Alloy Films Research Articles

Tunable piezoresistivity for sensors with antiferromagnetic pure Cr and Cr-rich alloy thin films: Cr–V, Cr–W, Cr–Mn

Tunable piezoresistivity for sensors with antiferromagnetic pure Cr and Cr-rich alloy thin films: Cr–V, Cr–W, Cr–Mn

Room-Temperature Magnetic Antiskyrmions in Canted Ferrimagnetic CoHo Alloy Films.

Magnetic antiskyrmions, the anti-quasiparticles of magnetic skyrmions, possess alternating Bloch- and Néel-type spin spirals, rendering them promising for advanced spintronics-based information storage. To date, antiskyrmions are demonstrated in a few bulk materials featuring anisotropic Dzyaloshinskii-Moriya interactions and a limited number of artificial multilayers. Identifying novel film materials capable of hosting isolated antiskyrmions is critical for memory applications in topological spintronics. Herein, the formation of room-temperature antiskyrmions in single ferrimagnetic CoHo rare-metal alloy films of varying thicknesses, observed using Lorentz transmission electron microscopy is reported. Furthermore, rotating magnetic fields (H) are proposed to facilitate antiskyrmion nucleation and enhance their areal density by an order of magnitude compared to that in the same area under individual vertical H. In addition, experimental and phenomenological analysis confirm that antiskyrmion nucleation can be attributed to spin reorientation involving spontaneous canted magnetism, as evidenced by polarized neutron reflectometry. Micromagnetic simulations further show that the antiskyrmion density significantly depends on the magnitude of the rotating field. These findings expand the family of known antiskyrmion-hosting materials and provide insights into their formation mechanisms, thus serving as a basis for their application in topological spintronics.

Enhanced two-temperature model and its application in comprehensive analysis of femtosecond laser melting of gold, copper and their alloys.

Extensive research on ultrashort laser-induced melting of noble metals like Au, Ag and Cu is available. However, studies on laser energy deposition and thermal damage of their alloys, which are currently attracting interest for energy harvesting and storage devices, are limited. This study investigates the melting damage threshold (DT) of three intermetallic alloys of Au and Cu (Au3Cu, AuCu and AuCu3) subjected to single-pulse femtosecond laser irradiation, comparing them with their constituent metals. This is accomplished by extending an earlier-developed two-temperature model (TTM)-based code with several improvements, including precise modeling of temperature-dependent optical properties and ballistic electron transport. The dynamic optical model inclusive of ballistic effects is demonstrated to reproduce the experimental DT of pure metals with minimal variation and is therefore adopted for further investigation. Our simulations reveal that the alloy films have significantly lower incipient and complete melting thresholds compared to the pure metals due to their low thermal conductivity and high electron-phonon coupling strength. Theoretical studies on varying the thickness of metal and alloy films unveil the usual trend of a rapid increase in DT up to a certain thickness, followed by a saturation region. This universal DT profile is elucidated by proposing a first-of-its-kind analytical function. Excellent agreement between the coefficients of the function with optical and electron diffusion parameters derived from the comprehensive theory proposed here reinforces the robustness of the model. The novelty of this study also lies in introducing the concept of a critical film thickness for which the entire film attains the melting temperature at its complete melting threshold.

Guanine-based spin valve with spin rectification effect for an artificial memory element.

Non-volatile electronic memory elements are very attractive for applications, not only for information storage but also in logic circuits, sensing devices and neuromorphic computing. Here, a ferroelectric film of guanine nucleobase is used in a resistive memory junction sandwiched between two different ferromagnetic films of Co and CoCr alloys. The magnetic films have an in-plane easy axis of magnetization and different coercive fields whereas the guanine film ensures a very long spin transport length, at 100K. The non-volatile resistance states of the multiferroic spintronic junction with two-terminals are manipulated by a combined action of small external magnetic and electric fields. Thus, the magnetic field controls the relative orientation of the magnetization of the metallic ferromagnetic electrodes, that leads to different magnetoresistance states. The orientation and the magnitude of the electric field controls the orientation of the polarization of the guanine ferroelectric barrier, that leads to different electroresistance states, respectively. Moreover, we have observed a strong interfacial coupling of the two parameters. Consequently, positive and negative magnetoresistance hysteresis loops corresponding to spin rectification effects and non-hysteretic (erased) resistive states are manipulated with the electric field by switching the orientation of the electrical polarization of the organic ferroelectric.

