Stick–slip behavior of serrated flow during inhomogeneous deformation of bulk metallic glasses

Accurate compression tests with a piezoelectric load cell and an acquisition rate of up to 10 kHz were performed on a Zr-based bulk metallic glass in the temperature range 210–320 K at a strain rate of 10^?3 s^?1. Information about the stress drop magnitude and the associated size of shear displacements as a function of temperature and strain provides detailed insights into the shear band characteristics, which can be described by a stick–slip process. The average shear slip displacement is on average about 1–2 μm, irrespective of temperature, whereas the associated slip time (or stress drop time) increases from ～1 ms at 320 K to ～0.4 s at 213 K, yielding values on the deformation kinetics and the shear viscosity. Scanning electron microscopy investigations on shear surfaces and in situ acoustic emission measurements provide further understanding into the complex multistep shear slip process.     

Strain rate sensitivity of a high strain rate superplastic TiNp/2014 Al composite

The effects of deformation temperature and strain in hot rolling deformation on strain rate sensitivity of the TiN_p/2014 Al composite were studied by tensile tests conducted out at 773, 798, 818 and 838 K with the strain rates from 1.7 ×10^?3 to 1.7 × 10⁰ s^?1. It is shown that the curves of m value of the TiN_p/2014Al composite deformed at different temperatures can be divided into two stages with the variation of strain rate, and the critical strain rates are 10^?1 s^?1. The optimum deformation temperature of the TiN_p/2014 Al composite is near incipient melting temperature of 816 K and the optimum strain rate is a little higher than the critical strain rate. The effect of deformation temperature on strain rate sensitivity is relative to liquid phase helper accommodation. The effect of strain in hot rolling deformation on strain rate sensitivity attributes to change of microstructure and deformation mechanism.     

A constitutive theory for metallic glasses at high homologous temperatures

The elastic–viscoplastic constitutive theory of Anand and Su [Anand L, Su C. J Mech Phys Solids 2005;53:1362] for metallic glasses has been extended to the high homologous temperature regime. The constitutive equations appearing in the theory have been specialized to model the response of metallic glasses in the temperature range 0.7ϑ_g ⪅ ϑ ⪅ ϑ_g and strain rate range [10⁻⁵, 10⁻²] s⁻¹. The material parameters appearing in the theory have been estimated for the metallic glass Pd₄₀Ni₄₀P₂₀ from the experimental data of De Hey et al. [De Hey P, Sietsma J, Van Den Beukel A. Acta Mater 1998;46:5873]. The model is shown to capture the major features of the stress–strain response, and the evolution of an order-parameter for this metallic glass. In particular, the phenomena of stress overshoot and strain softening in monotonic experiments at a given strain rate and temperature, as well as strain rate history effects in experiments involving strain rate increments and decrements, are shown to be nicely reproduced by the model.     

Characterization of hot deformation behavior of Haynes230 by using processing maps

The hot deformation characteristics of Haynes230 has been investigated in the temperature range 1050–1250 °C and strain rate range 0.001–10 s^?1 using hot compression tests. Power dissipation map for hot working are developed on the basis of the Dynamic Materials Model. The map exhibits two domains of dynamic recrystallization (DRX): one occurring in the temperature range of 1200–1250 °C and in the strain rate range of 0.001–0.03 s^?1, which associated with grain coarsening; the other occurring in the temperature range of 1100–1200 °C and strain rate range of 0.001–0.01 s^?1, which are the optimum condition for hot working of this material. The average apparent activation energy for hot deformation is calculated to be 449 kJ/mol. The material undergoes flow instabilities at temperatures of 1050–1100 °C and at strain rates of 1–10 s^?1, as predicted by the continuum instability criterion. The manifestations of the instabilities have been observed to be adiabatic shear bands which are confirmed by optical observation.     

Studies of shear band velocity using spatially and temporally resolved measurements of strain during quasistatic compression of a bulk metallic glass

We have made measurements of the temporal and spatial features of the evolution of strain during the serrated flow of Pd₄₀Ni₄₀P₂₀ bulk metallic glass tested under quasistatic, room temperature, uniaxial compression. Strain and load data were acquired at rates of up to 400 kHz using strain gages affixed to all four sides of the specimen and a piezoelectric load cell located near the specimen. Calculation of the displacement rate requires an assumption about the nature of the shear displacement. If one assumes that the entire shear plane displaces simultaneously, the displacement rate is approximately 0.002 m s^–1. If instead one assumes that the displacement occurs as a localized propagating front, the velocity of the front is approximately 2.8 m s^?1. In either case, the velocity is orders of magnitude less than the shear wave speed (～2000 m s^?1). The significance of these measurements for estimates of heating in shear bands is discussed.     

