Metallic Glass Surface Research Articles

Bulk metallic glassy surface native oxide: Its atomic structure, growth rate and electrical properties

Formation of a native oxide layer on the surface of bulk metallic glasses (BMGs) influences significantly the nanoscale tribological properties and mechanical behavior of the BMGs used in nanodevices. However, our knowledge of the native oxidation process on the BMG surface and structure of the corresponding oxides remains limited because the oxide layer is very thin. Here we conducted a combined state-of-the-art experimental technique study of the atomic structure, oxidations states and electrical conductivity of the native surface oxides on a CuZrAl BMG formed at ambient conditions by aberration-corrected scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS) and conductive atomic force microscopy (AFM). This allowed shedding light on the atomic structure, metal oxidation state, growth behavior and nanoscale electrical properties of the surface oxide. The conductive AFM measurements reveal that the electrical conductivity of the native oxide layer transits from the initially metallic to a nonlinear one after some air exposure, and finally changes to insulative state. These findings represent a significant step forward in the knowledge of surface oxides and open up the possibility of fabricating nanoscale electrical devices based on BMGs with controllable conductivity.

Effect of hydrothermal condition on the formation of multi-component oxides of Ni-based metallic glass under high temperature water near the critical point

The specific feature of multi-component oxides synthesized by hydrothermal process under high temperature (633 K) and highly pressurized water (18.9 MPa) near critical point. Effects of hydrothermal processing duration times 24 hours and 72 hours, respectively, on the oxide formation of the Ni59Zr20Ti16Si2Sn3 metallic glass synthesized by powder metallurgy process were characterized by X-ray diffractometer, differential scanning calorimeter along with the particle size, morphology and crystalline phase of the oxides. The crystallization of the needle-shape NiTiO3, ZrTiO4 and ZrSnO4 ternary oxide phases observed on the surface of metallic glass at below glass transition temperature and the morphology of oxide phases changed to plate-shape around 2 μm in diameter by the increase processing time. This hydrothermal processing in subcritical water provides accelerated dense metal oxide crystals due to the reaction medium being at higher pressure than conventional oxidation processing.

Cavitation instability in bulk metallic glasses

Recent experiments have shown that fracture surfaces of bulk metallic glasses (BMGs) usually exhibit an intriguing nanoscale corrugation like fractographic feature mediated by nanoscale void formation. We attribute the onset of this nanoscale corrugation to TTZs (tension transformation zones) mediated cavitation. In our recent study, the spall experiments of Zr-based BMG using a single-stage light gas gun were performed. To uncover the mechanisms of the spallation damage nucleation and evolution, the samples were designed to be subjected to dynamic tensile loadings of identical amplitude but with different durations by making use of the multi-stress pulse and the double-flyer techniques. It is clearly revealed that the macroscopic spall fracture in BMGs originates from the nucleation, growth and coalescence of micro-voids. Then, a microvoid nucleation model of BMGs based on free volume theory is proposed, which indicates that the nucleation of microvoids at the early stage of spallation in BMGs is resulted from diffusion and coalescence of free volume. Furthermore, a theoretical model of void growth in BMGs undergoing remote dynamic hydrostatic tension is developed. The critical condition of cavitation instability is obtained. It is found that dynamic void growth in BMGs can be well controlled by a dimensionless inertial number characterizing the competition between intrinsic and extrinsic time scales. To unveil the atomic-level mechanism of cavitation, a systematic molecular dynamics (MD) simulation of spallation behaviour of a binary metallic glass with different impact velocities was performed. It is found that micro-void nucleation is determined TTZs while the growth is controlled by shear transformation zones (STZs) at atomic scale.

Preparation of hydroxyapatite layer on Ti-based bulk metallic glasses by acid and alkali pre-treatment

In this paper, a thin calcium phosphate layer was prepared on the surface of Ti40Zr10Cu36Pd14 bulk metallic glasses (BMGs). All samples were dipped in HNO3 aqueous solution and boiled in 1.2 mol·L−1 NaOH aqueous solution. After the two preliminary treatment steps, they were soaked in simulated body fluid (SBF) to form an apatite layer on the surface. After immersion for 12 days, a coating composed mainly of Ca–P particles was rapidly deposited on the surface of Ti40Zr10Cu36Pd14 bulk metallic glasses. The surfaces of chemically treated samples and the samples after immersion in SBF were characterized by X-ray diffraction (XRD), X-ray photoemission spectroscopy (XPS), field emission scanning electron microscopy (FESEM), and energy dispersive spectroscopy (EDS). Analysis results indicate that this apatite coating consists of Ca–P particles with different diameters. In addition, the Ca/P ratio of the apatite coatings immersed for 14 days is 1.62, which is close to that of hydroxyapatite (HA).

