La-based Metallic Glass Research Articles

Influence of short- to medium-range electronic and atomic structure on secondary relaxations in metallic glasses

Unusual secondary relaxations were revealed in metallic glasses (MGs) recently. In this paper, we explore the structural origins of secondary relaxations in La-based MGs alloyed with different species of elements. With the combination of synchrotron X-ray diffraction and ab initio molecular dynamics simulations, solute-atom-centered clusters with a string-like type of medium-range order are found in the MGs, the formation of which leaves dispersed low-electron-density regions. It is found the activation energy of fast secondary relaxation increases with the reduction of low-electron-density regions, while slow secondary relaxation relates to the distance of La in next nearest La-La atomic pairs and the size of the string-like solute-atom-centered clusters. The phenomena are interpreted within the framework of the generalized Maxell model and free volume model. Our results demonstrate fast secondary relaxation as the activation of a small concentration of liquid-like regions with extremely low viscosities preceding slow secondary relaxation, and provide evidence for the correlation of secondary relaxations with short- to medium-range electronic and atomic structure in MGs.

Rejuvenation through plastic deformation of a La-based metallic glass measured by fast-scanning calorimetry

We explore the glassy states achievable after a metallic glass is formed on liquid quenching. Samples of La55Al25Ni20 (at.%) metallic glass (rod and ribbon) are studied. The extent of structural relaxation at room temperature is characterized for this low-glass-transition temperature glass. Plastic deformation (uniaxial compression) rejuvenates the glass to states of higher enthalpy characteristic of glass formation at high cooling rate. Deformation increases the heterogeneity of the glass, widening the spectrum of relaxation times. The extent of rejuvenation in samples of low aspect ratio is compared with that under conditions of high constraint in notched samples. The deformation-induced rejuvenation is particularly susceptible to reduction on subsequent ageing. Fast-scanning calorimetry is useful in characterizing the dynamics of structural relaxation. The shadow glass transition is more evident on fast heating, and is observed in this glass for the first time. A new excess exothermic effect is observed before the glass transition.

Contribution of cryogenic thermal cycling to the atomic dynamics in a La-based bulk metallic glass with different initial states

Structural rejuvenation in metallic glasses (MGs) induced by cryogenic thermal cycling has been intensively studied. However, the effect of thermal cycling on the atomic dynamics in MGs is still missing. In this work, we present a systematic study on the atomic dynamics in a La-based MG affected by sub-Tg annealing and thermal cycling. We find that the thermal cycling has little effect on the atomic dynamics in both as-cast and annealed MGs although it could significantly increase the free volume and change mechanical properties of the as-cast MG sample. In contrast, both the structure and mechanical properties of the well-relaxed sample are almost unchanged by thermal cycling. The increased open volume, mainly in the form of atomic vacancies, does not greatly promote the atomic dynamics in the as-cast MG sample, indicating that the free volume induced by thermal cycling is not the key factor in stimulating the β-relaxation and the boson peak in the studied MG.

Bulk intrinsic heterogeneity of metallic glasses probed by Meissner effect

Structural heterogeneity has been proposed as a key intrinsic feature and mechanism underline unique properties and dynamic behavior of metallic glasses; however, it is still challenging in precisely describing and effectively characterizing the atomic-scale heterogeneity in metallic glasses. In this work, we employed the Meissner effect of superconductivity in magnetic susceptibility measurements as a sensitive bulk probe and successfully revealed the three-dimensional structural heterogeneity and its two-way evolution tuned by structural aging or rejuvenation in a La-based metallic glass. Compared with the resistivity measurements which only signal the most superconductive loop in inhomogeneous materials, the diamagnetic susceptibility signal of the Meissner effect maps the volumetric distribution of all superconductive regions and the corresponding structural heterogeneity in metallic glasses with high sensitivity. The experimental results reported in this article can be well interpreted based on a structural model of tunable “soft liquid-like” regions with soft vibration modes mixed with “hard solid-like” regions, validating the heterogeneity models of metallic glasses with new experimental data and approach.

Homogeneous elongation and distinguishing work hardening in La-based metallic glass composites upon dynamic tension

In-situ La74Al14Cu6Ni6 metallic glass matrix composites exhibit distinct work-hardening capability under quasi-static tension, attributed to the profuse deformation twins. Under dynamic tension, 2% tensile ductility and significant negative strain rate sensitivity (SRS) of the yielding strength was revealed. A modified Cooperative-Shear model was employed to explain the negative SRS behavior.

