Plasticity Of Bulk Metallic Glasses Research Articles

Thermal and plastic behavior of nanoglasses

Abstract

Improving the plasticity of bulk metallic glasses via pre-compression below the yield stress

The poor plasticity of bulk metallic glasses (BMGs) has severely limited their potential structural applications. In this paper, we reported that uniaxial pre-compression below the yield stress can be used to significantly improve the plasticity of BMGs. An as-cast Cu36Zr48Al8Ag8 BMG was pre-compressed under various pressures (400, 800 and 1600MPa) and durations (30, 60 and 120min). The pre-compression processes led to structural change that increases the free volume and produces nanocrystals in the BMG matrix, which subsequently improved the plasticity of the BMG. Detailed data analysis showed that increasing the applied pressure speeds up the structural relaxation process and nanocrystallization occurs only at high enough pressure. This investigation indicated that appropriate selection of the pressure and duration of a pre-compression process is critical for the improved plasticity of BMGs.

Linking high- and low-temperature plasticity in bulk metallic glasses: thermal activation, extreme value statistics and kinetic freezing

At temperatures well below their glass transition, the deformation properties of bulk metallic glasses are characterized by a sharp transition from elasticity to plasticity, a reproducible yield stress and an approximately linear decrease of this stress with increasing temperature. In the present work, it is shown that when the well-known properties of the undercooled liquid regime, in terms of the underlying potential energy landscape, are assumed to be also valid at low temperature, a thermal activation model is able to reproduce the observed onset of macroscopic yield. At these temperatures, the thermal accessibility of the complex potential energy landscape is drastically reduced, and the statistics of extreme value and the phenomenon of kinetic freezing become important, affecting the spatial heterogeneity of the irreversible structural transitions mediating the elastic-to-plastic transition. As the temperature increases and approaches the glass transition temperature, the theory is able to smoothly transit to the high-temperature deformation regime where plasticity is known to be well described by thermally activated viscoplastic models.

Structural heterogeneity induced plasticity in bulk metallic glasses: From well-relaxed fragile glass to metal-like behavior

To reveal the structural origin responsible for the sharp change of the fracture mode on the as-cast and thermally-relaxed status, we use nanomechanical testing to measure the stresses for the onset of plasticity of a metallic glass and develop a stochastic statistical model, which can be used to characterize structural heterogeneity (defect density and strength) inside the metallic glass. Our experiments and calculations found that, with increasing the structural relaxation, the defect density drops by two orders of magnitude. Correspondingly, the fracture of metallic glasses changes from a significantly plastic (metal-like) mode to an extremely brittle (fragile glass) one.

Zirconium based bulk metallic glass—Better resistance to slurry erosion compared to hydroturbine steel

The slurry erosion behavior of a zirconium based bulk metallic glass, Zr44Ti11Cu10Ni10Be25, was evaluated in this study. Slurry erosion tests were carried out using a non-circulating type test rig at impingement angles of 30°, 60°, and 90°. Commonly used hydroturbine steel was evaluated under the same test conditions. At 30° impingement angle, the metallic glass demonstrated nearly 2.6 times higher erosion resistance compared to the steel. At normal impingement, the metallic glass was marginally better. Lower erosion rates of the bulk metallic glass at oblique impingement angles was attributed to its significantly higher hardness and deformation induced partial devitrification. For normal impingement, material was removed in the form of fragments from highly strained platelets due to limited plasticity of bulk metallic glasses. The metallic glass demonstrated a brittle mode of erosion with higher erosion rate at higher angle of impingement. In contrast, hydroturbine steel showed ductile mode of erosion under the same test conditions.

