Brittle Metallic Glasses Research Articles

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.

Evaluation of the relationship between the effective strain and the springback behavior during the deformation of metallic glass ribbons

We evaluated the role of effective strain on the plasticity of metallic glasses related to springback phenomena in both ductile Zr-based metallic glass and brittle Hf-based metallic glass. Experimental investigations of the deformation behavior and the intrinsic mechanical properties of metallic glass were performed by room temperature L-bending deformation. The modeling analysis clearly shows that an inhomogeneous effective strain for initiating shear band is generated during deformation, with the brittleness of a metallic glass being dependent on the effective strain reaching the critical fracture strain of the glass. The combined experimental and modeling results reveal broad and general criteria that should allow for deformability of a wide-variety of glass-forming alloys by adjusting the effective strain during deformation.

Understanding the brittleness of metallic glasses through dynamic clusters

Abstract

On the stress-state dependent plasticity of brittle metallic glasses: Experiment, theory and simulation

In this work, the stress-state dependent plasticity of brittle Ti-based metallic glass was investigated by experiment, mechanics theory and simulation systematically. With different settings of arc-shaped edges, various stress gradients could be generated artificially. The systematical work showed that the global plastic deformability of a metallic glass depended strongly on the stress states under compressive loadings. By introducing the Mohr–Coulomb yielding criterion and the associated flow rule, we proposed an elastic–plastic model to analyze the deformation behaviors of all specimens. Combined with the experimental results and the numerical results by finite element method (FEM), it was demonstrated that the present simulation model could suitably describe the plastic deformation process of the specimens, and the different stress states might have a vital significance on the global plastic deformation behavior. The current results and analytical model could provide an approach to understand the further plastic deformation mechanism of brittle metallic glasses with different stress states.

Effect of Minor V Addition on Al<sub>88</sub>Y<sub>7</sub>Fe<sub>5 </sub>Amorphous Alloys

The effect of the addition of 0.2-0.65 at% of V on the glass forming ability and mechanical properties of Al88Y7Fe5 alloy were investigated. The addition of V in this range had little effect on the glass forming ability of the alloy, but lowered the tensile strength of the amorphous ribbon. The fracture surface of Al88Y7Fe5 amorphous ribbons was typical vein pattern for ductile metallic glasses, however, that of the alloy with 0.5% V addition changed to two different regions, i.e., vein pattern region and smooth region. At high magnification, the smooth region was composed of nanometer sized corrugations, which is typical for brittle metallic glasses.

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.

Cavitation in materials with distributed weak zones: Implications on the origin of brittle fracture in metallic glasses

Several experimental studies have shown that fracture surfaces in brittle metallic glasses (MGs) generally exhibit nanoscale corrugations which may be attributed to the nucleation and coalescence of nanovoids during crack propagation. Recent atomistic simulations suggest that this phenomenon is due to large spatial fluctuations in material properties in a brittle MG, which leads to void nucleation in regions of low atomic density and then catastrophic fracture through void coalescence. To explain this behavior, we propose a model of a heterogeneous solid containing a distribution of weak zones to represent a brittle MG. Plane strain continuum finite element analysis of cavitation in such an elastic-plastic solid is performed with the weak zones idealized as periodically distributed regions having lower yield strength than the background material. It is found that the presence of weak zones can significantly reduce the critical hydrostatic stress for the onset of cavitation which is controlled uniquely by the local yield properties of these zones. Also, the presence of weak zones diminishes the sensitivity of the cavitation stress to the volume fraction of a preexisting void. These results provide plausible explanations for the observations reported in recent atomistic simulations of brittle MGs. An analytical solution for a composite, incompressible elastic-plastic solid with a weak inner core is used to investigate the effect of volume fraction and yield strength of the core on the nature of cavitation bifurcation. It is shown that snap-cavitation may occur, giving rise to sudden formation of voids with finite size, which does not happen in a homogeneous plastic solid.

Atomic Scale Fluctuations Govern Brittle Fracture and Cavitation Behavior in Metallic Glasses

We perform atomistic simulations on the fracture behavior of two typical metallic glasses, one brittle (FeP) and the other ductile (CuZr), and show that brittle fracture in the FeP glass is governed by an intrinsic cavitation mechanism near crack tips in contrast to extensive shear banding in the ductile CuZr glass. We show that a high degree of atomic scale spatial fluctuations in the local properties is the main reason for the observed cavitation behavior in the brittle metallic glass. Our study corroborates with recent experimental observations of nanoscale cavity nucleation found on the brittle fracture surfaces of metallic glasses and provides important insights into the root cause of the ductile versus brittle behavior in such materials.

Deformation behavior and enhanced plasticity of Ti-based metallic glasses with notches

The plasticity of Ti-based metallic glasses with different aspect ratios can be improved by introducing two semicircular notches on the edges of the samples, owing to the interactions of shear bands (SBs) under conventional compression tests. The interaction of SBs can be ascribed to the easy initiation of SBs around the notches due to the large stress gradient, and the consequent blocking effect of notches on the propagation of shear bands. Additionally, the current findings provide a new way to understand the physical nature for the plastic deformation behavior of some brittle metallic glasses and supply an effective approach to enhance the plasticity to some extent.

