Plasticity Of Bulk Metallic Glasses Research Articles

Synchronously improving glass forming ability and mechanical properties by Pd alloying Zr-Al-Cu bulk metallic glass

In this work, the influences of Pd addition on the glass-forming ability (GFA), thermal behaviors, mechanical properties and microstructure of the Zr60Al10Cu30 bulk metallic glasses (BMGs) were systematically investigated. Our study shows that proper addition of Pd can simultaneously improve the GFA, strength and plasticity of BMGs. The BMGs with Pd contents of 2.5 at.% and 5 at.% exhibit high GFA of 10 mm and 12 mm, high strength of 1852 MPa and 1858 MPa, and good plasticity of 4.3 % and 5.9 %, respectively, all of them are much higher than those of Zr60Al10Cu30 BMGs. It is revealed that alloying BMGs with proper Pd promotes the formation of a higher degree of icosahedral clusters, which is responsible for improved GFA and strength, and simultaneously encourages a decrease in the volume of shear transition zone (STZ) and the amount of imperfect ordered packing (IOP) clusters, which are responsible for the enhanced plasticity. Our findings indicate that the synergistic effect of short-range ordered structures induced by alloying amorphous alloys plays an important role in simultaneously improving the GFA and mechanical properties, providing valuable insights for BMG design.

Cryogenic thermal cycling induced simultaneous improvement of strength and ductility in a Zr-based bulk metallic glass composite

Preparing bulk metallic glass composites (BMGCs) is an effective way to improve the plasticity of bulk metallic glasses (BMGs). However, the enhancement of plasticity is usually at the cost of deterioration in strength. In this study, a series of BMGCs with the nominal composition of Zr48Cu48Al4 was prepared. Finely dispersed B2 CuZr crystals are embedded in a glassy matrix and subsequently, these composites were cryogenically cycled in liquid nitrogen followed by heating to various temperatures below the glass transition temperature. It was found that the samples after cryogenic thermal cycling (CTC) treated at an intermediate upper temperature shown a simultaneous improvement of yielding strength from 1680 MPa to 1832 MPa and plasticity from 5.6% to 9.1%. The enhancement of yielding strength is ascribed to the existence of the B19′ phase resulted from the partial martensitic transformation during the CTC treatment, while the improvement of plasticity is attributed to the higher free volume content and the larger “blocking effect” from the B19’ after the samples were treated by CTC. The combination of CTC treatment and precipitation of crystals provides a new strategy for improving the mechanical properties of the BMGs.

Extreme rejuvenation of a bulk metallic glass at the nanoscale by swift heavy ion irradiation

Swift heavy ions can be used as a tool to tune material properties by generating high aspect ratio, nanometric trails of defects, or new disordered phases. This work explores different aspects of using this tool for rejuvenating and enhancing plasticity in bulk metallic glasses. An amorphous alloy with a nominal composition of Pd40Ni40P20 was irradiated with GeV-accelerated Au ions. Irradiation-induced out-of-plane swelling steps up to approximately 100 nm in height are measured at the boundary between irradiated and non-irradiated areas. Changes of the relaxation enthalpy have been investigated using differential scanning calorimetry. Low-temperature heat capacity measurements substantiate an irradiation-induced increase of the boson peak height with increasing fluences. Accompanying transport measurements using radioactive Ag atoms as tracer also revealed increased diffusion rates in the irradiated samples dependent on the total fluence. Nano-indentation measurements show enhanced plasticity in the ion-irradiated glass which can be correlated with an increased heterogeneity as indicated by variable resolution fluctuation electron microscopy. The whole volume of the derived data substantiates a prominent enhancement of the excess volume in the solidified ion tracks and the irradiation-induced modifications are discussed and analyzed in the framework of strong glass rejuvenation within the nanometric ion tracks.