The optimal determination of forging process parameters for Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy with thick lamellar microstructure in two phase field based on P-map

The deformation behavior of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy with thick lamellar α microstructure is investigated by using the Processing-map (P-map). The results show that the P-map can predict the regime of flow instability and reveal deformation mechanisms well. Through analyzing P-maps and observing the microstructure evolution of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy in forging process, the phenomena of flow instability are found to occur at the temperature and strain rate ranges of (750–880 °C, 0.005–10.0 s^?1) and (880–950 °C, 0.17–10.0 s^?1), which include macrocracks, adiabatic shear bands and prior β boundary cavities. The preferable temperature and strain rate for hot working of the Ti-alloy are (790–900 °C, 0.001–0.003 s^?1) and (900–950 °C, 0.001–0.017 s^?1). In these two deformation domains, the globularization of α lamellae occurs, and the combination of the globularization of α lamellae and α + β → β phase transformation happen, respectively. For forging of Ti–6.5Al–3.5Mo–1.5Zr–0.3Si alloy in α + β phase field, the optimum temperature can be selected from the temperature range of 850–950 °C and the optimum stain rate is 0.001 s^?1 based on the volume fraction of α phase for obtaining the needed properties of forgings in design of forging processes.     

Yielding and shear banding of metallic glasses

In this study the yielding and subsequent shear banding evolution process of Zr-, Cu-, Fe- and Mg-based metallic glasses (MGs) were investigated using carefully designed cyclic microcompression tests. It was found that yielding starts from a stable plastic flow with a viscosity of the order of 10¹² Pa s, resembling that of the glass transition. This provides critical evidence that yielding is caused by a stress-induced glass transition with internal randomly distributed liquid-like cores get connected. Up to a critical point the liquidized layer penetrates the entire sample with an initiation viscosity of 10⁸ Pa s, comparable with that of the liquid-like cores. Along the liquid-like layer shear band propagation involves shear band sliding and is succeeded by shear band arrest. Dynamic softening leads to an increase in the velocity of shear band sliding, with resultant shear offset, which can be successfully captured by a linear softening model. Once the elastic energy in the shear band is dissipated its internal structure begins to recover, with the solid-like matrix being reconstructed, resulting in shear band arrest. A simple diagram elucidating the yielding and shear banding dynamics is constructed, which sheds light on the fundamental nature of the deformation mechanisms of MGs.     

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.     

Effect of strain rate on microstructure of polycrystalline oxygen-free high conductivity copper severely deformed at liquid nitrogen temperature

The microstructure of polycrystalline oxygen-free high conductivity copper subjected to severe uniaxial single compression at liquid nitrogen temperature and strain rates ranging from 10^?2 to 10⁵ s^?1 is characterized using transmission electron microscopy, X-ray diffraction and differential scanning calorimetry. A difference in strain rate leads to a change in the density, character and arrangement of dislocations, as well as the size and configuration of dislocations cells/(sub)grains in the deformed sample. A threshold strain rate of 10³ s^?1 is identified for the formation of localized deformation bands, which characterizes heterogeneity of deformation at high strain rates. These bands are composed of grains that are significantly smaller than those outside them, as well as those obtained at strain rates lower than 10³ s^?1. Under particular conditions, grains as small as several nanometers can be generated in the vicinity of these bands, through the activation of rotational dynamic recrystallization. Amorphization is identified as a deformation mechanism in structures consisting of grains smaller than ～13 nm, and this offers an explanation for the “inverse Hall–Petch effect”. A model that illustrates the initiation and propagation of an amorphous phase during deformation is proposed. Deformed samples exhibit the tendency of an increase in strength with the value of the Zener–Hollomon parameter, which captures strain rate and temperature rise during deformation. This study suggests that a strain rate in the order of 10² s^?1 should be adopted in severe plastic deformation techniques to produce nanometer-sized grains.     

Composition dependence of indentation deformation and indentation cracking in glass

Crack initiation and deformation behaviors of oxide glasses belonging to different chemical systems were studied using the Vickers indentation test. The crack initiation resistance is chiefly governed by the extents to which densification and isochoric shear flow develop in a process zone beneath and within the contact area. Densification is favored in glasses with relatively small Poisson’s ratio (ν), whereas shear is favored at large ν. Glasses were ranged according to their resistance to the formation of corner cracks as follows: Resilient, for 0.15 ? ν ? 0.20; Semi-Resilient, for 0.20 ? ν ? 0.25; and Easily-Damaged for 0.25 < ν < 0.30. Radial-median cracks occur at low load (?50 mN) in Easily-Damaged glasses, while cone cracks predominate in Resilient glasses under higher loads. A critical value for ν (～0.22 depending on the Young’s modulus/hardness ratio) was identified, at which the intensity of the indentation stress field tends to vanish, preventing crack formation on loading, while the driving force on unloading remains very small.     