Efficient dye-sensitized solar cells using niobium-palladium binary alloy films counter electrode

Efficient dye-sensitized solar cells using niobium-palladium binary alloy films counter electrode

Electrodeposition and electrocatalytic performance of Pd-Ni alloy films from aqueous solutions for enhanced electrochemical hydrogen evolution

Electrodeposition and electrocatalytic performance of Pd-Ni alloy films from aqueous solutions for enhanced electrochemical hydrogen evolution

Tailoring the structure of Al50Mo50 alloy films towards realizing the well match between mechanical and electrical performance by controlling the growth mode

Tailoring the structure of Al50Mo50 alloy films towards realizing the well match between mechanical and electrical performance by controlling the growth mode

Dual-Nano Composite Design with Grain Boundary Segregation for Enhanced Strength and Plasticity in CoCrNi-CuZr Thin Films.

Enhancing both strength and plasticity simultaneously in nanostructured materials remains a significant challenge. While grain refinement is effective in increasing strength, it typically leads to reduced plasticity due to localized strain. In this study, we propose a novel design strategy featuring a dual-nano composite structure with grain boundary segregation to enhance the deformation stability of nanostructured materials. This strategy is demonstrated using a CoCrNi-CuZr multiprincipal element alloy film, which shows a dual-nano composite structure with high-density nanotwins and crystalline-amorphous nanocomposite, along with elemental segregation at the columnar grain boundaries. Our strategy achieves homogeneous plastic deformation in a (CoCrNi)91(CuZr)9 thin film, an 18% increase in strength compared to nanocrystalline CoCrNi, and a 67% increase compared to amorphous CuZr thin films. These results provide valuable insights into designing high-strength, ductile alloys through the engineering of dual-nano composite structures.

Synergistic Contribution of the Strain and Magnetic Field in Ferromagnetic NiMnIn Heusler Alloy Films for the Hydrogen Evolution Reaction.

The external field-assisted hydrogen evolution reaction (HER), beyond modifying electrocatalysts themselves, has garnered significant research attention. However, achieving synergy between multiple fields to enhance the HER performance remains challenging and is not well-explored. Here, NiMnIn Heusler alloy thin films are fabricated using pulsed laser deposition on flexional Cu substrates. Due to the different adsorption free energy under tensile strain (TS) and compressive strain (CS), the most significant enhancement of the electrocatalytic HER activities for the NiMnIn film is observed in the 1.9% TS state, while the CS state of the NiMnIn film shows negative effects. Furthermore, when an external magnetic field is coupled to the 1.9% tensile-strained NiMnIn film, the total overpotential is reduced by 28.6% at a current density of -50 mA cm-2. This improvement is attributed to a synergistic effect that enhances the H adsorption stability and accelerates the H2 desorption process. This work demonstrates the effectiveness of combining multiple external fields and presents a promising strategy for advancing electrocatalysis.

Research on the Microstructure, Mechanical Properties and Strengthening Mechanism of Nanocrystalline Al-Mo Alloy Films.

In this work, the Al-Mo nanocrystalline alloy films with Mo contents ranging from 0-10.5 at.% were prepared via magnetron co-sputtering technology. The composition and microstructure of alloy thin films were studied using XRD, TEM, and EDS. The mechanical behaviors were tested through nanoindentation. The weights of each strengthening factor were calculated and the strengthening mechanism of alloy thin films was revealed. The results indicate that a portion of Mo atoms exist in the Al lattice, forming a solid solution of Mo in Al. The other part of Mo atoms tends to segregate at the grain boundaries, and this segregation becomes more pronounced with an increase in Mo content. There are no compounds or second phases present in any alloy films. As the Mo element content increases, the grain size of the alloy films gradually decreases. The hardness of pure aluminum film is 2.2 GPa. The hardness increases with an increase in Mo content. When the Mo content is 10.5 at.%, The hardness of the film increases to a maximum value of 4.9 GPa. The fine grain (∆Hgb), solid solution (∆Hss), and nanocrystalline solute pinning (∆Hnc,ss) are the three main reasons for the increase in the hardness of alloy thin films. The contribution of ∆Hgb is the largest, accounting for over 60% of the total, while the contribution of ∆Hss accounts for about 30%, ranking second. The rest of the increase is due to ∆Hnc,ss.

