Rate-dependent inhomogeneous creep behavior in metallic glasses

Creep deformation can be classified as homogeneous flow and inhomogeneous flow in bulk metallic glass (BMG). In order to understand the conversion conditions of the two types of creep deformation, the effect of loading rate on the creep behavior of a Ti₄₀Zr₁₀Cu₄₇Sn₃ (at.%) BMG at ambient temperature was investigated using nanoindentation and molecular dynamic simulation. Results indicate that at low loading rates, many serrations appear in loading stage, leading to inhomogeneous serrated flow in the creep stage. When the loading rate is high enough, the creep deformation tends to be homogeneous. The related mechanism responsible for the rate-dependent creep behavior is attributed to the number of pre-existing major shear bands which is influenced significantly by the loading rate.     

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.     

Superplasticity in a bulk amorphous Pd-40Ni-20P alloy:a compression study

Compressive deformation behavior of a cast Pd₄₀Ni₄₀P₂₀ bulk metallic glass in the supercooled liquid region (589–670 K) was investigated at strain rates ranging from 10⁻⁴ to 10⁻² s⁻¹. The material exhibited excellent mechanical formability in the supercooled liquid region. However, in contrast to a Newtonian behavior generally observed in oxide glasses, the present alloy also showed a non-Newtonian behavior, depending upon the temperature and applied strain rate. Specifically, the alloy is like a Newtonian fluid at high temperatures, but becomes non-Newtonian at low temperatures and high strain rates. Structures of the amorphous material, both before and after deformation, were examined using X-ray diffraction and high-resolution transmission electron microscopy. The non-Newtonian behavior is proposed to be associated with the glass instability during deformation.     

Effect of strain rate on compressive behavior of a Pd40Ni40P20 bulk metallic glass

Mechanical deformation of Pd₄₀Ni₄₀P₂₀ was characterized in compression over a wide strain rate range (3.3×10⁻⁵ to 2×10³ s⁻¹) at room temperature. The compression sample fractured with a shear plane inclined 42 degree with respect to the loading axis, in contrast to 56 degree for the case of tension. This suggests the yielding of the material deviates from the classical von Mises yield criterion, but follows the Mohr-Coulomb yield criterion. Fracture stress as well as strain was found to decrease with increasing applied strain rate. The compressive stress (1.74 GPa) was also found to be higher than the tensile fracture stress at a quasi-static strain rate. Close examination of the stress–strain curves revealed that localized shear might have occurred at a compressive stress of about 1.4 GPa, much lower than the “apparent” yield stress of 1.74 GPa. However, the stress of 1.4 GPa for shear band initiation is almost the same as the fracture stress measured at a dynamic strain rate of 5×10² s⁻¹. These results suggested that the fracture of a bulk metallic glass is sensitive to the applied loading rate.     

Investigation of primary and secondary creep deformation mechanism of TiAl

Creep deformation behaviors in lamellar TiAl alloys have been investigated. As in the case with metals, the normal primary creep stage was observed. As creep strain increased within the primary regime, dislocation density decreased, and creep activation energy increased from 300 kJ/mol, the activation energy of the self-diffusion of Ti in TiAl, to about 380 kJ/mol, that of steady state creep deformation. During primary creep deformation of lamellar TiAl, as the initial dislocation density decreased, the α₂ -phase was found to transform to a γ-phase, generating new dislocations which contributed to the creep deformation. In other words, this phase transformation is the source of the dislocation generation for continuous creep deformation. Therefore, we suggest that phase transformation is the rate controlling processes having an activation energy of about 400 kJ/mol, which is higher than that of self-diffusion. A small amount of prestrain was found to be responsible for the reduction of initial dislocation density. In addition, this prestrained specimen showed significantly reduced primary creep strain, and the creep activation energy in the primary stage was measured to be about 380 kJ/mol. These results clearly confirm the suggested creep deformation mechanism of lamellar TiAl alloys.     

