Structural Rejuvenation Research Articles

Long-term elasto-static compressive loading drives rejuvenation of a metallic glass

The current work focuses on the impact of mechanical protocol which can lead to either rejuvenation or relaxation of metallic glasses (MGs) through elasto-static compressive loading (ECL) treatment. Here we demonstrate that, even after imposing a long-term ECL process, the Zr52.5Cu17.9Ni14.6Al10Ti5 MG subjected to an elasto-static stress of 85% of the yield stress at room temperature shows varying degrees of rejuvenation without any relaxation. The MG undergoes an increasing trend in structural rejuvenation with an increased stored energy and a more disordered structure. Nanoindentation results reveal that the variation of mechanical responses emerges from the complementary effects of the thermal activation process and the structural heterogeneity of glassy phase. The spital heterogeneities can effectively reduce the activation barriers of the shear transformation zones (STZs) and widen their distributions, which will perturb the shear-banding processes from single motion to collective movements, leading to shear band multiplication/branching and an increase in the elastic energy density cut-off. As a result, the plasticity is significantly enhanced and the yield strength is decreased. These findings shed light on that ECL process is an effective way to enhance the structural heterogeneity and the level of rejuvenation of a monolithic MG, which may allow the design of MGs with desirable mechanical properties.

Young blood-mediated cerebromicrovascular rejuvenation through heterochronic parabiosis: enhancing blood-brain barrier integrity and capillarization in the aged mouse brain.

Age-related cerebromicrovascular changes, including blood-brain barrier (BBB) disruption and microvascular rarefaction, play a significant role in the development of vascular cognitive impairment (VCI) and neurodegenerative diseases. Utilizing the unique model of heterochronic parabiosis, which involves surgically joining young and old animals, we investigated the influence of systemic factors on these vascular changes. Our study employed heterochronic parabiosis to explore the effects of young and aged systemic environments on cerebromicrovascular aging in mice. We evaluated microvascular density and BBB integrity in parabiotic pairs equipped with chronic cranial windows, using intravital two-photon imaging techniques. Our results indicate that short-term exposure to young systemic factors leads to both functional and structural rejuvenation of cerebral microcirculation. Notably, we observed a marked decrease in capillary density and an increase in BBB permeability to fluorescent tracers in the cortices of aged mice undergoing isochronic parabiosis (20-month-old C57BL/6 mice [A-(A)]; 6 weeks of parabiosis), compared to young isochronic parabionts (6-month-old, [Y-(Y)]). However, aged heterochronic parabionts (A-(Y)) exposed to young blood exhibited a significant increase in cortical capillary density and restoration of BBB integrity. In contrast, young mice exposed to old blood from aged parabionts (Y-(A)) rapidly developed cerebromicrovascular aging traits, evidenced by reduced capillary density and increased BBB permeability. These findings underscore the profound impact of systemic factors in regulating cerebromicrovascular aging. The rejuvenation observed in the endothelium, following exposure to young blood, suggests the existence of anti-geronic elements that counteract microvascular aging. Conversely, pro-geronic factors in aged blood appear to accelerate cerebromicrovascular aging. Further research is needed to assess whether the rejuvenating effects of young blood factors could extend to other age-related cerebromicrovascular pathologies, such as microvascular amyloid deposition and increased microvascular fragility.

Direct Evidence of Electric Double Layer (EDL) Repulsive Force Being Responsible for the Time-Dependent Behavior of Clay Gels in the Structural Rejuvenation Mode.

A strong EDL repulsive force is needed to accentuate the time-dependent behavior of charge and shape anisotropic clay gels at the stepdown shear rate. This force was strengthened by P2O74- adsorption, increasing the negative charge density of the clay particles. At the stepdown shear rate of 10 s-1, it is strong enough to disrupt the flow-aligned structure attained at 1000 s-1 and orient the particles to form more bonds. The resultant outcome is stepdown shear stress increasing with time until these structure disruption and bond formation processes reach an equilibrium state. The number of lower energy approach configurations (-ve face - +ve edge) for bonding is reduced by the strengthened EDL repulsive force, slowing the bonding process. The time to reach the equilibrium stepdown shear stress value increased initially and then decreased and became zero at a high negative charge density where the charge anisotropy of the particles no longer exists. The need of a sufficiently strong EDL repulsive force for the display of time-dependent behavior is true for all clay gels: Laponite, hectorite, NaMnt, sepiolite, and kaolin gels. The untreated NaMnt gel already displayed time-dependent behavior as its EDL repulsive force is sufficiently strong. The same EDL-control time-dependent behavior was obtained if pH was used to vary the negative charge density of the clay particles.

