Shear Transformation Zone Volume Research Articles

Thermal cycling-induced evolution of structure and local mechanical properties in metallic glass

The influence of thermal treatment on the long-term mechanical properties of two different kinds of metallic glasses was investigated in this study. As model systems, bulk metallic glasses (BMGs) with the composition of Pd43Cu27Ni10P20 cycled 60, 200, and 500 times between 77 K and room temperature and Zr44Ti11Cu10Ni10Be25 cycled 300, 500, and 700 times have been employed. To ensure that meaningful steady-state properties have been reached, experiments have been performed three years after sample preparation. It was found that the thermal cycling of the Pd-based BMGs had induced rejuvenation when compared to the non-cycled sample, including a higher relaxation enthalpy, lower modulus, hardness, activation volume, and shear transformation zone volume. However, for Zr-BMGs, a rejuvenation to relaxation behavior is observed. Also, while Zr-BMGs showed significantly changes in yielding behavior upon thermal cycling, fluctuations in Pd-BMGs were negligible. We speculate that the observed substantial differences between these two samples might originate because Pd-BMG has a denser structure and higher fragility, making it more difficult to induce excess free volumes.

Mechanism of chip segmentation transition from shear slip to shear fracture of Zirconium-based bulk metallic glass in mechanical machining

An in-depth understanding of material cutting deformation forms the foundation for manufacturing Zirconium-based bulk metallic glass (Zr-based BMG) parts. This study explores the transition in chip segmentation mode from shear slip to shear fracture in Zr-based BMG during mechanical machining. The investigation covers chip micromorphology, cyclic cutting force, machined surface quality, and the themo-mechanical properties under various chip segmentation modes linked to cutting conditions. A size-dependent transition in chip segmentation has been observed in the cutting of Zr-based BMG. At a specific cutting speed, a smaller uncut chip thickness favors chip formation via shear slip, resulting in a smooth primary shear zone (PSZ). Conversely, a larger uncut chip thickness leads to chip shear fracture, with the PSZ displaying fishbone-like and dimple patterns. The integration of the modified cutting kinematic model with the free volume-dependent shear transformation zone (STZ) model suggests that the transition from shear slip to fracture is due to an increase in uncut chip thickness, which causes a greater STZ volume in the PSZ. When the STZ volume surpasses a threshold, the intrinsic structural recovery of material from shearing cannot fully compensate for the free volume softening, leading to excessive free volume that ultimately triggers shear fracture. During the chip shear fracture process, the evolution from fishbone-like patterns to dimples in the PSZ is attributed to faster crack propagation at greater uncut chip thickness or higher cutting speeds. In practical applications, it is advisable to avoid chip shear fracture during machining to maintain high-quality machined surface quality.

Tailoring rejuvenation behavior of Zr-based metallic glass upon deep cryogenic cycling treatment

The rejuvenation behavior of a Zr55Cu30Ni5Al10 (at.%) metallic glass upon deep cryogenic cycling treatment was investigated. The sample was rejuvenated to a larger extent with increasing cyclic number, resulting in a higher enthalpy of relaxation and a lower density. The rejuvenation seems to be saturated after 200 cycles, which may originate from the limited content of free volume in the amorphous structure. The sample subjected to a higher cyclic number showed a lower hardness and better plasticity due to a larger shear transformation zone (STZ) volume. In this case, a lower shear plane formation energy facilitates the generation of multiple shear bands. Furthermore, it is found that the content of free volume has linear relationship with the plastic strain and the STZ volume. The results from molecular dynamics simulation also demonstrates that the atomic volume saturates with a higher degree of rejuvenation at a higher cooling rate of the simulated sample.

In situ investigation of the evolution of shear transformation zones with different elastic tensile strains in metallic glass

Shear transformation zones are usually thought as the fundamental unit characterizing microscopic atomic rearrangement motion during deformation in amorphous materials and the corresponding evolution process determines the macroscopic mechanical behaviors. However, the evolution process of shear transformation zones under tensile loading are difficult to be experimentally detected. Here, one in-situ tensile platform coupled with nanoindenter was introduced to conduct a series of nanoscale continuous loading and creep tests on Cu50Zr50 metallic glass film under different pre-tensile strains within the elastic deformation region. The effects of loading rate and pre-tensile strain on the continuous loading and creep behaviors were systematically investigated. Moreover, the corresponding shear transformation zone volume, the activation volume and the relaxation time spectrum during tensile deformation were determined based on the Cooperative Shearing model and Maxwell-Voigt model. It was found that both of the shear transformation zone volume, activation volume and the peak intensity for the relaxation time spectrum increase with pre-tensile strain. Finally, one scheme is proposed to illustrate the microscopic mechanism for the evolution of shear transformation zones during tensile deformation. The current study offers one effective experimental strategy to in-situ investigate the mechanical behaviors and to reveal microscopic deformation mechanism in amorphous materials.

