Annihilation Of Free Volume Research Articles

Effects of highly-textured and untextured polycrystalline layers on deformation mechanisms in amorphous submicron-layered materials

Amorphous alloys have high strength, but softening can lead to room temperature brittleness, limiting their applications. Typically, soft means low hardness and yield strength, and the lower the yield stress, the higher the fracture toughness. Two amorphous/crystalline multilayer samples with submicron single layer thickness were prepared. The crystalline layer of one sample is composed of highly-textured (111) nanocrystalline grains, and the other sample is untextured. Indentation experiments proved that the samples were plastically deformed, while the amorphous layer did not form shear bands with decreased layer thickness. The thickness reduction of the amorphous layer in the sample with highly-textured crystalline layers is greater than in the sample with untextured crystalline layers. The strain-hardening ability of the textured crystalline layer is higher, while the corresponding amorphous layer is deformed more severely with free volume annihilation and greater flow with more compact atoms. The amorphous layer corresponding to the untextured crystalline layer absorbs dislocations, and activates shear transformation zones (STZs), increasing the free volume near the interface. The annihilation of the free volume is less than the sample with highly-textured crystalline layers.

Unique irradiation damage behavior and deformation mechanisms in crystalline/amorphous Ag/Cu-Zr nanolaminates

Nanolaminated materials due to the abundant interfaces that are efficient sinks for irradiation defects have received broad attention. In this work, Ag/Cu-Zr crystalline/amorphous nanolaminates (C/ANLs) with a wide modulation ratio η from 0.1 to 9.0 were prepared using sputtering magnetron and implanted to a total influence of 1×1017 ions cm-2 He ions to investigate their irradiation damage behavior and mechanical properties. The irradiated Ag/Cu-Zr C/ANLs enable an interesting distribution behavior of He bubbles that only aggregate in the crystalline Ag layers but are absent in the amorphous Cu-Zr layers and interfaces, and become increasingly obvious with reducing η. This is attributed to the synergistic effect of C/A interfaces and amorphous layers as efficient sinks for irradiation defects. The irradiation-induced hardening is found to go through a minimum at η = 3.0, owing to the transformation of strengthening mechanism from He bubbles strengthening in Ag layers as η ≤ 3.0 to free volume annihilation in Cu-Zr layers as η > 3.0. Moreover, the strain rate sensitivity (SRS) m of irradiated C/ANLs firstly decreases rapidly from positive to negative and then increases slowly with raising η, differing from the monotonic η-dependent SRS m of as-deposited C/ANLs. Relevant deformation mechanisms are elucidated based on the dislocation-bubble interactions in terms of pinning effect on dislocations bow-out and dynamic strain aging effect. Our findings could provide valuable insights for the microstructural design of C/ANLs for applications in nuclear reactors.

Reversing relaxation-induced embrittlement by high-temperature thermal cyclic annealing in Zr-based metallic glass

Annealing usually induces structural relaxation and reduction of liquid-like regions, leading to embrittlement in bulk metallic glasses (BMGs). Here, we find that the short-term high-temperature thermal cycling (HTC) annealed Zr41.2Ti13.8Cu12.5Ni10Be22.5 (Vit1) BMG shows an improved plasticity without sacrificing their yield strength. And only relaxation behavior with increased hardness is observed after HTC, which is different from the rejuvenation effect usually observed in cryogenic thermal cycling (CTC). We revealed that this embrittlement reversal is attributed to enhanced fluctuations of full width at half maximum (FWHM) of the hardness’ distribution at micrometer and larger scales induced by short-term HTC. The enhanced fluctuations of mechanical heterogeneities across the diameter on a cross-section of the short-term HTC sample may increase the number and decrease the size of shear transformation zones (STZs) activated at high stress and promote the deflection of shear bands (SBs) during their propagation to form multiple SBs. The enhanced plasticity after HTC induced relaxation contradicts the common sense that relaxation accompanying with annihilation of free volume or liquid-like region usually causes embrittlement. Present results indicate that short-term HTC could be a powerful mean to tune the mechanical performance, shedding new lights on the interplay among relaxation, mechanical/structural heterogeneity, and plasticity of MGs.