Distinct avalanche dynamics detected in metallic glasses with high energy state revealing the crack-like shear banding mechanism

When a sufficiently high stress is applied to a metallic glass, causing plastic deformation, the material undergoes structural reconfiguration through dissipative slip avalanche events that release local stresses. By utilizing isothermal annealing and cold rolling techniques to tune the energy levels of metallic glasses, it has been observed that structural rejuvenation is accompanied by structural relaxation, as evidenced by distinct changes in avalanche dynamics. We present detailed statistics of the avalanche dynamics during shear band formation in energy-tuned metallic glasses, ranging from structurally relaxed to rejuvenated states. By analyzing shear band characteristics and examining scaling exponents, avalanche durations, and stress relaxation rates, we can establish a connection between the local activation of shear transformation zones and the formation of macroscopic shear bands. The statistics of avalanche duration indicate that an increase in soft zones within metallic glasses can alleviate stress release and stabilize plastic flow, as evidenced by the characteristics of shear bands. We attribute the significant transition of serrated flow, observed at different energy levels (i.e., as-cast, relaxed, and rejuvenated states) to the variations in nucleation and multiplication of shear bands that originate from local weak spots. Analysis of the distinct avalanche dynamics suggests that in lower energy level metallic glasses, the nucleation and propagation of shear bands exhibit localized crack-like behavior, while in higher energy level metallic glasses, they display diffused crack-like characteristics. Indeed, our results strongly support that the decreased avalanches observed in the high energy level metallic glasses originate from the nucleation of numerous small shear bands, which directly compete with the propagation of the main local shear band. These findings deepen our fundamental understanding of the relationship between the microscopic mechanism of slip avalanche dynamics and shear banding, providing a pathway to control the plasticity of metallic glasses.

Endoscopic, "Scarless" Composite Flap Face and Necklift.

Deep-plane or composite flap facelift techniques allow deep structural rejuvenation of the face by release of retaining ligaments in the sub-SMAS plane with elevation and fixation of the SMAS/platysma/skin flap as a single, "en-bloc" unit. This means that in cases with mild to moderate skin laxity, the preauricular incision serves the purpose of access only. The author therefore developed an endoscopic-assisted, en-bloc composite flap face and necklift without a preauricular incision. The technique uses the prezygomatic and premasseteric (facial) and subplatysmal (neck) spaces as ideal optical cavities for endoscopic dissection and ligament release. Verticalization of fixation vectors and modified concho-mastoid traction sutures are used to minimize preauricular skin reduncancy. The surgical procedure is described in detail. Clinical experience in 41 consecutive cases and comparison to other techniques with respect to relevant anatomy are also presented. This endoscopic en-bloc composite flap facelift technique consistently and safely produced results comparable to conventional, "open" composite flap facelifts done by the same surgeon on similar candidates during a prior period.

Time-dependent clay gels: Stepdown shear rate behavior, microstructure, ageing, and phase state ambiguity

To study the time-dependent properties of clay gels in the structural rejuvenation mode, a combination of the stepdown shear rate method and an ageing test is advocated. In this mode, the structural development process at two different initial equilibrium states during the low stepdown shear rate and the rest period of the ageing test were followed. The roles played by the electric double layer (EDL) repulsive force in this structural development process with and without shear, for platelet sodium montmorillonite or NaMnt, hectorite and Laponite gels, and fibrous sepiolite gels were assessed. The response of the stepdown shear stress and ageing yield stress reflects the action of these forces on the structural rejuvenation kinetics and the type of structure formed. The platelet clay gels with their strong EDL repulsive particle interaction commenced the structural development process immediately and formed an open network structure. This process is short in shear and takes years when without. The fibrous sepiolite gel with its weak EDL repulsive interaction did not display structural rejuvenation in shear and only showed a significant yield stress increase after the first few hours of ageing. An additional issue concerning the stability of the isotropic liquid (IL) phase state of dilute Laponite dispersions was addressed. These dispersions aged for >9 years, displaying that yield stress and network structure confirmed that the IL phase is unstable. A low energy path available for Laponite particles to aggregate was discussed. A new method for determining the flocculate phase state was also presented.

