Universality of slip avalanches in a ductile Fe-based bulk metallic glass

Intermittent serrated flows of a novel ductile Fe60Ni20P13C7 bulk metallic glass(BMG)at variant strain rates were investigated by statistics analysis.Peak and clutter distribution of slip-avalanche magnitudes are displayed during stable plastic flows at strain rates of 2×10-4 s-1 and 5×10-5 s-1,respectively,which means that serration behavior depends on the strain rate.However,the remarkable agreement between measured slip-avalanche magnitudes and the scaling behavior,i.e.a universal complementary cumulative distribution function(CCDF)predicted by mean-field theory(MFT)model,indicates that the plasticity of the present Fe-based BMGs can be tuned by imposed strain rates:Smax~6)ε-λ.This tuned plasticity is elucidated with expended free-volume model.Moreover,the scaling behavior of serrated flows for other strain rates can be predicted as well.     

Rate Dependence of Serrated Flow and Its Effect on Shear Stability of Bulk Metallic Glasses

The rate dependence of serrated flow and its effects on the stability of shear banding were systematically investigated in a prototypic bulk metallic glass.It was found that with the increase of external strain rate,the serra-ted flow is gradually suppressed and could completely disappear at a critical strain rate.The serration size,character-ized by the mean stress drop amplitude,decreases inversely with the strain rate,while the waiting time for serration decreases with the strain rate in a power-law manner.The rate dependence of the serrated flow has important effects on the dynamics and stability of shear banding process,and leads to an optimal plasticity achieved around the critical strain rate for the disappearance of serrated flow.These results are discussed and interpreted in terms of the mi-croscopic deformation theory and the stick-slip dynamics of shear banding for bulk metallic glasses.     

Elastic deformation behavior of CuZrAlNb metallic glass matrix composites with different crystallization degrees

The room temperature brittleness has been a long standing problem in bulk metallic glasses realm.This has seriously limited the application potential of metallic glasses and their composites.The elastic deform-ation behaviors of metallic glass matrix composites are closely related to their plastic deformation states. The elastic deformation behaviors of Cu48-xZr48Al4Nbx(x=0, 3 at.%) metallic glass matrix composites (MGMCs) with different crystallization degrees were investigated using an in-situ digital image correlation (DIC) technique during tensile process.With decreasing crystallization degree, MGMC exhibits obvious elastic deformation ability and an increased tensile fracture strength.The notable tensile elasticity is attrib-uted to the larger shear strain heterogeneity emerging on the surface of the sample.This finding has impli-cations for the development of MGMCs with excellent tensile properties.     

Serration Behavior in Zr-Cu-Al Glass-forming Systems

The metallic glass matrix composites(MGMCs)and bulk metallic glasses(BMGs)were studied by statistical analysis during plastic deformation at the strain rates of 2×10~(-2),2×10~(-3),and 2×10~(-4) s~(-1),respectively.No serration events occur in both MGMCs and BMGs during compression tests at the strain rate of 2×10~(-2) s~(-1).When deformed at the strain rate of 2×10~(-3) s~(-1),the BMG displays a larger plasticity,which is due to the larger serration events followed by a series of small serrations caused by the continuous movement of free volume.The amplitudes and elastic-energy densities increase with increasing the strain rates owing to many serrations in MGMCs.It is deduced that the Young′s modulus decreases from the normalized stress drop and fluctuations are observed on stressstrain curves,which is attributed to a lower coefficient according to the stick-slip model.     

Stored Mechanical Work in Inhomogeneous Deformation Processes of a Pd-Based Bulk Metallic Glass

We investigated the effect of anelastic rearrangements in a Pd-based metallic glass during inhomogeneous plastic deformation producing shear bands at room temperature. Therefore, we subjected bulk metallic glasses to uniaxial stresses and characterized the influence of deformation on the global configurational energy state with ultrasonic and calorimetric methods. The results provide evidence that even in an inhomogeneous plastic deformation process at room temperature, a certain amount of energy can be stored which is thermally relaxed below the glass transition temperature.     

