Metallic Glass Specimens Research Articles

Microstructural evolution of laser-welded Ti-based bulk metallic glass

A fiber laser beam is used to butt-weld of identical Ti43·3Zr21·7Ni7·5Be27.5 (at.%) bulk metallic glass (BMG) specimens. Various tests, such as SEM, TEM, XRD, DSC, and Vickers hardness, are conducted on the welded specimens to analyze the crystalline phases generated at the HAZ of each specimen during welding. The heat-affected zone (HAZ) can be obviously identified in the welded specimens owing to the clustering of the crystalline phases. The HAZ is composed of a heterogeneous group of crystalline phases, such as Ti–Zr–Ni polygonal phase, Ti and Ti–Zr dendritic phase. These crystalline phases decrease the glass transition temperature (Tg) of the welding fusion zone (WFZ) and increase the hardness at the HAZ.

Recreating the shear band evolution in nanoscale metallic glass by mimicking the atomistic rolling deformation: a molecular dynamics study.

Rolling processes are extensively used to induce network of shear bands (SBs) in the bulk metallic glasses, which in turn enhances the overall plasticity of the specimen. However, the atomic-level understanding of shear band formation/propagation mechanism during mechanical processing is still limited. In this perspective, we have developed a molecular dynamics (MD) simulation model to recreate the rolling deformation process and investigate the SB formation in Cu-Zr metallic glass (MG) specimen. Results have shown that dense and concentrated primary SBs along with secondary branching are formed during cryo-rolling, whereas a scattered and thicker SBs are formed during hot rolling process. Meanwhile, Voronoi cluster analysis revealed that the high five-fold symmetry clusters tend to decrease, while the crystalline-like cluster increases during the hot rolling process. These findings from the study are in good agreement with previous experimental studies substantiated in literature, which shows that the model correctly predicts the shear-banding phenomenon.

Size-temperature equivalence in tensile deformation of metallic glass

Abstract Tensile fracture of irradiation-free metallic glass specimens with diameters ranging from 100 nm to 500 μm is investigated at different temperatures. A gradual change in fracture morphology from vein-pattern to completely smooth fracture surface to necking is observed with decreasing sample size and testing temperature. The size-temperature equivalence in the entire length scale can be described by considering the thermal effects in shear localization of metallic glasses. We construct an empirical model based on the shear band heating and velocity formulations to qualitatively describe the size and temperature effects on the fracture morphology. Our results suggest that the widely reported size-dependent transition from shear-localized to homogeneous flow in metallic glasses is fundamentally different from the high temperature homogeneous viscous flow. The plastic deformation in nanoscale samples is spatially localized in embryonic shear bands, which never mature to the propagation stage due to lack of heat content.

Creep behavior of polycrystalline Al (metal)-Cu50Zr50 (metallic glass) cold welds

Creep properties of cold weld polycrystalline Al (metal)-Cu50Zr50 (metallic glass) specimens are investigated by molecular dynamics (MD) simulations. The EAM (Embedded Atom Method) potential is used for modelling the interactions between Al, Cu and Zr atoms. Cold welding is carried out at a velocity of 30 m/s and interference (contact time) of 1.3 nm. The Creep behavior study of welded sample is carried out at different range of stress (100 MPa–500 MPa) and temperatures (100 K–500 K) for analysing the effect of stress and temperature on creep behavior. The creep curve shows very short primary creep and significant steady-state creep. Also with an increase in temperature or stress the steady-state creep rate increases. At lower temperature (100 K) the creep deformation mechanism is by dislocation slip and at a higher temperature (500 K) and higher stress (500 MPa) it is by diffusion. The stress exponent ‘n’ has two values, at lower stress n = 1.56 and at higher stress n = 5.82 respectively.

Study on stochastic nature of plasticity of Cu/Zr metallic glass micropillars

Abstract Micro-sized metallic glass pillars with a diameter ranging 2.5∼7.5 μm were subjected to compression experiments using a nanoindentation tester equipped with a flat punch, to characterize their deformation behaviors. The strain-stress response and scattering mechanical properties were studied in statistics. The intermittent and trigger-aftershock shear avalanches exhibited a stochastic nature of plasticity of metallic glass specimens under microscale. The size distribution of strain bursts versus the stress where they occurred indicated the flow dynamics as a self-organized behavior with collective motion of multiple shear bands, and the maximum burst size increased with applied stress following in a power-law manner independent of burst order and pillar size. According to the experimental observation, universal features were proposed to characterize the current jerky deformation, meanwhile a Monte Carlo simulation method was developed to predict the stochastic plasticity of metallic glass under microscale.