Structural rejuvenation and relaxation of a metallic glass under the periodically thermal-mechanical loading

The current work focuses on effect of the thermomechanical protocol which can lead either to aging or rejuvenation of glass through decoupling of the thermal and mechanical processes. We demonstrated that the La-based metallic glass shows a kind of material hypomnesia because a clearer rejuvenation trend is observed just after imposing strain oscillations of increasing amplitude than after a long period from that, as it is proved by the results of stress relaxation. There is a threshold value of the oscillation amplitude that separates the effect of the protocol between acceleration of aging and rejuvenation. The activated energy spectrums imply plasticity and mechanical heterogeneity of the metallic glass promoted by simple mechanical stimulation. Mechanical perturbations applied not long ago in time are vital for hypomnesia metallic glasses, in which different histories lead to various degrees of aging or rejuvenation. The correlation between the thermomechanical properties of metallic glasses and the previous application of strain oscillations of various amplitude is unveiled in this work, revealing an effective tool for regulating the structural state of metallic glasses through an easy-operated method.

Rejuvenation-deformation relationship of a well-aged metallic glass during Newtonian to non-Newtonian flow

Stress overshoot is a sign of structural rejuvenation of metallic glass, which usually occurs in non-Newtonian flow with large strain rate. Here we show a heavily-aged Zr-based metallic glass that rejuvenates by low-strain-rate Newtonian flow without showing any stress overshoot. Its rejuvenation does not increase the energy level of its basin of potential energy landscape, but reduces the basin's mechanical stability. Synchrotron X-ray total scattering gives direct evidence that under Newtonian flow, the rejuvenation is contributed by complex structural rearrangements over short- and medium-range order scales. We further find that due to the structurally inhomogeneous response, there is an optimal strain rate for rejuvenation, above which the energy level decreases, but the characteristic size of the medium-range order continues to increase. Thus, the mechanical stability and structural disorder do not decrease synchronously. Our findings refresh the understanding of the relationship between mechanical deformation and structural rejuvenation in metallic glasses.

Unexpected creep behavior in a rejuvenated metallic glass

Rejuvenation, bringing metallic glasses (MGs) to the younger and higher energy states, provides an alternative avenue to explore the interplay between the property and microstructures of MGs. In this study, the creep behavior of the Zr69.5Cu12Ni11Al7.5 MGs was experimentally examined by controlling the energy state in terms of structural rejuvenation and thermal annealing. It is found that compared to the as-cast counterpart, the annealed MG at a lower energy state exhibits a higher hardness, a smaller displacement, and a lower creep rate due to the decreased free volume and the inhibited activation of the shear transformation zone. Conversely, the rejuvenated MG at a high energy state displays lower hardness and increased free volume content, yet it demonstrates superior creep resistance compared to its as-cast counterpart, which deviates from conventional understanding. This unexpected phenomenon occurs as the initial high-content free volume annihilates during creep, and strain hardening takes precedence over strain softening as the prevailing process during creep deformation, leading to a superior creep performance in extremely rejuvenated MGs.

Nanoscale strain mapping and symmetry analysis of Zr50Cu40Al10 metallic glass rejuvenated by high-pressure torsion via 4D scanning transmission electron microscopy

Structural rejuvenation by the application of high-pressure torsion suppresses the shear localization of metallic glasses. However, it remains unknown how the local atomic structure of the glass changes under such conditions. In this study, we investigated the strain and order of the local atomic structure after the application of high-pressure torsion by obtaining their spatial distribution through diffraction mapping, finding that anisotropic local strain was introduced, and the local atomic structure was disordered.

Optimally rejuvenated model binary glasses

Using the creation relaxation algorithm developed for the atomistic modeling of the high-dose irradiation limit of crystalline systems, we explore the limits of the structural rejuvenation of a highly excited model binary glass. This high-energy athermal amorphous structure exhibits a direct transition to homogeneous plastic flow and a microstructure that is largely insensitive to this flow, being characterized by a porous system-spanning network of minimally frustrated structural motifs. The observed homogeneous plasticity is mediated by the same string-like structural excitations, which mediate structural relaxation and microplasticity at finite temperature in more relaxed structures. This highly rejuvenated structural asymptote is not far from the structural state of regions, which have experienced athermal shear localization in more relaxed samples, suggesting an optimally rejuvenated glassy structure will always be limited by that produced by shear localization.