Interpretation of viscous deformation of Zr-based bulk metallic glass alloys based on Nabarro-Herring creep model

Superplastic-like viscous deformation of bulk metallic glass alloys around the glass transition temperature (Tg) was analyzed based on the Nabarro-Herring creep model, a classical creep model, where the diffusional motion of atoms or vacancies through the lattice (atomic configuration) is considered. The amorphous matrix of bulk metallic glasses that has a randomly-packed atomic configuration was assumed to behave in a manner similar to the grain boundary in polycrystalline metals so as to approximate the diffusivity of the major constituent element. In spite of rough approximation of the parameters in the Nabarro-Herring creep equation, a reasonable value of the diffusion path (d) could be obtained from the experimentally-obtained metal flow data, including the steady state stress and the strain rate. Due to the absence of vacancy sources such as grain boundaries in homogeneous metallic glasses, the diffusion path, which, in polycrystalline materials, generally is the average distance between vacancy sources such as grain boundaries, was considered in this work as the average distance between tunneling centers in bulk metallic glass alloys. The calculated diffusion path was comparable to the density of tunneling centers around Tg, proposed by M. H. Cohen and G. S. Grest based on free volume theory. The calculated diffusion path showed monotonous decrease with temperature over Tg for Zr-based bulk metallic glass alloys. Based on this analysis, a schematic model for viscous deformation of bulk metallic glass was proposed.     

In situ tensile deformation of Fe-rich metallic glass at elevated temperatures using hard X-ray diffraction

The present work focuses on the study of the mechanical properties of the Fe_72.5Cu₁Nb₂Mo₂Si_15.5B₇ metallic glass using in situ hard X-ray diffraction techniques. In situ tensile deformation tests provided detailed information about the mechanical properties of the investigated Fe-based metallic glass. Analyzing series of two-dimensional XRD patterns in reciprocal space yields strain tensor components of the amorphous alloy providing insight about its deformation mechanisms. Comparing tensile tests performed at different temperatures indicates that the deformation mechanism gradually changes from purely elastic to completely plastic regime. The Poisson ratio ν of the investigated alloy increases with increasing deformation temperature however the fracture strength σ_f shows opposite behavior.     

Creep deformation of fully lamellar TiAl controlled by the viscous glide of interfacial dislocations

Creep mechanisms of fully lamellar TiAl with a refined microstructure (γ lamellae: 100–300 nm thick, α₂ lamellae: 10–50 nm thick) have been investigated. A nearly linear creep behavior (i.e. the steady-state creep rate is nearly proportional to the applied stress) was observed when the alloy was creep deformed at low applied stresses (<400 MPa) and intermediate temperatures (650–810°C). Since the operation and multiplication of lattice dislocations within both γ and α₂ lamellae are very limited in a low stress level as a result of the refined lamellar microstructure, creep mechanisms based upon glide and/or climb of lattice dislocations become insignificant. Instead, the motion of interfacial dislocation arrays on γ/α₂ and γ/γ interfaces (i.e. interface sliding) has found to be a predominant deformation mechanism. According to the observed interfacial substructure caused by interface sliding and the measured activation energy for creep, it is proposed that creep deformation of the refined lamellar TiAl in the intermediate-temperature and low-stress regime is primarily controlled by the viscous glide of interfacial dislocations.     

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.     

Electrochemical investigation of the coating/substrate interface stability for styrene/acrylate copolymer films applied on iron

Water-borne n-butyl acrylate (nBA)/styrene (Sty), 2-ethylhexyl acrylate (EHA)/Sty and nBA/EHA/Sty copolymer dispersions were applied on iron to investigate the latex/substrate interface stability. Scanning Kelvin Probe (SKP) studies were performed to detect the progress of cathodic delamination as function of the latex particle composition. The interface stability decreased with increasing polymer glass transition temperature (T_g) and in the order EHA/Sty > nBA/EHA/Sty > nBA/Sty. The surface energy of the adhesives reflected the SKP results. Electrochemical Impedance Spectroscopy (EIS) was applied to evaluate water uptake of and diffusion kinetics within the polymers. Calculated diffusion coefficients did not change with the T_g.     

Internal stress plasticity due to chemical stresses

Internal stress plasticity occurs when a small external stress biases internal mismatch strains produced by, e.g., phase transformation or thermal expansion mismatch. At small applied stresses, this deformation mechanism is characterized by a deformation rate which is proportional to the applied stress and is higher than for conventional creep mechanisms. In this work, we demonstrate the operation of internal stress plasticity due to internal chemical stresses produced by chemical composition gradients. We subject specimens of β-phase Ti-6Al-4V to cyclic charging/discharging with hydrogen (by cyclic exposure of specimens to gaseous H₂), under a small external tensile stress. As expected for internal stress plasticity, the average strain rate during chemical cycles at 1030°C is larger than for creep at constant composition (hydrogen-free or -saturated), and a linear stress dependence is observed at small applied stresses. Additionally, we present an analytical model which couples elastic and creep deformation with a transient diffusion problem, wherein the diffusant species induces swelling of the host lattice. Without the use of any adjustable parameters, the model accurately predicts both the observed strain evolution during hydrogen cycling of Ti-6Al-4V and the measured stress dependence of the deformation.     

