Monolithic Bulk Metallic Glass Research Articles

Enhanced plasticity in laser additive manufactured Nb-reinforced bulk metallic glass composite

The microstructures and mechanical properties of 50 wt% Nb-reinforced Zr-based bulk metallic glass composites, (BMGCs), manufactured via laser directed energy deposition (LDED) with two different laser powers, P , are investigated and compared with those of LDED manufactured monolithic bulk metallic glass (BMG). Results show that both BMGCs have a multi-component microstructure that includes dual-sized Nb particles and devitrification-induced nano crystals at the melt-pool boundaries whereas the monolithic BMG contains only the latter. The nanocrystals at the melt-pool boundaries embrittle the LDED manufactured monolithic BMG, which reflects as a higher yield strength but negligible plastic deformation in uniaxial compression. In contrast, both BMGCs undergo significant plastic deformation as the Nb-reinforcements, besides improving plasticity by arresting propagating shear bands, reduces the enthalpy of relaxation, Δ H rel , of the matrix and decreases the volume fraction of the nanocrystallites, V nc , at the melt-pool boundaries. However, amongst the two, the BMGC manufactured at higher P exhibits much higher shear band plasticity, despite having higher V nc and lower Δ H rel . This is rationalized by analyzing the load transfer characteristics of the Nb-BMG matrix interfaces in the two BMGCs. • LDED manufactured Nb-reinforced Zr-based BMGCs have significantly improved plasticity. • Nb induced plasticity improvement is achieved through limiting the structural relaxation of amorphous matrix. • Higher thermal diffusivity of Nb can minimize the devitrification induced brittle phases and thus enhance plasticity. • Partially dissolving Nb in the amorphous matrix by sufficient laser power can maximize shear band arresting capabilities.

A flow model in bulk metallic glasses

Based on the microscopic mechanisms applicable to bulk metallic glasses (BMGs), such as the free-volume theory and fracture theory of brittle solids, a constitutive equation for the crack-like propagation velocity of shear bands in BMGs is deduced, νSBcl=ν0a0exp(−ES/kT). Furthermore, a flow model of monolithic BMGs is successfully established, logνSB=log(ν0a0)+logeES/(kTl)−logeES/(kT). The model explains clearly the fundamental deformation of BMGs at the temperature far below the liquidus temperature, i.e., the mechanism of serrated flows. Not only does the model accurately predict the transition between serrated and non-serrated flows, but the Arrhenius equation and activation energy received by the current model for most BMGs are consistent with the experimental results. The current study paves a way to plastically process BMGs, avoiding catastrophic plasticity instability.

Mechanics of thin film coated Bulk Metallic Glass composites

Bulk Metallic Glasses (BMGs) exhibit superior mechanical properties such as high strength (≈2 GPa), high yield strain (≈2%) and high corrosion resistance. But, they lack ductility which prevents them from being used as structural materials. One of the methods explored by material scientists to improve their strain to failure is to coat BMGs with a thin layer of a ductile polycrystalline metal such as Copper. In this work, we numerically investigate the mechanical behavior of thin film coated BMGs through Finite Element (FE) Simulations. We simulate the effect of an extrinsic copper coating on the deformation and failure of monolithic BMG cylinders under uniaxial loading. We employ the Anand and Su (2005) constitutive model for BMGs implemented by us in ABAQUS FE software via user material subroutines: UMAT and VUMAT. An isotropic, elastoplastic material model is used for representing the Copper coating with properties of pure Copper. Three-dimensional, two-dimensional plane strain and axisymmetric simulations of uniaxial loading of monolithic BMG and Copper coated BMG composite at quasi-static strain-rate are performed using the Explicit/Dynamics formulation in ABAQUS. The simulations take into consideration a ductile damage and failure mechanism involving cracking inside the shear bands in the BMG matrix. We observe the formation of shear bands which transform into cracks in the BMG matrix. The simulations are able to correctly predict the deformation and fracture mechanism of the BMG matrix and copper coating under uniaxial loading. Results show that a thin copper coating of about 1–5% of the diametrical thickness of the cylinder increases the strain to failure of the coated BMG composite by about 2% which is in qualitative agreement with experimental observations reported in literature. The present results have implications for designing coated BMGs with improved malleability or strain to failure enabling their deployment in structural applications.