Structural rejuvenation of a well-aged metallic glass

Rejuvenation of glassy structures in general is characterized by the exothermic enthalpy prior to the glass transition. In the present work, we find that this situation is not applicable to a heavily-aged Zr-based metallic glass that rejuvenates by anelastic deformation before yield. Instead, its rejuvenation can be precisely captured by the low-temperature boson heat capacity peak as well as the effective enthalpy change with the glass-to-liquid transition. These results demonstrate that a structurally stable glass could rejuvenate by decreasing mechanical stability of its basin of potential energy landscape, but without changing the basin's energy level. The underlying mechanism points toward the redistribution of the atomic free volume with a constant system-averaged value. We further find that the rejuvenation limit of this glass is its steady-flow state with self-similar inherent structures at both short- and long-time scales. Our findings refresh the understanding of glass rejuvenation and suggest that the boson peak is a better probe for the structural rejuvenation of glasses.

Ultrasonic-promoted defect activation and structural rejuvenation in a La-based metallic glass

Ultrasonic-assistant rejuvenation in a La-based metallic glass was demonstrated by means of nanoindentation for the first time. A more pronounced nanoindentation creep behavior along with mechanical softening was detected. It is proposed to be associated with the activation of flow defects with long characteristic relaxation time at a low loading rate and short characteristic relaxation time at a high loading rate during anelastic deformation process based on the Maxwell-Voigt model. Such rejuvenation behavior also leads to an unstable plastic flow and a relatively low energy barrier for atomic diffusion during the fast growth process of crystalline phases accompanied with the reduced glass transition and crystallization temperatures. Our studies might provide insights into the microscopic deformation mechanism of glassy materials, which could help in seeking the novel applications of amorphous alloys such as ultrasonic-assistant cold joining or molding.

Rejuvenation by compressive elasto-static loading: The role of static stress on a Zr-based metallic glass

Calorimetric, mechanical, and microstructural characterizations are used to investigate the rejuvenation of Zr55Cu30Al10Ni5 (at%) bulk metallic glass (BMG) by elasto-static loading (ESL). The as-cast BMG samples are compressed at room temperature for 24 h while being subjected to static stresses that ranged from 40 %, 70 %, 80 %, and 90 % of the yield stress. The BMG undergoes structural rejuvenation as a result of the compressive ESL treatment, increasing the free volume content at all applied static stresses. The amount of energy that has been stored, as determined by the relaxation exotherms that precede glass transition, appears to be a function of static stress and reaches a maximum of 13.2 J g−1 at the highest applied stress, which is twice the stored energy of the as-cast sample. A steady increase in plastic strain, a decline in hardness, and a fall in Young's modulus are linked to enthalpy rejuvenation of the BMG. According to microstructural investigations, ESL introduces structural heterogeneities into the BMG that are linked to structural dilatation at the short- and medium-range atomic scales. This study emphasizes rejuvenation as the main effect of ESL treatment on the BMG under study.

Unified upper temperature for cryogenic thermal cycling treatment in Fe-based bulk metallic glasses

Fe-based bulk metallic glasses have high fracture strength and large elastic limit. However, the extremely poor plasticity at room temperature severely renders their wide use as structural materials. In this work, three Fe-based bulk metallic glasses were selected to explore the effect of the upper temperature for cryogenic thermal cycling treatment on the plasticity of the specimens after rejuvenation. It was found that all three Fe-based bulk metallic glasses exhibit the largest plasticity when the upper temperature is about 0.62Tg, where Tg is glass transition temperature. The underlying mechanism is understood by the evolution of the structural heterogeneity dominates by the competition between structural rejuvenation and physical aging during the CTC treatment. When the upper temperature is relatively low, the structural rejuvenation dominates, which causes an increase of the plasticity of bulk metallic glasses; Otherwise, a too high upper temperature leads to a physical aging, resulting in the decline of plasticity. This work will provide process reference for the cryogenic thermal cycling treatment of Fe-based bulk metallic glasses.

Achieving structural rejuvenation in metallic glass by modulating β relaxation intensity via easy-to-operate mechanical cycling

Structural rejuvenation is an effective measure to optimize the mechanical properties of metallic glasses (MGs). Sophisticated solutions to rejuvenation include thermal cycling, laser shocking, and multiaxial stress loading. Here, we propose an easy-to-operate mechanical cycling as an alternative strategy to tailor the mechanical relaxation, deformation, and structural heterogeneity of MGs. Structural rejuvenation in a La-based MG is achieved via mechanical cycling even at very few cycles (102 tension load cycles) and low frequencies (10−3 Hz). The results manifest intuitively the competition between structural relaxation and rejuvenation, which constitutes the structural evolution in MGs. A theoretical model is constructed which reveals a scenario that mechanical cycling wakes up frozen flow defect, accelerating creep and, thus, enhancing the β relaxation in MGs. Therefore, this handy anti-ageing methodology provides an alternative pathway to optimize the mechanical properties of MGs. It also contributes to a more comprehensive understanding of the structure-property relationship in amorphous materials, especially with regard to the correlation between structural rejuvenation and relaxation behavior in such topologically disordered materials.