Effects of cryogenic temperatures on mechanical behavior of a Zr60Ni25Al15 bulk metallic glass

The effects of cryogenic temperatures on mechanical behavior of a Zr60Ni25Al15 bulk metallic glass (BMG) with high glass-forming ability were investigated. With lowering the temperature from 293K to 77K, the compressive yield stress of the BMG increases from 1791MPa to 2217MPa, and the compressive plastic strain enhances from 3.5% to 16.7%. Similarly, the BMG exhibits an enhanced tensile fracture stress of 2036MPa and a distinct tensile plastic strain of 0.14% at 77K, compared to those of 1743MPa and 0 at 293K, respectively. In addition, the elastic moduli and Debye temperature of the BMG increase monotonously with decreasing temperature. The mechanisms of enhanced strength and plasticity of BMGs at cryogenic temperatures are discussed in terms of the inter-atomic bonding and the mobility and coalescence of free volumes.

Recovering compressive plasticity of bulk metallic glasses by high-temperature creep

Most bulk metallic glasses fail mechanically in a brittle manner, without much plasticity. Annealing at a temperature below the glass transition temperature typically results in structural relaxation and even more enhanced brittleness. However, we report here that significant plasticity can be recovered if the sample is subjected to stress during annealing, resulting in thermomechanical creep. The structural analysis indicates that the high-temperature creep alleviates the effect of the structural relaxation and thus leads to structural rejuvenation and improved plasticity.

Isomechanical groups in bulk metallic glasses

Isomechanical groups are, as defined by Frost and Ashby [H.J. Frost and M.F. Ashby, Deformation–Mechanism Maps, Pergamon Press, Oxford, 1982], separate classes of materials that exhibit similar deformation and transport properties when normalised by an appropriate parameter. Fundamentally, this separation results from significant differences in material structure and bonding. Here, such an analysis is applied to 40 bulk metallic glasses (BMGs) grouped into three classes according to their Poisson's ratio, which is known to be an indicator of intrinsic toughness. Through rigorous statistical analysis, it is found that isomechanical groups are present and that they may result from (1) variation in the tendency for directional bonding, and (2) how liquid-like the structure is, which may be characterised by a quantification of local volumetric strain. These results suggest that, although experimentally observed properties from BMGs in different isomechanical groups are all typically considered within the same framework, differences in atomic packing and inter-atomic bonding mean that they should in fact be treated separately. These fundamental differences in bonding and structure may explain the known large variation in the tendency for toughness and plasticity in BMGs.

Design of ductile bulk metallic glasses by adding “soft” atoms

We propose a strategy for the design of ductile bulk metallic glasses (BMGs) through minor substitution using relatively large atoms, which make the bonding nature become more metallic and with it less shear resistant. Such a locally modified structure results in topological heterogeneity, which appears to be crucial for achieving enhanced plasticity. This strategy is verified for Ti-Zr-Cu-Pd glassy alloys, in which Cu was replaced by In, and seems to be extendable to other BMG systems. The atomic-scale heterogeneity in BMGs is somewhat analog to defects in crystalline alloys and helps to improve the overall plasticity of BMGs.

On valence electron density, energy dissipation and plasticity of bulk metallic glasses

Abstract In conventional crystalline alloys, valence electron density (VED) is one of the most significant factors in determining their phase stability and mechanical properties. Extending the concept to metallic glasses (MGs), it is found, not totally surprisingly, that their mechanical properties are VED-dependent as in crystalline alloys. Interestingly, the whole VED region can be separated into two zones: Zone 1 consists of Mg-, Ca-, and RE-based (RE for rare earth) alloys; Zone 2 consists of the rest of MGs. In either zone, for each type of MGs, Poisson's ratio generally decreases as VED increases. From the energy dissipation viewpoint proposed recently, the amorphous plasticity is closely related to the activation energy for the operation of shear-transformation-zones (STZs). Smaller STZ activation energy suggests higher ductility because STZs with lower activation energy are able to convert deformation work more efficiently into configurational energy rather than heat, which yields mechanical softening and advances the growth of shear bands (SBs). Following this model, it is revealed that the activation energies for STZ operation and crystallization are certainly proportional to VED. Thus, it is understood that, in Zone 2, MGs have a smaller VED and hence lower activation energies which are favorable for ductility and Poisson's ratio. In Zone 1, MGs have the lowest VED but apparent brittleness because either of low glass transition temperature and poor resistance to oxidation or of a large fraction of covalent bonds.