Thermally stable Mo-Co-B thin film metallic glass as a potential diffusion barrier in Cu/Si contact system

Thin film metallic glass (TFMG) has demonstrated significant potential for applications in the semiconductor industry. In this work, fully amorphous Mo-Co-B thin films with a smooth surface and dense microstructure were prepared through magnetron sputtering. The thermal stability and diffusion barrier performance of the Mo-based TFMG were evaluated through vacuum annealing at temperatures ranging from 300 to 800 °C for 30 min. Nanoindentation tests were conducted to investigate the evolution of mechanical properties with annealing for the Mo-based thin film. When subjected to annealing at temperatures below 700 °C, the Mo-based thin film retained its fully amorphous structure. It displayed enhanced hardness and Young's modulus due to the annihilation of free volume with annealing. Increasing the annealing temperature to 700 °C and 800 °C resulted in surface oxidation and crystallization of the Mo-based TFMG, respectively, leading to a continuous decline in its mechanical properties. When employed as the diffusion barrier between Cu and Si, the Mo-based barrier layer effectively blocked the diffusion of Cu atoms through the barrier at temperatures below 800 °C. The failure of the barrier occurred at 800 °C due to the crystallization of the amorphous phase. Such excellent diffusion barrier performance of the thermally stable Mo-Co-B TFMG makes it a promising candidate as the barrier material in Cu metallization.

Strain rate response and rheological characteristics of ZrCuNiAl bulk metallic glass in supercooled liquid region

In this study, the plastic deformation behavior of Zr55.7Cu22.4Ni7.2Al14.7 bulk metallic glass (BMG) in the supercooled liquid region (SLR) was studied by high temperature compression experiments at various strain rates. The results show that the high temperature plastic deformation behavior of the BMG in SLR is highly sensitive to the strain rate. With the decrease in strain rate, the steady-state flow stress and peak stress decreases, and the viscosity increases. Meanwhile the stress overshoot gradually disappears, the deformation of the BMG specimen changes from non-Newtonian flow to near-Newtonian flow, and there is a critical strain rate of 5 × 10−4 s−1. The surface deformation profile changes from an "S" shaped truncated cone to a true truncated cone. The plastic deformation of BMGs at room temperature is completed by the initiation and expansion of the shear bands, while the high temperature plastic deformation is caused by the generation and annihilation of free volume.

Effect of oxygen impurity on corrosion behavior of a Zr-based bulk metallic glass in 0.5 M H2SO4 and 0.5 M NaOH solutions

• Effect of oxygen impurity on corrosion of a Zr-based BMG in H 2 SO 4 and NaOH is studied. • Oxygen impurity has no effects on the corrosion in 0.5 M H 2 SO 4 . • Oxygen impurity can improve the corrosion resistance in 0.5 M NaOH. • Oxygen-induced annihilation of free volume may account for the inhibited corrosion. Effects of unavoidable oxygen impurity in industrial-grade Zr-based bulk metallic glasses (BMGs) on their properties are the issues that must be considered. The studies on oxygen-related corrosion behaviors of Zr-based BMGs are rarely reported. This paper aims to clarify the effects of oxygen impurity on corrosion of the as-cast Zr 61 Ti 2 Cu 25 Al 12 BMGs in 0.5 M H 2 SO 4 and 0.5 M NaOH solutions. The results show that oxygen impurity has no effects on the corrosion in 0.5 M H 2 SO 4 , but is unexpectedly beneficial for improving the corrosion resistance in 0.5 M NaOH by inhibiting the cathodic reaction. The latter phenomenon could be correlated with the oxygen-induced annihilation of free volume. The finding herein further confirms that oxygen impurity is not harmful, even beneficial, for corrosion resistance of industrial-grade Zr 61 Ti 2 Cu 25 Al 12 BMGs.