A study of the interactive effects of strain,strain rate and temperature in severe plastic deformation of copper

The deformation field in machining was controlled to access a range of deformation parameters—strains of 1–15, strain rates of 10–100,000 s^?1 and temperatures of up to 0.4 T_m—in the severe plastic deformation (SPD) of copper. This range is far wider than has been accessed to date in conventional SPD methods, enabling a study of the interactive effects of the parameters on microstructure and strength properties. Nano-twinning was demonstrated at strain rates as small as 1000 s^?1 at ?196 °C and at strain rates of ?10,000 s^?1 even when the deformation temperature was well above room temperature. Bi-modal grain structures were produced in a single stage of deformation through in situ partial dynamic recrystallization. The SPD conditions for engineering specific microstructures by deformation rate control are presented in the form of maps, both in deformation parameter space and in terms of the Zener–Hollomon parameter.     

Plastic deformation in nanostructured bulk glass composites during nanoindentation

Nanoindentation experiments of a Ti₄₅Zr₁₆Be₂₀Cu₁₀Ni₉ bulk metallic glass and partially vitrified nano-composite metallic glass matrix have been performed under a constant maximum load of 10 mN and constant loading rate of 0.08 mN s^?1 with the aim of comparative study of their micro-plastic deformation behavior. Remarkable difference in deformation behavior was found in load–displacement curves of nanoindentation and pile-up morphologies around the indents. The difference in shear banding behavior has been attributed to the presence of nanosized icosahedral particles in amorphous matrix.     

Deformation behavior of isothermally forged Ti–5Al–2Sn–2Zr–4Mo–4Cr powder compact

The isothermal deformation behavior of hot isostatic pressed (HIPed) Ti–5Al–2Sn–2Zr–4Mo–4Cr(Ti-17) powder compact was investigated by compression testing in the temperature range of 810–920 °C and constant strain rate range of 0.001–1 s^?1. The true stress–true strain curves of the powder compact exhibit flow oscillation and flow softening phenomenon in both beta field and beta + alpha field. The flow softening behavior is related to the globularization of the primary acicular microstructure and deformation heating. The apparent activation energy for deformation in beta field is estimated to be 149 kJ mol^?1, indicating that the deformation is controlled by diffusion. The high apparent activation energy of 537 kJ mol^?1 for deformation in beta + alpha field may be related to the dynamic recrystallization of the primary acicular microstructure. Constitutive equations with the form of Arrhenius-type hyperbolic-sine relationship are proposed to delineate the peak flow stress as a function of the strain rate and the temperature for isothermal forging HIPed Ti-17 powder compact.     

Continuous dynamic recrystallization during the transient severe deformation of aluminum alloy 7475

Effect of strain rate and its discontinuous changes on the deformation and microstructural behavior of a coarse-grained 7475 Al alloy were studied in multidirectional forging at 763 K. Deformation at a higher strain rate of 3 × 10^?2 s^?1, controlled by homogeneous dislocation motion, leads to partial grain refinement taking place only near the original grain boundaries. Deformation at a lower rate of 3 × 10^?4 s^?1, controlled mainly by grain boundary sliding, in contrast, results in full development of strain-induced grains through grain fragmentation due to microshear band formation. Under conditions of discontinuous changes in strain rate, the flow stresses and grain size developed by subsequent severe deformation do not approach those appearing during continuous change at a constant strain rate. The nature of such strain-induced events is irreversible and athermal. The mechanisms of continuous dynamic recrystallization operating during severe deformation are discussed in detail.     

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.     

Dependence of tensile deformation behavior of TWIP steels on stacking fault energy,temperature and strain rate

Three experimental high manganese twinning induced plasticity (TWIP) steels were produced based on thermodynamic stacking fault energy (SFE) calculations, following the thermodynamic modeling approach originally proposed by Olson and Cohen (Metall Trans 7A (1976) 1897). At room temperature, the SFE γ_SFE of the three materials varied from 20.5 to 42 mJ m^?2. In order to study the correlation between the SFE and the mechanical behavior of the TWIP steels, as manifested by the propensity of the material to deformation-induced phase transformations or twinning, tensile tests were performed at temperatures ?50 °C ? T ? 80 °C using strain rates varying between 10^?3 s^?1 and 1250 s^?1. The mechanical behavior of TWIP steels reveals clear temperature dependence, related to the prevailing deformation/strain hardening mechanism, i.e., dislocation slip, deformation twinning or ε-martensite transformation. At high strain rates an increase in temperature due to adiabatic deformation heating also contributes to the SFE, shifting γ_SFE either towards or away from the optimum value for twinning.     

