Reliability of the plastic deformation behavior of a Zr-based bulk metallic glass

It is important to interpret the scattering of the plastic deformation behavior data for the structural-applications of bulk metallic glasses (BMGs), however, few studies have focused on statistical analysis of the variation and reliability of the plastic deformation behavior of BMGs. In this work, statistical analyses show unavoidable large variations in the maximum nominal strains of as-cast BMGs, although they exhibited greatly-enhanced average values of the maximum nominal strains with reduced sample sizes and in the presence of stress gradients. The large variations are attributed to the intrinsic variability of the atomic arrangements stemming from the solidification processes. Nevertheless, the investigations show enhanced cut-off nominal strains (safety threshold) in the specimens with stress gradients. The findings suggest that, despite large variations in the plastic deformation behavior, BMGs are still reliable in practical structural-applications where the materials always deform under more complex stress states.     

Oxidation features of plastic deformed Zr-based bulk metallic glass

The surface morphology of in situ deformed, pre-deformed, undeformed and crystallized Zr_47.9Ti_0.3Ni_3.1Cu_39.3Al_9.4 bulk metallic glass in air at elevated temperatures are investigated to improve the understanding of the effect of plastic deformation on the oxidation. The plastic deformation is performed through instrumental macroindentation, while could provide a well developed shear offset pattern around the indents at lower temperatures. The oxidation experimental results show that the in situ formed and pre-existed shear bands are more susceptible to oxidation. The plastic deformation region during homogenous deformation has also a significant oxidation sensitivity compared with undeformed material. To clarify the oxidation mechanism of shear band, specimens with surface scratches were also studied. The mechanism for the preferential oxidation of shear bands and of the plastic deformation region is discussed.     

Tribological behaviors of Zr-based bulk metallic glass versus Zr-based bulk metallic glass under relative heavy loads

As a novel engineering material, bulk metallic glass (BMG) has received much attention. However, the knowledge concerning the tribological behavior of BMG versus BMG under relatively heavy loads is still insufficient. In this study, Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 metallic glass pins and discs were prepared by copper-mold suction casting. The dry sliding friction and the wear characteristics of the as-cast Zr-based BMG versus Zr-based BMG were tested under loads of 100, 125 and 150 N, respectively, using a pin-on-disc tribological apparatus at room temperature. The worn surfaces were studied by scanning electron microscopy in order to identify the wear mechanisms. The results showed that both the coefficient of friction and the wear rate increased with both the normal load and the rotational sliding velocity. X-ray diffraction patterns recorded after the tribological experiments indicated that no sliding-induced crystallization occurred. Transmission electron microscopy was also used to confirm the amorphous of the BMGs after sliding tests. In addition, the wear mechanisms changed with the experimental conditions. For a normal load of 100 N, the main mechanisms were abrasive wear, slight grooves and micro-cracks. For higher loads, adhesive wear was predominant, accompanied by abrasive wear and deeper grooves and more micro-cracks. When the rotational sliding velocity was increased, the dominant wear changed from slight grooves to viscous flow and adhesive wear.     

A study on the surface severe plastic deformation behavior of a Zr-based bulk metallic glass (BMG)

A surface treatment process, which can generate the severe plastic deformation in the near-surface layer of crystalline materials, is successfully applied on the Zr₅₀Cu₄₀Al₁₀ bulk metallic glass (BMG). The experiment is implemented using 20 WC/Co balls with a velocity of about 5 m/s to bombard the surface of the samples in a purified argon atmosphere. The plastic-flow deformation in the unconstrained sample edge was observed, which exhibits the good intrinsic ductility of this material under the experimental condition. In the sub-surface layer, the bombardment-induced shear-band operations generate the extrusion and intrusion marks on the side face. Differential scanning calorimetry (DSC) shows that the free volumes of the deformed BMG have increased, and possible crystallization may occur during the process. X-ray diffraction (XRD) and synchrotron high-energy X-ray diffraction techniques were used to inspect the possible crystal phase. A nanoindentation test shows that on the side surface, the hardness increases and, then, decreases with the distance from the processed surface. Four-point-bending-fatigue behavior has been studied and related to the modified surface structure and the compressive residual stress introduced by the surface treatment.     

Boss formability of a Zr-based bulk metallic glass

Macroscopic solid-to-solid hot formability of a Zr-based bulk metallic glass has been investigated in this study by applying a boss forming process within supercooled liquid region (SLR). The morphology and microstructures after boss forming were first examined by using an optical microscopy (OM), a field emission scanning electron microscope (FE-SEM), X-ray diffraction (XRD), and transmission electron microscope (TEM). The variation of thermal properties before and after boss forming was also analyzed by using a differential scanning calorimeter (DSC). The glass transition temperature, T_g and the crystallization onset temperature, T_{x, onset} were found to decrease with increasing test time and temperature. Macroscopic extrusion formability was found to match well with the results predicted through a processing map based on a dynamic materials model (DMM). The specimens were found to form well under the conditions of high temperature and slow punch jig speed. A FEM simulation study has also been carried out to understand the causes of piping problem. A sound boss without a pipe could be formed by reducing the flow rate difference in the contact and the core sites.     