Tensile Properties of Melt-Extracted and Annealed Ni/Fe-Based Amorphous Metallic Fibers

Here, melt-extracted Ni- and Fe-based amorphous metallic fibers (AMFs) were annealed below their glass transition temperatures. The tensile behaviors and microstructures of the melt-extracted and the annealed AMF samples were studied. For melt-extracted Ni- and Fe-based samples, the difference of fracture angles can be attributed to their difference of parameter α in the unified tensile fracture criterion. The revolution in the microstructure and mechanical properties induced by annealing treatment has been interpreted in detail. Prolonging the annealing time or increasing the annealing temperature can lead to an increase in fracture stresses of both Ni- and Fe-based AMFs. It was demonstrated that the increase in the fracture stresses of annealed AMFs is caused by the free volume annihilation in annealing processing.

Deformation and fracture behaviors of Zr-based metallic glasses under different constraint shear angles

The deformation and fracture behaviors of Zr35Ti30Be27.5Cu7.5 metallic glasses (MGs) under different shear angles were systematically investigated via shear tests with different constraint shear angles of 30°, 45° and 60°. The fracture strength of the MGs subjected to constraint shear tests increases gradually with increasing shear angles. This positive relationship between the fracture strength and the applied shear angles was verified by the Mohr-Coulomb criterion, suggesting a good agreement between experimental results and theoretical predictions. Due to the large shear stress, the content of free volume generation is greater than the content of free volume annihilation in MGs with the small constraint shear angles and then, a large amount of free volume aggregates to form multiple shear bands, manifesting as a large number of serration events. The formation and interaction of multiple shear bands can prevent the shear bands from rapidly expanding into cracks, significantly improves the plasticity of MGs.

Maximizing the degree of rejuvenation in metallic glasses

As the reverse process of relaxation, rejuvenation is the structural excitation process that can bring metallic glasses (MGs) to a higher energy state and usually increases their free volume. Here, using a dilution procedure conducted by randomly removing atoms from the modeled glass matrix, the degree of rejuvenation can be systematically controlled and the maximum rejuvenation threshold of MGs is identified. The structural relaxation is activated during the rejuvenation process and the dynamic balance between free volume creation and annihilation defines the rejuvenation ability of MGs. The highest degree of rejuvenation correlates to the flow strain of the materials and the structure is similar to that found in shear bands.

Shear band evolution related with thermal annealing revealing ductile-brittle transition of Zr35Ti30Be27.5Cu7.5 metallic glass under complex stress state

The ductile-brittle transition (DBT) caused by thermal annealing was systematically investigated by means of small punch test (SPT), accompanied by finite element simulation, uniaxial compression and nanoindentation in Zr-based metallic glasses (MGs) annealed for 1 h in a wide temperature range, i.e., sub-Tg, Tg ∼ Tx and above-Tx. In multiaxial loading mode, three deformation mechanisms, i.e., (ⅰ) circumferential shear bands (CSBs) + radial shear bands (RSBs), (ⅱ) RSBs and (ⅲ) radial cracks (RCs), can be found separately on MGs annealed sub-Tg, Tg ∼ Tx and above-Tx, leading to a transition in deformation mode from ductility to brittleness. First, for MGs with sub-Tg, Tg ∼ Tx annealing, the reduction in the number of CSBs and RSBs, as well as the disappearance of CSBs, are responsible for the ductile-brittle transition, which is attributed to the annihilation of free volume caused by structural relaxation and the reduction of released elastic energy. Based on simulation results, the decrease in plasticity of MGs is accompanied by the reduction in stress distribution area, which is in agreement with the change in the number and distribution area of shear bands. The initiation of multiple shear bands and their strong interaction dramatically improve the plasticity of MGs under the synergistic effect of large stress distribution area and high content of free volume. Second, the embrittlement of MGs with above-Tx annealing is mainly attributed to the rapid formation and extension of RCs induced by the occurrence of crystallization. Meeanwhile, simulation results indicate that stress distribution area decreases gradually and facilitate the formation of stress concentration, leading to the rapid extension of RCs and the occurrence of catastrophic fracture of MGs. In addition, MGs with sub-Tg, Tg ∼ Tx and above-Tx annealing not only exhibit such DBT in uniaxial compression, but also show a soft-hard transition in nanoindentation test.

