Bulk Metallic Glass Surface Research Articles

Electropolishing of complex-shaped bulk metallic glasses in NaCl-ethylene glycol electrolyte

Electropolishing (EP) has been widely used to obtain good surface roughness of metallic structures; however, precisely-shaped cathodes are still necessary for the EP of bulk metallic glass (BMG) devices and components. In this work, the EP behavior of a Zr-based BMG was investigated using a plain steel cathode in NaCl-ethylene glycol electrolyte. The recast layer of wire electrical discharge machined (WEDMed) BMG surfaces with crystallization was effectively removed through the EP process, restoring the surface properties of BMGs. The effects of processing parameters, such as voltage, temperature, time, and electrolyte concentration, on the EP performance were explored, and the optimal EP parameters were determined. The EP mechanisms were discussed, where pitting formation was found to consist of three stages. Finally, the EP of a curved BMG part was successfully achieved based on the optimized processing parameters, further validating the feasibility to electropolish complex-shaped BMG structures using direct current (DC) power and NaCl-ethylene glycol electrolyte.

Corrosion mechanisms of Zr-based bulk metallic glasses in NaBr solution

In this paper, the corrosion mechanisms of Zr55Cu35Al10 and Zr56Co28Al16 bulk metallic glasses (BMGs) in 3.5 wt.% NaBr solution was systematically investigated by the kinetic potential polarization and electrochemical impedance spectroscopy. Both Zr55Cu35Al10 and Zr56Co28Al16 BMGs were subjected to pitting damage during the polarization process in NaBr solution due to the presence of Br− ions. However, Zr56Co28Al16 BMG exhibits better corrosion resistance compared to Zr55Cu35Al10 BMG. Zr55Cu35Al10 BMG experiences selective dissolution of alloying elements under the attack of Br−, and Cu element was enriched in the corrosion pit. The surface of Zr56Co28Al16 BMG shows a more uniform distribution of elements after corrosion. Simultaneously, the substitution of Cu by Co changes the composition of the passive films and increases the relative content of ZrO2, which is a principal factor contributing to the enhanced stability of Zr56Co28Al16 BMG's passive film.

The Effect of Electroplating Nickel on the Mechanical Properties of Brittle Mg-Based Bulk Metallic Glasses

Magnesium-based bulk metallic glasses (BMGs) are typical intrinsic brittle lightweight BMG alloys, and their improvement in plasticity has attracted widespread attention in the field of BMGs. We used the electroplating method to modify the surface of Mg59.5Cu22.9Ag11Gd6.6 BMGs and investigated the geometric confinement effect of the Ni coating on the mechanical properties of the BMG. The results show that under the plating conditions of adding 1 g/L nano Al2O3 to the plating solution, adjusting the plating temperature to 50 °C, and plating time to 3 h, a smooth and dense nickel coating with a thickness of about 150 μm can be formed on the surface of the Mg-based BMG. The uniaxial compression tests showed that the average fracture strength of the BMG was increased from 565 MPa to 598 MPa by a 50 μm Ni coating, and the fluctuation range of strength was decreased from 429 MPa to 265 MPa, a reduction of 36%. The Weibull analysis showed that the Weibull modulus m was increased from 4.3 to 4.8 by the coating, and the safety stress was increased from 54 MPa to 235 MPa, indicating that electroplating nickel could improve the reliability of the Mg-based BMG alloy. However, no significant improvement of the compression plasticity was found, which indicated that improving the room temperature plasticity of brittle Mg-based BMG alloys by the geometric confinement of electroplating Ni was limited. The influence of the thickness of the Ni coating on the maximum stress level and stress distribution in the BMG samples was analyzed by ANSYS finite element simulation. It was found that when the thickness of the coating was 30% of the radius of the cylindrical compressed sample, the stress distribution caused by the Ni coating was the most uniform, and the maximum stress level was relatively reduced, which is beneficial for improving the geometric confinement effect. As a result, the Mg-based BMG sample coated with a Ni coating of 150 μm thickness exhibited ~0.3% macroscopic compressive plasticity. This is of great significance for understanding the plastic deformation mechanism of brittle BMGs improved by geometric confinement.