Accelerated evolution of hydrogen from metals during plastic deformation

Plastic deformation accelerates the release of hydrogen from iron, Type 304L stainless steel, nickel, Inconel 718, and 5086 aluminum. The release rate is strain dependent: it increases rapidly when plastic deformation begins, reaches a maximum, and then decreases with additional strain with a final large release at fracture. The release rate is constant during Lüder’s extension for iron, and fluctuates coincidentally with the serrated flow of 5086 aluminum. The release rate during deformation also depends on temperature and strain rate. The accelerated release rate during deformation is discussed in terms of hydrogen-dislocation interactions and is interpreted as being caused by the egress of of dislocations and their associated hydrogen atmospheres during plastic deformation.     

Effect of pre-existing shear bands on mechanical properties and serration behaviors in bulk metallic glasses

Pre-existing (multiple) shear bands were introduced into the ductile Zr56 Co28 Al16 and Zr65 Ni10 Cu15 Al10 bulk metallic glasses (BMGs) through the lateral-deformation, respectively.It was found that the pre-exi-ting shear bands can further enhance the compressive plasticity of ductile BMGs.According to the serra-tion analysis on the plastic deformation of the as-cast and the pre-deformed samples, the serration events in the stress-strain curves during deformation display a self-organized critical (SOC) behavior.Compared with the as-cast BMGs, a larger power-law scaling exponent calculated based on serrated flow behaviors becomes larger for the pre-deformed BMGs, implying that the shear banding stability of BMGs is effec-tively enhanced.This should be caused by the pronounced interactions of shear bands during plastic de-formation for the pre-deformed BMGs.However, by introducing a large amount of multiple shear bands into the glassy matrix, it also becomes easier for shear bands to propagate along the pre-existing shear bands, leading to a lower cut-off elastic energy density for the pre-deformed BMGs.More multiple shear bands with stronger interactions for the pre-deformed BMGs could provide a larger chance to activate the shear-band cracking but less local elastic energies are remained for the subsequent crack-linking.     

Accelerated evolution of hydrogen from metals during plastic deformation

Plastic deformation accelerates the release of hydrogen from iron, Type 304L stainless steel, nickel, Inconel 718, and 5086 aluminum. The release rate is strain dependent: it increases rapidly when plastic deformation begins, reaches a maximum, and then decreases with additional strain with a final large release at fracture. The release rate is constant during Lüder’s extension for iron, and fluctuates coincidentally with the serrated flow of 5086 aluminum. The release rate during deformation also depends on temperature and strain rate. The accelerated release rate during deformation is discussed in terms of hydrogen-dislocation interactions and is interpreted as being caused by the egress of of dislocations and their associated hydrogen atmospheres during plastic deformation.      

Extracting compressive stress-strain curve based on stick-slip shear banding process in bulk metallic glasses

Based on the stick-slip process, an effective method to extract the stress-strain curve directly from the crosshead displacement-load raw data in compression of bulk metallic glasses was proposed.The method was tested in two bulk metallic glass samples with different plasticities and shear band morphologies.The extracted stress-strain curves were found to well resemble the stress-strain curve measured by a laser ex-tensometer.In addition, the extracted curve could resolve fine structures of serrated flow much better than that measured by extensometer, thus facilitating the study of shear banding process.Results obtained by this method made the stick-slip dynamics of shear banding valid, and this method could be employed to obtain the real strain of small-sized metallic glass samples where extensometer cannot be applied.     

