hypophosphite or phosphorous acid as phosphorus sources.
Fe-P is used for corrosion protection or as a soft magnetic alloy in the electronic industry [51]. Due to its high hardness, it can be used as well for tribological and mechanical applications.

3.3. Fe-Zn

Fe-Zn alloys are valuable substitutes for pure zinc coatings when the corrosion resistance of protective coatings must be enhanced at reasonable costs.
In the Fe-Zn system, many different intermetallics with high zinc content can form, starting from the Γ phase that is stable for Zn percentages around 70 at.% (~73 wt.%). As a result, a wide two-phase field (α-Fe + Г) exists in the Fe-Zn phase diagram [52]. When the alloy is electrodeposited, usually from acidic sulfate- or chloride-based electrolytes [53,54,55] or from alkaline ammine based baths [56], some non-equilibrium phases appear. The most notable ones are the iron-supersaturated zinc phase (η phase, with Fe content up to 20 wt.%) or the Γ-like phase (with Fe percentage between 20 and 40 wt.%). The first phase is hexagonal close-packed (hcp) in structure and the latter is body-centered cubic (bcc), like the Γ phase, with larger variability in iron content. If the alloy is annealed, these metastable structures transform into the equilibrium counterparts predicted by the phase diagram [57,58]. This can pose some problems when the alloy is annealed, for example during a paint bake step.
Electrodeposited iron–zinc alloys are usually applied to steel panels used mainly in the automotive industry. At compositions between 8 and 18 wt.% Fe, they are able to provide galvanic protection as well as a barrier action to structural steel. In some cases, double layers are used, with a first thick layer of material containing around 10 wt.% Fe for perforation protection and a second thin layer at about 80 wt.% Fe applied for paint adhesion purposes [59]. The alloys present good corrosion properties with both the as-deposited structure and the post annealing phase composition. Interestingly, Fe-Zn is also biocompatible and its use has been proposed for the realization of biomedical devices like degradable stents [60].

3.4. Co-Cu

Co-Cu alloys are of great scientific and possibly applicative interest due to their remarkable magnetic properties. In Co-Cu alloys, both the spin and orbital moments are enhanced with respect to pure Co, giving origin to a high magnetic moment.
The solubility of Cu in Co below 500 °C is practically null according to the phase diagram [27]. The electrodeposition process, however, produces metastable supersaturated Co-Cu alloys. According to the bath formulation and the conditions employed for the deposition, solid solutions with variable Cu content can be obtained. The metastable structure, containing 19 at.% Cu, obtained by Yuasa et al. [61] from a sulfate-based electrolyte is characterized by a face-centered cubic (fcc) structure. This constitutes a major difference with respect to other techniques like sputtering, where usually mixtures of fcc and hcp structures are observed. Exposure to high temperatures leads to the complete separation in two phases, ε-Co and α-Cu. In some specific cases, depending on annealing conditions and Co concentration, the formation of γ-Co has also been observed [62]. Also, large plastic deformations can promote the recrystallization of metastable Co-Cu alloys [63], with the formation of rows of aligned Co particles in the alloy.
Co-Cu alloys, due to their interesting magnetic properties, may find potential applications in magnetic recording devices and magnetic sensors [64,65,66,67]. The use of electrodeposition to produce these alloys is crucial because, as shown by quantomechanical calculations [61], an enhanced magnetic moment due to the magnetovolume effect is obtained only in the case of fcc Co-Cu alloys (with other techniques, as previously stated, both fcc and hcp structures are usually obtained). Co-Cu alloys are also high performing from the mechanical point of view. Pratama et al., for example, deposited Co-Cu from a tartrate based electrolyte, obtaining the best yield strength among nanostructured Co and Cu alloys [68].