On low-temperature strength and tensile ductility of bulk metallic glass composites containing stable or shape memory β-Ti crystals

Bulk metallic glass composites (BMGCs) with in-situ formed crystals can display tensile ductility at room temperature, overcoming brittleness of monolithic bulk metallic glasses (BMGs). However, the ductility and toughness of BMGCs severely deteriorate at low temperatures, which has been a longstanding bottleneck hindering their low-temperature applications. Here, the low-temperature tensile properties of two Ti-based BMGCs containing stable β-Ti crystals with dislocation-mediated plasticity (Fe1) or shape memory β-Ti crystals (Fe0) were investigated. With temperature decreasing from 298 to 77 K, the strength of Fe1 increases but its tensile ductility decreases. In comparison, the shape memory Fe0 BMGC exhibits a simultaneous enhancement of strength and tensile ductility as temperature decreases from 298 to 143 K. The superior low-temperature properties of Fe0 are attributed to its combined structure of a glassy matrix and shape memory crystals. As temperature decreasing, the glassy matrix possesses an increased strength which softens as shear bands form, and the shape memory crystals are more inclined to deformation-induced martensitic transformation. Therefore, the combination of characteristic properties of metallic glasses and shape memory crystals in shape memory BMGCs can successfully overcome the low-temperature brittleness. These findings are believed to facilitate the development of BMGCs with superior low-temperature properties and promote their applications at low temperatures.

Effect of alloying oxygen on the microstructure and mechanical properties of Zr-based bulk metallic glass

Due to the high solubility limit of oxygen in Zr, dissolution of oxygen in the Zr-based alloys is unavoidable. However, as an alloying element, the effect of oxygen on the microstructure and mechanical properties of monolithic Zr-based bulk metallic glasses (BMGs) is rarely reported. Here, we systematically investigated the evolution of microstructure and mechanical properties of monolithic Zr61Cu25Al12Ti2 BMGs with alloying oxygen content ranging from 650 at. ppm to 5600 at. ppm. With increasing oxygen content from 650 at. ppm to 3500 at. ppm, the compressive plasticity only decreased slightly, from 2.00±0.85% to 1.60±0.91%; however, it plunged to 0.77±0.33% when the oxygen content was further increased to 5600 at. ppm. The threshold oxygen content level for the ductile-brittle transition was revealed at 3500 at. ppm, and the embrittlement at high oxygen content is closely related with a number of changes in microstructure: sharply increases in the size of shear transformation zone (STZ) and in the degree of local ordering, including the imperfect ordered packing (IOP) atomic clusters and large medium-range ordering (MRO) domains. The above findings not only advance the understanding of the effect of alloying oxygen on the microstructure and mechanical properties, but also help to design Zr-based BMGs with excellent performance, promoting their wider commercial application.

Computational Modeling of Compressive Behavior of Wire-Reinforced Bulk Metallic Glass Matrix Composites

The effect of wire type and volume fraction on the compressive behavior of wire-reinforced bulk metallic glass (BMG) matrix composites were investigated using the finite element method. The composites and test procedure were simulated in the ABAQUS software, and analyses on stress propagation and extracted compressive stress–strain curves were performed, which showed an acceptable agreement with experimental results. The results exhibited that Ta, Mo, W and Stainless Steel type 302 as the reinforcement wires significantly improved the plastic strain of the composites. Meanwhile, W wires sustained the composite strength in the same level of monolithic BMG, without sacrificing noticeable plastic strain of the composite in comparison with other reinforcements. The different volume fractions of W wires showed that the trend of plastic strain progress with increase in the second phase reaches a maximum at 65% of reinforcements. This behavior is caused by the fracture mechanism of the composites which is in its optimum condition when the reinforcement volume fraction is equal to 65%. The mechanical properties of the wire-reinforced BMG composites can be modeled by using the iso-strain Voigt model, which allows the prediction of the mechanical properties of a composite from the volume-weighted averages of the components properties. The above-mentioned findings corroborate the experiments elsewhere and may be considered as a valuable starting point to lead the future researches.