Strong dependency of the tribological behavior of CuZr-based bulk metallic glasses on relative humidity in ambient air

Thanks to their outstanding mechanical properties, Bulk Metallic Glasses (BMGs) are new alternatives to traditional crystalline metals for mechanical and micromechanical applications including power transmission. However, the tribological properties of BMGs are still poorly understood, mostly because their amorphous nature induces counter intuitive responses to friction and wear. In the present study, four different BMGs (Cu47Zr46Al7, Zr46Cu45Al7Nb2, Zr60Cu28Al12, and Zr61Cu25Al12Ti2) underwent ball-on-disc friction tests against 100Cr6 steel balls (American Iron and Steel Institute (AISI) 52100) at different relative humidities (RHs) ranging from 20% to 80%. Controlling humidity enabled to observe a high repeatability of the friction and wear responses of the BMG. Interestingly, the friction coefficient decreased by a factor of 2 when the humidity was increased, and the wear rate of BMGs was particularly low thanks to a 3rd-body tribolayer that forms on the BMG surface, composed of oxidized wear particles originating from the ball. The morphology of this tribolayer is highly correlated to humidity. The study also identifies how the tribolayer is built up from the initial contact until the steady state is achieved.

Corrosion interpretation of the novel rare-element bearing bulk metallic glass: Electrochemical, thermodynamic, and surface analysis of the (Cu50Zr43Al7)100-xErx

Bulk metallic glasses (BMGs) have evoked considerable attention in recent years owing to unique properties. However, the surface and anticorrosive properties of this kind of material has not been widely investigated. In this paper, a novel BMG comprises Erbium (Er)-based rare element and high anticorrosive capacity has been introduced. Valorization of Er element (2, 4, 6% at.) in the Cu50Zr43Al7 protective film has been explored from the anticorrosive viewpoint. XRD (X-ray diffraction), GIXRD (Grazing Incidence X-ray Diffraction), AFM (Atomic force microscopy), SEM/EDS (scanning electron microscope/energy-dispersive X-ray spectroscopy), and XPS (X-ray photoelectron spectroscopy) analyses in cooperation with the detailed electrochemical experiments i.e., EIS (Electrochemical Impedance Spectroscopy) and potentiodynamic polarization tests were utilized for surface and corrosion studies. The EIS outcomes have demonstrated that the presence of the Er (at 2% at.) leads to higher corrosion resistance (Rct = 17490 Ω cm2, Rtotal = 7590 Ω cm2) at different time intervals than the other specimens. The proposed mechanism based on surface studies (GIXRD, SEM/EDS, AFM, and XPS) and thermodynamic interpretation imply that the formation of the Er2O3 as a robust protective film along with other types of metal-oxides on the BMG surface is responsible for the high corrosion resistance of this sample.

Enhanced mechanical properties of Zr65Cu15Ni10Al10 bulk metallic glass by simultaneously introducing surface grooves and multiple shear bands

Recent investigations have demonstrated that the plasticity of bulk metallic glasses (BMGs) at room temperature can be improved by artificially introducing both macroscopic and microscopic defects. In this work, both surface grooves and multiple shear bands (MLSBs) are concurrently introduced into the surface of a Zr65Cu15Ni10Al10 BMG employing an ultrasonic bonding machine, whose sizes are also tailored by tuning gas pressures and times. After surface treatment, MLSBs appear around grooves due to the pre-plastic deformation, while numerous nano-scale crystals resulting from thermal-induced nanocrystallization appear within the grooves, leading to the emergence of hierarchical structural heterogeneities. As a result, the strength and plasticity are effectively enhanced compared to as-cast samples when suitable experimental parameters are adopted. The plastic deformation mechanisms change from single main shear banding to multiple shear banding. Therefore, the serrated plastic behaviors exhibit a transition from chaotic to self-organized critical dynamics after surface treatment, confirming the modification of the shear banding instability. The present study provides a viable method to tailor the mechanical properties of BMGs and sheds light on their dynamic plastic deformation behaviors.