Effects of irradiation spectrum on the microstructural and mechanical properties of bulk metallic glasses

Zr52.5Cu17.9Ni14.6Al10Ti5 (BAM-11) and Cu60Zr20Hf10Ti10 bulk metallic glass (BMG) specimens were ion irradiated to characterize and compare their irradiation induced microstructural and mechanical property evolution. For the ion irradiations, samples of each BMG were exposed to 9 MeV Ni3+ and 5.5 MeV C+ ions to a midrange (∼1.5 μm depth) dose of 0.5 displacements per atom (dpa) at temperatures ranging from room temperature to 360 °C in order to examine potential effects associated with low (5.5 MeV C) and high (9 MeV Ni) average primary knock on energies. BAM-11 BMG samples were also irradiated by neutrons (E > 0.1 MeV) at ∼70 °C to a fluence of 1.4 × 1020 n/cm2 (dose of 0.1 dpa). Importantly, no pronounced irradiation spectrum effects were observed for the nanoindentation mechanical properties of the two BMGs following ion irradiation. However, significant softening was observed in the BAM-11 BMG sample irradiated by neutrons, while annealing of the unirradiated samples led to a marked increase in hardening. Results of the nanoindentation experiments indicate that softening is caused by irradiation induced creation of free volume defects, while the hardening is caused by their annihilation.

Investigation of the mechanical and microstructural evolution of a Cu based bulk metallic glass during ion irradiation

Ion irradiation and annealing experiments were performed on Cu60Zr20Hf10Ti10 bulk metallic glass (BMG) specimens to investigate their irradiation- and temperature-induced microstructural and mechanical property evolution. For the ion irradiations, samples were exposed to 9 MeV Ni3+ ions to a midrange (~1.2 μm depth) dose of 10 displacements per atom (dpa) at temperatures ranging from room temperature to 360 °C (the corresponding peak dose at ~2.8 μm depth was ~25 dpa). Bulk X-ray diffraction (XRD) and transmission electron microscopy (TEM) characterization revealed that the alloy did not crystallize during irradiation up to 290 °C but did partially crystallize at 360 °C. XRD analysis revealed that the crystallization which occurred in the sample irradiated at 360 °C was caused by thermal effects instead of irradiation displacement damage. Subsequent Rietveld refinement analysis of the XRD measurements revealed the presence of two distinct crystal phases, namely a CuTiZr hexagonal structure belonging to the P63/mmc space group and a CuTi tetragonal structure belonging to the P4/mmm space group. Nanoindentation experiments revealed that no pronounced hardness changes occurred in the specimens irradiated at room temperature and 290°C, although significant hardening was observed in the sample irradiated at 360 °C. The significant increase in the hardness at 360°C was ascribed to thermally induced partial crystallization of the alloy instead of the ion irradiation. In general, the results of the nanoindentation experiments and XRD characterization suggest that although the Cu BMG exhibits good stability during irradiation at temperatures up to 290 °C it is not suitable for irradiation environments where the temperature is 360 °C for extended periods of time. The Lam and Chong extrapolation method, which has been used to study the indentation size effect (ISE) in amorphous alloys, was employed to quantify how irradiation and temperature affect this type of behavior in the BMG. However, the poor linear fitting of the indentation hardness data by this model indicate that a new ISE model is likely needed to quantify indentation hardening in BMGs.