3.5. Cu-Ag

Silver is the metal characterized by the highest electrical conductivity (6.3 × 107 S m−1), with copper being the second one (5.96 × 107 S m−1) [69]. Thus, silver is the ideal material for electronic applications that require low resistivity, but due to its high cost it is usually substituted by copper. Copper, however, is a soft and ductile metal, with low mechanical properties. This is a shortcoming in applications where mechanical strength or wear resistance are additional requirements together with high conductivity. A possible solution is given by solid solution hardening of Cu, obtained by adding an alloying element like Ag that causes a minimal reduction of electrical conductivity. By doing this, a considerable increase in hardness can be achieved with a minor decrease in electrical conductivity, provided that the addition of Ag is restricted to a small percentage. The same effect can be achieved in the case of Ag-rich Ag-Cu alloys.
The heat of mixing of Cu and Ag is positive all over the composition range, and for this reason a wide miscibility gap exists in the phase diagram. Two almost pure phases, namely (Cu) and (Ag), are stable at room temperature over the whole composition range. When electrodeposition is used to produce a Cu-Ag alloy, a metastable solid solution can be obtained. However, in many cases partial demixing can occur and the minor presence of an Ag-rich phase is observed. The structure actually obtained strongly depends on the deposition conditions and the electrolyte composition. Cyanide-based electrolytes [70,71] usually yield all the three phases (Cu, Ag, and the alloy) at Ag percentages of a few units. Acidic copper modified solutions give solid solutions, but only with Ag content below 1% and facing significant problems related to film morphology control at high thickness [72,73,74]. Other electrolytes can be used including, for example, the pyrophosphate-iodide bath [75]. Alloys deposited with this bath exhibit significant supersaturation of the Cu matrix, although nano-sized Ag particles are revealed by TEM analysis, even with Ag percentage as low as 4.4 at.%. If the alloys are exposed to high temperatures, typically above 300 °C, the existing metastable phase decomposes in the two (Cu) and (Ag) components. The as-deposited alloy may be harder (628 HV) than the same alloy after annealing at 400 °C (~400 HV), suggesting that solid-solution hardening combined with nanoparticles precipitation may be very effective in increasing the mechanical strength of Cu-Ag alloys [75].
Cu-Ag alloys can be used mainly as a material for contacts and switches [76]. Their low resistivity and good mechanical behavior constitute an ideal combination of properties also for contact probes. Another advantage of adding Ag in Cu alloys is the increase in electromigration resistance, for which the results are interesting for the realization of interconnects [77].

3.6. Ag-Ni

Ag-Ni alloys present an interesting mix of electrical, magnetic and catalytic properties. Besides being characterized by high conductivity [78], they exhibit giant magnetoresistance in suitable conditions [79,80] and binding energy of adsorbed hydrogen suitable for hydrogen oxidation and evolution [81,82].
From the thermodynamic point of view, the miscibility of Ni in Ag is negligible [83]. Supersaturated solid solutions can be, however, obtained by molecular beam epitaxy or other evaporation methods, but the most cost-effective technique is electrodeposition [84]. Electrolytic baths used for this purpose can be based on cyanides [49], thiosulfate [85], thiourea [86,87], uracyl [88], gluconate-thiourea [89,90] or citrate [91]. Both direct-current and pulse methods are used for depositing Ag-Ni alloys according to the specific electrolyte employed, and the complete range of average compositions between 0 and 100% Ni can be obtained. In these alloys, a supersaturated Ag-rich solid solution can be identified by XRD, whereas the Ni-rich phase sometimes is not detected even in alloys with high Ni content (Ni > 90 at.%), probably because of the nanocrystalline or amorphous nature of this phase [87]. Upon heating, the system completely decomposes in two almost pure isolated phases, (Ag) and (Ni).
Ag-Ni alloys, like Cu-Ag alloys, are used for electrical contacts and switches [83]. The alloys are, in fact, characterized by good conductivity coupled with mechanical properties far better than those of pure silver, a combination of properties ideal for electric components that must be wear-resistant. Other applications include magnetic recording and sensors [82] as well as electrocatalysis [92].

3.7. Other Alloys

Many alloys besides the above cited ones consist of metastable phases when electrodeposited. They are listed in Table 1 together with their main possible application fields and the electrolytes used for their deposition.
It can be noticed from the examples reported in Table 1 that not all the alloys are plated in aqueous solutions. Al-Mn is a typical example of a metastable alloy obtained from non-aqueous electrolytes. The use of ionic liquids or molten salts makes possible the codeposition of metals having low reduction potentials, too low for water-based electrolytes. Electrodeposition of supersaturated Al-M alloys (M being a transition metal) is a promising method for improving corrosion resistance of aluminum alloys [107].
Another important point that must be addressed is the deposition of alloys with more than two metals. Some metastable alloys formed by three or more metals exist and have been electrodeposited. They are not treated in detail in the present review, considering that additional limitations arise in defining proper deposition conditions for growing compact layers made of multicomponent solid solutions. An example is given by Sn-Ni-Fe soft magnetic alloys [108].