Surface engineering of a Zr-based bulk metallic glass with low energy Ar- or Ca-ion implantation.

In the present study, low energy ion implantation was employed to engineer the surface of a Zr-based bulk metallic glass (BMG), aiming at improving the biocompatibility and imparting bioactivity to the surface. Ca- or Ar-ions were implanted at 10 or 50 keV at a fluence of 8 × 10(15)ions/cm(2) to (Zr0.55Al0.10Ni0.05Cu0.30)99Y1 (at.%) BMG. The effects of ion implantation on material properties and subsequent cellular responses were investigated. Both Ar- and Ca-ion implantations were suggested to induce atom displacements on the surfaces according to the Monte-Carlo simulation. The change of atomic environment of Zr in the surface regions as implied by the alteration in X-ray absorption measurements at Zr K-edge. X-ray photoelectron spectroscopy revealed that the ion implantation process has modified the surface chemical compositions and indicated the presence of Ca after Ca-ion implantation. The surface nanohardness has been enhanced by implantation of either ion species, with Ca-ion implantation showing more prominent effect. The BMG surfaces were altered to be more hydrophobic after ion implantation, which can be attributed to the reduced amount of hydroxyl groups on the implanted surfaces. Higher numbers of adherent cells were found on Ar- and Ca-ion implanted samples, while more pronounced cell adhesion was observed on Ca-ion implanted substrates. The low energy ion implantation resulted in concurrent modifications in atomic structure, nanohardness, surface chemistry, hydrophobicity, and cell behavior on the surface of the Zr-based BMG, which were proposed to be mutually correlated with each other.

In vitro biocompatibility response of Ti–Zr–Si thin film metallic glasses

In this study, the bio-electrochemical response of the Ti–Zr–Si thin film metallic glasses (TFMGs) in simulated body fluid with different contents of titanium is measured via potentiostat. According to the results of bio-corrosion potential and current, as well as the polarization resistance, it is concluded that the Ti66Zr25Si9 TFMGs possess the highest bio-electrochemical resistance. With increasing content of titanium, the corrosion resistance becomes progressively higher. The passive current results reveal that amorphous alloys can form a more protective and denser passive film on the metallic glass surface than the crystalline materials. In addition, the mechanical performance of the Ti–Zr–Si TFMGs is better than the crystalline counterparts. As a result, the Ti-based TFMGs are considered to be potential materials for bio-coating applications.

Fracture morphology pattern transition dominated by the crack tip curvature radius in brittle metallic glasses

The dynamic evolutions of different fracture morphologies in the fracture surface of brittle Mg-based metallic glasses (MGs) were systematically and quantitatively studied by SEM and AFM. It shows that the evolution of the fracture morphology pattern is dominated by an effective dynamic parameter of critical crack tip curvature radius. Based on crack tip plastic zone and shear transformation zone (STZ) theories, a theoretical model for the evolution of the crack tip curvature radius by considering the activation of STZs during fracture was proposed. The model can simulate the crack propagation process, and predict the critical crack tip curvature radius and the fracture pattern transition between the dimples and the periodic corrugations solely occurring in brittle MGs. These results have implications for understanding the microscopic fracture mechanism and the structural origin of fracture in MGs.

Effects on structure and properties of Zr55Al10Cu30Ni5 metallic glass irradiated by high intensity pulsed ion beam

Abstract High intensity pulsed ion beam technology was used for the surface irradiation treatment of metallic glass Zr 55 Al 10 Cu 30 Ni 5 and W metal. The ion beam was mainly composed of C n + (70%) and H + (30%) at an acceleration voltage of 250 kV under different energy densities for different number of pulses. XRD analysis showed that the metallic glass remained amorphous in its main structure after HIPIB irradiation, without apparent presence of crystalline phases. SEM analysis concluded that there was no apparent irradiation damage on the surface of metallic glass at the low irradiation frequency (3 times and 10 times); “petal”-shaped irradiation damage appeared on the surface of metallic glass after multi-irradiation (100 times and 300 times), and the composition of the petal center included Fe and Cr, the composition of an ion diode cathode, in addition to the composition of Zr-based metallic glass. TEM analysis of irradiated metallic glass showed that a small amount of nanocrystalline Zr 2 Ni-type phase (face centered cubic) was produced only at 300-time irradiation. Cracks appeared on the surface of W after 100-time and 300-time irradiation; shedding phenomenon even appeared on the surface of W at the energy densities of 1.4 J/cm 2 and 2.0 J/cm 2 . The surface nano-hardness of irradiated metallic glass decreased.

