Self-organized intermittent plastic flow in bulk metallic glasses

Under stress, bulk metallic glasses irreversibly deform through shear banding processes that manifest as serrated flow behavior. These serration events exhibit a shock-and-aftershock, earthquake-like behavior. Statistical analysis shows that the shear avalanches can self-organize to a critical state (SOC). In analogy to the smooth macroscopic-scale crystalline plasticity that arises from the spatio-temporal averages of disruptive earthquake-like events at the nanometer scale, shear avalanches in glassy metals are another model system that can be used to study SOC behavior. With our understanding of SOC behavior, we further demonstrate how to enhance the plasticity of glassy (brittle) materials. It is expected that the findings can be extended to other glassy or brittle materials.     

Analysis of plastic strain and deformation mode of a Zr-based two-phase bulk metallic glass in compression

A ductile phase-separated Zr-based bulk metallic glass (BMG) deformed to different strains at room temperature and low strain rate was characterized. The BMG samples were compressed to nominal strains of 3%, 7%, and 10%, after which the samples were unloaded for morphological observation using scanning electron microscopy. The morphological observation was subsequently used for the interpretation of the measured load–displacement curves. It was found that the BMG exhibited apparent uniform deformation initially (at plastic strain <1%) and, then, visible local shear bands began to developed. Afterwards, a principal shear band was soon developed and dominated the deformation process until fracture. In this study, we also found that the local shear strain varies along the principal shear plane and decreases monotonically from the shear band initiation site.     

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.     

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.     

Viscous flow and microforming of a Zr-base bulk metallic glass

The thermal plastic deformation behavior of the (Zr₅₃Cu₃₀Ni₉Al₈)_99.5Si_0.5 bulk metallic glass (BMG) was studied by means of thermo mechanical analysis (TMA) and high-temperature compression test in the supercooled liquid region. Using the result obtained from TMA, the deformation behavior for (Zr₅₃Cu₃₀Ni₉Al₈)_99.5Si_0.5 BMG rod was investigated by compression test at different strain rates (5 × 10^?3～5 × 10^?1 s^?1) and temperatures in the supercooled liquid region, specifically at 733 K, 738 K, 743 K, 748 K, and 753 K. The value of flow stress with a constant strain rate of 5 × 10^?3 s^?1 decreases with increasing temperature and reaches a relatively low flow stress of 36 MPa at the test temperature of 753 K. Conversely, the value of flow stress increases with the strain rate in compression. In this paper, an imprinted hologram pattern with 0.6 μm depth was demonstrated which showed extremely good microforming ability of this (Zr₅₃Cu₃₀Ni₉Al₈)_99.5Si_0.5 BMG in the supercooled liquid region.     

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.     

The characterization of plastic deformation in Ce-based bulk metallic glasses

The plastic deformation behavior of Ce₆₈Al₁₀Cu₂₀Nb₂ and Ce₇₀Al₁₀Cu₂₀ bulk metallic glasses (BMGs) at room temperature was studied by depth-sensing nanoindentation and microindentation. It is shown that the two BMGs exhibit a continuous plastic deformation without distinct serration at the all of the studied loading rates during nanoindentation. An obvious creep displacement was observed during the holding-load segment at the maximum load for the two alloys, and the magnitude of creep during holding-load increases with loading rate. The subsurface plastic deformation zone of the two BMGs after indentation at various loading rates was investigated through bonded interface technique using depth-sensing microindentation. A highly developed shear banding pattern can be observed in the plastic deformation region, though the global load–depth curves illuminate a “homogeneous flow”. The plastic deformation behavior of the Ce-based BMGs during indentation measurements is discussed in terms of localized viscous flow.     

Micromechanisms of serrated flow in a Ni50Pd30P20 bulk metallic glass with a large compression plasticity

Serrated flow is a characteristic feature of plastic deformation of bulk metallic glasses (BMGs) with a large compression strain. However, the underlying mechanisms of the discrete plasticity in the disordered solids have been debated for many years. Here, we report mechanical behavior and microstructural evolution of a Ni₅₀Pd₃₀P₂₀ BMG subjected to uniaxial compression testing. Extensive nanocrystallization within shear bands and in the vicinity of fracture surfaces was observed and various crystal defects, including dislocations, twins and kink bands, were detected in the resultant nanocrystals. These observations suggest a microscopic mechanism of the serrated flow of the BMG, i.e. the stress drop is caused by local strain-softening and the arrest of shear bands is associated with in situ nanocrystallization.     

