Analyses of shear band emission in a Mg-based bulk metallic glass deformed at different nanoindentation rates

Nanoindentation behavior of Mg₅₇Cu₃₁Y_6.6Nd_5.4 bulk metallic glass was characterized at different loading rates. The load–displacement curves exhibit significant displacement serrations, apparently associated with discrete shear band emission, at low loading rates but disappear at high rates. Analyses based on displacement serration, strain rate serration and hardness serration were carried out to determine the critical strain rate beyond which the transition from inhomogeneous to homogeneous deformation actually took place. It was concluded that the hardness serration analysis probably provides the most reasonable result as the other two were limited by the instrument noises. Based on a shear band nucleation model, the critical nucleus size was estimated to be a sphere of about 25 nm in diameter.     

块体非晶合金绝热升温与锯齿流变机制

对Zr41．2Ti13．8Ni10Cu12．5Be22．5块体非晶合金压缩条件下的力学行为进行了研究,利用应变能理论,以面积比(As／A)为参量,计算出了流变过程中剪切带形成时变形区域的温度,变化规律结果表明：随着变形的增大,弹性应变能增加,形成剪切带时的温度逐渐升高,当剪切带面积比As／A值小于1／4时,升高的温度将达到或超过玻璃转变温度,导致变形区域粘度降低,从而促进剪切带继续扩展并导致最终断裂．此局部温度变化规律揭示了块体非晶合金锯齿流变直至断裂的机制．     

Strain rate-dependent deformation in bulk metallic glasses

Metallic glasses have metastable structures. As a result, their plastic deformation is dependent upon structural dynamics. In the present paper, we present data obtained from Zr-base and La-base metallic glasses and discuss the kinetic aspects of plastic deformation, including both homogeneous and heterogeneous deformation. In the case of homogeneous deformation (typically occurring in the supercooled liquid region), Newtonian behavior is not universally observed and usually takes place only at low strain rates. At high strain rates, non-Newtonian behavior is usually observed. It is demonstrated that this non-Newtonian behavior is associated with in situ crystallization of the amorphous structure. In the case of heterogeneous deformation (occurring at room temperature), deformation is controlled by localized shear banding. The plastic deformation of a La-base metallic glass is also investigated using instrumented nanoindentation experiments over a broad range of indentation strain rates. At low rates, the load-displacement curves during indentation exhibit numerous serrations or pop-ins, but these serrations become less prominent as the indentation rate is increased. Using the tip velocity during pop-in as a gauge of serration activity, we find that serrated flow is only significant at indentation strain rates below a certain critical value.     

A nanoindentation study of serrated flow in bulk metallic glasses

Plastic deformation of two Pd- and two Zr-based bulk metallic glasses (BMGs) is investigated through the use of nanoindentation, which probes mechanical properties at the length scale of shear bands, the carriers of plasticity in such alloys. These materials exhibit serrated flow during nanoindentation, manifested as a stepped load-displacement curve punctuated by discrete bursts of plasticity. These discrete “pop-in” events correspond to the activation of individual shear bands, and the character of serrations is strongly dependent on the indentation loading rate; slower indentation rates promote more conspicuous serrations, and rapid indentations suppress serrated flow. Analysis of the experimental data reveals a critical applied strain rate, above which serrated flow is completely suppressed. Furthermore, careful separation of the plastic and elastic contributions to deformation reveals that, at sufficiently low indentation rates, plastic deformation occurs entirely in discrete events of isolated shear banding, while at the highest rates, deformation is continuous, without any evidence of discrete events at any size scale. All of the present results are consistent with a kinetic limitation for shear bands, where at high rates, a single shear band cannot accommodate the imposed strain rapidly enough, and consequently multiple shear bands must operate simultaneously.     