Superhydrophobic metallic glass surface with superior mechanical stability and corrosion resistance

Superhydrophobic surface with mechanical stability and corrosion resistance is long expected due to its practical applications. We show that a micro-nano scale hierarchical structured Pd-based metallic glass surface with superhydrophobic effect can be prepared by the thermoplastic forming, which is a unique and facile synthesis strategy for metallic glasses. The superhydrophobic metallic glass surface without modification of low surface energy chemical layer also exhibits superior mechanical stability and corrosion resistance compared with conventional superhydrophobic materials. Our results indicate that the metallic glass is a promising candidate superhydrophobic material for applications.

Wettability of Electrochemically Nano-textured Metallic Glasses

ABSTRACTDifferent surface patterns with nanometer length-scales were obtained by electrochemical processing of metallic glasses. The wetting behaviour of these surface patterns was studied using sessile drop technique with distilled water droplet. It is demonstrated that the hydrophilic-hydrophobic nature of the metallic glass surface can be controlled through nano topography. The contact angle was found to increase from 70° for the flat metallic glass surface to 112° for the nano-dendritic structure. Atomic force microscopy was utilized to explain the difference in contact angle in term of the surface roughness for different nano-textures.

Wettability of nanotextured metallic glass surfaces

The wettability of different nanotextured metallic glass surfaces is investigated. Wettability is quantified by the sessile drop technique using a distilled water droplet. It is demonstrated that hydrophilic–hydrophobic nature of the metallic glass surface can be controlled through nanotopography. The contact angle was found to increase from 70° for the flat metallic glass surface to 110° for a nanorod patterned surface. The difference in contact angle is explained in terms of the surface topography/roughness measured using atomic force microscopy .

Superhydrophobic Zr-based metallic glass surface with high adhesive force

Micro/nano hierarchical structures were constructed on Zr35Ti30Be26.75Cu8.25 metallic glass surface by silicon moulding and subsequently chemical etching. The as-formed surface exhibited both superhydrophobicity and high adhesive force towards water. The superhydrophobicity is rationalized based on the modified Cassie-Baxter model [A. B. D. Cassie and S. Baxter, Trans. Faraday Soc. 40, 546 (1944)]. The origin of the robust adhesion is described in terms of intermolecular capillary forces. The present results not only provide a method to fabricate superhydrophobic metallic glasses surface but also explore an important industrial application as dry adhesives and transport of liquid microdroplets.

Simulated body fluid electrochemical response of Zr-based metallic glasses with different degrees of crystallization

The bio-electrochemical response in simulated body fluid of the Zr53Cu30Ni9Al8 metallic glasses with different degrees of partial crystallization was systematically examined and discussed. Through thermal annealing, the volume fractions of the crystalline phases are determined to be 0, 34, 63, and near 100%. Based on the bio-corrosion voltage and current, as well as the polarization resistance, it is concluded that the fully amorphous alloy exhibits the highest bio-electrochemical resistance. With an increasing degree of partial crystallization, the corrosion resistance becomes progressively degraded. The passive current reveals that the fully amorphous metallic glasses can form a more protective and denser passive film on the metallic glass surface. The formation of reactive nanocrystalline phases in the amorphous matrix would reduce the bio-corrosion resistance.

Effect of thermal stability of the amorphous substrate on the amorphous oxide growth on Zr–Al–(Cu,Ni) metallic glass surfaces

In the present study, we propose that the glass stability of an amorphous oxide film is highly dependent on the interface stability between the amorphous oxide and the amorphous substrate. The interface stability is closely linked to the thermal stability of the amorphous substrate, which is changed by the chemical composition shift during formation of the oxide layer. Therefore, significant improvement of the oxidation resistance in metallic glass systems can be achieved by controlling the compositional changes underneath the oxide layer.