CuZr-based bulk metallic glass and glass matrix composites fabricated by selective laser melting

Monolithic bulk metallic glass and glass matrix composites with a relative density above 98 % were produced by processing Cu46Zr46Al8 (at.%) via selective laser melting (SLM). Their microstructures and mechanical properties were systematically examined. B2 CuZr nanocrystals (30–100 nm in diameter) are uniformly dispersed in the glassy matrix when SLM is conducted at an intermediate energy input. These B2 CuZr nanocrystals nucleate the oxygen-stabilized big cube phase during a remelting step. The presence of these nanocrystals increases the structural heterogeneity as indirectly revealed by mircrohardness and nanoindentation measurements. The corresponding maps in combination with calorimetric data indicate that the glassy phase is altered by the processing conditions. Despite the formation of crystals and a high overall free volume content, all additively manufactured samples fail at lower stress than the as-cast glass and without any plastic strain. The inherent brittleness is attributed to the presence of relatively large pores and the increased oxygen content after selective laser melting.

Tailored hardening of ZrCuAl bulk metallic glass induced by 2D gradient rejuvenation

The work hardening behavior of bulk metallic glasses has not been previously ascribed to their intrinsic structure but rather to the introduction of other components that act as hardening elements. Here, we present clear evidence of a 2D gradient rejuvenation state that can induce tailored hardening of a monolithic bulk metallic glass. We show that the local free volume content related to the rejuvenation state controls the shear band angle and the maximum effective shear stress. Hence, shear band propagation is prohibited, and the formation of a complete shear plane transecting the whole specimen is blocked. The generation of plastic strain is accompanied by an increase in the critical shear stress, resulting in sustainable apparent hardening. In this way, we present a bulk metallic glass that has a tailored hardening mechanism and establish an experimental link between a gradient rejuvenation state and mechanical properties.

The role of minor alloying in the plasticity of bulk metallic glasses

Micro- or minor alloying of metallic glasses is of technological interest. An originally ductile Pd-based monolithic bulk metallic glass (Pd40Ni40P20) was selectively manipulated by additions of Fe or Co. The alloying effects were extreme, showing either exceptional ductility upon Co addition or immediate catastrophic failure upon Fe addition when tested under uniaxial compression or 3-point bending. The amorphous structure was characterized prior to deformation with respect to its medium-range order (MRO) using variable resolution fluctuation electron microscopy (VR-FEM). We observe striking differences in the MRO between the ductile and brittle metallic glasses, with the ductile glasses exhibiting a rich structural diversity and MRO correlation lengths up to 6 nm. The MRO diversity seems to enable easier shear banding and hence enhance the deformability.

Correlations between the ductility and medium-range order of bulk metallic glasses

Understanding ductility or brittleness of monolithic bulk metallic glasses requires detailed knowledge of the amorphous structure. The medium-range order (MRO) of ductile Pd40Ni40P20 and brittle Zr52.5Cu17.9Ni14.6Al10Ti5 (Vit105) was characterized prior to and after notched three-point bending tests using variable-resolution fluctuation electron microscopy. Here, we show the presence of a second larger MRO correlation length in the ductile material, which is not present in the brittle material. The second larger correlation length accounts for an increased heterogeneity, which seems to enable an easier shear band formation and thus explain the difference in deformability.

Enhancing the plasticity of a Ti-based bulk metallic glass composite by cryogenic cycling treatments

Abstract The effect of cryogenic cycling treatments on the thermal and mechanical properties of a Ti-based bulk metallic glass (BMG) composite was investigated. Even though the macroscopic structures of the dendrite-phase and glass-matrix remain unchanged, the treated samples exhibited a rejuvenation behavior in terms of a higher relaxation enthalpy, which leads to an enhancement of plasticity as high as 90%. Different from the monolithic BMGs, in which rejuvenation has been attributed to the nanoscale structural heterogeneity, rejuvenation in BMG composite is induced by the crystal/amorphous heterogeneous microstructures. The increasing defects around the interfaces may promote formation of multiple shear bands and accommodation of plastic deformation. This work shows clearly that the cryogenic cycling treatments could be an effective and controllable way to modulate mechanical properties of BMG composites.