Tribological studies of a Zr-based bulk metallic glass

In the present study, the tribological behavior of a Zr_52.5Cu_17.9Ni_14.6Ti₅Al₁₀ bulk metallic glass (BMG) was investigated using pin-on-disk sliding measurements under an argon atmosphere, rubbing against a type 303 stainless steel counterface. The tested pins and disk were examined using X-ray diffractometry, optical microscopy, profilometry, scanning electron microscopy and transmission electron microscopy. The results showed that the wear of the BMG pins was substantially larger compared with previous tests performed against a zirconia counterface. Strain softening was found in the near-surface region of the glassy pin due to the highly localized shearing. Frictional heating contributed to the occurrence of viscous flow and material transfer on the worn surface of the wear pin and the disk, respectively. Thus, the pin exhibited a severe adhesive-dominated sliding wear.     

Flow serration and shear-band viscosity during inhomogeneous deformation of a Zr-based bulk metallic glass

Uniaxial compressive behavior of Zr_64.13Cu_15.75Ni_10.12Al₁₀ bulk metallic glass at room temperature was characterized with high-sensitivity strain gauges directly attached to test samples. Displacement–time curves exhibited micron-size serrations (or bursts) after the onset of yielding, apparently associated with discrete shear band formation. Each displacement burst disclosed three-step (acceleration, steady-state, and deceleration) process in shear band propagation. The viscosity of a propagating shear band was found to be relatively low and, actually, in a similar range usually measured in the supercooled liquid region. A detailed analysis of the experimental results using a self-consistent Vogel–Fulcher–Tamann (VFT) equation based on free volume model suggested that shear band propagation was mainly resulted from free volume accumulation.     

Zr基块体非晶合金的微塑性成形性能

采用自行研制的微成形系统进行热压缩实验,分别研究成形温度、成形时间和冲头速度等对尺寸为d1 mm×1.5 mm的Zr41.2Ti13.8Cu12.5Ni10Be22.5块体非晶合金(Vit.1)在过冷液相区微塑性成形性能的影响规律。进一步研究了不同坯料尺寸对Vit.1块体非晶合金在过冷液相区超塑性成形性能的影响程度,结果表明流动应力随坯料尺寸的减小而降低。在此基础上,利用闭式模锻方法成形了分度圆直径为d1 mm的微型齿轮,采用SEM观察成形件的表面形貌,结果表明采用微成形方法可以获得尺寸精度较高的Vit.1块体非晶合金微型齿轮。     

Nanoscale creep deformation in Zr-based metallic glass

Nanoscale creep deformation in Zr₆₁Al_7.5Cu_17.5Ni₁₀Si₄ thin films was investigated via instrumented nanoindentation testing. Over three decades of indentation strain rate with varying penetration depth were used to evaluate the effects of applied strain rate and initial creep depth on the creep deformation within small volumes of the metallic glass. A critical penetration depth was identified, below which the creep deformation was dependent upon the applied strain rate, and above which the strain rate sensitivity of the creep deformation reached a plateau value. It was proposed that an interface diffusion mechanism dominated the creep deformation within the shallow depth regime, whilst a transition from the interface diffusion dominant mechanism to the intrinsic creep behavior of the Zr-based metallic glass occurred within the deep depth regime.     

Structural rejuvenation in a bulk metallic glass induced by severe plastic deformation

Using high-energy X-ray diffraction we examined the atomic structure in bulk metallic glass samples which underwent severe plastic deformation by the high-pressure torsion (HPT) technique. We obtained the atomic pair distribution function (PDF) and determined the changes in the PDFs due to deformation. The observed changes in the PDF clearly show structural disordering, which suggests structural rejuvenation by heavy deformation. However, the changes cannot be explained simply in terms of creating excess free volume, and they indicate that much more extensive atomic rearrangements take place as a consequence of deformation. Also, we suggest that the observed structural change may well be an outcome of local heating due to deformation and may not directly provide the knowledge of the atomistic mechanism of strain localization.     

Ball indentation tests for a Zr-based bulk metallic glass

Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ bulk metallic glass was characterized using ball indentation tests. Comparison of the data with the expanding cavity model revealed that the deformation is pressure insensitive for compressive loading. The plastic flow curves obtained from indentation tests showed perfectly plastic response and no strain rate sensitivity up to 15% strain.     