Structural Signature of β-Relaxation in La-Based Metallic Glasses.

The secondary β-relaxation is an intrinsic feature in glassy materials. However, its structural origin is still not well understood. Here we report that the β-relaxations in La50Al15Ni35 and La50Al15Cu35 metallic glasses (MGs) mainly depend on the vibration of small Ni and Cu atoms in local cages. By using advanced synchrotron X-ray techniques and theoretical calculations, we elucidate that the tricapped-trigonal-prism-like polyhedra with more large La atoms in shells favor the local vibration of center Ni atoms, leading to the pronounced β-relaxation event. In contrast, the in-cage vibration of Cu atoms is somehow suppressed by the appearance of more shell Cu atoms. Nevertheless, they could easily diffuse out of the cages compared with Ni, thus triggering the onset of α-relaxation. This work provides a pathway to understand the different structural relaxation behaviors in MGs and other disordered materials from their local atomic packing and dynamics points of view.

Extraordinary creep relaxation time in a La-based metallic glass

The structural relaxation times of metallic glasses (MGs) are generally believed to decrease with increasing flow rate, also known as shear thinning. In this work, an extraordinary loading rate-independent creep relaxation time, in contrast to the decreasing Maxwell relaxation time with increasing loading rate, is observed for a La-based MG with pronounced Johari–Goldstein relaxation under nanoindentation. The Johari–Goldstein relaxation in the flow of MGs is suggested to be a form of local Newtonian flow and may be not crucial in shear thinning, whereas the main α relaxation is proposed to predominate in the shear thinning of MGs, based on the disparity between the creep relaxation time and the Maxwell relaxation time. These results improve the current understandings on the structural relaxation and its role in the shear thinning and plastic deformation of MGs.

On the origin of heat effects and shear modulus changes upon structural relaxation and crystallization of metallic glasses

It is well known that structural relaxation below the glass transition and crystallization of metallic glasses are accompanied by the heat release and increase of the shear modulus. In the present work, these effects are studied in detail for sixteen Zr-, Pd- and La-based metallic glasses and analyzed within the framework of the Interstitialcy theory. The obtained results imply that the increase of the shear modulus and heat effects occurring upon structural relaxation and crystallization are conditioned by the same mechanism – a change of the elastic energy related to the system of interstitialcy defects frozen-in upon glass production. In particular, the heat of crystallization constitutes a direct manifestation of the dissipation of the elastic energy related to this defect system, which disappears upon crystallization.

Slow β relaxation in La-based metallic glasses based on mechanical spectroscopy measurements

Dynamic mechanical relaxations of La-based metallic glasses were investigated by mechanical spectroscopy. In the framework of the mixing enthalpy of constituent atoms, it was found that β relaxation was less evident by the addition of Cu to replace Ni in the LaCuNiAl glassy alloy. By introducing Cu into the LaNiAl metallic glass, the mixing enthalpy was less negative, which led to weaker β relaxation of the metallic glasses. The α relaxation of the La-based metallic glasses could be described by a Kohlrausch-Williams-Watts (KWW) function with a Kohlrausch exponent βKWW around 0.5. It should be noted that physical aging above the glass transition temperature Tg induced a decrease of β relaxation intensity in the La-based metallic glass.

Thermomechanical Adhesion between Metallic Glass and Die Materials

Thermoplastic forming is a promising method for fabricating metallic glass (MG) products with complex shapes. This method can avoid the difficulties encountered in other manufacturing processes, such as very high cooling rate required by casting and catastrophic cracking in machining. However, during thermoplastic forming the adhesion between dies and MGs restricts the production. It is therefore important to explore the underlying adhesion mechanisms during forming and establish guidelines for selecting proper die materials. In this paper, we comprehensively studied the adhesion between La-based MG and some widely-used die materials (electroless Ni-P, Si, alumina and silicon nitride) in the thermoplastic forming process. It was found that, among these die materials, alumina has the best performance, which is attributed to its strong chemical bonds and low surface energy. The study concludes that the surface energy and the type of chemical bonds can be proper indicators for selecting die materials.

