Deformation Of Bulk Metallic Glass Research Articles

Structural and stress state dependence of small-scale deformation in bulk metallic glass

Bulk Metallic Glasses (BMGs) are attractive for myriad structural applications at multiple length scales (nano-micro-macro) but show limited bulk plasticity due to the tendency for shear localization. However, there is limited understanding of the effect of structural state and stress state on the small-scale deformation behavior of BMGs. Here, the micro-scale deformation behavior of a model Ni-based BMG was studied in as-cast and corresponding relaxed state under multiaxial nano-indentation, uniaxial micro-pillar compression, and micro-cantilever beam bending. The relaxed BMG showed 6 % higher hardness, 22 % higher yield strength, and 26 % higher bending strength compared to its as-cast counterpart. The increase in hardness, yield strength, and bending strength for the relaxed alloy compared to its as-cast counterpart demonstrates the relation of intrinsic free volume present in a BMG to its resistance for initiation of plastic events at this scale. Both the as-cast and corresponding relaxed samples showed stable notch opening and blunting during micro-cantilever bending tests rather than unstable crack propagation. However, pronounced notch weakening was observed for both the structural states, with the bending strength lower by ∼ 25 % for the notched samples compared to the un-notched samples. This work may stimulate further investigations into the deformation behavior of BMGs in response to complex stress states pertinent to real-world structural applications at small scale.

Investigation of the softening behavior in severely deformed micromachined sub-surface of Zr-based bulk metallic glass via nanoindentation

In this study, the orthogonal micromachining of a Zr-based bulk metallic glass (BMG) was carried out at different cutting speeds and undeformed chip thicknesses. The work softening behavior of the micromachined sub-surface was characterized via nanoindentation. It was observed that the serrated flow in the load-displacement curves during nanoindentation depends on the previous machining conditions. The softening in the machined subsurface was studied up to a depth of 300 µm and the minimum and the maximum reductions in nanohardness with respect to unmachined region were 13% and 37%, respectively. Furthermore, the shear transformation zone volume of the machined surface was determined and its correlation with the cutting force coefficients Kc and Kt was observed. Differential scanning calorimetry (DSC) reveals an increase in the free volume due to micromachining of BMG. This work presents a nanoindentation-based framework and insight into the changes occurring in the sub-surface due to severe plastic deformation in BMG.

Expanding the homogeneous regime of deformation in bulk metallic glass by electromigration-induced rejuvenation

The origin of plasticity in bulk metallic glasses (BMGs) is highly elusive due to their complicated amorphous atomic structures. Nonetheless, there is a general consensus that the homogeneous regime of deformation on the deformation map of BMGs is constrained by high temperature and low strain rate. Here we report an expanded homogeneous regime in a Zr55Cu30Al10Ni5 BMG, which is achieved by applying a pulsed electric current to the BMG specimen subjected to external loading. The BMG specimen exhibits homogeneous elongation and necking, rather than shear banding as it should without pulsed current. The results indicate that the pulsed current produces an additional effective “temperature increment” ∼0.15Tg (Tg, the glass transition temperature) apart from the Joule heating effect. This expanded homogeneous regime is attributed primarily to electromigration-induced dynamic rejuvenation which promotes homogeneously distributed deformation by enlarging the shear transformation zones. These findings could benefit understandings of plasticity in glassy materials.

Structural heterogeneity originated plasticity in Zr–Cu–Al bulk metallic glasses

The brittleness at room temperature has become the major issue for bulk metallic glasses (BMGs) as structural engineering materials and a significant challenge is to improve and research the plasticity. Study on structural heterogeneity may be a key to understand the origin of plasticity in BMGs. In the current research, correlation of plasticity with structural heterogeneity which is studied by β-relaxation, fragility, serration dynamics, and free volume has been studied in a Zr–Cu–Al BMGs system. These quantities are unified with each other. It has evidenced that the degree of β-relaxation behavior has a close correlation with the plasticity and fragility. A good plastic deformation in BMG has a more heterogeneous serrations with a higher serration barrier. Compared with a poor plasticity BMG, the ductile has a more heterogeneous structure with more free volume. All of these can owe to the structural heterogeneity originated plasticity. This finding is believed to have implications for understanding the origin of plasticity from the perspective of structural heterogeneity in BMGs.