Micro‐and Nanoscale Metallic Glassy Fibers

AbstractWhen bulk materials are made into micro‐and nanoscale fibers, there will be attractive improvement of structural and functional properties, even unusual experimental phenomena [Ref. 3]. The main drawback of various applications of metallic fibers is poor ability of present fabrication methods for controlling their dimensions and surface properties [Ref. 4,5]. Metallic glassy fibers (MGFs) are desired because of unique mechanical and physical properties and glass‐like thermoplastic processability of metallic glasses (MGs). Here, we report a synthetic route for production of micro‐to nanoscale MGFs (the diameter ranges from 100 µm to 70 nm) by driving bulk metallic glass rods in their supercooled liquid region via superplastic deformation. Compared with existing metallic fibers, the MGFs have precisely designed and controlled properties and size, high structural uniformity and surface smoothness, and extremely flexibility. Remarkably, the method is simple, efficient, and low cost, and the MGFs can be continuous prepared by the method. Furthermore, the MGFs circumvent brittleness of MGs by size reduction. We proposed a parameter based on the thermal and rheological properties of MG‐forming alloys to control the preparation and size of the fibers. The MGFs with superior properties might attract intensive scientific interest and open wide engineering and functional applications of glassy alloys.

Shear stability of metallic glasses

The effect of sample size on the shear deformation and compressive plasticity of different metallic glasses were investigated. The experimental results showed that the deformation and fracture behaviors of samples prepared from chemically identical Zr-, Ti-, Fe-, or Mg-based metallic glass ingots were strongly dependent on the sample size and machine stiffness, and a super-high compressive plasticity was achieved in the Zr-based metallic glasses with sample size of 1.0 mm in width. It is also found that the sample size can significantly influence the density of elastic energy dissipated in the shear band: with sample size decreasing and machine stiffness increasing, the density of the elastic energy dissipated in the shear band of metallic glasses is prominently decreased, thus the shear deformation turns to be more stable, resulting in the improvement of plasticity in ductile metallic glasses and the transition from fragmentation fracture to shear fracture in brittle metallic glasses. This finding is important for the potential applications of the present metallic glasses and for designing new metallic glasses with better mechanical properties.

Shear band evolution during large plastic deformation of brittle and ductile metallic glasses

Four monolithic metallic glasses (MMGs) with different plasticities varying from brittle to ductile behavior under unconstrained loading were subjected to small punch testing. All specimens undergo large plastic deformation with multiple cobweb-like shear bands under these conditions. The process of shear band evolution was carefully controlled and investigated. Plasticity of MMGs is characterized by equivalent plastic strain ε* (product of shear band density and critical shear offset). Thus, this article provides an experimental basis for a better understanding of the shear band evolution during plastic deformation of MMGs.

Microscale deformation behavior of amorphous/nanocrystalline multilayered pillars

Metallic glasses have recently been extended their research and application in micro-electro-mechanical systems (MEMS). However, the brittle nature of metallic glasses in the bulk and thin film forms inevitably imposes limitation. The current study applies the new idea to adopt a thin layer of nanocrystalline metal film beneath the brittle binary ZrCu thin film metallic glass (TFMG) layer. This metal film needs to be sufficiently strong in modulus and strength and needs to be deposited with the appropriate film orientation. The face-centered cubic Cu {111} film appears to be too soft, the body-centered cubic Mo {110} film behaves to be too brittle, but the hexagonal close-packed Zr {0001} film matches all above requirements. The shear bands initiated in the ZrCu thin film metallic glass layer can be absorbed and accommodated by the nanocrystalline Zr {0001} layer via the nano-twinning mechanism. The original brittle ZrCu TFMG, with the inclusion of a Zr layer beneath, can behave highly ductile with semi-uniform plastic deformation of 55%, even more ductile than most pure metals. The amorphous-crystalline interface exhibits good strain compatibility after appreciable plastic deformation. This finding can impose great impact on the TFMG/metal multilayer structures useful for MEMS design.

Correlation between onset of yielding and free volume in metallic glasses

Based on the free volume model, a critical value of reduced free volume is proposed to be the sufficient condition for the onset of yielding in metallic glasses. The corresponding stress for the onset of yielding is found to be less than the macroscopically measured yield strength, which is consistent with experimental observations. The difference in deformability between brittle and plastic metallic glasses can be explained in terms of the free volume evolution in the localized shear zone.