Doping Elemental 2D Semiconductor Te through Surface Se Substitutions.

The development of two-dimensional (2D) semiconductors is limited by the lack of doping methods. We propose surface isovalent substitution as an efficient doping mechanism for 2D semiconductors by revealing the evolution of the structure and electronic properties of 2D Se/Te. Because of the different electronegativity of Se and Te, Se substitution for Te at the specific lattice sites introduces electric dipoles and leads to charge redistribution, which lowers the work function and tunes the Te films from p-type to n-type semiconductors. This differs from the gap enlargement of alloy films with random Se-Te substitutions. This doping method minimizes the change of lattice structure and surface roughness, which benefits structure stacking. Further increasing the Se content leads to the formation of two types of 2D semiconducting Se polymorphs.

Vertical β-Ga2O3 Schottky Barrier Diode with the Composite Termination Structure

In this work, β-Ga2O3 based Schottky barrier diodes (SBDs) with a composite termination structure of the ZnNiO thin alloy films are reported. β-Ga2O3 SBDs with the junction termination extension structure of the wide-bandgap p-type ZnNiO achieve a breakdown voltage (Vbr) of 2040 V and a specific on-resistance Ron,sp of 3.48 mΩ·cm2, contributing a power figure of merit (PFOM, Vbr 2/Ron,sp) of 1.20 GW cm−2. Meanwhile, we demonstrated high-performance vertical β-Ga2O3 SBDs with the bevel junction termination extension (BJTE) structure, which the Ron,sp,Vbr and BFOM are 4.21 mΩ·cm2, 2704 V, and 1.74 GW cm−2, respectively. Technology computer aided design simulations show that BJTE structure significantly optimizes the surface electric field of β-Ga2O3 drift layer. These devices make a significant step to achieve high performance β-Ga2O3 power devices by implementing a composite termination structure.

Enhancing NiCoB alloy coatings with hBN nanoparticles: exploring structural, mechanical, and corrosion improvements through pulse electrodeposition

ABSTRACT NiCoB/hBN composite coatings deposited onto a 304 stainless steel substrate by pulsed current electrodeposition method. The morphology, chemical composition, structural, mechanical and corrosion properties of the coatings were investigated. Optimal microhardness, corrosion resistance, and a lower coefficient of friction were achieved when the hBN concentration in the plating bath was 8 g L−1 . Vickers microhardness increased from 610 to 980 HV and the coefficient of friction decreased from 0.53 to 0.4. Grain size decreased as the amount of added hBN increased, affecting the microstructure of the coating. Increasing the concentration of hBN in the plating solution increased the percentage of hBN particles in the film, reducing the percentage of total metal from 95.18% to 89.76%. The magnitude of the Nyquist parabola of the composite film from the bath containing 8 g L−1 hBN increased by an average of 5 times compared to that of the NiCoB metal alloy film.