Effect of mechanically induced structural rejuvenation on the deformation behaviour of CuZr based bulk metallic glass

Severe plastic deformation of bulk metallic glasses leads to rejuvenation of the amorphous structure. Here we present results demonstrating an improved plasticity for a Cu45Zr45Al5Ag5 bulk metallic glass alloy rejuvenated by severe plastic deformation via high-pressure torsion (HPT). Nanoindentation as well as atomic force microscopy measurements performed on the cross-section of HPT samples provide evidence for the enhancement in plasticity. A robust algorithm developed to reliably detect and quantify the discontinuities (pop-ins) in the nanoindentation load–displacement curves is applied to a large dataset. The results related to the rejuvenated structure demonstrate (i) a reduction of the number of detectable pop-ins as well as (ii) a reduction of their displacement offset. Furthermore, (iii) a pronounced increase in the total creep value is observed and correlates with a plasticity criterion as well as a reduced hardness and Young’s modulus that are heterogeneously distributed across the samples due to localized strain softening during HPT. Finally, (iv) atomic force microscopy topographies obtained from selected indents support the findings, showing less pronounced steps in the plastic pile-up. The experimental results indicate that rejuvenation by HPT processing leads to structural and dynamical heterogeneities, increased atomic mobility and an improved plastic deformation response.

Severe Plastic Deformation of Amorphous Alloys

Bulk metallic glasses having a disordered amorphous structure have been in the focus of recent intensive materials research due to their special mechanical properties, however, these materials exhibit brittleness in conventional unconstrained deformation modes. High-pressure torsion as a special severe plastic deformation method, which applies constraints on the material, can induce significant plasticity in metallic glasses. Apart, the deformation can promote structural changes in the glass, such as anisotropy and nanocrystallization. The inhomogeneity generated by torsional shear deformation in bulk metallic glasses can be detected in internal surfaces. The final structure and morphology of the deformed material depend on the processing parameters (deformation rate, shear strain, temperature and pressure) of the high-pressure torsion apparatus.

Experimental and computational analysis of initiation and propagation of shear bands in bulk metallic glasses

Shear bands (SBs) play a key role in the mechanical deformation of Bulk Metallic Glasses (BMGs). The key to obtaining an insight into the shear band mechanism is to understand the root cause of shear band initiation and propagation. SB initiation is a difficult event to capture, considering the negligible macroscopic ductility of BMGs. It is the premature shear localisation in the nano scale which is responsible for the noticeable brittle nature in the macro-scale. Plastic deformation of BMGs occurs due to the formation of highly localised SBs of approximately10–20 nm in thickness. These then propagate at a velocity of around 100 ms−1 and results in catastrophic failure in both tension and compression due to the micro-cracks formed through the coalescence of the nano voids. This study develops a technique to capture the early initiation of SBs through a wedge indentation test. The work found that incremental loading (a definite load applying incrementally by 1 kN in stages) did not affect the overall mechanical response of the BMG material, indicating that SB evolution can be examined with an incremental indentation test. A predictive numerical model for SB initiation and propagation in BMGs is presented using two approaches: a strain gradient model and an integral-type non-local approach employing the Vermeer–Brinkgreve modelling strategy(Strömberg, Ristinmaa 1996). Comparisons are made between the outcomes of physical tests and numerical modelling to obtain an insight into the relative merits.

Effect of mechanical-annealing on the mechanical properties of a Zr-based bulk metallic glass

The mechanical-annealing referred in this work is also named pre-strain, which is widely investigated in TRIP steel, stainless steel, magnesium alloy and aluminum alloy. In this case, we used preloading to input energy into a bulk metallic glass (BMG) to observe the changes in the structure and mechanical properties. We selected Zr[Formula: see text]Co[Formula: see text]Al[Formula: see text] BMG as a model material owning to its outstanding glass forming ability and excellent mechanical properties. The samples were kept at a constant pressure of 1900, 1700 and 1500 MPa (below the yield strength) for 40, 55 and 70 h. The study found out that the density of those samples increased after being pre-loaded. Then, the samples underwent aging treatment at room-temperature for more than 30 days after unloading. After re-compressing the samples, the results show that the yield strength and fracture strength of the samples decreased, and the amplitude of the serrated plastic flow increased during the plastic stage. Our finding might have some implications for understanding the plastic deformation of BMGs.

Deformation Behavior of Bulk Metallic Glasses and High Entropy Alloys under Complex Stress Fields: A Review.

The plastic deformation of bulk metallic glasses (BMGs) depends significantly on applied stress states, and more importantly, in practical applications of BMGs as structural materials, they always deform under complex stress fields. The understanding of deformation behavior of BMGs under complex stress fields is important not only for uncovering the plastic deformation mechanisms of BMGs, but also for developing BMG components with excellent mechanical performance. In this article, we briefly summarize the recent research progress on the deformation behavior of BMGs under complex stress fields, including the formation and propagation of shear bands, tunable macroscopic plasticity, and serrated plastic flows. The effect of complex stress fields on the plastic deformation mechanisms of BMGs is discussed from simple stress gradient to tailored complex stress fields. The deformation behavior of high entropy alloys (HEAs) under complex stress states has also been discussed. Challenges, potential implications and some unresolved issues are proposed.

