Brittle Bulk Metallic Glass Research Articles

The rheological behavior of brittle BMGs of Cu44.25Ag14.75Zr36Ti5 and Ti32.8Zr30.2Cu9Ni5.3Be22.7 in the supercooled liquid region

Bulk metallic glasses (BMGs) show limited plasticity at room temperature. However, BMGs usually exhibit superplasticity and high plastic forming ability in their supercooled liquid region (SLR). The rheological behavior of BMGs in SLR is vitally important to their thermoplastic forming process. In contrast to the ductile BMGs, thermoplastic deformation behavior of brittle BMGs is rarely reported. In present work, the rheological behavior of two brittle BMGs with high glass forming ability Cu44.25Ag14.75Zr36Ti5 and Ti32.8Zr30.2Cu9Ni5.3Be22.7 was investigated on Gleeble3500. The two BMGs can deform homogeneously depending on the temperature and strain rate. According to the high value of m (strain rate sensitivity index), which is the most important mechanical characteristic of a superplastic material, the two BMGs show superplasticity in their SLR with m ≥ 0.3. Based on the free volume model, their activation volumes are calculated as 0.263∼0.486 nm3 and 1.261∼1.650 nm3, indicating the minimum displacement clusters with average 26∼48 and 91∼127 atoms, for Cu44.25Ag14.75Zr36Ti5 and Ti32.8Zr30.2Cu9Ni5.3Be22.7, respectively. Thus, the two investigated brittle BMGs can be thermoplastic processed in the SLR and the deformation maps are given. BMG Cu44.25Ag14.75Zr36Ti5 shows better machinable property than Ti32.8Zr30.2Cu9Ni5.3Be22.7. Compared to the ductile BMGs, no Newtonian flow is found for the two investigated brittle BMGs.

Enhancing the bioactivity and ductility of bulk metallic glass by introducing Fe to construct semi-degradable biomaterial

Porous Ti–Zr–Cu–Pd–Sn bulk metallic glass (BMG) produced by Spark Plasma Sintering (SPS) in our previous work demonstrates bone-like mechanical properties, effectively mitigating the issue of stress shielding within the implant. Nevertheless, concerns persist regarding the BMG's brittleness and its lack of bioactivity, both of which pose concealed risks in practical applications. In light of these challenges, a semi-degradable biomaterial, the MG-Fe composites, has been meticulously crafted via SPS in this work. The incorporation of ductile Fe phase in the MG matrix significantly enhances its plasticity. Moreover, the degradation of Fe results in the deposition of Ca–P compounds, imbuing the MG-Fe composites with a degree of bioactivity. Furthermore, by introducing a gradient porous structure, researchers have managed to fine-tune the mechanical properties of the MG-Fe composites. This innovative design imparts plastic and ductile compression deformation behavior to the gradient porous MG-Fe composites, offering a potential solution to the issue of brittle fracture behavior observed in conventional brittle BMGs. In addition, the introduction of the gradient porous structure serves to further accelerate the degradation rate of Fe. This advancement holds the potential to strike a dynamic balance with the growth rate of human bone, further elevating the bioactivity of the MG-Fe composites.

Super plasticity of bulk metallic glasses at room temperature: A friction self-locking state

This study examines the compressive deformation and fracture characteristics of Fe74Mo6P13C7 bulk metallic glasses (BMGs) with aspect ratios ranging from 1.0 to 2.5. The compressive plastic strain of BMGs exhibits a consistent upward trend as the aspect ratio decreases. As the aspect ratio decreases, the influence of the surrounding friction becomes increasingly pronounced. The finding was made that the exceptional ability of BMGs to exhibit super plasticity at ambient temperature is attributed to the phenomenon of friction self-locking occurring within the shear planes. The deformation behavior of brittle BMGs is significantly influenced by the applied stress conditions. However, it is worth noting that even brittle BMGs can display ductile characteristics when subjected to multiaxial constraints. This finding is not only essential for comprehending the deformation and fracture of amorphous materials, but it can also serve as a valuable reference for rock mass control, structural stability, earthquake mitigation, and disaster reduction engineering.

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.

Numerical study of stationary cracks in bulk metallic glass composites under Mode I, small scale yielding conditions

In this study, finite element analyses of Mode I loading of stationary cracks in in-situ bulk metallic glass composites (BMGCs) are performed under plane strain, small scale yielding (SSY) conditions. In the first part of this work, a thermodynamically consistent finite strain constitutive theory for BMGCs is employed to represent the entire domain. In this homogenized model, the soft crystalline dendrites are considered to obey J2 flow theory of plasticity with isotropic power law hardening, while the bulk metallic glass (BMG) matrix is taken to follow a free-volume based constitutive model. The effects of volume fraction Vf and hardening of the crystalline dendrites on the macroscopic stress and plastic strain fields are systematically studied. The predicted trend of the fracture toughness versus Vf corroborates well with experiments. In the second part, multi-scale SSY analyses are performed, with discrete soft dendrites in the BMG matrix close to notch tip explicitly simulated, while the surrounding region is represented by the homogenized BMGC model. It is found that low strength and/or low hardening dendrites with high Vf dissipate more plastic work in the neighborhood of the notch tip, thereby shielding the relatively brittle BMG matrix.