The Effects of Minor Element Addition on the Structural Heterogeneity and Mechanical Properties of ZrCuAl Bulk Metallic Glasses

The present study reveals the role of Nb and Ni minor addition on the nanomechanical properties and nanostructure of ZrCuAl bulk metallic glass (BMG). For this purpose, atomic force microscopy (AFM) was used to evaluate the viscoelastic response of the BMG surface at the nanoscale, while the nanoindentation technique was applied to show the mechanism of plastic deformation. The results indicated that minor Nb addition decreased the relaxation of enthalpy accompanied by the weakening of structural heterogeneity. On the other hand, Ni addition improved the stored energy of the material and intensified the distribution of loosely packed regions in the microstructure. Moreover, the mechanical test unveiled that Ni addition enhanced the viscoelastic response; however, it came at the expense of creep resistance. The evaluation of the magnitude of the derivative in the nanoindentation test also demonstrated that the Ni-added sample exhibited a multiple shear-band mode for plastic deformation.

Effect of Laser Surface Structuring on Surface Wettability and Tribological Performance of Bulk Metallic Glass

Bulk metallic glasses (BMGs) have been extremely popular in recent decades, owing to their superior properties. However, how to improve the surface functions and durability of BMGs has always been a key engineering issue. In this work, a facile laser-based surface structuring technique was developed for modulation and control of the surface functionalities of Zr-based BMG. For this technique, a laser beam was first irradiated on the surface to create periodic surface structure, followed by heat treatment to control surface chemistry. Through experimental analyses, it was clearly shown that laser surface structuring turned the BMG surface superhydrophilic, and subsequent heat treatment turned the surface superhydrophobic. We confirmed that the combination of laser-induced periodic surface structure and modified surface chemistry contributed to the wettability transition. The laser-heat-treated surface also exhibited improved antifriction performance with the help of lubrication medium. This work provides a feasible method for surface modification of BMG, suggesting applications in the areas of medicine, biology and microelectronics.

Thermoplastic embossing device to probe rheological changes of supercooled metallic liquids during rapid heating.

The setup and working principle of a device designed for thermoplastic embossing and investigating rheological changes during fast-resistive heating of bulk metallic glasses (BMGs) is presented. The device was developed and custom-built at the Leibniz Institute for Solid State and Materials Research Dresden (Leibniz IFW Dresden) and is integrated into a universal testing machine. By varying the electrical-current pulse signal, the rate at which BMGs are resistively heated is controlled. Next to temperature and electrical resistance, the displacement of the punch, which penetrates the sample during rapid heating, is monitored. Additional pre-heating controlled by thermocouples allows for stable heating conditions to minimize heat extraction from the resistively heated specimen, which could eventually lead to damage of the device. The main focus of this device is to study fundamental phenomena under kinetic constraints evoked by fast heating conditions and to structure the surface of BMGs by thermoplastic embossing. A case study, which is carried out with Zr52.5Cu17.9Ni14.6Al10Ti5 BMG, demonstrates the device's performance covering heating rate ranges of three magnitudes and the access to rheological changes during the devitrification and subsequent crystallization of the BMG during rapid heating. Furthermore, the present device was successfully used to nanostructure the surface of the BMG at elevated heating rates.

Strong adhesion induced by liquid-like surface of metallic glasses

Self-healing with the capability to be self-adhesive, which can recover from physical damage, is essential for space applications. However, regulatable adhesion under extreme space conditions has only been realized in low-dimensional materials and still poses a challenge on the discovery of suitable materials. Under an ultrahigh vacuum of 10−7 Pa, we found a strong adhesion between bulk Cu46Zr46Al8 metallic glasses with a maximum adhesion strength of 32.8 kPa, which is two orders of magnitude higher than that of the corresponding crystalline. This adhesion is suggested to be induced by a liquid-like layer on a bulk metallic glass surface, which has a high diffusion coefficient of 6.9 × 10−11 m2⋅s−1, even at a relatively low temperature of 263 K. By investigating the dynamics for this liquid-like layer, a special fractional Stokes–Einstein relationship was found. Inspired by this strong adhesion, metallic glasses can be proposed as one of the promising self-healing materials for future space applications.