Ultra-low Gilbert damping and self-induced inverse spin Hall effect in GdFeCo thin films

Ferrimagnetic materials have garnered significant attention due to their broad range of tunabilities and functionalities in spintronics applications. Among these materials, rare earth-transition metal GdFeCo alloy films have been the subject of intensive investigation due to their spin-dependent transport properties and strong spin–orbit coupling. In this report, we present self-induced spin-to-charge conversion in single-layer GdFeCo films of different thicknesses via an inverse spin Hall effect. A detailed investigation of spin dynamics was carried out using broadband ferromagnetic resonance measurements. The anisotropy constant and the effective g-factor are found to decrease with thickness, and they become nearly constant for thicknesses beyond 25 nm. A remarkably low damping constant of 0.0029 ± 0.0003 is obtained in the 43 nm-thick film, which is the lowest among all previous reports on GdFeCo thin films. Furthermore, we have demonstrated a self-induced inverse spin Hall effect, which has not been reported so far in a single-layer of GdFeCo thin films. Our analysis shows that the inverse spin Hall effect becomes increasingly dominant over the spin rectification effect with increasing film thickness. The in-plane angular-dependent voltage measurement of the 43 nm-thick film reveals a spin pumping voltage of 1.64 μV. The observation of spin-to-charge current conversion could be due to the high spin–orbit coupling element Gd in the film as well as the interface between GeFeCo/Ti and substrate/GdFeCo of the films. Our findings underscore the potential of GdFeCo as a prime ferrimagnetic material for emerging spintronic technologies.

The study on the magnetic FeCoNiCuAl high-entropy alloy film with excellent corrosion resistance

The study on the magnetic FeCoNiCuAl high-entropy alloy film with excellent corrosion resistance

(Keynote) Controlling CO2 Electrolyzer Reactivity Using Alloy and Polymer-Modified Electrodes

This talk addresses the reactivity associated with CO2 reduction. Electrodeposition of metals from plating baths containing 3,5-diamino-1,2,4-triazole (DAT) as an inhibitor yields highly porous materials exhibiting enhanced activity for electrochemical reactions. Electrodeposition of Cu or CuAg and CuSn, alloy films from such plating baths yields high surface area catalysts for the active and selective electroreduction of CO2 to multi-carbon hydrocarbons and oxygenates. EXAFS shows the co-deposited alloy film to be homogenously mixed. Alloy films containing Sn exhibit the best CO2 electroreduction performance, with the Faradaic efficiency for C2H4 and C2H5OH production reaching nearly 60 and 25%, respectively, at a cathode potential of just –0.7 V vs. RHE and a total current density of ~–300 mA/cm2. Alloy films containing Sn exhibit greater efficiency for CO production relative to either Cu alone or CuAg at low overpotentials. In-situ Raman and electroanalysis studies suggest the origin of the high selectivity towards C2 products to be a combined effect of the enhanced destabilization of the Cu2O overlayer and the optimal availability of the CO intermediate due to the Ag or Sn incorporated in the alloy. Sn-containing films exhibit less Cu2O relative to either the Ag-containing or neat Cu films, likely due to the increased oxophilicity of the admixed Sn. Additionally, modification of the Cu electrode with certain polymers (Figure 1) yields substantially enhanced reactivity, due in part to control of the Cu2O layer and elevated surface pH, measured in situ. These electrodes exhibit enhance ethanol production. Polymer-composite electrodes exhibit enhanced reactivity over a wide range of proton-involved electrochemical reactions, such as methanol oxidation and oxygen reduction.