Elastic Properties and Glass Forming Ability of the Zr<sub>50</sub>Cu<sub>40</sub>Ag<sub>10</sub> Metallic Alloy

The elastic properties of the Zr50Cu40Ag10 metallic alloy, such as the bulk modulus B, the shear modulus G, the Young’s modulus E and the Poisson’s ratio σ, are investigated by molecular dynamics simulation in the temperature range T=250–2000 K and at an external pressure of p=1.0 bar. It is shown that the liquid–glass transition is accompanied by a considerable increase in the shear modulus G and the Young’s modulus E (by more than 50%). The temperature dependence of the Poisson’s ratio exhibits a sharp fall from typical values for metals of approximately 0.32–0.33 to low values (close to zero), which are characteristic for brittle bulk metallic glasses. Non-monotonic temperature dependence of the longitudinal and transverse sound velocity near the liquid-glass transition is also observed. The glass forming ability of the alloy is evaluated in terms of the fragility index m. Its value is m≈64 for the Zr50Cu40Ag10 metallic glass, which is in a good agreement with the experimental data for the Zr-based metallic glasses.

Size-dependent failure of the strongest bulk metallic glass

Upon reducing the sample size into micrometer scale, an obvious brittle-to-ductile transition accompanied by a drastic change of failure mode from shattering to shear-banding was observed when compressing the brittle but strong Co55Ta10B35 bulk metallic glass (BMG). The shattering failure under macroscopic compression is dominated by splitting cracking, which completely differs from shear-banding and originates from extrinsic defects like inclusions. To reveal the critical conditions for shear-banding and splitting cracking, various micropillar specimens with intentionally introduced holes as extrinsic defects were tested, and the stress distributions at the failure moment were analyzed with finite element simulation. The shear plane criterion was found to be quite effective to estimate the nominal stress required for the failure dominated by shear-banding. However, brittle splitting cracking does not occur although the maximum tensile stress reaches the critical value, which is different from traditional brittle solids. To initiate splitting cracking, a high-tensile-stress region over a critical distance, which depends on defect size and fracture toughness of the BMG, is required. The critical conditions for shear failure and splitting cracking demonstrated in this approach can be used to estimate the failure conditions of various BMG components with complex geometries in a wide range of length scales, and to design tough composites based on brittle BMGs. As an example, a design criterion to avoid brittle splitting fracture of porous BMG materials is proposed.

Size-Dependent Failure of the Strongest Bulk Metallic Glass

Upon reducing the sample size into micrometer scale, an obvious brittle-to-ductile transition accompanied by a drastic change of failure mode from shattering to shear-banding was observed when compressing the brittle but strong Co55Ta10B35 bulk metallic glass (BMG). The shattering failure under macroscopic compression is dominated by splitting cracking, which completely differs from shear-banding and originates from extrinsic defects like inclusions. To reveal the critical conditions for shear-banding and splitting cracking, various micropillar specimens with intentionally introduced holes as extrinsic defects were tested, and the stress distributions at the failure moment were analyzed with finite element simulation. The shear plane criterion was found to be quite effective to estimate the nominal stress required for the failure dominated by shear-banding. However, brittle splitting cracking does not occur although the maximum tensile stress reaches the critical value, which is different from traditional brittle solids. To initiate splitting cracking, a high-tensile-stress region over a critical distance, which depends on defect size and fracture toughness of the BMG, is required. The critical conditions for shear failure and splitting cracking demonstrated in this approach can be used to estimate the failure conditions of various BMG components with complex geometries in a wide range of length scales, and to design tough composites based on brittle BMGs. As an example, a design criterion to avoid brittle splitting fracture of porous BMG materials is proposed.

A comparative study of the rate effect on deformation mode in ductile and brittle bulk metallic glasses

The rate effect on the deformation mode of bulk metallic glasses (BMGs) is investigated by a comparative study between ductile and brittle compositions via real-time photographing. The strain-rate controls the deformation mode transition from shear-dominated sliding under quasi-static compression to cracking-dominated fracture under dynamic compression. In ductile BMGs, progressive sliding occurs at lower strain rate and contributes to a stable deformation manner. In brittle BMGs, however, unstable deformation occurs through rapid sliding even under quasi-static compression. Both ductile and brittle BMGs undergo unstable deformation under dynamic compression. The rate-dependent and composition-dependent tendency of stable or unstable deformation is well characterized by a model based on the ratio of the applied strain energy to the critical dissipation energy at the local shear banding region.