4. Conclusions and Future Outlook

Metastability can be exploited to synthesize alloys characterized by exceptional properties. Solid solution hardening, for example, can provide high mechanical strength retaining sufficient ductility. In addition, metastable alloys can present attractive anticorrosion or magnetic properties. On the other side, thermal stability of metastable alloys is limited, because decomposition into stable crystalline phases is promoted at high temperatures. One of the most valuable techniques to produce thin films of metastable solid solutions is electrodeposition. If compared to alternative techniques like physical vapor deposition or rapid melt quenching, electrodeposition is cost-effective and highly versatile, with thickness up to 1 mm or more. The most significant examples of binary electrodeposited metastable alloys have been reviewed, highlighting their peculiarities and industrial applicability. Some alloys, like Ni-P, have already reached a considerable industrial value, while some others still need to be developed. In order to extend the applicability of many electrodeposited metastable alloys, some critical points must be addressed. First, a deeper knowledge of the deposition mechanisms of supersaturated alloys should be achieved, in order to properly tune their properties. Advanced characterization techniques, like in situ scanning probe microscopy, may provide better understanding of interface interactions and deposition mechanisms. Experimental findings can be combined with modeling of atomic interactions and growth kinetics, in view of finding new promising systems/alloys and predicting their structural properties. Another important aspect is represented by the chemistry of some of the electrolytes used for metastable alloys plating. Many electrolytes are based on cyanides, perchlorates or other toxic components, although non-toxic formulations have been developed for alloys with established industrial applications, as reported in Section 3.1, Section 3.2, Section 3.3, Section 3.4, Section 3.5 and Section 3.6. In the age of green chemistry, it is evident that environmentally sustainable alternatives to harmful baths must be investigated, possibly also considering prospects given by ionic-liquid electrolytes. Moving beyond these concerns, new opportunities may come from multicomponent alloys, including the so-called high entropy alloys. These systems are characterized by extraordinary complexity, but they potentially offer unique properties.

Author Contributions

Writing—review and editing, R.B., L.N. and L.M. All authors have read and agreed to the published version of the manuscript.

Funding

This research received no external funding.

Data Availability Statement

No new data were created or analyzed in this study. Data sharing is not applicable to this article.

Conflicts of Interest

The authors declare no conflicts of interest.

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Crystals 14 00761 g001
Figure 1. Schematic representation of the top layer of an equiatomic binary alloy characterized by the existence of a miscibility gap; deposition time increases from (a–c). Full and open circles represent atoms in the top layer and sub-surface layer, respectively; different colors are used to distinguish the alloying elements.
Figure 1. Schematic representation of the top layer of an equiatomic binary alloy characterized by the existence of a miscibility gap; deposition time increases from (a–c). Full and open circles represent atoms in the top layer and sub-surface layer, respectively; different colors are used to distinguish the alloying elements.
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Figure 2. Molecular dynamic simulation of a bulk Ni-P glass quenched at 1011 K s−1 (a) and of a Ni-P electroless layer ((b) represents the top view, while (c) is the bottom view). Nickel atoms are represented in red and phosphorus atoms in blue. Reprinted with permission from [32].
Figure 2. Molecular dynamic simulation of a bulk Ni-P glass quenched at 1011 K s−1 (a) and of a Ni-P electroless layer ((b) represents the top view, while (c) is the bottom view). Nickel atoms are represented in red and phosphorus atoms in blue. Reprinted with permission from [32].
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Figure 3. XRD patterns of electrodeposited Ni-P at increasing P content, showing the progressive amorphization of the microstructure. Reprinted with permission from [34].
Figure 3. XRD patterns of electrodeposited Ni-P at increasing P content, showing the progressive amorphization of the microstructure. Reprinted with permission from [34].
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Figure 4. XRD patterns of annealed electrodeposited Ni-P at increasing P content, showing the formation of Ni3P. Reprinted with permission from [34].
Figure 4. XRD patterns of annealed electrodeposited Ni-P at increasing P content, showing the formation of Ni3P. Reprinted with permission from [34].
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Table 1. Notable metastable alloys obtained via electrodeposition.
Table 1. Notable metastable alloys obtained via electrodeposition.
AlloyType of ElectrolyteApplicationReferences
Au-NiAcidic cyanide-citrateContacts and switches,
substrate protection		[93,94,95]
Al-MnMolten saltsCorrosion protection, protective coatings[96]
Al-MoIonic liquidCorrosion protection[97]
Al-VIonic liquidCorrosion protection[98]
Cu-PbAlkaline citrateMetal bearing coating[99]
Ag-PbAlkaline cyanideMetal bearing coating[99]
Fe-AgAcidic perchlorateMagnetic applications[100]
Fe-CuSulfateMagnetic applications[101]
Fe-PtAcidic chloride; acidic sulfate; non-aqueousMagnetic properties[102,103,104]
Co-WCitrateWear protection[105]
Ni-WCitrateWear protection[106]
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Bernasconi, R.; Nobili, L.; Magagnin, L. Properties and Applications of Supersaturated Metastable Alloys Obtained via Electrodeposition. Crystals 2024, 14, 761. https://doi.org/10.3390/cryst14090761

AMA Style

Bernasconi R, Nobili L, Magagnin L. Properties and Applications of Supersaturated Metastable Alloys Obtained via Electrodeposition. Crystals. 2024; 14(9):761. https://doi.org/10.3390/cryst14090761

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Bernasconi, Roberto, Luca Nobili, and Luca Magagnin. 2024. "Properties and Applications of Supersaturated Metastable Alloys Obtained via Electrodeposition" Crystals 14, no. 9: 761. https://doi.org/10.3390/cryst14090761

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