Superplastic deformation and crystallization behavior of Cu54Ni6Zr22Ti18 metallic-glass sheet

Superplastic deformation and crystallization behavior of a Cu₅₄Ni₆Zr₂₂Ti₁₈ metallic glass were investigated. A maximum elongation of 650% was obtained at 733 K at 1 × 10⁻² s⁻¹ from the sheet fabricated by squeeze copper-mold casting method. At low strain rates, the strain-rate-sensitivity exponent value was close to 1, suggesting that Newtonian-like behavior governed the plastic flow. At a high strain rate around 10⁻² s⁻¹, a transition from Newtonian to non-Newtonian behavior took place with decrease in m value. Large strain hardening by crystallization occurred during the course of deformation. The strain hardening was found to be caused by crystallization according to the analyses of the relation of true stress vs. testing time, T-T-T diagram and DSC characteristics. The time periods up to the strain before strain hardening at 733 K for the Cu₅₄Ni₆Zr₂₂Ti₁₈ metallic glass were similar to that of the Zr₆₅Al₁₀Ni₁₀Cu₁₅ metallic glass at 696 K as 180–300 s (3–5 min). This coincidence could be explained by comparison of their T-T-T diagrams showing that the incubation times for crystallization of the Cu BMG at 733 K and for Zr BMG at 696 K are similar.     

Hot Deformation Behavior and Hardness of a CoCrFeMnNi High-Entropy Alloy with High Content of Carbon

A CoCrFeMnNi high-entropy alloy with a high content of carbon was synthesized, and its hot deformation behavior was studied at the temperatures 800–1000 ℃ at the strain rates ranging from 0.001 to 0.1 s~(-1).As-prepared alloy is a face-centered cubic-structured solid solution, with a large amount of carbides residing at grain boundaries.True stress–strain curves were employed to develop the constitutive equation of apparent activation energy.The apparent activation energy( Q) was found to be 423 kJ mol~(-1), indicating a dynamic flow softening behavior.The size of dynamic recrystallized(DRXed) grains increases with increasing the temperature or decreasing the strain rate.A processing map was sketched on the basis of the flow stress.The temperature range of 900–1000 ℃ and 10~(-3)–10~(-2.6) s~(-1) strain rate were found to be the optimum hot-forging parameter.With increasing temperature or decreasing strain rate, the volume fraction of fine carbides(≤ 1 μm) increases.A lot of coarse carbides can be found in the matrix after deformation at 800 ℃, which leads to a high hardness value of 345 HV.The carbides after deformation at 1000 ℃ are mainly nano-sized M_7C_3 and M_(23)C_6, which can promote the nucleation of DRX.     

Temperature,strain rate and reinforcement volume fraction dependence of plastic deformation in metallic glass matrix composites

A systematic study of mechanical properties is presented for Zr-based bulk metallic glass matrix composites, spanning a wide range of strain rates and temperatures, as well as various levels of reinforcement volume fraction. All of the experimental materials exhibit mechanical properties dominated by deformation of the amorphous matrix phase, including inhomogeneous flow and fracture at low temperatures, as well as homogeneous flow of both Newtonian and non-Newtonian character at high temperatures. In the homogeneous flow regime, the composites exhibit clear strengthening as the volume fraction of reinforcement increases. This strengthening effect is quantitatively explained in both the Newtonian and non-Newtonian regimes, and is found to arise from two contributions: (i) load transfer from the amorphous matrix to the reinforcements; and (ii) a shift in the glass structure and properties upon precipitation of the reinforcements. An additional source of apparent strengthening – in situ precipitation of reinforcement during deformation – is also discussed.     

纳米压入下镁基非晶合金的间歇性塑性流动

镁基非晶合金通常表现出显著的宏观脆性,因此用常规拉伸、压缩等方法对该合金的变形行为进行研究具有很大困难。本研究利用具有高时间分辨率和高空间分辨率的纳米压痕技术观察了不同加载速率下镁基非晶合金的锯齿流变行为。结果表明,低加载速率促进锯齿的形成,而高加载速率则抑制锯齿的形成。其原因是在低加载速率下,单一剪切带足以耗散外加应变;而在高加载速率条件下,由于单一剪切带不能将应变耗散掉,因此需要更多的剪切带参与变形。为了进一步解释这一锯齿流变行为,本研究采用遍历处理对每个锯齿的应变突变进行了统计分析。结果表明,在不同的加载速率下,小的应变突变服从幂律分布,且幂指数为1．45;而大的应变突变则呈现指数衰减规律。最后,借助硬度对应变速率的敏感性,估算了镁基非晶合金在纳米压人条件下剪切转变区的体积,为4．5nm^3。     