Simulations of Shear bands in Metallic Glasses with Mesoscale Modeling

Metallic glasses (MGs) have many attractive advantages such as high yield strength, high hardness, high wear resistance, high fracture toughness, and low friction coefficient. However, the localized deformation and poor ductility due to shear bands prohibit the further applications of MGs. The localized deformation could be controlled by the residual stresses in MGs. In this study, a mesoscale model which combines the kinetic Monte Carlo algorithm (kMC) and the finite element method (FEM) is constructed to investigate the effects of residual stresses on deformation behaviors of MGs. The developed computational framework is implemented by integrating in-house Matlab program for kMC and commercial finite element software, Abaqus. In the mesoscale model, a shear transformation zone (STZ) with nanoscale volume is the basic deformation unit and is represented by an element in the FEM model. Each element whether is transformed into an STZ is determined by the kMC algorithm according to its energy state. The developed mesoscale model can simulate the deformation process of MGs with the time and length scales which are greater than those by atomistic modeling. Finally, a sanity check was performed to verify the accuracy of FEM in solving Eshelby’s inclusion problem. Then the developed computational framework is applied to simulate the uniaxial tension test of the metallic glass specimen with different residual stresses. Simulation results demonstrate that the residual stresses could dominate the occurrences of STZs and change the stress–strain curve of MGs.

Evolution of the microstructural and mechanical properties of BAM-11 bulk metallic glass during ion irradiation and annealing

Ion irradiation and annealing experiments were performed on Zr52.5Cu17.9Ni14.6Al10Ti5 “BAM-11” bulk metallic glass (BMG) specimens to evaluate their irradiation- and temperature-induced microstructural and mechanical property evolution. For the ion irradiations, samples were exposed to 9 MeV Ni3+ ions to a midrange (∼1.5 nm depth) dose of 10 displacements per atom (dpa) at temperatures ranging from 25 to 360 °C. A separate set of samples were annealed at 150 °C, 200 °C and 300 °C with respective heating times of 96, 72, and 48 h. Bulk X-ray diffraction (XRD) and transmission electron microscopy (TEM) characterization revealed that the alloy did not crystallize during irradiation up to 290 °C, although partial crystallization occurred at 360 °C throughout the unirradiated and irradiated regions of the sample. Transmission electron microscopy (TEM) characterization suggested that some of the irradiated region retained an amorphous structure, supporting the idea of partial crystallization. XRD analysis revealed that the crystallization which occurred in the sample irradiated at 360 °C was caused by thermal effects, and not irradiation displacement damage, and may be due to slight impurity contamination in the ingot. Nanoindentation experiments showed that only a slight amount of hardening was observed in the specimen irradiated at room temperature and 290 °C. However, significant hardening occurred in the sample irradiated at 360 °C (∼18% increase) as well as the unirradiated annealed specimens. For the sample irradiated at the highest temperature, the substantial increase in hardness was attributed to the partial crystallization of the alloy due to thermal, rather than irradiation effects. Overall, the results of the XRD and nanoindentation characterizations indicate good stability during irradiation at 25–290 °C but suggest that the BAM-11 BMG is not suitable for irradiation environments where temperatures exceed 300 °C for prolonged periods of time. Three different extrapolation models were employed to study how irradiation and annealing modify the indentation size effect (ISE) in the BAM-11 BMG. The poor linear fitting, as exhibited by all of the underlying equations, indicate that a new ISE model is needed to quantify nanoindentation mechanical properties in BMGs.

Additive manufacturing of an iron-based bulk metallic glass larger than the critical casting thickness

Fe-based bulk metallic glasses (BMG) are of increasing research interest, driven in part by a unique combination of mechanical, magnetic and chemical properties. However, the maximum thickness and geometry of BMGs achievable in traditional manufacturing processes is limited. This work examines the capabilities of laser based powder bed additive manufacturing (AM) to produce relatively large Fe-based bulk metallic glass specimens. AM fabricated specimens exceed the critical casting thickness of the material by a factor of 15 or more in all dimensions. Resulting microstructural and mechanical properties are reported. Despite decreasing quench effect with increasing build thickness, X-ray diffraction analysis suggests that a fully amorphous structure was maintained throughout the build. However, a low concentration of sparsely distributed nano-grain clusters was discovered using a high-resolution electron backscatter diffraction scan. The results pave the way for novel applications of metallic glasses achievable through appropriate material design and optimization of existing additive manufacturing processes.

Anisotropy in a bulk metallic glass induced by uni-directional heat flow

In this work, a controlled uni-directional heat flow was realized during the bulk metallic glasses preparation by using a vertical Bridgman solidification apparatus. Fully amorphous Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass specimens were prepared. The obvious difference in hardness between the cross and longitudinal sections of the as-prepared Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk metallic glass specimens was identified, indicating strong mechanical anisotropy. The observed anisotropy can be ascribed to the uni-directional heat flow during the process of Bridgman solidification. The mechanical anisotropy may be explained by the free volume theory.