Shape memory effect in metallic glasses

Shape memory effect in metallic glasses

Oxygen impurity improving corrosion resistance of a Zr-based bulk metallic glass in 3.5 wt% NaCl solution

The effect of oxygen impurity on corrosion behavior of as-cast Zr61Ti2Cu25Al12 bulk metallic glasses (BMGs) is clarified using electrochemical measurements, microstructure characterization and surface analyses. We find unexpectedly that oxygen impurity (≤1200 ppm) improves pitting corrosion resistance when a fully amorphous structure is attained. The oxygen-induced annihilation of free volume enhances passive film protectiveness and hence accounts for the improved corrosion property. In contrast, adding more oxygen impurity (1800 ppm) deteriorates pitting corrosion resistance by assisting the formation of highly-reactive crystalline phases. The finding herein provides an avenue for designing new low-cost Zr-based BMGs with superior properties by oxygen microalloying.

Structural evolution of heavy rare Earth-based metal glass under high pressure

The structural evolution of Er55Al25Co20 metallic glasses (MGs) at high pressure was studied through x-ray diffraction with synchrotron radiation. The compression ratio, differential structure factor, pair distribution function g(r), and relative resistance as functions of pressure were analyzed and discussed. A reversible polyamorphic transition with a clear hysteresis was detected in the Er55Al25Co20 MGs. The irreversible annihilation of free volume and voids led to a densification of the specimens. Electronic resistance measurements demonstrated that the transition was strongly correlated with the electronic structural evolution. The results provide a new insight into understanding the mechanisms of polyamorphism in MGs.

Atomic dynamics transition in a Cu-Zr-Al metallic glass

We reveal a slow-to-fast dynamics transition in the as-cast Cu46Zr46Al8 (at.%) metallic glass (MG) at a temperature below the glass transition upon heating, which depends on the competition between free volume annihilation and thermally-activated atomic mobility, well consistent with the bond-length change of Cu-Cu pairs. The activation of α-relaxation at high temperatures could be related to the bond-length extension of Cu-Cu and Zr-Zr like-atom pairs together with the shortening of Cu-Zr unlike-atom pairs. Our results provide new insights into the correlation of dynamics with atomic structure in MGs and will be helpful to the understanding of ageing behaviors in MGs.

Reevaluating the efficacy of moderate annealing in nuclear waste vitrification for sustainable high-level waste management

The reliable immobilization of nuclear waste, particularly high-level waste (HLW), is a key issue for sustainable utilization of nuclear energy. Annealing process is adopted in nuclear waste vitrification to improve mechanical properties of products; however, its efficacy on chemical stability of treated HLW glass remain uncertain. This study aims to reevaluate the effects of moderate annealing on chemical durability of sodium borosilicate glass matrix incorporated with simulated HLW. Two types of leaching tests, i.e. acid leaching and the modified Product Consistency Test (PCT), were performed to examine the leaching behavior of HLW glassy products controlled by corrosion mechanisms of ion exchange and hydrolysis, respectively. The acid leaching results showed that annealed glass samples released lanthanum (La) species at a faster rate than that of quenched samples, but the effects of annealing on the release behavior of alkaline and alkaline-earth elements were limited. In contrast, the PCT results showed minor difference between quenched and annealed samples, which suggests that the small structure modification caused by moderate annealing was not comparable with the dominant hydrolysis reaction occurred in the PCT process. According to the free-volume theory, as well as evidences derived from Raman characterization and thermodynamic measurements, La was inferred to exist in the free-volume region in the HLW glass, and the annihilation of free volume during annealing could mobilize La from its stable surrounding environment, and thus reducing the chemical stability of nuclear waste glass.

Coupling effects of self-limiting lithiation, reaction front evolution and free volume evolution on chemical stress in amorphous wire-based electrodes

The lithiation of silicon (Si) involves the evolution of reaction front, self-limiting lithiation, and visco-plastic deformation. During the lithiation of crystalline Si, solid-state amorphization occurs to lower Gibbs free energy, and lithiated Si-electrode in lithium-ion battery is mainly present in amorphous phase. In this work, we develop a viscoplastic constitutive relationship for the lithiation-induced deformation of amorphous materials from the theory of free volume, and establish a chemo-mechanical model for the lithiation-induced deformation of a-Si electrode from the frameworks of phase-field theory, stress-assisted thermal activation process and the viscoplastic constitutive relationship. The chemo-mechanical model takes into account three important chemophysical phenomena of the self-limiting lithiation, evolution of reaction front/interphase zone and plastic flow. Using the newly developed chemo-mechanical model, we investigate the lithiation-induced deformation of an a-Si nanowire. The numerical results reveal that both the stress-assisted thermal activation process and plastic flow retard the motion of the reaction front from free surface to the center of the a-Si nanowire. The annihilation and creation of free volume significantly reduces the Cauchy stress.