Stability of shear banding process in bulk metallic glasses and composites

Abstract

Scattering mechanical performances for brittle bulk metallic glasses

Scattering mechanical performances of brittle La- and Mg-based BMGs are found in the present study. Upon dynamic loading, there exist largely scattered fracture strengths even if the strain rates are under the same order, and the BMG systems are the same. The negative strain rate dependence for La- and Mg-based BMGs is obtained, i.e., a decreased fracture strength is dominating from quasi-static to dynamic compression. At cryogenic temperatures, distinguishingly low fracture strengths are available for these two brittle BMGs, and decreased tolerance to accommodate strains makes BMGs more and more brittle. It is concluded that the scattering mechanical performances of brittle BMGs should be carefully evaluated before actual applications.

Wallner lines, crack velocity and mechanisms of crack nucleation and growth in a brittle bulk metallic glass

Mode I fracture experiments were conducted on brittle bulk metallic glass (BMG) samples and the fracture surface features were analyzed in detail to understand the underlying physical processes. Wallner lines, which result from the interaction between the propagating crack front and shear waves emanating from a secondary source, were observed on the fracture surface and geometric analysis of them indicates that the maximum crack velocity is ∼800ms−1, which corresponds to ∼0.32 times the shear wave speed. Fractography reveals that the sharp crack nucleation at the notch tip occurs at the mid-section of the specimens with the observation of flat and half-penny-shaped cracks. On this basis, we conclude that the crack initiation in brittle BMGs is stress-controlled and occurs through hydrostatic stress-assisted cavity nucleation ahead of the notch tip. High magnification scanning electron and atomic force microscopies of the dynamic crack growth regions reveal highly organized, nanoscale periodic patterns with a spacing of ∼79nm. Juxtaposition of the crack velocity with this spacing suggests that the crack takes ∼10−10s for peak-to-peak propagation. This, and the estimated adiabatic temperature rise ahead of the propagating crack tip that suggests local softening, is utilized to critically discuss possible causes for the nanocorrugation formation. Taylor’s fluid meniscus instability is unequivocally ruled out. Then, two other possible mechanisms, viz. (a) crack tip blunting and resharpening through nanovoid nucleation and growth ahead of the crack tip and eventual coalescence, and (b) dynamic oscillation of the crack in a thin slab of softened zone ahead of the crack-tip, are critically discussed.

Correlation between fractal dimension and strength of brittle bulk metallic glasses

In this paper, the strength of brittle bulk metallic glasses (BMGs) was investigated, and a corresponding theoretical model was proposed by modification of Griffith theory using fractal geometry. It was shown that the fracture energy and strength of brittle BMGs not only are related to the plastic zone size, the length scales of atomic order range and the surface energy but also has significant relationship with the fractal dimension in fracture surface. The strength of typical BMGs was also calculated by our improved model, which is in excellent agreement with the experimental data. This study provides a compelling approach for investigating and understanding the high strength of BMGs.

General relationship between strength and hardness

► The hardness is not an intrinsic property but reflects the hardening state in CG. ► The ratio of hardness to strength can also be reflected by indentation morphology. ► The ratio of hardness to strength increases with increasing parameter α . ► H V = 3 σ UTS is valid for materials with relatively high strength and better toughness. Both hardness and strength are the important properties of materials, and they often obey the three times empirical relationship in work-hardened metals and some bulk metallic glasses (BMGs). But the relationships between strength and hardness are quite different for those coarse-grained (CG) and ultrafine-grained materials, brittle BMGs and ceramics. In the present work, some Cu alloys with different microstructures, Zr-, Co-based BMGs and Al 2 O 3 were employed to analyze the general relationship between hardness and strength. Several different relationships could be gotten from the experimental results of different materials available, and three types of indentation morphologies were observed. Indentation with “sink-in” morphology always represents a state of material and one third of hardness is in the range from yield strength to ultimate tensile strength. The other two indentation morphologies induced the fully hardening of material, so hardness could represent the intrinsic mechanical property of materials. The ratios of hardness to strength are found to be affected by the piled-up behaviors and their ability of shear deformation. Combined effect of the two aspects makes hardness approximately be three times of strength in the work-hardened crystalline materials and the shearable BMGs, but higher than three times of strength in the brittle-, annealed BMGs and ceramics.