Mechanical cycling-induced evolution of structure and local mechanical properties in a PdCuNiP bulk metallic glass

To improve the ductility of bulk metallic glasses (BMGs), external mechanical stimulations can be applied. Thereby, cyclically loading the samples at stress levels below the macroscopic elastic limit leads to permanent structural changes, which can alter the plasticity in bulk metallic glasses. Here, the effect of systematic variations of Pd43Cu27Ni10P20 BMG's structural state, achieved through cyclically loading the material to 70, 80, and 90% of the yield strength, on a range of mechanical properties like modulus, hardness, yield strength, strain rate sensitivity, activation volume, maximum shear stress and stiffness at different probing volume is studied. It is observed that samples rejuvenated from the non-cycled state to the 70% of yield stress state, relaxed when cycled up to 80% of yield stress, and finally rejuvenated again when cycled up to 90% of yield stress. This rejuvenation to relaxation transition might be due to the random atomic rearrangement during the cycling.

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.

Temperature dependence of shear transformation zone volume and plasticity in bulk metallic glass

In this paper, a theoretical formulation for shear transformation zone (STZ) volume, shear band density, and plasticity are presented and temperature dependence of these parameters are discussed. In particular, the strain energy barrier for a shear cooperative transformation is modeled following the Johnson and Samwer approach for amorphous metallic glasses; positive temperature dependence of the STZ volume and negative temperature dependence of the shear band density are formulated, based on which a significant improvement in plasticity at lower temperatures is predicted. Experimental data on the plasticity for Vit105 bulk metallic glass - obtained from the present compression tests and also from the literature - show negative temperature dependence, which is consistent with the prediction of the analytical formulation.

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.

Unified upper temperature for cryogenic thermal cycling treatment in Fe-based bulk metallic glasses

Fe-based bulk metallic glasses have high fracture strength and large elastic limit. However, the extremely poor plasticity at room temperature severely renders their wide use as structural materials. In this work, three Fe-based bulk metallic glasses were selected to explore the effect of the upper temperature for cryogenic thermal cycling treatment on the plasticity of the specimens after rejuvenation. It was found that all three Fe-based bulk metallic glasses exhibit the largest plasticity when the upper temperature is about 0.62Tg, where Tg is glass transition temperature. The underlying mechanism is understood by the evolution of the structural heterogeneity dominates by the competition between structural rejuvenation and physical aging during the CTC treatment. When the upper temperature is relatively low, the structural rejuvenation dominates, which causes an increase of the plasticity of bulk metallic glasses; Otherwise, a too high upper temperature leads to a physical aging, resulting in the decline of plasticity. This work will provide process reference for the cryogenic thermal cycling treatment of Fe-based bulk metallic glasses.

Effect of Nb and Ta addition on mechanical properties of Zr-based bulk metallic glasses and composites

To modify the poor plasticity of bulk metallic glasses (BMGs) under room temperature, the present work reported that microalloying can improve the plasticity of BMGs, and the deformation mechanism at room temperature was elucidated. The selection of minor elements, Nb and Ta have a positive heat of mixing with the based compositions. The maximum plasticity of 32.9% is shown in the Nb1 sample. The self-organize to a critical state (SOC) occurred in all BMGs, the scaling exponent β value maintains the constant 0.24. The variation of the rotation angle of primary shear bands (SBs) is in agreement with plasticity. Meanwhile, the yield strength is linearly dependent on the local fracture toughness of BMGs. Summarily, the compressive plasticity of BMGs at room temperature can be improved by adding elements that can form positive mixing heat with the components of based compositions.

Effect of sample size and cooling rate on the plastic deformation behavior of bulk metallic glasses: A comparative study

Understanding the differences between the cooling-rate and size effects on the plastic deformation behavior of bulk metallic glasses (BMGs) is important not only for uncovering the plastic deformation mechanisms but also for achieving better plasticity of BMGs. In the present work, both the cooling-rate and size effects of a Zr-based BMG were investigated by quasi-static compressive experiments. The findings show that the macroscopic plasticity of BMGs can be remarkably enhanced by increasing the casting cooling rate, but moderately improved by decreasing the sample size. The intrinsic atomic arrangement stemming from cooling rate is the primary factor which affects the plastic deformation behavior rather than the extrinsic size effect, and the size effect is also dependent on the intrinsic cooling rate. The findings provide more explanations for the different size effects of BMG communities at different conditions, and are also of significance for achieving better plasticity in BMGs and BMG structures.