Shear Transformation Zone and Its Correlation with Fracture Characteristics for Fe-Based Amorphous Ribbons in Different Structural States

Fe-based amorphous alloys often exhibit severe brittleness induced by annealing treatment, which increases the difficulties in handling and application in the industry. In this work, the shear transformation zone and its correlation with fracture characteristics for FeSiB amorphous alloy ribbons in different structural states were investigated. The results show that the bending strain decreases sharply with the annealing temperature increase, accompanied by decreased shear band density and the induced plastic deformation zone. Furthermore, the microscopic fracture surface features transform from a micron-scale dimple pattern to nano-scale dimples and periodic corrugations. According to nano-indentation results, the strain rate sensitivity and shear transformation zone volume change significantly upon annealing treatment, which is responsible for the deterioration of bending ductility and the transition of microscopic fracture surface features.

Synthesis and mechanical properties of highly structure-controlled Zr-based metallic glasses by thermal rejuvenation technique

A novel thermal rejuvenation treatment facility for Zr-based bulk metallic glass (BMG) was developed, consisting of a rapid heating and indirect liquid nitrogen quenching process. The re-introduction of free volume into thermally rejuvenated BMG results in more disordered state. The rejuvenation improves ductility, implying that the re-introduced free volume aids in the recovery of the shear transformation zone (STZ) site and volume. Actually, it is confirmed that relaxation significantly reduces STZ volume; however, it is recovered by thermal rejuvenation. Molecular dynamics simulations also indicate that rejuvenation enhances homogeneous deformation. The current findings indicate that the thermal rejuvenation method is extremely effective for recovering or improving the ductility of metallic glass that has been lost due to relaxation.

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.

Indentation creep dynamics in metallic glasses under different structural states

In this study, the effects of the structural energy states (as-cast and relaxed) of Zr50Cu40Al10 metallic glass were investigated using depth-sensing nanoindentation. In terms of cooperative shear model, we performed experimental characterization of shear transformation zone for plastic flow of metallic glass. The shear transformation zone volume of the relaxed metallic glass is larger than that of as-cast one, while the strain rates sensitivity displays the opposite tendency. The plastic deformation in the creep stage of nanoindentation originates from the “delayed plasticity” inherited from the loading process. The creep depth shows strain rates and microstructural states dependence. The characteristic relaxation time and creep depth limit were determined by fitting the displacement versus time curves with Kelvin model are employed to characterize the creep dynamics of metallic glasses. It was found that the softening effects in the plastic deformation originates from the collective movements of the deformation carriers (shear transformation zones). The different softening behaviors were discussed from the perspective of soft zones in the framework of the free volume model. Our experimental as well as the theoretical analysis deepens the fundamental understanding of plasticity deformation mechanism in metallic glasses.

Effect of nanoindentation experimental parameters on the estimation of the plastic events in metallic glasses employing various analysis methods

The term Shear Transformation Zone (STZ) refers to the atomic cluster that dissipates external stress through cooperative shear. The STZ is the process that describes the local atomic rearrangement mechanism of metallic glasses (MGs). Based on the cooperative shear model (CSM), the size of an STZ has been estimated using the rate jump method (RJM), nano-scale creep method (NCM), first pop-in statistical method (FSM) and serration-flow statistic method (SSM), respectively. In the nanoindentation process, different sizes of STZs can be elicited by employing different experimental conditions (indentation depth and loading rate). In addition, different characterization methods exhibit various degrees of response to changes in the experimental conditions. In order to study the heterogeneity of the STZ of MGs using different nanoindentation methods, in this study, three types of Zr-based MGs with different Poisson's ratios were used as the research objects, and the sizes of the STZs of these amorphous alloys were calculated using the previously mentioned methods. By applying different maximum loads and loading rates, the effect of the nanoindentation test parameters on the size of the metallic glasses' STZs were characterized using different nanoindentation methods. Under the same testing conditions, the volume of the STZs was calculated using each method that exhibited improvement with the increase of the Poisson's ratio. The calculated results obtained using the RJM, NCM and SSM were found to be much greater than those obtained using the FSM. With the increase in the loading rate, the results obtained using NCM decreased, whereas the statistic method results slightly increased. At the maximum load, the STZ volume calculated using the JRM, NCM and SSM increased significantly. The FSM was not affected by the maximum load. Finally, these differences are discussed from the aspects of the calculation principles and the activation and evolution of the STZ.