Serrated flow during inhomogeneous bending deformation of bulk metallic glasses: From self-organized critical to chaotic dynamics

In this research, the dynamic behavior of serrated flows observed in the flexural stress-deflection curves during three-point bending (3 PB) tests on Zr-, Ti- and La-based bulk metallic glasses (BMGs) is statistically analyzed. Furthermore, the effect of strain rate on the serrations is studied. Based on the ductility of alloy, two distinct types of dynamics are detected. For more ductile alloys (Zr- and Ti-based BMGs) the stress drop magnitude of serrations obeys a power-law distribution of shear avalanches, which is an indicator of the self-organized critical (SOC) state in dynamics. In this case, the size of avalanches has no characteristic scale. In contrast, the size of serrations corresponding to the less ductile alloy (La-based BMG) exhibits a characteristic length scale and implies that the underlying dynamics is chaotic. Results indicate that the plasticity in less ductile alloy can be related to the stick-slip process of a few shear bands, but in the case of more ductile alloys, it can be attributed to the simultaneous nucleation and interaction of a large number of shear bands. Above a critical strain rate, the serrations on flexural stress-deflection curves do not appear and a transition occurs from the serrated to non-serrated plastic flow. The comparison of apparent activation energy for the onset of serrated flow and the STZ activation energy indicates that these two processes are interrelated. Using these values, it is possible to determine the dynamic behavior of serrated flows in BMGs.

Ductile bulk metallic glass by controlling structural heterogeneities

A prerequisite to utilize the full potential of structural heterogeneities for improving the room-temperature plastic deformation of bulk metallic glasses (BMGs) is to understand their interaction with the mechanism of shear band formation and propagation. This task requires the ability to artificially create heterogeneous microstructures with controlled morphology and orientation. Here, we analyze the effect of the designed heterogeneities generated by imprinting on the tensile mechanical behavior of the Zr52.5Ti5Cu18Ni14.5Al10 BMG by using experimental and computational methods. The imprinted material is elastically heterogeneous and displays anisotropic mechanical properties: strength and ductility increase with increasing the loading angle between imprints and tensile direction. This behavior occurs through shear band branching and their progressive rotation. Molecular dynamics and finite element simulations indicate that shear band branching and rotation originates at the interface between the heterogeneities, where the characteristic atomistic mechanism responsible for shear banding in a homogeneous glass is perturbed.

Experimental studies of shear bands in Zr-Cu metallic glass

Shear bands are the key feature that control deformation in bulk metallic glasses (BMGs). This study provides a comprehensive analysis of plastic deformation of a Zr-Cu-based BMG at room temperature. Experiments were conducted to observe the evolution of shear bands in the material. It was shown that shear bands formed discretely in the material which allows for material deformation to occur across it. Additionally, individual shear bands were characterised to obtain a better understanding of shear-band-induced plasticity. Assessment of mechanical properties, such as hardness and elastic modulus, indicate the deformed regions of the material were weaker than undeformed regions. No compositional or structural changes were found in shear-band of the studied BMG suggesting generation of local free volume in the deformed region.

Low-temperature slip along intergrain boundaries

Equations are derived for slip in a disordered atomic layer which describe diffusive creep as well as high-speed slip at low temperatures. An exact solution for the slip velocity is found in the form of a functional of the distribution function of the threshold shear stresses in the slip layer. The relationship between the microscopic parameters of the theory and the macroscopic properties of metallic glass is established in terms of the Mott intergrain slip model. The calculated rate of deformation of bulk metallic glass is compared with published experimental data.

Ductile fracture in notched bulk metallic glasses

The deformation of bulk metallic glasses (BMGs) is generally driven by highly localized shear. Due to such inhomogeneous flow, failure occurs in a catastrophic brittle manner through rapid shear banding, often associated with very limited plastic strain macroscopically. Here, we demonstrate metal-like ductile fracture in Zr-based BMGs under tension, a completely opposite situation, by suppressing shear banding. In the absence of shear bands, nucleation of cavies/voids and subsequent void growth and coalescence dominate the initial plastic failure process, enabling BMGs to display the essential characteristics of ductile fracture, with deep dimples and cup-and-cone morphology. This ductile fracture only occurs in amorphous alloys, but not in the fully crystallized counterpart. Furthermore, the characteristic decohesion strength of the ductile fracture in Zr-based BMGs was found to be 1.75 GPa, one of the highest among engineering metals and alloys. These present findings reveal the previously hidden ductile behavior of BMGs, suggesting an alternative method to enhance the ductility of BMGs by removing shear banding.