Effect of yttrium addition on flow behavior of Cu-Zr-Al bulk metallic glass in the supercooled liquid region

The high temperature deformation behaviors and thermal workability of Cu_(43)Zr_(48)Al_9 and(Cu_(43)Zr_(48)Al_9)_(98)Y_2 bulk metallic glasses in the supercooled liquid region were investigated by the uniaxial compression tests. The results showed that the high temperature deformation behaviors were highly sensitive to strain rate and temperature, and the flow stress decreased with the increase of temperature, as well as with the decrease of strain rate. Additionally, the(Cu_(43)Zr_(48)Al_9)_(98)Y_2 bulk metallic glass displayed smaller flow stress under the same condition. The flow behavior changed from Newtonian to non-Newtonian with increase of the strain rate, as well as the decrease of temperature, which could be explained by the transition state theory. We found that(Cu_(43)Zr_(48)Al_9)_(98)Y_2 bulk metallic glass had better flow behavior than the Cu_(43)Zr_(48)Al_9 bulk metallic glass in the supercooled liquid region. In addition, the processing maps of the two bulk metallic glasses were constructed considering the power dissipation efficiency. The optimum domain for thermal workability of the bulk metallic glass was located using the processing map, where the power dissipation efficiency was larger than 0.8. It was shown that the(Cu_(43)Zr_(48)Al_9)_(98)Y_2 bulk metallic glass, which had larger area of optimum domain, had excellent thermoplastic forming.     

Specimen Size Effects on Zr-Based Bulk Metallic Glasses Investigated by Uniaxial Compression and Spherical Nanoindentation

Specimen size effects on the mechanical behavior of Zr-based bulk metallic glasses (BMGs) were investigated by compression and nanoindentation tests. In compression, even at the 1- to 10-mm scale, stable shear band propagation and extensive plastic deformation can be achieved in small (2 mm) specimens, in contrast to large (6.5 mm) specimens, which fail catastrophically after limited plastic deformation. The yield strength is independent of specimen size in this range, and plastic deformation remains highly localized in a few shear bands even in those specimens that exhibit stable shear sliding. The fracture surfaces of small specimens are smooth, without the vein patterns normally observed as characteristic features on the fracture surfaces of BMGs. During spherical nanoindentation, it is found that the upper bound of the maximum shear stress to initiate plasticity (yielding) in a Zr-based BMG is almost constant for indenter radii smaller than ~90 μm. However, the lower bound of this maximum shear stress decreases with increasing indenter radius, probably due to the increased probability of finding defects underneath larger indenters.     

Rate-Dependent Temperature Increases in Shear Bands of a Bulk-Metallic Glass

Using an infrared (IR) camera, we observed in situ the dynamical shear-banding processes of the geometrically constrained specimens of a Zr-based bulk metallic glass in a quasi-static compression at various strain rates, measured the temperature evolutions within the specimens, and calculated the temperature increases in shear bands. Strain-rate-dependent serrated plastic flow is a result of shear-banding operations. The average temperature increases in the specimens are observed during the plastic deformation and their magnitudes are strain rate dependent. The temperature increases in shear bands are related to strain rates. The higher the strain rates, the larger the temperature increases in a shear band. The shear strain in a shear band may be responsible for the strain-rate-dependent temperature increase in a shear band. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.

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Self-organized Criticality Behavior in Bulk Metallic Glasses

Serrated flows are known as repeated yielding of bulk metallic glasses (BMGs)during plastic deformation under different loading conditions,which are associated with the operation of shear banding.According to the statis-tics of some parameters,the shear avalanches can display a self-organized critical state,suggesting a large ductility of BMGs.The emergence of the self-organized criticality (SOC)behavior in different BMGs is due to the tempera-ture,strain rate,and chemical compositions.The SOC behavior is accompanied with the following phenomena:the interactions occur in the shear bands;the incubation time is longer than the relaxation time;the time interval is lac-king of typical time scale;and the spatial or temporal parameters should display a power-law distribution.     

Effect of plastic deformation and high pressure working on the structure and microhardness of metallic glasses

The effects of rolling, bending and high pressure working on the structure and microhardness of Fe, FeNi and Co based metallic glasses have been investigated. Some special methods of specimen deformation and microhardness measurement were used. The structure has beeninvestigated by X-ray analysis and positron annihilation methods. A large importance of structural relaxation process occurring at room temperature during some hours after deformation has been established. The correlation between structural changes and those of mechanical properties after deformation has been found. The strain hardening and strain softening of metallic glasses has been found to depend upon the pattern of stressed state under the deformation.     