Plastic deformation and failure mechanisms in nano-scale notched metallic glass specimens under tensile loading

In this work, numerical simulations using molecular dynamics and non-local plasticity based finite element analysis are carried out on tensile loading of nano-scale double edge notched metallic glass specimens. The effect of acuteness of notches as well as the metallic glass chemical composition or internal material length scale on the plastic deformation response of the specimens are studied. Both MD and FE simulations, in spite of the fundamental differences in their nature, indicate near-identical deformation features. Results show two distinct transitions in the notch tip deformation behavior as the acuity is increased, first from single shear band dominant plastic flow localization to ligament necking, and then to double shear banding in notches that are very sharp. Specimens with moderately blunt notches and composition showing wider shear bands or higher material length scale characterizing the interaction stress associated with flow defects display profuse plastic deformation and failure by ligament necking. These results are rationalized from the role of the interaction stress and development of the notch root plastic zones.

Improvement of dynamic notch toughness for the Zr56Co28Al16 bulk metallic glass by local pre-deformation

Mode I dynamic three-point bending fracture tests are conducted on both as-cast and local pre-deformed Zr56Co28Al16 bulk metallic glass specimens. Similar to quasi-static cases, the notch toughness of the pre-compressed specimens is improved compared with that of as-cast ones upon dynamic loadings, which is mainly attributed to the fact that the pre-compression can produce numerous shear bands, especially for main shear band, guiding and deflecting the cracks. In addition, the crack patterns and fractography are analyzed to evaluate the influence of local pre-existing shear bands on the dynamic fracture toughness.

Contribution of Nb towards enhancement of glass forming ability and plasticity of Ni-Nb binary metallic glass

In this present study, effect of Nb content on glass forming ability and plasticity of Ni-Nb metallic glass has been investigated using molecular dynamics simulation with embedded atom method (EAM). Ni-Nb metallic glass specimens having different atomic percentages of Nb content (i.e. ~15, 20, 25, 30, 38, 40.5, 45 and 50at.%) are prepared by fast cooling process (cooling rate~1012Ks−1) from their liquid phase. It is observed that increase in Nb content up till 38at.% causes improvement of glass forming ability of Ni-Nb binary system. On further increment of Nb content, glass forming ability is found to be slightly reduced or remain almost same. Voronoi cluster analysis has been performed to evaluate the variation in short range ordering with increasing Nb content in Ni-Nb glass. Simulated tensile tests are also performed, and it reveals that higher Nb content actually helps in augmentation of the plastic behavior of the Ni-Nb metallic glass.

Notch sensitivity in nanoscale metallic glass specimens: Insights from continuum simulations

Recent experiments have shown that nano-sized metallic glass (MG) specimens subjected to tensile loading exhibit increased ductility and work hardening. Failure occurs by necking as opposed to shear banding which is seen in bulk samples. Also, the necking is generally observed at shallow notches present on the specimen surface. In this work, continuum finite element analysis of tensile loading of nano-sized notched MG specimens is conducted using a thermodynamically consistent non-local plasticity model to clearly understand the deformation behavior from a mechanics perspective. It is found that plastic zone size in front of the notch attains a saturation level at the stage when a dominant shear band forms extending across the specimen. This size scales with an intrinsic material length associated with the interaction stress between flow defects. A transition in deformation behavior from quasi-brittle to ductile becomes possible when this critical plastic zone size is larger than the uncracked ligament length. These observations corroborate with atomistic simulations and experimental results.

The limited annealing treatment of the Zr55Cu30Al10Ni5 metallic glass below its glass transition temperature

Annealing treatment below the glass transition temperature of Zr55Cu30Al10Ni5 bulk metallic glass (BMG) specimens are investigated. Every 10K temperature interval isothermal limited annealing treatment time will be selected to draw up time–temperature–transformation (T–T–T) curve. The apparent activation energy of crystallization is about 325±7kJ/mol, higher than the values of supercooled liquid region conditions. Surface hardness of the samples before and after the limited annealing treatment are tested by nanoindentation, where the spherical-like crystallization morphology indicate the higher standard deviation of surface hardness of the BMG after the limited annealing treatment. The crystallization kinetics of the Avrami exponent is 4 in accordance with spherical-like crystallization morphology. A short review on mathematical setting of the problem could distinctly give the differences of the different crystallization behaviors at different temperature regions. Avrami exponent is from 4 to 3, explaining the crystallization morphology is from spherical-like to dendritic, could be ascribed to the concepts of symmetry breaking.