Influence of friction on the residual morphology, the penetration load and the residual stress distribution of a Zr-based bulk metallic glass

In this paper, frictionbetween the Cube-Corner indenter and the sample surface of a Zr-based bulk metallic glass(BMG) was analyzed and discussed by the experimental method, the theoretical method and the finite element simulation. Linear residua are observed on the surfaceof the indenter for the first time, which gives the direct evidence that strong interaction processes exist between the indenter surface and the sample surface because of strong friction and local high contact press. A simplified model was developed to correct the penetration load with the consideration of friction.Effects of friction on the penetration load-depth curves, plastic flow, surfacedeformation and residual stress distribution of the sample with different friction coefficients were investigated by the finite element simulation.

A Novel Micro-Scale Plastic Deformation Feature on a Bulk Metallic Glass Surface under Laser Shock Peening

Laser shocking peening is a widely applied surface treatment technique that can effectively improve the fatigue properties of metal parts. We observe many micro-scale arc plastic steps on the surface of Zr47.9Ti0.3Ni3.1Cu39.3Al9.4 metallic glass subjected to the ultra-high pressure and strain rate induced by laser shock peening. The scanning electronic microscopy and atomic force microscopy show that the arc plastic step (APS) has an arc boundary, 50–300 nm step height, 5–50 μm radius and no preferable direction. These APSs have the ability to accommodate plastic deformation in the same way as shear band. This may indicate a new mechanism to accommodate the plastic deformation in amorphous metallic glass under high pressure, ultra-high strain rates, and short duration.

Effects of Ar ion irradiation on the diffusion bonding joints of Zr55Cu30Ni5Al10 bulk metallic glass to aluminum alloy

1keV low energy argon ion beam was used to irradiate the surface of Zr55Cu30Ni5Al10 bulk metallic glass. Bulk metallic glass specimens irradiated for different times were used to join with Aluminum alloy by argon ion activated diffusion bonding. Compared with the joints using the as-cast bulk metallic glass, the tensile strength of the joints using irradiated specimens had a drastic improvement. Moreover, a crystalline layer was obtained in the BMG side of the joint interface using irradiated specimens. The improved joint quality was attributed to the local heating resulting from a crystallization behavior of irradiated surface layer which increases the probability of jumping the barrier to form the atom bonds, as well as the significant enhancement of the atomic diffusivity after crystallization.

Anti-irradiation performance against helium bombardment in bulk metallic glass (Cu47Zr45Al8)98.5Y1.5

In order to compare the resistance to He2+ ion induced irradiation between metallic glass and polycrystal W metal, this paper used different fluences of He2+ ion-irradiated metallic glass (Cu47Zr45Al8)98.5Y1.5 and polycrystal W with an energy of 500keV. The SRIM simulation calculation results showed that the range (1.19μm) of He2+ in metallic glass was greater than the one (0.76μm) in polycrystal W. The SEM analysis showed that there was no significant irradiation damage phenomenon on the surface of metallic glass, and there was only a damage layer 1.45μm away from the surface when the fluence reached 2×1018ions/cm2. For W, there were surface peeling, flaking and other surface damages at a fluence of 1×1018ions/cm2; when the fluence increased to 2×1018ions/cm2, multilayer detachment phenomenon appeared. The surface root mean square roughness of metallic glass (Cu47Zr45Al8)98.5Y1.5 first increased and then decreased with the increase of fluence. The surface reflectivity of (Cu47Zr45Al8)98.5Y1.5 decreased with the increase of fluence. Through detection by XRD, it was found that (Cu47Zr45Al8)98.5Y1.5 always maintained amorphous phase after different fluences of radiation. The resistance to He2+ irradiation of metallic glass (Cu47Zr45Al8)98.5Y1.5 was superior to the one in polycrystal W.

Shear bands mediate cavitation in brittle metallic glasses

Recent experimental studies have revealed nanoscale cavities and periodic corrugations on the fracture surfaces of brittle metallic glasses. How such cavitation in these materials leads to brittle failure remains unclear. Here we show, using atomistic and continuum finite element simulations, that a shear band can mediate cavity nucleation and coalescence owing to plastic flow confinement caused by material softening. This leads to brittle fracture as cavities nucleate and coalesce within a shear band, causing the crack to extend along it.

Micropatterning of Fe-based bulk metallic glass surfaces by pulsed electrochemical micromachining

Abstract