Effect of Fe addition on glass forming ability and mechanical properties in Zr–Co–Al–(Fe) bulk metallic glasses

Abstract A series of Zr56Co28−xAl16Fex (x = 0, 1, 2, 4, 8 and 14, respectively) bulk metallic glasses (BMGs) are cast using water-cooled copper mold casting technique. It is found that the addition of Fe impacts the structural, thermal and mechanical properties remarkably. Enhanced glass forming ability (GFA) and compressive plasticity are obtained for x ≤ 4. Higher Fe contents deteriorate the GFA and the mechanical performance. It is further revealed that Fe addition can significantly affect the internal states thus allowing to tune the yield strength and the plasticity of BMGs. Among the investigated alloys, Zr56Co26Al16Fe2 BMG has good GFA and excellent mechanical properties, e.g. a high fracture strength of over 2 GPa and a pronounced compressive critical fracture strain of 15.1%.

The Role of Disorder and the Elastic Robustness of Bulk Metallic Glasses

ABSTRACTDespite significant atomic-scale heterogeneity, bulk metallic glasses well below their glass transition temperature exhibit a surprisingly robust elastic regime and a sharp elastic-to-plastic transition with a yield stress that depends approximately linearly on temperature. The present work attempts to understand these features within the framework of thermally activated plasticity. The presented statistical thermal activation model, in which the number of available structural transformations scales exponentially with system size, results in two distinct temperature regimes of deformation. At temperatures close to the glass transition temperature thermally activated Newtonian plastic flow emerges, whilst at lower temperatures the deformation properties fundamentally change due to the eventual kinetic freezing of the available structural transformations. In this regime, a linear temperature dependence emerges for the stress which characterises the elastic to plastic transition. For both regimes the transition to macroscopic plastic flow corresponds to a transition from a barrier energy dominated to a barrier entropy dominated statistics. The work concludes by discussing the possible influence that kinetic freezing might have on the low temperature heterogeneous and high temperature homogeneous plasticity of bulk metallic glasses.

Macroscopic tensile plasticity of bulk metallic glass through designed artificial defects

Catastrophic fracture with nearly zero tensile elongation at room temperature in bulk metallic glasses (BMGs) makes their future application as structural materials seem bleak. The instable localized shear banding with work softening under unconfined uniaxial tension should be responsible for the zero elongation. Here we show how to control the shear band and make it contribute to the macroscopic tensile plasticity of BMGs through designed artificial defects. By promoting the initiation of shear bands and hindering their instable propagation through introducing artificial defects, appreciable macroscopic tensile plasticity was successfully achieved in a conventional Zr-based BMG. Moreover, by designing the distribution of the defects to inhibit the intrinsic shear fracture, the nominal tensile strength and elongation of the BMG have been enhanced simultaneously. The principles for toughening BMG through designed artificial defects are proposed; accordingly, some strategies for designing future porous BMG materials with high mechanical performance were presented. (C) 2011 Elsevier B.V. All rights reserved.

Effects of Ni addition on the glass-forming ability, mechanical properties and corrosion resistance of Zr–Cu–Al bulk metallic glasses

The thermal stability, glass-forming ability (GFA), mechanical properties and corrosion resistance of Zr60Cu25−xAl15Nix (x = 0–25 at.%) bulk metallic glasses (BMGs) were systematically investigated. The additional Ni greatly enhances the thermal stability and GFA of BMGs. For alloy with x = 15, a fully glassy rod with a critical diameter up to 18 mm can be obtained by copper mold casting. In addition, the strength and plasticity of BMGs are effectively improved by adding Ni, and they show remarkable correlations with glass transition temperature and Poisson's ratio or G/B value, respectively. A Zr60Cu5Al15Ni20 BMG exhibits largest compressive plastic strain of 5.5%, yielding strength of 1755 MPa and Young's modulus of 83 GPa at ambient temperature. Moreover, the electrochemical potentiodynamic polarization curves demonstrate that replacement of Cu by Ni improves the corrosion resistance in chloride-ion-containing solutions significantly.