Investigation of a novel sub-surface work hardening phenomenon in micro-turning of Zr-based bulk metallic glass

This study investigates the influence of the micro-turning operation on the subsurface deformation-induced transformation. The variations in nanohardness, shear transformation zone (STZ) volume, and free volume in the machined subsurface of the Zr-based bulk metallic glass (BMG) have been characterized. Nanoindentation studies reveal that the machined subsurface is strain hardened, a unique phenomenon that has not been reported in Zr-based BMG machining. The work-hardened zone extends until a depth of ∼20 μm from the surface. The quantitative evaluation of the deformation-induced hardening has been done experimentally by nanoindentation followed by the free volume characterization via Differential Scanning Calorimetry (DSC). The Shear Transformation Zone (STZ) volume approach has been used to understand the changes in the machined subsurface induced due to microturning. The STZ volume in the machined region is lower than that in the undeformed zone for all the machining conditions investigated in this work. The enthalpy change obtained using DSC analysis for estimating the change in free volume content of the machined regions correlates with the STZ volume.

Nanoscale creep behavior and its size dependency of a Zr-based bulk metallic glass manufactured by selective laser melting

Nanoscale creep behaviors of metallic glasses (MGs) have gathered considerable interests in recent years, owing to their distinct atomistic mechanisms of plasticity. Additive manufacturing (AM) is a burgeoning technique for manufacturing MGs, the nanoscale creep characteristics and creep mechanism of the AM-ed MGs, however, remain ambiguous. In this work, the nanoscale creep behavior and creep size effect of a selective laser melted (SLM-ed) Zr-based MG were investigated by using nanoindentation. The creep stress exponent (n) and shear transformation zone (STZ) volume, as the indicators of the creep mechanism, were estimated. The n values are in the range of 2–3, and STZ volumes are around 2 nm3. The creep resistance of the MG was found to decrease with the increasing applied peak loads. A potential mechanism for this creep size effect was revealed: the smaller of the STZ volume, as well as the greater ratio of plastic flow under the higher maximum load, are responsible for the decreasing tendency of creep resistance. This research gives a comprehensive understanding of the atomistic mechanisms in the AM-ed MG during the nanoscale creep deformation process and can serve as a reference for improving the plasticity and further engineering applications.

Influence of Defects to Zr65Cu18Ni7Al10 Bulk Metallic Glass Properties Under Dynamic Compression

The Zr65Cu18Ni7Al10 bulk metallic glass with smaller diameter exhibits higher fracture strength under dynamic compression, which is ascribed to concentration of flow defect. The density of shear bands in the sample surface will increase with decreasing of the diameter, whereas, average distance and width of tear ridges in the fracture surface will increase with larger diameter. In addition, the volume of shear transformation zone can be estimated, which presents a ductile-to-brittle transition with the change of diameter. The physical graph of shear transformation zone can be obtained from the experimental analysis.

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

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.

Time-dependent deformation mechanism of metallic glass in different structural states at different temperatures

There is limited understanding of time-dependent plastic deformation behavior of amorphous alloys as a function of their structural state. Here, the creep behavior of Zr52.5Ti5Cu17.9Ni14.6Al10 bulk metallic glass was investigated in its as-cast and relaxed states using nanoindentation technique with applied load in the range of 500–1500 mN and temperature in the range of room temperature to 573 K. The creep displacement increased with increasing load and temperature since the creep process is thermally activated and diffusion rate is enhanced at elevated temperature and higher load. The creep strain rate sensitivity, which is a measure of the creep mechanism, increased with increase in temperature and decrease in applied load. This was attributed to the transition from localized to more homogeneous creep. Reduction in free volume for the relaxed alloy resulted in lower creep displacement and larger strain rate sensitivity compared to its as-cast counterpart. The results suggest that diffusion-based deformation dominate at higher temperature in contrast to shear transformation mediated plasticity at room temperature. The volume of shear transformation zone for the metallic glass was calculated using cooperative shearing model and correlated with the structural state for fundamental insights into the deformation process as a function of temperature and load.