Superconductivity of Electrodeposited Rhenium Alloys

Superconductivity of Electrodeposited Rhenium Alloys Rhenium (Re) is a type I superconductor with an intrinsic superconducting transition temperature, or the critical temperature (Tc), at 1.7 K. Electrodeposited Re has been found to be in its amorphous state with an enhanced critical temperature of 6 K[1]. This enhancement enables superconductivity beyond the boiling point of liquid helium, i.e. 4.2K, facilitating the applications of Re in the connectivity for quantum devices. In addition, Re also maintains its ductility without undergoing ductile-to-brittle transition as temperature decreases, owing to its hexagonal close-packed (hcp) crystal structure[2], which further supports its accessibility to cryogenic electronic application, such as interconnects in quantum devices. However, this enhancement in T directly results from the amorphous or nanocrystalline grain structure and rapidly degrades upon Re recrystallization at elevated temperatures.In order to improve the thermal stability of the T of nanocrystalline Re or even further increase the Tc, rhenium films doped with other elements have been electrodeposited. For example, while alloying Re with Fe inhibits the recrystallization, the superconductivity of ReFe is found significantly suppressed. On the other hand, while Co doping suppresses the superconductivity to a much less degree, it has no impact on the recrystallization. Between these two alloying elements, both Fe and Co are ferromagnetic and have similar atomic radius. Yet, they have completely different crystal structures and are also different with respect to the Re host.The present work continues from previous studies to perform systematic comparison between dopants with various atomic radius, electronegativity, magnetism, and intrinsic crystallographic configurations to identify the determining factor for alloy superconductivity and the thermal stability. For example, Figure 1 shows the XRD patterns of as-deposited ReRu alloy films with controlled Ru contents as well as the same films after thermal annealing at various elevated temperatures. The electrodeposition is carried out on rotating Cu disk electrodes. The concentration of Re and Ru in electrolyte and the time of deposition are varied in order to achieve different composition and to keep film thickness between 300 and 400 nm. Film composition and thickness are measured using x-ray fluorescence spectroscopy (XRF). The film grain structure and superconductivity as well as their thermal stability will be discussed in detail in the presentation. Reference Pappas, D.P., et al., Enhanced superconducting transition temperature in electroplated rhenium. Applied Physics Letters, 2018. 112(18).Naor, A., et al., Properties and applications of rhenium and its alloys. Ammtiac Quarterly, 2010. 5(11).Malekpouri, B., K. Ahammed, and Q. Huang, Electrodeposition and superconductivity of rhenium-iron alloy films from water-in-salt electrolytes. Journal of Alloys and Compounds, 2022. 912: p. 165077.De, S., et al., Electrodeposition of superconducting rhenium-cobalt alloys from water-in-salt electrolytes. Journal of Electroanalytical Chemistry, 2020. 860: p. 113889. Figure 1. XRD patterns of rhenium-ruthenium alloy films (a) before annealing, after annealing at (b) 150 °C, (c) 200 °C, (d) 300 °C. XRD standards of pure Re and Cu are included in (c) and (d). Figure 1

Role of Glycine in the Electroless Fe-Ni-B Alloy Process for Power Semiconductor Devices

Pyrometallurgically produced Invar Fe-Ni alloys possessing an Fe content of 55–70 mass% exhibit a low coefficient of thermal expansion (CTE). Therefore, one can expect electroless deposited Invar Fe-Ni alloy films to also exhibit a low CTE comparable to those of semiconductor chips and insulating substrates used in semiconductor devices. However, a process for producing an Invar Fe-Ni alloy film by electroless plating has not been established and the thermal expansion properties of the obtained film have not been sufficiently investigated.We prepared electroless Fe-Ni-B alloy thin films using a citrate-pyrophosphate bath as a complexing agent and dimethylamine borane (DMAB) as a reducing agent, whereupon the thermal stress behavior of the thin films was evaluated [1]. In a previously reported plating process [1], the deposition rate of the Invar alloy plated film was 0.6 µm·h−1, which is too slow for practical applications. This deterioration in reaction rate of the electroless plating is caused by the high concentration of Fe2+, known as a stabilizer [2], in the plating bath. It is therefore necessary to improve the process to obtain the ~5–10 μm thickness required for high-density semiconductor packaging.In general, an increase in bath temperature and pH value improves the deposition rate, but this process limits the optimization of these operation conditions owing to the promotion of Fe2+ oxidation and the absence of stable complexes. Therefore, control of the deposition rate was attempted in this study by selecting a complexing agent in the plating bath. Here, we chose glycine [2], which is known to form a complex with Ni2+ in an electroless Ni plating bath and improve the deposition rate of electroless Ni. The glycine was added to the electroless Fe-Ni-B alloy plating bath, and the effect on the deposition of the Fe-Ni-B alloy film was investigated.Table 1 shows the electroless plating bath and plating conditions. The total metal-ion concentration was 50 mmol·L−1. A pure copper plate was used for the substrate, and a contact plating method was used where an aluminum plate was brought into contact with the substrate to initiate the deposition reaction. A 1 h plating time was used in each case.Fig. 1 shows Fe content and deposition rate of the electroless Fe-Ni-B alloy film obtained from the plating bath without or with addition of 1 to 100 mmol·L− 1 glycine, where ratios of Fe2+ / (Fe2+ + Ni2+) were 0.3 and 0.8. In both ratios of Fe2+ / (Fe2+ + Ni2+), Fe content was significantly reduced by adding of glycine of 10 mmol·L-1 or more. The deposition rate increased when glycine was added to the plating bath compared to that in the bath without glycine. When 100 mmol·L-1 of glycine was added, the deposition rate decreased compared to that when 10 mmol·L-1 was added.Using the stability constants of the complexes formed in the bath, it is estimated that when the glycine concentration is up to 10 mmol·L-1, it predominantly coordinates with Ni2+, whereas upon the addition of 100 mmol·L-1, it also forms complexes with Fe2+. As previously mentioned, the electroless plating process utilizing Ni2+ complex ions with glycine as a ligand exhibits a faster rate compared to deposition reactions involving other stable complex ions [2]. It is assumed that the Ni-glycine complex ion species with a high reduction rate becomes dominant in the plating bath containing 10 mmol·L-1 of glycine, so that the reduction rate of the Fe and Ni alloy is accelerated accompanied by acceleration of Ni2+ reduction and reached the maximum value.It turns out that by using glycine as a second complexing agent and optimizing its addition amount in an electroless Fe-Ni-B alloy plating bath, complex ions with a high reduction rate are formed, resulting in the improving of the deposition rate. Acknowledgement This work was supported by JSPS KAKENHI Grant Number JP24K08121.