Brittle metallic glass deforms plastically at room temperature in glassy multilayers

Bulk metallic glasses are emerging as a new class of materials that can have applications ranging from structural materials to materials for future nanotechnology. However, catastrophic mechanical failure is a serious issue hindering the use of these materials in engineering applications. Here we introduce an approach to understanding and solving the problem of brittleness of metallic glasses. We have shown that even a very brittle metallic glass (La based) can be forced to deform plastically at room temperature if it is made in the form of multilayers involving other metallic glasses, i.e., a two-phase glass. The mechanically soft glassy layer (La based) having a lower critical shear stress acts as a nucleation or an initiation site for shear bands and the mechanically hard glassy layer (Zr based) acts as an obstacle to the propagation of shear bands. This process results in the multiplication of shear bands. Since the shear bands are associated with a local rise in temperature, a large number of shear bands can raise the overall temperature of the soft layer and eventually can drive it to the supercooled liquid state, where deformation of metallic glass is very large and homogeneous. The results reported here not only clarify the mechanism of large plastic deformation in two-phase glassy alloys but also suggest the possibility of a different kind of two-phase bulk glassy alloys exhibiting large plastic deformation at room temperature.

Fracture mechanism of some brittle metallic glasses

A systematic study on the fracture surface of brittle Mg-, Fe-, and Co-based metallic glasses under compressive loading is approached and a fracture mechanism is proposed. Experimentally, the metallic glass samples are compressed into many small fragments, displaying an explosion fracture feature. Therefore, an energy equilibrium model is employed to describe the fracture processes of those brittle metallic glasses. Furthermore, some regular nanoscale steps, which were scarcely discovered, are found on the mirror region on their fracture surfaces. It is suggested that such nanoscale steps are associated with the energy distribution in metallic glasses and are created by the shear waves generated by the instability of crack propagation during the explosion rupture processes. Based on the comparison of experimental observations with numerical calculations, we recommend a novel model for interpreting the development of nanoscale steps on the dynamic fracture surfaces of these brittle metallic glasses, which appropriately describes the experimental findings.

Nanoscale oscillatory fracture propagation in metallic glasses

We describe the oscillatory crack propagation for small propagation velocities at the atomistic scale that was recently observed for brittle metallic glasses [G. Wang, Y.T. Wang, Y.H. Liu, M.X. Pan, D.Q. Zhao, W.H. Wang, Appl. Lett. 89 (2006) 121909; G. Wang, D.Q. Zhao, H.Y. Bai, M.X. Pan, A.L. Xia, B.S. Han, X.K. Xi, Y. Wu, W.H. Wang, Phys. Rev. Lett. 98 (2007) 235501]. Based on a simple model of crack propagation [Y. Braiman, T. Egami, Phys. Rev. E, 77 (2008) 065101(R)], we derived and analyzed expressions for the feature size, oscillation period, and maximum strain accumulated in the material.

Interfacial Microstructure of Aluminum/Metallic Glass Lap Joints Fabricated by Magnetic Pulse Welding

Magnetic pulse welding was applied to the lap joining of crystalline pure aluminum and metallic glass (Zr48Cu36Al8Ag8 and Cu50Zr42:5Al7:5) and interfacial microstructure was examined. The metallic glass fractured in brittle manner in conventional plate layout. However, Al/metallic glass joints were successfully obtained without fragmentation of brittle metallic glass by setting a soft aluminum plate as a shock absorber behind the metallic glass. The welding interface exhibited characteristic wavy morphology as well as that of metal/metal joints. Thin layer with medium chemical composition of the metallic glass and Al was also produced at the welding interface. No chemical composition change took place in the area of 2 mm and more away from each matrix/intermediate layer interface. Also, no hardness change of Al matrix was detected in the area of 2 mm and more away from Al/intermediate layer interface. Hardness of metallic glass matrix close to the welding interface was not constant, however, TEM observation revealed that the metallic glass matrix retained the amorphous structure after the welding. These obtained results are considered to indicate that the metallic glass did not crystallize after the magnetic pulse welding. [doi:10.2320/matertrans.ME200835]

Size-dependent shear fracture and global tensile plasticity of metallic glasses

The tensile ductility or brittleness of metallic glasses is found to depend strongly on the critical shear offset. Based on experimental observations, the tensile shear fracture processes of metallic glasses can be divided into three stages: multiplication and coalescence of the free volume, formation of void and the final fast propagation of a shear crack. Accordingly, the size effect on the tensile shear deformation processes of metallic glass can be well understood: with decreasing specimen size smaller than the equivalent critical shear offset, the shear deformation of metallic glass is changed from unstable to stable, which leads to a transition from global brittleness on the macroscale to large global plasticity or even necking on the microscale. These results are fundamentally useful in understanding the physical nature of tensile shear deformation of various metallic glasses and even in the design of new metallic glass materials with good plasticity.

Transitions from oscillatory to smooth fracture propagation in brittle metallic glasses

We present a simple model to explain the transition from oscillatory to smooth crack propagation in brittle metallic glasses. We demonstrate that the smooth fracture propagation that is characteristic for higher temperature or higher crack opening velocities (for type 1 crack propagation) becomes unstable and oscillatory behavior is being observed. The characteristic feature size of the crack propagation may be at the nanometer scale and grows as the opening velocity decreases.