Deformation behavior of Zr55.9Cu18.6Ta8Al7.5Ni10 bulk metallic glass matrix composite in the supercooled liquid region

A Zr_55.9Cu_18.6Ta₈Al_7.5Ni₁₀ bulk metallic glass (BMG) composite with an amorphous matrix reinforced by micro-scale particles of Ta-rich solid solution was prepared by copper-mold casting. Isothermal compression tests of the BMG composite were carried out in the range from glass transition temperature (∼673 K) to onset crystallization temperature (∼769 K) determined by differential scanning calorimetry (DSC). The compressive deformation behavior of the BMG composite in the supercooled region was investigated at strain rates ranging from 1 × 10⁻³ s⁻¹ to 8 × 10⁻² s⁻¹. It was found that both the strain rate and test temperature significantly affect the stress–strain behavior of the BMG composite in the supercooled liquid region. The alloy exhibited Newtonian behavior at low strain rates but became non-Newtonian at high strain rates. The largest compressive strain of 0.8 was achieved at a strain rate of 1 × 10⁻³ s⁻¹ at 713 K. The strain rate change method was employed to obtain the strain rate sensitivity (m). The deformation mechanism was discussed in terms of the transition state theory based on the free volume.     

Inelastic deformation of nanocrystalline Au thin films as a function of temperature and strain rate

The mechanical behavior of nanocrystalline Au thin films with average grain size of 64 nm was investigated at strain rates 10^?5–10 s^?1, and temperatures between 298 and 383 K. The yield strength was highly sensitive to both temperature and strain rate: at room temperature it increased by ～100% within the range of applied strain rates, while it decreased by as much as 50% in the given temperature range at each strain rate. The ductility and activation volume trends pointed to two distinct regimes of plastic deformation: namely, creep-driven and dislocation-mediated plasticity, with the transition occurring at increasing strain rate for increasing temperature. The activation volume for creep-influenced deformation increased monotonically from 6.4b³ to 29.5b³ between 298 and 383 K, signifying grain boundary (GB) diffusion processes and dislocation-mediated creep, respectively. Dislocation climb, as an accommodation mechanism for GB sliding, provided an explanation for the increased activation volume at higher temperatures. The activation volumes calculated at high strain rates decreased from 19.7b³ to 11.4b³ between 298 and 383 K. A model for thermally activated dislocation depinning was applied to explain this abnormal decreasing trend in the activation volume, resulting in activation energy of 1.2 eV.     

Indentation creep of an Fe-based bulk metallic glass

In this study, the room temperature creep behavior of Fe₄₁Co₇Cr₁₅Mo₁₄C₁₅B₆Y₂ bulk metallic glass was investigated using nanoindentation technique with the maximum applied load ranging from 1 mN to 100 mN under different loading rates (0.01–2.5 mN s^?1). The creep stress exponent was derived from the recorded displacement–holding time curve. It was found that the stress exponent increases rapidly from 2.87 to 6.37 with increasing indentation size, i.e. exhibiting a positive indentation size dependence. Furthermore, as the indentation loading rate increases from 0.01 mN s^?1 to 2.5 mN s^?1, the stress exponent decreases gradually from 4.93 to 0.94. The deformation mechanism causing the nanoindentation creep is discussed in the light of the “shear transformation zone” (STZ) which provides qualitative explanation for the observed plasticity in amorphous alloy.     

Distribution characterization of boundary misorientation angle of 7050 aluminum alloy after high-temperature compression

Compression tests of 7050 aluminum alloy have been conducted at different temperatures (340, 380, 420, and 460 °C) and different strain rates of 0.1, 1, 10, and 100 s^?1. The microstructure characteristics of the alloy after deformation are investigated using OM, electron backscatter diffraction (EBSD) technique and TEM. Results show that the volume fraction of recrystallized grains and the average misorientation angle increase with the increase of deformation temperature with the strain rate of 0.1 s^?1. When the 7050 aluminum alloys were deformed at 460 °C, the volume fraction of recrystallized grains and average misorientation angle decrease with increasing strain rate. The primary softening mechanism of the 7050 aluminum alloy deformed at 340, 380, and 420 °C with the strain rate of 0.1 s^?1 is dynamic recovery. Dynamic recrystallization is the main softening mechanism of the alloy deformed at 460 °C and different strain rates. The softening mechanism of the alloy is not sensitive to strain rate.