Understanding the serrated flow and Johari-Goldstein relaxation of metallic glasses

An increasing tendency of serrated flow with increasing loading rate of a La-based (La) metallic glass (MG) with pronounced Johari-Goldstein relaxation in nanoindentation is investigated, which is abnormal compared to precedent observations where the serrated flow normally decays with increasing loading rate as discovered in the referenced Zr-based (Zr) and Pd-based (Pd) MGs. Based on systematic analysis, a unified dimensionless number is proposed to elucidate the different origins of the abnormal serrated flow of the La MG and the normal serrated flow of the Zr and Pd MGs. The abnormal serrated flow behavior of the La MG provides critical implications in understanding the role of Johari-Goldstein relaxation in the flow of metallic glasses.

Generalized model for laser-induced surface structure in metallic glass**Project supported by the National Natural Science Foundation of China (Grant Nos. 10572002, 10732010, and 11332002).

The details of the special three-dimensional micro-nano scale ripples with a period of hundreds of microns on the surfaces of a Zr-based and a La-based metallic glass irradiated separately by single laser pulse are investigated. We use the small-amplitude capillary wave theory to unveil the ripple formation mechanism through considering each of the molten metallic glasses as an incompressible viscous fluid. A generalized model is presented to describe the special morphology, which fits the experimental result well. It is also revealed that the viscosity brings about the biggest effect on the monotone decreasing nature of the amplitude and the wavelength of the surface ripples. The greater the viscosity is, the shorter the amplitude and the wavelength are.

Fracto-emission in lanthanum-based metallic glass microwires under quasi-static tensile loading

Plastic deformation in metallic glasses is highly localized and often associated with shear banding, which may cause momentary release of heat upon fracture. Here, we report an explosive fracture phenomenon associated with momentary (∼10 ms) light emission (flash) in Lanthanum-based (LaAlNi) metallic glass microwires (dia. ∼50 μm) under quasi-static tensile loading. The load-displacement data as well as the visual information of the tensile deformation process were acquired through an in situ measurement set-up, which clearly showed nonlinear stress (σ)–strain (ϵ) curves prior to yielding and also captured the occurrence of the flash at high fracture stresses (∼1 GPa). Through the postmortem fractographic analysis, it can be revealed that the fracto-emission upon quasi-static loading could be mainly attributed to the localized adiabatic work accumulated at a very large elastic strain confined within the microscale sample volume, followed by a localized high temperature rise up to ∼1000 K at the fracture surface through localized energy dissipation. Our findings suggest that the La-based metallic glass microwires could be useful for energetic microchips, micro-ignition devices, and other functional applications.

Hand in hand evolution of boson heat capacity anomaly and slow β-relaxation in La-based metallic glasses

Glassy materials lacking the periodicity of crystals possess varieties of dynamic behaviors. Boson peak and slow β-relaxation (or Johari–Goldstein β-relaxation) preceding the α-relaxation are the two intrinsic and markedly different dynamic behaviors of glasses, and their structural origins are long-standing issues. Through alloying a model La-based metallic glass (MG) with different types of elements we find that the low-temperature boson heat capacity anomaly evolves hand in hand with high-temperature slow β-relaxation associated with free volume. With the free volume model and the generalized Maxwell model describing the heterogeneous structure, we demonstrate that the slow β-relaxation of MGs is a series of localized microscopic flows, which naturally develops from defective regions where boson peak arises. The coupling of boson peak and slow β-relaxation indicates their common structural origin of localized loosely packed regions in MGs.

Free-volume dependent atomic dynamics in beta relaxation pronounced La-based metallic glasses

The atomic dynamics and its structure dependence in a glass are fundamental issues but still little understood for decades. Through state-of-the-art experiment and simulation, we reveal that the annihilation of excess free volume in a β-relaxation pronounced La–Al–Ni metallic glass slows down the atomic motion by one order of magnitude upon annealing, which is mainly caused by the change of subatomic cavities and enhanced network structure resulting from the presence of Ni atoms, showing significant difference from a β-relaxation unpronounced La–Al–Cu glass. Our findings provide insights into the dynamics and atomic structure of La-based metallic glasses and will be helpful in understanding the microscopic dynamic behavior of metallic glasses in general.