Permeation Behavior of La, Ce, Pr, Nd, Gd, Tb, and Dy through a Ni–RE Alloy Diaphragm in Molten LiCl–KCl Systems

The authors have been investigating a new recycling process for Nd and Dy from wasted Nd–Fe–B magnets utilizing selective permeation phenomenon through alloy diaphragms in molten salts. In this process, alloy diaphragms such as Ni–rare earth (RE) alloy film, functions as a bipolar electrode through which RE ions permeate from the anolyte to the catholyte in three steps: (a) reduction of RE ions to form alloys on the anolyte side of the diaphragm, (b) diffusion of RE atoms inside the diaphragm, and (c) oxidation of RE atoms to dissolve into the catholyte as RE ions on the catholyte side of the diaphragm [1]. Because the alloying and dealloying potentials for each RE element are different, a specific RE element can selectively permeate through the alloy diaphragm under an appropriate condition. Actually, the selective permeation of Dy or Nd has already been demonstrated [2, 3]. However, in actual recycling from end-of-life products, the wasted Nd–Fe–B magnets may contain a variety of impurities, such as base metals and other RE elements. Although most of impurities can be easily separated from RE elements due to differences in electrochemical properties, RE impurities such as La, Ce, Pr, Gd, and Tb are more difficult to separate from Nd and Dy. Thus, in this study, the permeation behavior of La, Ce, Pr, Gd, and Tb was investigated in the presence of Nd and Dy in LiCl–KCl eutectic melts.A schematic drawing of the experimental setup is described in Figure 1. The electrolytic cell was made of alumina and separated to the anole and cathode compartments by a Ni film of thickness 0.1 mm which is used as the starting material for the alloy diaphragm. The electrolyte was a molten LiCl–KCl eutectic dissolved at 723 K under Ar flow, and RECl3 (RE = La, Ce, Pr, Nd, Gd, Tb, and Dy) anhydrous were added to the anolyte. In the catholyte, DyCl3 was added in order to prevent the dendritic deposition of RE metal on the cathode. The potentials of each side of the diaphragm were controlled by two potentio-galvanostats (PGS-A and B). The permeation experiment was performed under two experimental conditions that were expected to enable the selective permeation of Nd and Dy, respectively, which were determined based on our previous studies [2, 3]. In the first experiment, the potential of the anolyte side of the diaphragm (WE-A) was controlled at 0.65 V vs. Li+/Li by PGS-A. At this potential, a part of added RE ions can be reduced and form RE–Ni alloys (mostly RENi2). The catholyte side potential of the diaphragm (WE-B) was not controlled initially. As the alloy formation proceeded, the potential of WE-B negatively shifted. When the potential of WE-B reached 0.80 V vs. Li+/Li, this value was fixed. At this potential, all the added RE elements can be dissolved from the alloy phase if they exist on the catholyte side surface. In the second experiment, the anolyte and catholyte side of the diaphragm were controlled at 0.50 V and 0.70 V vs. Li+/Li, respectively. At this condition, most of the added RE elements can form RENi2, and only a part of RE elements can be dissolved from the diaphragm on the catholyte side.The permeated amount of RE elements were evaluated by ICP-AES analysis. As a result, Gd and Tb showed a similar permeation tendency to Dy, whereas Ce and Pr behaved like Nd in both experimental conditions. La hardly permeated probably because it does not form RENi2 alloy phase. These tendencies are discussed based on the applied potential and the equilibrium potential for RENi2/RENi3 (RE = Ce, Pr, Nd, Gd, Tb, and Dy) and La7Ni16/LaNi3. Keywords Rare earth, Recycle, Alloy diaphragm, Separation, Molten salt electrolysis References [1] T. Oishi et al., Kagaku Kogaku Ronbunshu, 36, 299 (2010) [in Japanese].[2] T. Oishi et al., J. Electrochem. Soc., 167, 163505 (2020).[3] T. Oishi et al., J. Electrochem. Soc., 168, 103504 (2021). Acknowledgments This work was partly supported by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. Figure 1