Thermomechanical instability analysis of inhomogeneous deformation in amorphous alloys

Recent experiments have shown that inhomogeneous deformation in amorphous alloys critically depends on the environmental temperature and the applied strain rate, and that the temperature field inside the shear-band can rise up to the glass transition temperature. A free-volume-based, thermo-viscoplastic constitutive law is developed in which the thermal transport equation includes contributions from the heating from plastic work and the heat conduction. For homogeneous deformation, the instantaneous temperature rise during the strain softening stage can lead to thermal softening and promote the initiation of shear bands. A linear stability analysis is carried out to examine the conditions for the unstable growth of temperature fluctuations. It is found out that the short-wavelength fluctuations, the amplitudes of which would decay at low strain rate and moderately high environmental temperature (but still much lower than the glass transition temperature), become unstable at high strain rate and low temperature, so that the resulting shear-band spacing will be shorter. A deformation mechanism map is constructed to delineate this transition of inhomogeneous deformation from coarse to fine shear-band arrangements. The theoretical results agree well with a nanoindentation experiment where there is varying applied strain rate and environmental temperature and with a microindentation experiment in which the evolution of the effective strain rate during loading influences the spatial distribution of the shear-band spacing observed using the bonded interface technique.     

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.     

Deformation and flow in bulk metallic glasses and deeply undercooled glass forming liquids—a self consistent dynamic free volume model

Bulk glass forming metallic liquids such as those of the Zr–Ti–Ni–Cu–Be Vitreloy alloy family have been shown to have Newtonian Viscosity which is well described by the Vogel–Fulcher–Tamann (VFT) equation over roughly 15 orders of magnitude from the high temperature equilibrium melt to the deeply undercooled liquid near and below the experimentally observed glass transition. Experiments have also shown flow becomes non-Newtonian and ultimately unstable against spatial localization into shear bands as the strain rate at a given temperature is increased. This transition from homogeneous to inhomogeneous flow and flow localization has been discussed by several authors and attributed to the influence of strain softening, strain rate sensitivity, and thermal softening.which collectively result in the destabilization of the uniform flow field. The present paper presents a simple self consistent model of uniform steady state flow which is based on the tradition Free Volume Model of the glass transition, the VFT-equation, and a simple treatment of free volume production and annihilation during flow. The model is used to analyze the flow data and shown to give a simple one-parameter fit to experimental steady state Newtonian and non-Newtonian flow data over a broad range of temperatures and strain rates. The model gives simple analytic expressions for the steady state constitutive flow law, the strain rate sensitivity exponent (SRSE), and an implicit equation for the strain rate and temperature dependent viscosity, which is solved numerically. An approximate analytic expression for the non-Newtonian effects is proposed. Generalizing the model to time dependent flow, it is argued that observed shear localization and serrated flow in bulk glass forming liquids arises primarily from transient response phenomena.     

Effect of the nanoindentation rate on the shear band formation in an Au-based bulk metallic glass

This study investigated the nanoindentation behavior of Au₄₉Ag_5.5Pd_2.3Cu_26.9Si_16.3 bulk metallic glass samples at loading rates ranging from 0.03 to 300 mN s⁻¹. Notable shear band pop-in events were observed. The pop-in size was observed to increase linearly with the load and decreased exponentially with the strain rate. A free-volume mechanism was proposed for interpreting these observations quantitatively. The results and analyses also shed light on the shear band nucleation and evolution processes in bulk metallic glasses.     

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.     