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.     

小冲杆试验法评价大块非晶合金的超塑性性能

采用小冲杆试验法(SPT)研究了大块非晶合金(Zr41.2Ti13.8Cu12.5Ni10Be22.5)在不同温度和不同压头速度下的变形行为,通过SPT载荷-位移曲线结合理论分析,确定大块非晶(BMG)材料超塑性本构关系.结果表明,BMG材料的SPT载荷-位移曲线对压头速度和实验温度的敏感性较高,而SPT载荷-位移曲线的形状与材料的应变速率敏感系数、粘度、压头速度等因素有关.在应变速率敏感系数和压头速度固定的条件下,SPT最大破裂载荷与粘度成正比;最大破裂位移仅随应变速率敏感系数的增大而增大,而与压头速度和粘度的大小无关.获得了Zr41.25Ti13.75Ni10Cu12.5Be22.5 BMG应变速率敏感系数和表观粘度,确定了该合金的超塑性本构关系.     

Mean stress effects on flow localization and failure in a bulk metallic glass

The effect of stress state on strain localization and subsequent failure of a bulk metallic glass alloy is examined. It is shown that failure is associated with a critical tensile mean stress of 0.95 GPa. This is in contrast with previous work utilizing superimposed compressive mean stresses, which found that failure resulted at a critical effective stress. Interestingly, the critical tensile mean stress measured in this study causes the same dilatation as a 274 K temperature increase, nearly to the glass transition temperature. The effect of mean stress on elastic variation of the average free volume is added to a strain localization model. This model describes the compressive mean stress behavior very well, and predicts a strong sensitivity to tensile mean stresses.     

Local order influences initiation of plastic flow in metallic glass: Effects of alloy composition and sample cooling history

Bulk metallic glasses (MGs) with tunable plasticity and strength have been reported recently. Using Cu–Zr and Cu–Zr–Al MG models, here we illustrate how and why alloy composition and cooling history influence the initial flow behavior in the early stage of plasticity. Starting from Cu₄₆Zr₅₄, either increasing the Cu concentration, or substituting Al for a few percent of Zr, increases the resistance to the initiation of plastic flow, the softening after the local yielding, and the propensity for strain localization. These effects are shown to be intrinsic to the uniform, fully amorphous MGs and rooted in their internal structure. Our quantitative monitoring of the local environment, especially the role of full-icosahedral clusters in shear transformations, identifies the fertile and resistant structural entities controlling deformation. The structural mechanisms have implications for macroscopic plasticity, and the alloy dependence of the MG structure reveals a microscopic origin underlying the varying mechanical properties.     

Instrumented indentation of a Pd-based bulk metallic glass: Constant loading-rate test vs constant strain-rate test

Most of the previous nanoindentation experiments on bulk metallic glasses (BMGs) were made under a constant ‘loading rate,’ although ‘strain rate’ is a more useful parameter than loading rate to analyze the inhomogeneous plasticity in the BMG according to the classic free-volume theory. Here, we explore the strain-rate dependency of plastic characteristics in a Pd-based BMG through nanoindentation tests under a variety of constant strain rates (0.01–0.25 s⁻¹). The results are compared with those from nanoindentations under various constant loading rates (0.05–5 mN/s) and discussed in terms of the influences of strain rate on the plastic flow characteristics in the BMG.     

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.     

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.     

Thermomechanical characterization of Cu47.5Zr47.5Al5 bulk metallic glass within the homogeneous flow regime

We present a systematic study of the high temperature deformation behavior of a Cu_47.5Zr_47.5Al₅ ternary bulk metallic glass over a wide range of strain rates within the homogeneous flow regime. The apparent viscosity and the effective strain rate determined by thermomechanical analysis in the low stress regime strongly depend on the isothermal annealing temperature and the applied compressive force. Three distinct flow modes, viz. inhomogeneous, non-Newtonian and Newtonian flow, can be distinguished from compression tests. The strain rate–stress data, deduced from both thermomechanical analysis and quasi-static compression tests, were used to construct a Norton-type plot indicating a transition from Newtonian to non-Newtonian flow. The significance of these findings for the expected macroscopic shaping capability based on the dynamic materials model as well as the change of the amount of atomic-scale flow defects such as free volume is also investigated.