Chemically dealloyed MgCuGd metallic glass with enhanced catalytic activity in degradation of phenol

Nanoporous Cu with tunable pore size (20–50 nm) are synthesized through chemical dealloying of the Mg₆₅Cu₂₅Gd₁₀ metallic glass in sulfuric acid solution. X-ray diffraction (XRD) and scanning electron microscopy (SEM) demonstrated the formation of mixing structures consisting of amorphous matrix and fcc-Cu ligaments with nanoporous structure in the dealloyed samples. The nanoporous alloy obtained shows superior catalytic activity in degrading phenol-containing wastewater, e.g., the degradation rate increases by 2–4 times as compared to the un-dealloyed Mg-based metallic glass. It was also found that surface wettability plays an important role in degradation, which results in a better catalytic performance in the sample with coarser nanoporous structure although it has relatively less specific surface area as compared to the samples with finer pores. Finally, the mechanism for degradation of phenol is discussed.     

Ti20Zr20Hf20Cu20Be20高熵非晶合金应力弛豫行为研究

高熵非晶合金力学弛豫行为的研究,对于理解玻璃转变、塑性变形、弛豫机理等科学问题和拓展其工程应用极为关键。本文采用应力分析方法对Ti20Zr20Hf20Cu20Be20高熵非晶合金条带进行了研究,旨在揭示高熵非晶合金应力弛豫行为。通过分析其在恒定应变下较宽时间窗口和温度窗口内的应力衰减过程,发现在低于Tg的玻璃态下,高熵非晶合金中存在着弛豫解耦现象,分别是慢弛豫和快弛豫过程。其中慢弛豫呈现扩展指数衰减模式,弛豫时间存在对温度的Arrhenius动力学依赖关系,与长程原子重排运动有关,快弛豫过程对应于微观局部内应力的逐步消散过程。不论该高熵非晶合金变形处于弹性阶段还是发生了屈服,应力弛豫过程受应变的影响都较小。本研究揭示了高熵非晶合金中新的弛豫解耦现象和与之相关的独特动力学机制,拓宽了我们对高熵非晶合金弛豫动力学过程以及其本征特性的认知。     

Dynamic Deformation Behaviors of an In Situ Ti-Based Metallic Glass Matrix Composite

Quasi-static and dynamic deformation behaviors, fracture characteristics, and microstructural evolution of an in situ dendrite-reinforced metallic glass matrix composite: Ti₅₀Zr₂₀V₁₀Cu₅Be₁₅ within a wide range of strain rates are investigated. Compared with the quasi-static compression, the yielding stress increases, but the macroscopic plasticity significantly decreases upon dynamic compression. The effects of the strain rate on strain hardening upon quasi-static loading and flow stress upon dynamic loading are evaluated, respectively. The Zerilli-Armstrong (Z-A) model based on dendrite-dominated mechanism is employed to further uncover the dependence of the yielding stress on the strain rate.     

Steady state flow of the FeCoNiCrMn high entropy alloy at elevated temperatures

Steady state flow behavior of the FeCoNiCrMn high-entropy alloy at temperatures ranging from 1023 to 1123 K was systematically characterized. It was found that the stress exponent (i.e., the reciprocal of strain-rate sensitivity) was dependent on the applied strain rate, and specifically the stress exponent is high (∼5) in the high strain rate regime, but decreases with decreasing strain rate. Microstructural examinations of the samples before and after deformation were performed to understand the interplay of the microstructures with the corresponding properties. Based on the observations, it was proposed that, at high strain rates, the deformation of the current high-entropy alloy was controlled by dislocation climb and the rate limiting process was the diffusion of Ni. At low strain rates, however, the deformation appeared to be controlled by the viscous glide of dislocations. Moreover, at the slowest strain rate (i.e., the longest thermal exposure time), new phases evolved, which caused elemental redistribution and weakening of the material.     