Temperature Dependence of Shear Transformation Zone Volume and Plasticity in Bulk Metallic Glass

Temperature Dependence of Shear Transformation Zone Volume and Plasticity in Bulk Metallic Glass

Substantially enhanced plasticity of bulk metallic glasses by densifying local atomic packing

Introducing regions of looser atomic packing in bulk metallic glasses (BMGs) was reported to facilitate plastic deformation, rendering BMGs more ductile at room temperature. Here, we present a different alloy design approach, namely, doping the nonmetallic elements to form densely packed motifs. The enhanced structural fluctuations in Ti-, Zr- and Cu-based BMG systems leads to improved strength and renders these solutes’ atomic neighborhoods more prone to plastic deformation at an increased critical stress. As a result, we simultaneously increased the compressive plasticity (from ∼8% to unfractured), strength (from ∼1725 to 1925 MPa) and toughness (from 87 ± 10 to 165 ± 15 MPa√m), as exemplarily demonstrated for the Zr20Cu20Hf20Ti20Ni20 BMG. Our study advances the understanding of the atomic-scale origin of structure-property relationships in amorphous solids and provides a new strategy for ductilizing BMG without sacrificing strength.

Shape memory effect in metallic glasses

Shape memory effect in metallic glasses

Improved tensile plasticity of bulk metallic glasses by heightening microstructural heterogeneity and energy state

Improved tensile plasticity of bulk metallic glasses by heightening microstructural heterogeneity and energy state

Cooling under Applied Stress Rejuvenates Amorphous Alloys and Enhances Their Ductility

The effect of tensile stress applied during cooling of binary glasses on the potential energy states and mechanical properties is investigated using molecular dynamics simulations. We study the three-dimensional binary mixture that was first annealed near the glass transition temperature and then rapidly cooled under tension into the glass phase. It is found that at larger values of applied stress, the liquid glass former freezes under higher strain and its potential energy is enhanced. For a fixed cooling rate, the maximum tensile stress that can be applied during cooling is reduced upon increasing initial temperature above the glass transition point. We also show that the amorphous structure of rejuvenated glasses is characterized by an increase in the number of contacts between smaller type atoms. Furthermore, the results of tensile tests demonstrate that the elastic modulus and the peak value of the stress overshoot are reduced in glasses prepared at larger applied stresses and higher initial temperatures, thus indicating enhanced ductility. These findings might be useful for the development of processing and fabrication methods to improve plasticity of bulk metallic glasses.

Strain rate-dependent avalanches in bulk metallic glasses

According to a simple mean-field model, two distinct types of slip avalanches during serrated plastic flows in bulk metallic glasses are extracted to distinguish the slipping modes of shear bands under various strain rates. Small avalanches propagate progressively, while large avalanches follow a simultaneous propagation. A weakening parameter and the critical size are defined for characterizing the two completely disparate shearing modes. The larger weakening and lower critical size mean more activated STZs, providing us a new method to explore more plasticity in bulk metallic glasses.

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.

The role of minor alloying in the plasticity of bulk metallic glasses

Micro- or minor alloying of metallic glasses is of technological interest. An originally ductile Pd-based monolithic bulk metallic glass (Pd40Ni40P20) was selectively manipulated by additions of Fe or Co. The alloying effects were extreme, showing either exceptional ductility upon Co addition or immediate catastrophic failure upon Fe addition when tested under uniaxial compression or 3-point bending. The amorphous structure was characterized prior to deformation with respect to its medium-range order (MRO) using variable resolution fluctuation electron microscopy (VR-FEM). We observe striking differences in the MRO between the ductile and brittle metallic glasses, with the ductile glasses exhibiting a rich structural diversity and MRO correlation lengths up to 6 nm. The MRO diversity seems to enable easier shear banding and hence enhance the deformability.

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.

Selective Laser Melting (SLM) of in-situ beta phase reinforced Ti/Zr-based bulk metallic glass matrix composite

In-situ ductile dendrite reinforcement is the most effective method to improve the plasticity of bulk metallic glasses. In this study, an in-situ beta phase reinforced Ti/Zr-based bulk metallic glass matrix composite (BMGC), (Ti0.65Zr0.35)90Cu10, was deposited via selective laser melting. Its microstructural characteristics and indentation deformation behaviors were investigated. There were fine and coarse microstructures in the BMGC, due to the existence of heat-affected zone, and they were finer than the microstructures of the BMGCs prepared using traditional technologies; the amorphous and beta phases could be coordinately deformed during the indentation process.