Modulation and Control of Wettability and Hardness of Zr-Based Metallic Glass via Facile Laser Surface Texturing.

Bulk metallic glass (BMG) has received consistent attention from the research community owing to its superior physical and mechanical properties. Modulating and controlling the surface functionalities of BMG can be more interesting for the surface engineering community and will render more practical applications. In this work, a facile laser-based surface texturing technique is presented to modulate and control the surface functionalities (i.e., wettability and hardness) of Zr-based BMG. Laser surface texturing was first utilized to create periodic surface structures, and heat treatment was subsequently employed to control the surface chemistry. The experimental results indicate that the laser textured BMG surface became superhydrophilic immediately upon laser texturing, and it turned superhydrophobic after heat treatment. Through surface morphology and chemistry analyses, it was confirmed that the wettability transition could be ascribed to the combined effects of laser-induced periodic surface structure and controllable surface chemistry. In the meantime, the microhardness of the BMG surface has been remarkably increased as a result of laser surface texturing. The facile laser-based technique developed in this work has shown its effectiveness in modification and control of the surface functionalities for BMG, and it is expected to endow more useful applications.

Study on tribological characteristics of Zr-based BMG via nanoscratch techniques

This study aims at investigating tribological characteristics of zirconium-based bulk metallic glass (BMG) using nanoindentation and nanoscratch techniques. The nanomechanical properties of Zr65Cu15Al10Ni10 (at. %, Zr-based BMG) are first assessed via nanoindentation tests and simulations. The tribological performance of Zr-based BMG under various scratch conditions are investigated via nanoscratch tests with multiple ramp and constant normal loads. It is found that coefficient of friction (COF) irregularly increases with normal load due to the formation of fracture and chipping, which play crucial roles on the frictional behaviors of Zr-based BMG. The critical load for fracture and chipping formation is identified for the considered test conditions. Wear mechanism of Zr-based BMG under nanoscratching are investigated by observing the morphologies of scratch tracks through scanning electron microscope (SEM). The findings of this study can be beneficial for tailoring the surface of Zr-based BMG by using micro-machining and patterning techniques.

Numerical Study of a Buckling Restrained Brace (BRB) in Steel Structures and Comparison with a Convergent Ordinary Brace (OCB) Under Static and Dynamic Loading

Bulk metallic glass (BMG) has good mechanical strength, high hardness, wear resistance, and corrosion resistance with promising application in various industries. However, for the industrial production of BMG, the main issue is how to overcome limitations of joining with other materials. The present study focuses on solder processing at low operating temperature to avoid exceeding the recrystallization temperature. A feasible joining process for BMG was developed using lead-free solders. The BMG surface is pre-plated with copper, nickel, or titanium as a wetting layer. The reaction temperature is set between the glass transition temperature of BMG and the melting point of the solder. After a reflowing and aging process, the joint sample was examined using SEM, EDS, EPMA, and XRD. The Cu–Zr based BMG can be successfully joined with Sn-58Bi solder after plating Cu on the BMG surface. A diffusion layer was observed and the thickness increased with longer aging time. The main components of the diffusion layer are ZrO2 and Cu10Zr7.