Formation of the B2–ZrCo phase and micro-hardness evolution in Zr–Co–Al BMGs via conventional and flash annealing

As a promising strategy to improve the ductility and work-hardening, the generation of bulk metallic glass (BMG) composites embedded with a B2 phase has sparked a great interest. In this study, we systematically explored the influence of annealing parameters on formation of the B2–ZrCo phase and micro-hardness evolution, and thus, we summarized the optimal annealing processes which can maximize the B2–ZrCo phase formation and maintain the glass matrix. We examined the microstructures of those annealed alloys via XRD, SEM plus EDS, DSC, FIB, TEM and Struers Durascan micro-hardness tester. Firstly, the amorphous alloys of Zr 55 Co 31 Al 14 and Zr 56 Co 28 Al 16 with 3 mm in diameter were heated to 753 K and 843/853 K for various times at a heating rate of 20 K/min via DSC and then naturally cooled to room temperature. It is found that for annealed Zr 56 Co 28 Al 16 alloys, the ZrCoAl phase is the dominant phase regardless of the annealing temperature and time, while for annealed Zr 55 Co 31 Al 14 alloys, at lower annealing temperature, the ZrCoAl phase is the dominant phase, whereas with increasing of annealing temperature and holding time, the B2–ZrCo phase is the dominant phase. In order to further understand the influence of annealing temperature on B2–ZrCo phase formation, we transiently heated those two amorphous alloys to 923 K and then naturally cooled to room temperature within DSC. It is found that the B2–ZrCo phase is still the dominant phase for annealed Zr 55 Co 31 Al 14 alloy, while the ZrCoAl phase is still the dominant phase for annealed Zr 56 Co 28 Al 16 alloy. However, the conventional annealing processes have notable disadvantages and significantly limit the retaining of amorphous matrix content due to its low heating rate. Based on the above analysis, the amorphous Zr 55 Co 31 Al 14 alloy is a promising candidate to fabricate the Zr–Co–Al BMG composite incorporating high volume fraction of B2–ZrCo phase. The flash annealing was used to transiently heat the glassy Zr 55 Co 31 Al 14 alloy at a heating rate of 1, 5 and 15 K/s via Gleeble. It is found that higher heating rate like 15 K/s can selectively promote formation of the B2–ZrCo phase and suppress the ZrCoAl phase and maintain a large volume fraction of amorphous content. As for the evolution of micro-hardness, we found that for all annealing conditions, the micro-hardness increases first and then decrease, and the micro-hardness of annealed Zr 56 Co 28 Al 16 alloy is higher than that of annealed Zr 55 Co 31 Al 14 alloy. The mixing enthalpy between the Zr–Al benefits the formation of a highly dense random packed structure which enhances the micro-hardness. The mechanism of micro-hardness evolution has been discussed in terms of volume fraction and types of crystalline phases, annihilation of free volume, transformation-induced hardening. • We examine the relationship between the formation of B2–ZrCo phase and annealing temperature and holding time, the composition of an alloy via conventional and flash annealing. • In conventional annealing, we found that for annealed Zr 56 Co 28 Al 16 , the intermetallic phases like ZrCoAl is the dominant phase regardless of the annealing temperature and holding time, while for annealed Zr 55 Co 31 Al 14 , the intermetallic phases dominants at low annealing temperature whereas the B2–ZrCo phase dominants at high annealing temperature. • In flash annealing, we found that the high heating rate like 15 K/s both maintains a large quantity of amorphous matrix and facilitates the formation of B2–ZrCo phase at high annealing temperature. Also, the high heating rate refines the size of crystalline phases. • The micro-hardness of annealed Zr–Co–Al alloys increased compared to the rapidly quenched Zr–Co–Al alloys and the mechanism for the micro-hardness evolution has been discussed.