Effect of deep cryogenic cycling treatment on the microstructure and mechanical properties of Ti-based bulk metallic glass

This study investigates the effect of deep cryogenic cycle treatment (DCT) on the microstructure and mechanical behaviors of Ti33Zr30Cu9Ni5.5Be22.5 bulk metallic glass (BMG). The results revealed that after the DCT, the relaxation enthalpy increased, whereas the Ti33Zr30Cu9Ni5.5Be22.5 BMG still maintained a complete amorphous nature. As the cycle numbers increased, the room-temperature plasticity of the Ti33Zr30Cu9Ni5.5Be22.5 BMG represented a tendency to increase first and then decrease. When the cycle numbers increased to 30, the Ti33Zr30Cu9Ni5.5Be22.5 BMG obtained the maximum compressive plasticity of 7.8%, which is 5.6 times that of the as-cast specimen, accompanied by a higher strength. Moreover, the nanoindentation results demonstrated that the Ti33Zr30Cu9Ni5.5Be22.5 BMG that underwent the DCT possessed a larger pop-in size, a lower initial pop-in force and a lower hardness. The shear transformation zone (STZ) theory was employed to elucidate the plasticising mechanism induced by the DCT for the Ti33Zr30Cu9Ni5.5Be22.5 BMG. The larger STZ volumes and the lower shear band formation energy induced through the DCT promoted the densification of multiple shear bands, which is the principal reason for the excellent plasticity after the DCT.

Chemical heterogeneous structure and internal record of deformation behavior in Cu-Fe-Zr metallic glasses

Although chemical structural heterogeneities were induced into metallic glasses (MGs) to improve their ductility, the nature of the influence of the heterogeneous structure on their deformation behavior is still unclear. Here, by using microalloying elements to tailor the microstructure in Cu-Fe-Zr MGs, the formation of heterogeneous structure and the correlation between the heterogeneous structure and the deformation behavior are discussed. The results show that a honeycomb-like heterogeneous structure is formed in the (Cu0.5Fe0.5)40Zr59X1 (XNb and Ta) alloys. From a viewpoint of dynamic, the nature of the microalloying effect on the microstructure has been analysed. The deformation behavior of (Cu0.5Fe0.5)40Zr59X1 MGs with a high population of nanoscale glassy particles (NGPs) is investigated by nanoindentation and bending test. These glassy particles play an interesting role in the internal record of the local structure evolution. It was found that NGPs contiguous to shear bands (SBs) coarsen by Ostwald ripening. However, NGPs within SB would dissolve due to direct mechanical shearing and mixing. It indicates that the volume of shear transformation zone decreases obviously in the heterogeneous MGs due to the high fraction of interfaces. This work is of significance in understanding the relationship between structure and property of MGs.

Effect of deep cryogenic cycling treatment on shear transformation zone volume and size of Zr-based metallic glass

Effect of deep cryogenic cycling treatment on shear transformation zone volume and size of Zr-based metallic glass

A new deformation mechanism of amorphous/amorphous laminates at deep nanoscale

Abstract The plastic deformation and mechanical response of ZrCu/ZrCuNiAlSi amorphous/amorphous nanolaminates (A/ANLs) with various individual layer thickness (h) were investigated by nanoindentation test. As h decreases, deformation behavior transformed from shear-banding events to homogeneous co-deformation of the constitute layers. The homogeneous deformed A/ANLs exhibit the maximum shear transformation zone volume and unexpected minimum strain rate sensitivity m, suggesting a distinct deformation mechanism from others. Interface effected zones that involved in the deformation of the A/ANLs with minimum h but not those with higher h was proposed to play a dominating role in determining the homogeneous deformation and the unusual thermally activated parameters.

Investigation of shear transformation zone and ductility of Zr-based bulk metallic glass after plasma-assisted hydrogenation

The shear transformation zone (STZ) and ductility of Zr-based bulk metallic glass with varying hydrogen content were analyzed using serrated flow in nanoindentation, with each serration representing the formation and expansion of one or more shear bands. An exponential model was employed to extract the serrations and obtain a certain range of shear stress, and STZ size was estimated via thorough statistical data analysis. The experimental results demonstrated a striking decrease in the serration number and width of the load-displacement curve after hydrogenation, which is similar to the effects of the nanoindentation loading rate. The calculated STZ volume and atomic number were in good agreement with those reported in the literature. The STZ volume, size, activation energy, and activation volume also increased with the increased hydrogen content, leading to shear band formation and proliferation as well as increased shear band density, plastic zone, and plasticity. At the microscopic level, the hydrogen addition accelerated free volume formation and enhanced flow ability between the atoms, as characterized by the improvement of plasticity. At the macroscopic level, hydrogen addition promoted shear band nucleation and multiplication and increased shear band density, plastic zones, and plasticity. The metallic glass plasticity was greatly improved after hydrogenation as assessed via compression tests at room temperature, confirming the rationale of the experimental results.