Mapping the cyclic plastic zone to elucidate the mechanisms of crack tip deformation in bulk metallic glasses

Developing damage-tolerant bulk metallic glasses (BMGs) requires knowledge of the physical mechanisms governing crack propagation. While fractography suggests that fatigue crack propagation occurs in an incremental manner, conclusive evidence of alternating crack tip blunting and resharpening is lacking. By mapping the strain fields in both the monotonic and cyclic plastic zones, it is shown that the characteristic compressive stresses required to resharpen the crack tip are developed in a BMG upon unloading. This result confirms the mechanism of fatigue crack propagation in BMGs. Broader implications of these findings are that the effect of shear banding is rather diffuse and plastic deformation ahead of a stress concentration, such as a crack tip, appears to extend well beyond the extent of visible shear bands on the sample surface.

Serrated flow behavior in a Pd-based bulk metallic glass under nanoindentation

This study presents the investigation on the serrated flow behavior of a Pd-based bulk metallic glass under nanoindentation. Firstly, a method of curve-fitting and statistical analysis was proposed that found to successfully extract the serration events from the deformation of bulk metallic glasses during nanoindentation. Then the serrated flow at various loading rates were systematically analyzed by using the proposed method. The results showed that the lower loading rates promote the serrated flow, while the individual serration was more likely to be triggered at higher loading rates. Combined with the intrinsic micromechanism of bulk metallic glasses, we conjectured the serrated flow features may be attributed to the evolution of liquid-like region in bulk metallic glasses.

Meso-mechanical constitutive model of bulk metallic glass matrix composites

Based on the existing experimental observations and extended Mori-Tanaka (M-T) method, a new meso-mechanical constitutive model is proposed to predict the monotonic tensile or compressive deformation of bulk metallic glass (BMG) matrix composites with toughening phases. In the proposed model, the deformation behaviors of the BMG matrix and toughening phase are described by using the extended free volume model and unified visco-plastic model, respectively, and corresponding algorithmic tangent operators are also derived. The proposed model is verified by predicting the monotonic tensile or compressive stress-strain responses of the BMG composites with an in-situ dendrite and different ex-situ toughening particles, respectively. Comparison of predicted results and experimental ones demonstrates that the proposed meso-mechanical constitutive model describes both the strain hardening and softening features of the BMG composites reasonably. The simulations agree with the experimentswell.

Numerical Modeling of Cyclic Deformation in Bulk Metallic Glasses

In this paper, a systematic numerical simulation was performed to elucidate the damage mechanisms in bulk metallic glasses (BMGs) subjected to the tension-compression cyclic loading, and then the relation between fatigue life, applied strain, and cycling frequency was therefore presented. The free volume was selected as an internal state variable to depict the shear-band nucleation, growth, and coalescence with the help of free volume theory, which was incorporated into the ABAQUS code via a user material subroutine UMAT. Under cyclic loading, the shear banding initiation mainly stems from the microstructure inhomogeneity in BMGs and, further, the effect of applied strain amplitude and cycling frequency was discussed. The present simulations will shed some light on the fatigue damage mechanisms and fatigue life evaluation of BMG structures.

Giant size effect on compressive plasticity of (Zr55Cu30Al10Ni5)99Nb1 bulk metallic glass

The effect of sample size on the compressive plasticity of (Zr55Cu30Al10Ni5)99Nb1 bulk metallic glass (BMG) has been studied on both as-cast and as-annealed samples. The results have revealed a size dependency of the mechanical behavior. The size effect has been considered according to two aspects: free volume and atomic mobility in shear bands. It is proposed that more shear softening in the largest sample as well as the more excess free volume in the sample experiencing higher cooling rate are responsible for the strong size effect. It is proposed that the amount of heat generated in shear bands is the most effective factor in deformation of BMGs

Influence of Stress-strain on the Microstructural Change in the Metallic Glass and Metallic Glass Matrix Composite

At room temperature, metallic glasses deform inhomogeneously by strain localization into narrow bands as a result of yielding due to an external force. When shear bands are generated during deformation, often nanocrystals form at the shear bands. Experimental results on the deformation of bulk metallic glass in the current study suggest that the occurrence of nanocrystallization at a shear band implies the loading condition that induces deformation is more triaxial in nature than uniaxial. Under a compressive stress state, the geometrical constraint strain imposed by the stress triaxiality plays a crucial role in the deformation-induced nanocrystallization at the shear bands.