How does the structural inhomogeneity influence the shear band behaviours of metallic glasses

ABSTRACTHere we propose a model to understand the influence of structural inhomogeneity on the shear band behaviours of metallic glasses. By considering the inhomogeneous structure and stress concentration, the model predicts that the strain for shear band nucleation in metallic glasses can be variable and far below the theoretical elastic limit. During sliding, the shear band will approach a dynamic equilibrium state of balanced free volume generation and annihilation. By considering the accumulation of irreversible structure change, the shear band will finally develop into fracture. Under fluctuating load, the shear band shows an ‘activate-arrest’ behaviour, which results from a delayed response to the external load change. These results reasonably explain and correlate the physics behind the elastic limit, stick-slip shear band behaviour, implicit shear events, and shear band fracture in metallic glasses. The study can provide another perspective and platform to understand the correlations between structural inhomogeneity and shear band behaviours in metallic glasses, and further explore other shear band related phenomena not only in metallic glasses but also in the class of shear-softened materials.

A Free Volume-Based Viscoplastic Model for Amorphous Silicon Electrode of Lithium-Ion Battery

The phase transition of Si from crystalline phase to amorphous phase with the formation of metastable amorphous structures of Li12Si7, Li7Si3, Li13Si4 and Li22Si5 after the first few cycles suggests that the Si in Si-based lithium-ion battery is mainly present in amorphous phase. In this work, we extend the free volume theory, which has been widely used in the deformation analysis of bulk metallic glass, to describe the cycling-induced visco-plastic deformation in amorphous Si-electrode during electrochemical cycling and establish a constitutive relationship with the flow unit of free volume for the plastic flow in lithiated Si. The plastic flow in lithiated Si is accompanied with the creation and annihilation of free volume. Using the constitutive relationship and incorporating the concentration-dependent mechanical properties of lithiated Si, we study the cycling-induced evolution of Cauchy stress and free volume in amorphous, thin-film Si-electrodes. The numerical results are in good accord with the experimental results reported in literature, which validates the approximation used in the analysis of the cycling-induced deformation of the amorphous, thin-film Si-electrodes.

The Glassy Structure Formation and Phase Evolution in Mechanically Alloyed and Spark Plasma-Sintered Al-TM-RE Alloys

The present study deals with the comparative study of amorphization tendency of Al86Ni6Y6, Al86Ni6Y6Co2 and Al86Ni6Y4.5Co2La1.5 alloy powders via mechanical alloying performed at 300 revolution per minute with ball-to-powder ratio of 15:1 and subsequently the devitrification tendency of 300 °C and 500 °C spark plasma-sintered bulk amorphous alloys. Mechanically alloyed Al86Ni6Y6, Al86Ni6Y6Co2 and Al86Ni6Y4.5Co2La1.5 powders yielded nearly fully amorphous structure after 140, 170 and 200 h, respectively. The requirement of prolonged milling was attributed to the soft and ductile nature of aluminum with high stacking fault energy. Amorphous powders were consolidated via spark plasma sintering at 300 and 500 °C by applying a constant pressure of 500 MPa. X-ray diffraction was performed on the 300- and 500 °C-sintered samples. XRD patterns of the 300 °C-sintered alloys exhibited very-low-intensity nanocrystalline FCC-Al peak overlaying an amorphous hump evincing retention of a large amount of the amorphous phase. Enhanced devitrification tendency was reported in the 500 °C-sintered alloys; however, a major difference in the devitrification tendency of the 500 °C-sintered Al86Ni8Y6, Al86Ni6Y6Co2 and Al86Ni6Y4.5Co2La1.5 alloys was that the quinary alloy exhibited higher tendency of devitrification, which was also corroborated by performing HRTEM and analytical TEM experiment. This could be attributed to the higher probability of coupling of atoms by short-range atomic shuffling during spark plasma sintering. Vickers hardness, and relative density estimated via Archimedes’ principle, varied depending on the degree of free volume annihilation and crystallization during sintering.