Precision die-casting of optical MU/SC conversion sleeve

The performance of MU/SC conversion sleeve produced by bulk metallic glass (Zr₅₅Al₁₀Ni₅Cu₃₀) was examined. A precision die-casting method was applied to improve size accuracy. The size accuracy of the conversion sleeve produced by the precision die-casting method was ±1 μm, and optical insertion loss (Li) was less than 0.3 dB for a standard value. The wear resistance of metallic glass is improved by surface oxidation treatment in air at 673 K. The MU/SC conversion sleeve produced from bulk metallic glass has superior characteristics for optical parts.     

一种铁基块体金属玻璃纳米压痕蠕变行为的加载速率敏感性(英文)

通过纳米压痕蠕变实验研究了加载速率对{[(Fe0.6Co0.4)0.75B0.2Si0.05]0.96Nb0.04}96Cr4块体金属玻璃室温蠕变变形的影响。结果表明,该铁基块体金属玻璃的蠕变变形随着加载速率的增加而增大。此外,根据经验幂率函数计算得到了材料室温蠕变应力指数,当加载速率从1mN/s增加到50mN/s时,应力指数从28.1逐渐下降到4.9,显示出显著的压痕加载速率敏感性。最后,基于自由体积理论和剪切转变区理论对该铁基块体金属玻璃的纳米压痕蠕变行为进行了探讨,并对实验结果和分析结果提供了半定量的解释。     

DEFORMATION BEHAVIOR AND CONSTITUTIVE EQUATION FOR Zr55Cu30Al10Ni5 BULK METALLIC GLASS IN SUPERCOOLED LIQUID REGION

Zr₅₅Cu₃₀Al₁₀Ni₅块体非晶合金(BMG)在过冷液态区内的单向压缩实验表明: 材料在过冷液态区内的形变行为强烈依赖于温度和变形速率. 随着应变速率的增加, 材料的流变特征由Newtonian流变转变为非Newtonian流变.利用扩展指数本构方程模型建立了非晶合金的流变应力、应变速率和温度的关系.     

Strain rate-dependent compressive deformation behavior of Nd-based bulk metallic glass

Compressive deformation behavior of the Nd₆₀Fe₂₀Co₁₀Al₁₀ bulk metallic glass was characterized over a wide strain rate range (6.0×10⁻⁴ to 1.0×10³ s⁻¹) at room temperature. Fracture stress was found to increase and fracture strain decrease with increasing applied strain rate. Serrated flow and a large number of shear bands were observed at the quasi-static strain rate (6.0×10⁻⁴ s⁻¹). The results suggest that the appearance of a large number of shear bands is probably associated with flow serration observed during compression; and both shear banding and flow serration are a strain accommodation and stress relaxation process. At dynamic strain rates (1.0×10³ s⁻¹), the rate of shear band nucleation is not sufficient to accommodate the applied strain rate and thus causes an early fracture of the test sample. The fracture behavior of the Nd₆₀Fe₂₀Co₁₀Al₁₀ bulk metallic glass is sensitive to strain rate.     

Dislocation and Structural Studies at Metal–Metallic Glass Interface at Low Temperature

In this paper, molecular dynamics (MD) simulation deformation studies on the Al (metal)–Cu₅₀Zr₅₀ (metallic glass) model interface is carried out based on cohesive zone model. The interface is subjected to mode-I loading at a strain rate of 10⁹ s^?1 and temperature of 100 K. The dislocations reactions and evolution of dislocation densities during the deformation have been investigated. Atomic interactions between Al, Cu and Zr atoms are modeled using EAM (embedded atom method) potential, and a timestep of 0.002 ps is used for performing the MD simulations. A circular crack and rectangular notch are introduced at the interface to investigate the effect on the deformation behavior and fracture. Further, scale size effect is also investigated. The structural changes and evolution of dislocation density are also examined. It is found that the dominant deformation mechanism is by Shockley partial dislocation nucleation. Amorphization is observed in the Al regions close to the interface and occurs at a lower strain in the presence of a crack. The total dislocation density is found to be maximum after the first yield in both the perfect and defect interface models and is highest in the case of perfect interface with a density of 6.31 × 10¹⁷ m^?2. In the perfect and circular crack defect interface models, it is observed that the fraction of Shockley partial dislocation density decreases, whereas that of strain rod dislocations increases with increase in strain.