A study of the strain rate microstructural response and wear of metals

Titanium (Ti) and copper (Cu) pins were slid against alumina in a pin-on-disk machine at a load of 50 N and sliding speeds varying from 0.1 to 4 ms⁻¹. The evolution of the microstructure in the subsurface of the material and the wear rate was co-related to the strain rate microstructural response of the material in uniaxial compression, at different strain rates (0.1–100 s⁻¹) and temperatures (298–673 K). The strain rates and temperatures in the plastically deforming zone near the surface of the pins were determined using noniterative methods. The strain rates were found to be in the region of 100 s⁻¹ near the surface and decreases as one moves into the sub-surface of the pin. The temperatures increased as the speed increased. These estimated strain rates and temperatures were superimposed on the strain rate microstructural response maps of these materials. The uniaxial compression test results of Ti showed adiabatic shear banding as a microstructural mechanism that evolves at high strain rates (≥10 s⁻¹) and lower temperatures (<575 K). Adiabatic shear bands are sites of easy crack nucleation and propagation. When Ti is slid at low speeds the near surface region of the pins deform in the adiabatic shear banding regions in the strain rate microstructural response map. At such speeds the wear rate is found to be high and reduces as the sliding speed is increased, when the material undergoes a more homogeneous deformation. The microstructural response of Cu under uniaxial compression showed that the material undergoes flow banding at intermediate strain rates (1 s⁻¹) and temperatures of up to 473 K. The subsurface microstructure of the pins slid at low speeds showed subsurface cracking and sheet like debris formation. This happen at lower speeds because the flow banding and crack nucleation is expected in the subsurface where the strain rates and temperatures are lower. The present test results show a clear relation to exist between the strain rate response of the material in uniaxial compression and its subsurface microstructural evolution and wear rate.     

Deformation behavior and indentation size effect of Au49Ag5.5Pd2.3Cu26.9Si16.3 bulk metallic glass at elevated temperatures

Instrumented nanoindentation was conducted on an Au₄₉Ag_5.5Pd_2.3Cu_26.9Si_16.3 bulk metallic glass from room temperature to supercooled liquid region. It was found that the hardness decreases as the depth of the indentation increases at a modest loading rate (e.g. ～0.5 mN s^?1), which is known as the indentation size effect (ISE). The transition from inhomogeneous to homogeneous flow was clearly observed at the glass transition temperature. However, the deformation behavior of the metallic glass in the supercooled liquid region showed strong loading rate dependence. The deformation mode changed from homogeneous to inhomogeneous, and even exhibited a reverse indentation size effect when the loading rate was sufficiently high (i.e., ≥10 mN s^?1 in the study). The different deformation behaviors and indentation size effects at various temperatures and loading rates are discussed in terms of free volume theory.     

Experimental characterization of plastic flow initiation in a bulk metallic glass via intermittent compression test

Intermittently plastic compression tests of the Zr_52.5Cu_17.9Ni_14.6Al₁₀Ti₅ bulk metallic glass were performed under a geometrically constrained condition at different cross-head displacement rates. A flow stress-overshoot phenomenon was observed during the macroscopically plastic deformation. It was found that there is a close correlation between the stress-overshoot intensity and the spatial density of shear bands. The stress-overshoot intensity can be used as a parameter to characterize the ability of plastic flow initiation caused by the spatial nucleation of shear bands in the bulk metallic glass. The micromechanisms of shear band multiplication were discussed.     

Structure relaxation and crystallization of Al83Ni10Ce7 metallic glass

Structure relaxation and crystallization of Al₈₃Ni₁₀Ce₇ metallic glass were studied by different scanning calorimetry (DSC) and X-ray diffraction (XRD). According to the DSC scan, it is interesting to find that the second exothermic peak changes with pre-annealing temperatures (below glass transition temperature), suggesting a change in the amorphous structure upon relaxation. Continuous heating crystallization and isothermal crystallization exhibit different crystallization mechanism of the present alloy. fcc-Al and a metastable phase precipitate simultaneously in the first stage crystallization during continuous heating; however, only a metastable phase precipitates during isothermal annealing below glass transition temperature (T_g).     

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.     

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.