Soldering Zr metallic glass to Ti alloy using pre-cladding ultrasonic processes

That increasing temperature is beneficial for solder-wetting but not for amorphous-maintaining the state causes difficulties in joining of Zr-based bulk metallic glass (BMG). In order to solve the contradiction, this paper presents a two-step process of pre-cladding ultrasonic soldering, in which a Zn-6Al-3Cu-20Sn solder layer was firstly vacuum-cladded on surface of a Ti-6Al-4V sample, and then a Zr-14Cu-10Ti-8Ni BMG sample was ultrasonic-soldered to the solder layer without flux. Microstructures of the Zr-14Cu-10Ti-8Ni/Zn-6Al-3Cu-20Sn/Ti-6Al-4V soldering joints were studied using optical microscope, SEM, EDS, and XRD, and shear strengths were tested. Results show that shear strength ranges from 20 to 50 MPa among the joints obtained under different soldering temperatures and ultrasonic times. Shear fracture of all the experimental ultrasonic soldered joints occurred at the interface between solder and Zr-based BMG. The fracture surface shows a mixture of solder tearing and solder peeling off from the surface of the Zr-based BMG. Pits or grooves formed on surface of Zr-based BMG by ultrasonic cavitation benefit not only chemical adhesion but also mechanical adhesion by an anchoring effect. This work confirmed that ultrasonic treatment is very effective for the soldered joint of Zr-based BMG.

Fabricating colorful bulk metallic glass surfaces by femtosecond laser processing

Here, we report on successful fabrication of colorful bulk metallic glass surfaces through the technique of femtosecond laser processing. The results manifest that with increasing the laser energy density from 0.5 J/mm2 to 8.0 J/mm2, the color of processed Zr based bulk metallic glasses varies from yellow to brown and then green. The mechanism of the surface color transformation can be fundamentally understood in terms of the diffraction efficiency of Bragg originating from various laser energy densities. Our findings provide a facile method for engraving colorful amorphous alloy surfaces, thereby improving the application potentials of such materials.

Slurry erosion behavior of two Zr-based bulk metallic glasses

Metallic glass, or named as amorphous alloy, is attracting in the engineering application as a wear resistance material since it has high hardness, yield strength, fracture toughness, and good corrosion resistance. Slurry erosion behaviors of the Zr-based bulk metallic glasses (BMGs) Zr55Al10Ni5Cu30 (Zr55) and Zr52.5Ti5Cu17.9Ni14.6Al10 (Vit105) were studied using sand-water two-phase flow. With the increase of impact angle, the volume loss rate of the BMGs roughly increased, indicating an erosion characteristic of brittle material. Compared with AISI 304 stainless steel, Vit105 reduced the erosion wear rate by about 35% at 30°. The volume loss rate increased obviously with the impact velocity from 3.51 to 10.08 m s−1. The relationship between the volume loss rate and the erosion time showed the volume loss rate of the BMGs dropped down after about 30 min erosion, suggesting a wear-in stage with strengthening wear resistance occurred. XRD patterns of these BMGs before and after erosion test illustrated the eroded Zr55 has no detectable crystallization, while some crystalline peaks presented at 10.08 m s−1 and no obvious crystalline peaks presented at 3.51 m s−1 in the patterns of Vit105. With the further DSC analysis, it can be concluded that the surface devitrification mainly depends on the impact velocity and BMG's thermal-stability. During the erosion, the variation of element concentration and the increase of oxygen content and oxides on the BMG surface indicated the mechanical degeration caused by sand particle was along with chemical degeration. With a higher ratio of hardness to elastic modulus (H/E), Vit105 shows better erosion wear resistance than Zr55. These work related to the wear-in stage of BMG with surface crystallization and element variation should be beneficial to improve the erosion resistance by the optimization of mechanical and thermal properties of amorphous alloys/coatings.

In vitro cytocompatibility of a Zr-based metallic glass modified by laser surface texturing for potential implant applications

Existing studies have shown the benefit of laser surface texturing (LST) in promoting the cytocompatibility of traditional metallic biomaterials. Researchers have also reported the potential of bulk metallic glasses (BMGs) as an alternative class of material for biomedical applications. However, investigations specifically focussed on studying the cytocompatibility of BMG surfaces processed with LST are still lacking. The present work demonstrated the feasibility of nanosecond LST as a method to modify the cytocompatibility of a Zr-based BMG material known as Vitreloy 105. Two different types of laser-induced surface patterns, namely grooves and dimples, were considered. Their respective influence on the resulting cell viability, attachment and morphology was studied and compared against the cytocompatibility of the original BMG surface. It was found that MG63 osteoblast-like cells on the groove-textured surface exhibited higher viability and better adhesion compared to those on the original and dimple-textured surfaces. Possible underlying mechanisms associated with LST, which can affect the in vitro cytocompatibility of Vitreloy 105 were discussed based on the induced changes to surface chemistry, wettability and roughness. It is suggested that the higher surface roughness, increased presence of metallic oxides and enhanced hydrophilicity of the groove-textured sample were the main contributors to its improved cytocompatibility.