Transmission Electron Microscope Characterizations of the Localized Corrosion Behavior of Biodegradable ZX21 Mg Alloy in Hanks’ Balanced Salt Solution

Magnesium (Mg) alloys attract a lot of attention as next-generation biodegradable implant materials due to their excellent biocompatibility and unique biodegradability. By utilizing Mg alloys as implants, Mg alloys can be degraded naturally in the human body, avoiding the second removal surgery. In addition, Mg alloys are promising for orthopedic applications because of their similar density and mechanical properties to bones, which mitigate the stress shielding effect. Among various Mg alloy systems, the ZX-series Mg alloys, which contain zinc (Zn) and calcium (Ca), show great potential for orthopedic applications since Zn and Ca are essential for metabolism and bone regeneration and improve the mechanical strength of the alloys. However, localized corrosion of a ZX-series Mg alloy was reported in our previous study [1], leading to a faster degradation rate and possible premature failure of the implant.To further study the origin and detailed mechanism of the localized corrosion behavior of ZX-series Mg alloys in simulated physiological environments, transmission electron microscope (TEM) characterizations were performed on a ZX21 (Mg-2.15 wt%Zn-0.97 wt%Ca) Mg alloy before and after corrosion in a Hanks’ balanced salt solution (HBSS) at 37°C. Two second phases, Ca2Mg6Zn3 and Mg2Ca, were found adjacent to each other in the cast ZX21 alloy. The potential difference between the two second phases leads to microgalvanic corrosion and the preferential dissolution of the lower-potential Mg2Ca, initiating localized corrosion on the alloy surface. The localized corrosion propagates a few micrometers deep in the alloy and underneath the surface corrosion film. By selected area electron diffraction (SAED) and energy dispersive X-ray spectroscopy (EDS) analysis in TEM, the corrosion products in the propagating localized corrosion regions are primarily composed of Mg(OH)2, which is different from the surface corrosion film (Mg(OH)2 + CaHPO4·2H2O) formed on the alloy surface [1]. Furthermore, the presence of Cl signal in the localized corrosion regions indicates that the Cl- ions in HBSS promote the propagation of localized corrosion.[1] P.W. Chu et al., Corrosion Behavior and Microstructure of the Surface Corrosion Film of Biodegradable WE43 and ZX21 Mg Alloys in Hanks’ Balanced Salt Solution, Materials Chemistry and Physics, vol. 312, pp. 128609-128620, 2024.