Correlation between internal states and plasticity in bulk metallic glass

We report a close correlation between the internal states and plasticity in a bulk metallic glass (BMG) and discover that the optimization of copper-mold casting current can induce large plasticity stably in an otherwise brittle BMG. It is possible to confirm that larger plasticity corresponds to the internal states with more average free volume (FV) as revealed by lower density, higher enthalpy change, and higher Poisson’s ratio. The enhanced plastic deformation mechanism is interpreted based on the FV model of BMGs, and our results may have some implications for understanding the role of the FV during plastic deformation of BMGs.

Plasticity improvement of an Fe-based bulk metallic glass by geometric confinement

In this paper, the effect of nickel coating on the plasticity of a brittle Fe-based bulk metallic glass (BMG) was investigated. By electrodepositing a nickel coating onto an Fe 75Mo 5P 10C 8.3B 1.7 BMG, the plasticity of the BMG is shown to increase dramatically from ~ 0.5% to ~ 5.0%. Without reducing the strength of the BMG, the findings demonstrate that the electrodeposited nickel is an effective way to improve prominently brittle BMGs. The increased compressive plastic strain is believed to be attributed to the nucleation, intersection and bifurcation of multiple shear bands due to the exterior impedance of the surrounding coating.

Effect of sample geometry on the deformation behavior of Zr-based bulk metallic glass

In this study, effect of geometrical confinement on the deformation behavior of Zr57.5Cu11.2Ni13.8Al17.5 bulk metallic glass (BMG) was investigated. For this purpose flat top conical samples with different semi-vertex angles (α) were prepared for quasi-static compression tests. It was observed that as the geometry of the sample is slightly changed from cylindrical shape (for which α = 0°) to conical shape (for which α > 0°), a brittle BMG sample tends to deform in a ductile manner due to the geometrical constraint-induced complex multi-axial stress distribution in the sample. In brittle Zr57.5Cu11.2Ni13.8Al17.5 specimen with α = 5°, plastic deformation of about 4% was observed due to the formation of extensive intersecting shear bands.

Size effects on the compressive deformation behaviour of a brittle Fe-based bulk metallic glass

The mechanical behaviour of brittle Fe-based bulk metallic glass (BMG) samples with different diameters and aspect ratios (ARs) has been investigated. For samples with an AR of 2, the fracture strength increases with a decrease in the sample diameter which can be well-explained by the Weibull analysis with a Weibull modulus of 34. For samples with a fixed diameter of 2 mm, a transition between three deformation modes, i.e. fragmentation, distensile cracking and confined shearing was observed as the AR was decreased. When the AR is below 0.19, large friction between the platen and specimen end suppresses the distensile cracking and prompts shear deformation, resulting in multiple shear bands and large compressive plasticity. Our analysis indicates that the deformation mode of brittle BMGs strongly depends on the applied stress conditions and can exhibit ductile characteristics under constraints. The present findings are important for engineering applications of brittle BMGs in which various geometries will be used.

Structural relaxation and self-repair behavior in nano-scaled Zr–Cu metallic glass under cyclic loading: Molecular dynamics simulations

Bulk metallic glasses are generally regarded as highly brittle materials at room temperature, with deformation localized within a few principal shear bands. In this simulation work, it is demonstrated that when the Zr–Cu metallic glass is in a small size-scale, it can deform under cyclic loading in a semi-homogeneous manner without the occurrence of pronounced mature shear bands. Instead, the plastic deformation in simulated samples proceeds via the network-like shear-transition zones (STZs) by the reversible and irreversible structure-relaxations during cyclic loading. Dynamic recovery and reversible/irreversible structure rearrangements occur in the current model, along with annihilation/creation of excessive free volumes. This behavior would in-turn retard the damage growth of metallic glass. Current studies can help to understand the structural relaxation mechanism in metallic glass under loading. The results also imply that the brittle bulk metallic glasses can become ductile with the sample size being reduced. The application of metallic glasses in the form of thin film or nano pieces in micro-electro-mechanical systems (MEMS) could be promising.

Synergistic effect of crystalline metal on the plasticity of bulk metallic glasses under uniaxial synchro-compression

A uniaxial synchro-compression test of a cylindrical bulk metallic glass (BMG) specimen along with a crystalline metallic ring having an inner diameter much larger than the specimen’s diameter and thereby without radial confinement on the BMG specimen was performed. The plastic deformation behavior of a conventional Zr65Cu17.5Ni10Al7.5 BMG under synchro-compression with a cupper ring was investigated. It was found that remarkably plastic deformation in the inherently brittle BMG could occur under the uniaxial stress state as the loading area of the cupper ring was sufficiently large, and multiplication of the shear band on the synergistically deformed BMG specimen could be accomplished. The synergistic effect of the crystalline metal on the plastic deformation behavior of the BMG was attributed to the reduced thermal softening extent inside the shear bands that resulted from the restraining effect of the copper ring on the spring-back action of the testing machine.