Surface smoothing of bulk metallic glasses by femtosecond laser double-pulse irradiation

Bulk metallic glasses (BMGs) with amorphous structure have excellent mechanical and chemical properties. In polishing process, it is hard but important to maintain amorphous state to keep original properties of BMGs, which are easily crystallized in traditional mechanical polishing and long-pulse laser polishing. In this paper, we first propose to use femtosecond laser double-pulse irradiation to smooth the surface of Zr-based BMG and make it free of laser-induced periodic surface structures and crystallization. Here, the smooth surface with half decreasing in roughness (Sa) to 111 nm is achieved by optimized parameters with laser pulse energy 0.06 μJ of the first pulse, energy ratio 40% of the first to the second pulse and time delay 20 ps. Obvious transition from a coarse surface to a smooth one occurs at near 10 ps time delay and 40% energy ratio. Mobility enhancement of molten materials guarantees the removal of undesirable surface structures through surface tension and gravity. Crystallization is avoided due to rapid heating and cooling under femtosecond laser double-pulse irradiation. A smoothed BMG surface obtained by this method verifies its feasibility in BMGs polishing.

Softening and hardening on a Zr-based bulk metallic glass induced by nanosecond laser surface melting

The study reported here confirms that laser surface melting (LSM) can be employed to modify the hardness and the shear banding behaviour of bulk metallic glasses (BMGs). More specifically, by conducting LSM operations on the Zr-based Vitreloy 105 BMG in ambient atmosphere using a nanosecond laser, it was found that surface hardening can be achieved, in addition to the well-known surface softening effect. Besides, it was found that the presence of compressive residual stress and an increased introduction of crystalline precipitates accompanied LSM-induced surface hardening. On the contrary, tensile residual stress and a reduced fraction of crystalline precipitates were observed for a softened surface post-LSM. Finally, differences in shear-banding mechanisms were detected near the surface of the laser irradiated regions. More specifically, overall reduced serrated flow but important surface shear bands events were observed following the LSM-based introduction of compressive residual stress. In contrast, more pronounced serrated flows and the likely distribution of shear banding activity well beneath the irradiated BMG surface was promoted when LSM resulted in the introduction of tensile residual stress.

Electro-Discharge Machining of Zr67Cu11Ni10Ti9Be3: An Investigation on Hydroxyapatite Deposition and Surface Roughness

This study attempts to simultaneously machine and synthesize a biomimetic nanoporous hydroxyapatite coating on the Zr67Cu11Ni10Ti9Be3 bulk metallic glass (BMG) surface. The aim is to investigate and optimize the hydroxyapatite deposition rate and the surface roughness during the electro-discharge coating of Zr67Cu11Ni10Ti9Be3 BMG. Scanning Electron Microscopy (SEM), X-ray powder Diffraction (XRD) and Energy-dispersive X-ray Spectroscopy (EDS) were employed to characterize and analyze the results. Response Surface Methodology using D-optimum custom design approach was utilized to generate the models and optimize the input parameters. A globule nanostructured and nanoporous coating of about 25.2 µm thick, containing mainly Ca, O, and K were ascertained. Further XRD analysis confirmed the deposition of biocompatible oxides (HA, CaZrO3, and ZrO2) and hard ZrC coating on the Zr67Cu11Ni10Ti9Be3 BMG surface. A significant improvement in cell viability was observed in the HA electro-discharge coated BMG specimens. The numerical models for the Hydroxyapatite Deposition Rate (HDR) and Surface Roughness (SR) were developed and experimentally validated using the optimized parameters setting suggested by the software. The achieved average predicted error of 4.94 and 5.09% for the HDR and SR respectively confirmed the excellent reproducibility of the developed models.