Correlation between internal states and plasticity in bulk metallic glass

We report a close correlation between the internal states and plasticity in a bulk metallic glass (BMG) and discover that the optimization of copper-mold casting current can induce large plasticity stably in an otherwise brittle BMG. It is possible to confirm that larger plasticity corresponds to the internal states with more average free volume (FV) as revealed by lower density, higher enthalpy change, and higher Poisson’s ratio. The enhanced plastic deformation mechanism is interpreted based on the FV model of BMGs, and our results may have some implications for understanding the role of the FV during plastic deformation of BMGs.

Cooling Process and Cast Structure of Zr-Al-Ni-Cu–Based Bulk Metallic Glasses Produced in Various Atmospheres

Cooling process in the production of Zr65Al7.5Ni10Cu17.5-based bulk metallic glasses (BMGs) was investigated in various chamber atmospheres in Cu mold casting. Two different cooling modes, consisting of the direct heat transfer between the melt and Cu mold in the high temperature and indirect transfer via cavity in the low-temperature regions, are suggested. In the later case, the cooling effect should depend on the chamber atmosphere, which results in the formation of glassy structure with large relaxation enthalpy casting under the ambient Ar and He atmospheres due to a good thermal conductivity. The less relaxed BMGs produced by an improved cooling effect are also expected to contain a large amount of free volume for significant deformability. Actually, it is clarified that the compressive plastic deformation is improved with an increase of the relaxation enthalpy. The present study indicates a necessity of development of the glassy structure, i.e., relaxation state, and provides a new technique of the formation of less relaxed glassy structure for the improvement of plasticity in BMGs.

Effect of frame stiffness on the deformation behavior of bulk metallic glass

It has been shown that the stability of shear banding and plasticity of bulk metallic glasses (BMGs) can be strongly influenced by the machine stiffness. Here, we demonstrated that the practice of adding a frame parallel to the sample is quantitatively equivalent to increasing the machine stiffness by the frame stiffness. A series of carefully designed experiments were conducted to verify such an effect, showing controllably enhanced plasticity of BMG samples.

Influence of Melt Temperature on Compressive Plasticity of a Cu-Zr-Al Bulk Metallic Glass

Cu47.5Zr47.5Al5 bulk metallic glasses (BMGs) were cast from the melt temperature 1143 to 1373 K. The structure, thermal and mechanical properties of the BMGs were investigated by XRD, DSC, HRTEM, dilatometric measurements, micro-hardness tests and uniaxial compression. The results indicate that the microstructure and mechanical performance of BMGs are closely affected by the casting temperature. Proper casting temperature ensures the BMGs with large relaxed excess free volume (REFV) and nano-crystallites, which favor the plastic deformation in Cu47.5Zr47.5Al5 BMGs. Regulating the preparing parameters is an important solution to good plasticity in BMGs.

Improved plasticity of bulk metallic glasses upon cold rolling

The intrinsic plasticity of Zr 44 Ti 11 Cu 9.8 Ni 10.2 Be 25 and Zr 55 Ti 5 Al 10 Cu 20 Ni 10 bulk metallic glasses (BMGs) are improved from 0.5% up to 15% plastic strain due to the introduction of microstructural inhomogeneities upon cold rolling at room temperature. This approach shows an easy way to overcome the intrinsic brittleness of the BMGs by modifying their physical properties, which enables easy nucleation and branching of multiple shear bands upon unconstrained loading during the compression test.

Characteristic length scales governing plasticity/brittleness of bulk metallic glasses at ambient temperature

In this letter, we propose a unified theory for the size-dependent plasticity of bulk metallic glasses (BMGs) at room temperature. Based on the principle of energy balance and the shear-banding kinetics, two characteristic length scales are derived. One is a sample-geometry dependent variable related to the elastic energy released to drive shear-band propagation and the other is a material-dependent constant related to the internal resistance to brittle fracture. It is shown that this unified theory is effective in explaining many unusual deformation and fracture behaviors of BMGs.