Size-independent strength and deformation mode in compression of a Pd-based metallic glass

We present quasi-static, room temperature compression data for Pd₄₀Ni₄₀P₂₀ metallic glasses, with specimen sizes ranging from the submicron to several millimeters in diameter. We observe no change in deformation mode over this range. At all sizes, plastic flow is localized in shear bands, which are accompanied by sudden strain bursts. This metallic glass shows only a modest increase in strength in going from bulk to micrometer-sized specimens. We show that stress gradients in tapered specimens can complicate measurement of the yield strength of metallic glasses in microcompression. Estimates of yield strength based on the minimum cross-sectional area implicitly assume that yielding is controlled by a maximum effective shear stress criterion. An alternative is the shear plane yield criterion, in which the minimum shear stress on the shear band trajectory determines yield. Application of this criterion in tapered microspecimens reinforces the notion that metallic glasses possess relatively size-independent mechanical properties.     

不均匀性抑制Zr基块状金属玻璃的脆化

研究了Zr_47.25Cu_47.25Al_5.5块状金属玻璃(BMG)在铸态和不同退火时间下的室温压缩塑性与微观结构之间的关系。结果表明,Zr_47.25Cu_47.25Al_5.5BMG具有3.96%的压缩塑性,经过0.5、1.5、3和6 h退火后,该合金的压缩塑性均有所提升。在0.5 h退火后,压缩塑性达到最高(5.84%)。铸态Zr_47.25Cu_47.25Al_5.5BMG的微观结构呈现出5 nm左右网状分布。经过6 h退火后,富Cu的网状区域尺寸为50 nm左右,并伴有少量晶化。这种微观结构的不均匀性使该铸态合金在室温具有一定的塑性,同时抑制了其在退火中因自由体积减小而导致的脆化。     

Microstructural studies of crystallization of a Zr-based bulk metallic glass

The evolution of the microstructure of a Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ alloy during isothermal annealing near the glass transition temperature was studied by transmission electron microscopy (TEM), small-angle neutron scattering (SANS), X-ray diffraction (XRD), and differential scanning calorimetry (DSC). In situ SANS experiments show that an interference maximum develops with isothermal annealing, which is attributed to decomposition in the undercooled liquid state. In agreement with these results, TEM observations show the formation of structural inhomogeneities in the glassy alloy in the early stages of annealing, which are correlated with the partially crystalline microstructure in the later stages. The TEM and XRD data show that finally two nanocrystalline phases form after long-term annealing as a result of the decomposition process in the early stages. Direct evidence for decomposition was obtained using Z-contrast imaging technique that showed a systematic variation of the Zr concentration between the two nanocrystalline phases.     

A thermoplastic forming map of a Zr-based bulk metallic glass

A thermoplastic forming (TPF) map of a Zr₃₅Ti₃₀Be_26.75Cu_8.25 bulk metallic glass was constructed through systematic hot-embossing experiments, spanning a wide range of strain rates and temperatures in the supercooled liquid region. By comparison with the corresponding deformation map, it is found that Newtonian flow, non-Newtonian flow and inhomogeneous flow regions correspond well to fully filled, partially filled and non-filled regions, respectively, in the hot-embossing TPF map. Furthermore, the spatio-temporally homogeneous flow facilitates the thermoplastic formability of the Zr-based bulk metallic glass, which is rationalized in terms of free volume theory as well as by finite element simulations. Finally, our results are corroborated by potential application tests.     

Enhancement of plastic deformation in FeCoNbB bulk metallic glass with superhigh strength

The effect of B on the glass-forming ability of FeCoNbB alloys was investigated. Bulk metallic glasses with critical diameters up to 3 mm and superhigh yield strength of 4860 MPa were synthesized in (Fe_0.5Co_0.5)_71−xNb₆B_23+x (x = 0, 2, 3, 4 and 5) alloy system. Besides, Cu was added to the Fe_33.5Co_33.5Nb₆B₂₇ alloy with the aim at enhancing mechanical properties, and it was found that proper amount of Cu addition could effectively improve the compressive plasticity from 1.4% to 3.7% without obvious strength decreasing. The enhancement of plasticity is ascribed to the formation of clusters in Cu-contained FeCoNbB bulk metallic glasses.     

Zr基块体非晶合金的摩擦磨损行为(英文)

采用销盘式摩擦实验研究Zr基块体非晶合金分别在空气与氩气环境中的摩擦磨损行为。结果表明,在16N和23N2种不同载荷下,非晶试样在氩气中的磨损量都比在空气中的低45%以上。通过X射线衍射仪、差示扫描量热分析仪、扫描电子显微镜和光学表面轮廓仪等检测分析手段对磨损试样摩擦面的形貌和微观结构进行表征,发现在空气中磨损试样的表面存在大量摩擦颗粒和犁沟,而氩气中的试样表面相对平滑;非晶试样的磨损机理在空气中以磨粒磨损为主,而在氩气中则为粘着磨损。