Investigation of the mechanical behavior of ion irradiated Ni-free Ti-based metallic glass by nanoindentation

The mechanical response of ion irradiated metallic glass specimens with nominal composition of Ti40Cu32Pd14Zr10Sn2Si2 was investigated by nanoindentation. Surfaces of specimens fabricated by the melt spinning process were irradiated with 4MeV Fe2+ ions at 25°C over a range of fluences from 1×1012 to 1×1015ions/cm2. Irradiations were also performed at temperatures up to 300°C. Nanoindentation experiments were performed to obtain values of reduced elastic modulus and hardness and to investigate the material's shear band behavior. Irradiation was found to result in a lower reduced elastic modulus and hardness, however was found to increase homogeneous deformation by suppression of shear band activity.

Deformation behavior of a Zr-based bulk metallic glass under a complex stress state

In this work, the deformation behavior of a Zr-based bulk metallic glass specimen under a tailored complex stress state was investigated. When the specially designed specimen was subjected to a uniaxial tensile loading, the in-plane stress distribution along the symmetrical plane was hyperbolic, with the presence of both compressive and tensile stresses. The evolution of deformation in the specimen was found to consist of three stages: the shear band initiation stage (stage I), the shear band multiplication stage (stage II) and the final rapid fracture stage (stage III). In a conventional tensile specimen with uniformly distributed stresses, the specimen fractures (stage III) catastrophically after stage I. However, in the specially designed specimen, the complex stress distributions result in not only more stable shearing processes in stage I, but also the evolution of stage II before stage III, which causes the formation of a large number of shear bands. The findings shed more light on the deformation mechanisms of bulk metallic glasses under complex stress states.

Effects of Ar ion irradiation on the diffusion bonding joints of Zr55Cu30Ni5Al10 bulk metallic glass to aluminum alloy

1keV low energy argon ion beam was used to irradiate the surface of Zr55Cu30Ni5Al10 bulk metallic glass. Bulk metallic glass specimens irradiated for different times were used to join with Aluminum alloy by argon ion activated diffusion bonding. Compared with the joints using the as-cast bulk metallic glass, the tensile strength of the joints using irradiated specimens had a drastic improvement. Moreover, a crystalline layer was obtained in the BMG side of the joint interface using irradiated specimens. The improved joint quality was attributed to the local heating resulting from a crystallization behavior of irradiated surface layer which increases the probability of jumping the barrier to form the atom bonds, as well as the significant enhancement of the atomic diffusivity after crystallization.

超音波疲労試験機を用いたZr<sub><font size="-1">55</font></sub>Al<sub><font size="-1">10</font></sub>Ni<sub><font size="-1">5</font></sub>Cu<sub><font size="-1">30</font></sub>金属ガラスの超高サイクル疲労特性

In this work, the ultra-high cycle fatigue behavior of Zr55Al10Ni5Cu30 metallic glass was investigated by using ultrasonic fatigue testing method. Test alloy rods with a diameter of 8mm were prepared by tilt-casting with copper mold. Test specimens were machined to hourglass shape type (the minimum diameter ; 2.0mm), and after machining the surface of specimens was manually polished with polishing paste. The specimens were tested at a frequency of 20kHz under a stress ratio of -1. The fatigue limit (σw) (half a total stress amplitude) and fatigue ratio (σw/σB) of the alloy showed 893MPa and 0.53, respectively. Furthermore, we did not see any fish-eye like crack-growth morphology on fracture surfaces. The initiation region of fatigue crack was always observed at the side surface of the specimens. A viscous-flow fracture region with large dimples between the fatigue crack-growth region and the final unstable fracture region was observed. This peculiar fracture morphology may be due to adiabatic heating of the metallic glass specimens during the fracture, resulting in transition to supercooled liquid, under extremely high frequency of fatigue stress using ultrasonic loading.