Serrated plastic flow behavior and microstructure in a Zr-based bulk metallic glass processed by surface mechanical attrition treatment

The serrated plastic flow, microstructure and residual stress of a Zr55 Cu30 Ni5 Al10 bulk metallic glass (BMG) undergone surface mechanical attrition treatment (SMAT) have been investigated by a combina-tion of compression tests with scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM) and the incremental hole-drilling strain-gage method.It is found that SMAT leads to various microstructural modifications and residual stress distribution in the surface layers of the Zr-based BMG due to the mechanically-induced nanocrystallization and generation of shear bands.As a re-sult, the BMG alloy exhibits a remarkable work-hardening like behavior and significant increase of plastic strain from less than 1% to 15%, and its plastic deformation dynamics yields a power-law distribution of shear avalanches.Based upon the analysis of the experimental results, it is indicated that this can be con-nected to the SMAT-induced microstructural modifications and the resulting residual compressive stress in the Zr-based BMG.     

Cu基块体非晶合金的高温均匀塑性流变行为研究

本文选用非晶形成能力高,且在玻璃转变区和过冷液相区有高热稳定性的Cu46Zr47-x Al7块体非晶合金为研究对象,利用差示扫描量热分析（DSC）、高温真空压缩实验等方法,对其高温均匀塑性流变行为进行研究。结果表明,Cu46Zr47Al7块体非晶合金的高温均匀塑性形变行为依赖于温度和应变速率,随着温度的降低或应变速率的增大呈现出牛顿流变向非牛顿流变的转变,该行为可以用基于自由体积的过渡态理论来进行定量描述。根据过渡态理论拟合出来的驱动体积大致相当于25~50个原子,驱动能为551kJ/mol,说明控制合金高温均匀形变的原子不会脱离周围原子而单独跃迁,其扩散涉及到多原子的协同运动。     

Effects of tungsten fiber on failure mode of zr-based bulk metallic glassy composite

The authors systematically investigated the effects of tungsten fiber on failure mode as well as deformation and fracture mechanisms in tungsten fiber-reinforced Zr_41.25Ti_13.75Ni₁₀Cu_12.5Be_22.5 bulk metallic glassy composite under uniaxial compression at room and high temperatures. At room temperature, the failure mode of the composite changes from shear fracture to longitudinal splitting failure with increasing fiber volume fraction. Similar to the observations in monolithic metallic glasses, the shear fracture angle of the composite is approximately equal to 39∼40 deg, indicating that the Mohr-Coulomb criterion is suitable to give the critical shear fracture condition of the composite. When the compression tests were performed below the glass transition temperature of Zr_41.25Ti_13.75Ni₁₀Cu_12.5Be_22.5 metallic glassT _g, the deformation behavior of the composite strongly depends on the strain rates and the test temperature, which is quite similar to the deformation behavior of monolithic metallic glasses in the supercooled liquid region. The corresponding failure mode of the composite changes from shear or splitting fracture to bending failure with decreasing strain rate or increasing test temperature. The failure modes at the temperature nearT _g are mainly controlled by the metallic glass matrix due to the decrease in its viscosity at high temperature. Based on these multiple failure modes, the effects of test temperature and tungsten fiber volume fraction on deformation and fracture mechanisms are summarized.     