Unusual fast secondary relaxation in metallic glass.

The relaxation spectrum of glassy solids has long been used to probe their dynamic structural features and the fundamental deformation mechanisms. Structurally complicated glasses, such as molecular glasses, often exhibit multiple relaxation processes. By comparison, metallic glasses have a simple atomic structure with dense atomic packing, and their relaxation spectra were commonly found to be simpler than those of molecular glasses. Here we show the compelling evidence obtained across a wide range of temperatures and frequencies from a La-based metallic glass, which clearly shows two peaks of secondary relaxations (fast versus slow) in addition to the primary relaxation peak. The discovery of the unusual fast secondary relaxation unveils the complicated relaxation dynamics in metallic glasses and, more importantly, provides us the clues which help decode the structural features serving as the ‘trigger' of inelasticity on mechanical agitations.

Thermal behaviors of liquid La-based bulk metallic glasses

Thermal behaviors of liquid La-based bulk metallic glasses have been measured by using the dilatometer with a self-sealed sample cell. It is demonstrated that the strong glass forming liquid not only has the small thermal expansion coefficient but also shows the slow variation rate. Moreover, the strong glass former has relatively dense atomic packing and also small density change in the liquid state. The results suggest that the high glass forming ability of La-based metallic glasses would be closely related to the slow atomic rearrangements in liquid melts.

Modulation of β-relaxation by modifying structural configurations in metallic glasses

β-relaxation and mechanical properties of La-based metallic glasses are investigated by modulation of quenching rate of the MGs. We find that the quenching rate has a remarkable effect on the β-relaxation behaviors and the mechanical properties of the metallic glasses. The β-relaxation shows a strong dependence on the cooling rate of the sample preparation, indicating that there is a correlation between the concentration of the potential flow units and the β-relaxation behaviors. The results imply that the β-relaxation is closely correlated with the inhomogeneous microstructure or the density of the potential flow units in the metallic glasses.

Compositional origin of unusual β-relaxation properties in La-Ni-Al metallic glasses.

The β-relaxation of metallic glasses (MGs) bears nontrivial connections to their microscopic and macroscopic properties. In an effort to elucidate the mechanism of the β-relaxation, we studied by dynamical mechanical measurements the change of its properties on varying the composition of La60Ni15Al25 in various ways. The properties of the β-relaxation turn out to be very sensitive to the composition. It is found that the isochronal loss peak temperature of β-relaxation, Tβ,peak, is effectively determined by the total (La + Ni) content. When Cu is added into the alloy to replace either La, Ni, or Al, the Tβ,peak increases with decrease of the (La + Ni) content. The trend is in accordance with data of binary and ternary MGs formed from La, Ni, Al, and Cu. Binary La-Ni MGs have pronounced β-relaxation loss peaks, well separated from the α-relaxation. In contrast, the β-relaxation is not resolved in La-Al and La-Cu MGs, showing up as an excess wing. For the ternary La-Ni-Al MGs, increase of La or Ni content is crucial to lower the Tβ,peak. Keeping the Al content fixed, increase of La content lowers the Tβ,peak further, indicating the more important role La plays in lowering Tβ,peak than Ni. The observed effects on changing the composition of La60Ni15Al25 lead to the conclusion that the properties of the β-relaxation are mainly determined by the interaction between the largest solvent element, La, and the smallest element, Ni. From our data, it is further deduced that La and Ni have high mobility in the MGs, and this explains why the β-relaxation in this La-based MGs is prominent and well resolved from the α-relaxation as opposed to Pd- and Zr-based MGs where the solvent and largest atoms, Pd and Zr, are the least mobile.

Tensile Plasticity in Metallic Glasses with PronouncedβRelaxations

Metallic glasses are commonly brittle, as they generally fail catastrophically under uniaxial tension. Here we show pronounced macroscopic tensile plasticity achieved in a La-based metallic glass which possesses strong β relaxations and nanoscale heterogeneous structures. We demonstrate that the β relaxation is closely correlated with the activation of the structural units of plastic deformations and global plasticity, and the transition from brittle to ductile in tension and the activation of the β relaxations follow a similar time-temperature scaling relationship. The results have implications for understanding the mechanisms of plastic deformation and structural origin of β relaxations as well as for solving the brittleness in metallic glasses.