Two-step annealing induced structural rejuvenation: A cause for memory effect in metallic glasses

The structural origin of the memory effect is still elusive in glassy materials. In this letter, the memory effect in a series of metallic glasses (MGs) is observed by two step annealing using differential scanning calorimetry (DSC) and molecular dynamics simulations. It demonstrates that the Kovacs peak can be reflected from not only the system potential energy but also the atomic dynamics, showing the memory effect could originate from the formation of more loose packing configurations around mobile atoms by structural rejuvenation in the second step of high temperature annealing. In contrast, the high-low temperature annealing protocol can only induce the monotonic relaxation. Therefore, our results pave a pathway to better understand the origin of memory effect in MGs and other glassy materials from their dynamics and local atomic structure.

Deformation-induced medium-range order changes in bulk metallic glasses

Bulk metallic glasses (BMGs) naturally have excellent strength and elasticity while structural rejuvenation into higher energy glassy states is often required to improve ductility. However, our understanding of the detailed atomic ordering changes that occur during rejuvenation processes, such as plastic deformation, remains limited. This study utilizes nanobeam electron diffraction in a transmission electron microscope as an effective method to reveal the structural changes that occur after deformation in two Zr-based BMGs. Our findings indicate that heavy deformation from indentation or fracture causes an increase in the size of fcc-like medium-range order (MRO) clusters in a harder icosahedral dominated matrix, which corresponds to local softening of the BMGs. By examining the structure evolution at different points in the fracture process, we reveal that the mechanism of growth of MRO clusters is likely driven by enhanced diffusion from local temperature rise and/or free volume generation rather than deformation-induced nucleation and growth of new MRO sites.

Effect of spatial heterogeneity on level of rejuvenation in Ni80P20 metallic glass

Understanding the relation between spatial heterogeneity and structural rejuvenation is one of the hottest topics in the field of metallic glasses (MGs). In this work, molecular dynamics (MD) simulation is implemented to discover the effects of initial spatial heterogeneity on the level of rejuvenation in the Ni80P20MGs. For this purpose, the samples are prepared with cooling rates of 1010 K/s–1012 K/s to make glassy alloys with different atomic configurations. Firstly, it is found that the increase in the cooling rate leads the Gaussian-type shear modulus distribution to widen, indicating the aggregations in both elastically soft and hard regions. After the primary evaluations, the elastostatic loading is also used to transform structural rejuvenation into the atomic configurations. The results indicate that the sample with intermediate structural heterogeneity prepared with 1011 K/s exhibits the maximum structural rejuvenation which is due to the fact that the atomic configuration in an intermediate structure contains more potential sites for generating the maximum atomic rearrangement and loosely packed regions under an external excitation. The features of atomic rearrangement and structural changes under the rejuvenation process are discussed in detail.

On the shear modulus and thermal effects during structural relaxation of a model metallic glass: Correlation and thermal decoupling

Pd40Ni40P20 (at.%) samples with different enthalpy states were fabricated through high-pressure torsion or sub-Tg annealing of the as-cast material. Subsequently, the underlying structural relaxation was studied by in-situ shear modulus measurements and modulated differential scanning calorimetry. The results show that high-pressure torsion leads to shear modulus softening and an increase of the nonreversible exothermic enthalpy, indicating a significant structural rejuvenation, while sub-Tg annealing causes shear modulus hardening and a decrease of the nonreversible exothermic enthalpy. The reversible endothermic effect which can reflect the fractional change of supercooled liquid with temperature was found to be almost identical for all samples, and independent of deformation or thermal history. The total heat flow can be well correlated with the shear modulus within the framework of interstitialcy theory. Furthermore, we demonstrate that the structural relaxation below Tg decouples into internal stress relaxation and β-relaxation. In addition, this work indicates that the processes of α-relaxation and β-relaxation in the metallic glass are of similar structural origin but occur on different spatial scales.