Optimisation of superplastic processing parameters for (Cu43Zr48Al9)98Y2 bulk metallic glasses

In this study,hot compression tests of (Cu_(43)Zr_(48)Al_9)_(98)Y_2 bulk metallic glasses (BMGs) were performed,and their micro structure and thermal properties after the deformation were studied to explore the appropriate range of their optimum processing parameters. The experimental results show that the superplastic deformation of (Cu_(43)Zr_(48)Al_9)_(98)Y_2 bulk metallic glasses depends mainly on the temperature and strain rate. It is suitable for superplastic processing when the alloys are in the state of Newtonian flow and do not crystallise. The appropriate processing parameters of (Cu_(43)Zr_(48)Al_9)_(98)Y_2 BMGs are the temperatures and strain rates, which are below the "dividing line". And when the temperature is above733 K, the strain rate must be1×10~(-3) s~(-1).     

A Brief Review of High Entropy Alloys and Serration Behavior and Flow Units

Multicomponent alloys with high entropy of mixing,e.g.,high entropy alloys (HEAs)and/or multiprin-cipal-element alloys (MEAs),are attracting increasing attentions,because the materials with novel properties are being developed,based on the design strategy of the equiatomic ratio,multicomponent,and high entropy of mixing in their liquid or random solution state.Recently,HEAs with the ultrahigh strength and fracture toughness,excel-lent magnetic properties,high fatigue,wear and corrosion resistance,great phase stability/high resistance to heat-softening behavior,sluggish diffusion effects,and potential superconductivity,etc.,were developed.The HEAs can even have very high irradiation resistance and may have some self-healing effects,and can potentially be used as the first wall and nuclear fuel cladding materials.Serration behaviors and flow units are powerful methods to understand the plastic deformation or fracture of materials.The methods have been successfully used to study the plasticity of amorphous alloys (also bulk metallic glasses,BMGs).The flow units are proposed as:free volumes,shear transi-tion zones (STZs),tension-transition zones (TTZs),liquid-like regions,soft regions or soft spots,etc.The flow units in the crystalline alloys are usually dislocations,which may interact with the solute atoms,interstitial types,or sub-stitution types.Moreover,the flow units often change with the testing temperatures and loading strain rates,e.g., at the low temperature and high strain rate,plastic deformation will be carried out by the flow unit of twinning,and at high temperatures,the grain boundary will be the weak area,and play as the flow unit.The serration shapes are related to the types of flow units,and the serration behavior can be analyzed using the power law and modified power law.     

Mechanical behavior of a bulk nanostructured iron alloy

Bulk, fully dense materials were prepared from Fe-10Cu with grain diameters between 45 nm and 1.7 μm. The materials were prepared by ball milling of powders in a glove box, followed by hot isostatic pressing (hipping) or powder forging. Larger grain sizes were obtained by thermal treatment of the consolidated powders. The bulk materials were relatively clean, with oxygen levels below 1500 wpm and other contaminants less than 0.1 at. pct. The mechanical behavior of these materials was unique. At temperatures from 77 to 470 K, the first and only mechanism of plastic deformation was intense shear banding, which was accompanied by a perfectly plastic stress-strain response (absence of strain hardening). There was a large tension-compression asymmetry in the strength, and the shear bands did not occur on the plane of maximum shear stress or the plane of zero extension. This behavior, while unusual for metals, has been observed in amorphous polymers and metallic glasses. On the other hand, the fine-grained Fe-10Cu materials behaved like coarse-grained iron in some respects, particularly by obeying the Hall-Petch equation with constants reasonably close to those of pure iron and by exhibiting low-temperature mechanical behavior which was very similar to that of steels. Transmission electron microscopy (TEM) studies found highly elongated grains within shear bands, indicating that shear banding occurred by a dislocation-based mechanism, at least at grain sizes above 100 nm. Similarities and differences between the fine-grained Fe-10Cu and metals, polymers, metallic glasses, radiation-damaged metals, and quench-damaged metals are discussed. This article is based on a presentation made in the symposium “Mechanical Behavior of Bulk Nanocrystalline Solids,” presented at the 1997 Fall TMS Meeting and Materials Week, September 14–18, 1997, in Indianapolis, Indiana, under the auspices of the Mechanical Metallurgy (SMD), Powder Materials (MDMD), and Chemistry and Physics of Materials (EMPMD/SMD) Committees.