Effect of in-situ crystalline phases on the mechanical properties and energy release behaviors of Zr55Ni5Al10Cu30 bulk metallic glasses

To explore the application potential of bulk metallic glasses (BMGs) as energetic structural materials (ESMs) and address the limitations in the preparation of large-sized monolithic BMGs, the effect of the in-situ crystalline phases on the mechanical properties and energy release behaviors of Zr55Ni5Al10Cu30 BMGs are evaluated. The results indicate that while the content of in-situ crystalline phases in Zr55Ni5Al10Cu30 alloy are effectively tailored by controlling the cooling rate of molten alloys, the mechanical properties and energy release behaviors of Zr55Ni5Al10Cu30 are determined by the content of in-situ crystalline phases. The alloy prepared using the highest cooling rate possesses the lowest crystalline phase content as well as the highest strength and deformability. By contrast, the complete crystalline alloy prepared by annealing exhibits the highest energy release characteristics under high-speed impact owing to the highest degree of brittleness and most oxidation reaction. The Zr55Ni5Al10Cu30 alloy with different crystalline phase content shows potential as an ESM. Its mechanical properties and energetic characteristics can be effectively tailored by controlling the content of in-situ crystalline phases via cooling rate modulation.

Micromechanical investigation of the role of percolation on ductility enhancement in metallic glass composites

In contrast to the catastrophic failure arising from the unconstrained shear band propagation in monolithic bulk metallic glasses (BMGs), the introduction of second phases in BMG composites (BMGCs) provides an extrinsic way of reducing the magnitude of strain localization, and thus may significantly improve the ductility. For this mechanism to be effective, a multitude of experiments have found that the volume fraction should reach a rather high threshold of about 30–40%, corresponding to the percolation limit with variations depending on the second phase morphology. However, we demonstrate here that an enhanced ductility can be realized in a BMGC made from crystalline Ni foam (completely percolated even with a volume fraction of about 11%), suggesting the resistance in the second phase to the impinging shear-force dipole from neighboring shear bands be the dominant factor for such a ductility improvement. The shortcomings of percolation approach, as well as the synergistic effects of microstructure and mismatches in mechanical properties, are investigated here in a thermo-mechanical finite element method based on the free volume model and plasticity-induced heating. The effectiveness of ductility improvement will be demonstrated by calculating the probability distributions of the shear-band effective strain.

Change dynamic behaviors by heightening its stored energy of monolithic bulk metallic glass

Bulk metallic glasses (BMGs) are usually brittle, and show serious negative strain rate sensitivity upon dynamic loading due to no sufficient time for generation of the shear bands, which fatally limit their practical applications. Investigations have been done on BMGs dynamic deformation behaviors, but in which yielding phenomenon was hardly reported before. In this work, by raising stored energy which appears as an exothermic heat of relaxation on heating the glass, the dynamic and quasi-static deformation behaviors of Cu45.75Zr46.75Al6.5Co1 BMG were explored. We found that, compared the BMG at higher energy state with the as-cast BMG, its quasi-static compressive plasticity is improved from 0.9% to 3.3%. Additionally, more meaningfully, under dynamic compression, an evident yield plateau appears on the stress–strain curves of all the treated specimens at different strain rates, also the typical negative strain rate sensitivity of BMG ultimate strength weakens significantly. Other results reveal that raising BMGs stored energy can bring their spatial heterogeneity enhancement which distinctly proliferates shear bands in deformation, thus avoids rapid spread of limited shear bands catastrophically. This work provides an efficient way to improve BMGs dynamic properties and may shed light on their dynamic behaviors.

Modulating heterogeneity and plasticity in bulk metallic glasses: Role of interfaces on shear banding

Structural heterogeneities in monolithic metallic glasses, i.e. free volume variations or stress/strain fields, are known to be pivotal for their plasticity but the mechanisms how they affect irreversible deformation remain poorly understood. We show that flash-annealing is a capable tool for modifying the free volume content and its distribution, which creates non-affine stress fields in a monolithic CuZr-based bulk metallic glass. Rather than the overall free volume, μm-scale regions of relaxed and rejuvenated glass govern plastic deformation. Complementary molecular dynamics simulations underpin the importance of the interfaces between relaxed and rejuvenated volumes for shear band proliferation. Not only can we reach unmatched states of metastability in metallic glasses with the present approach but also better understand the mechanisms underlying plasticity in monolithic but structurally heterogeneous metallic glasses.

Ti-Zr-V-Cu-Be BMGMCs Processed by Semi-Solid Processing

Compared with monolithic bulk-metallic glasses at room temperature, in-situ two-phase bulk metallic glass matrix composites (BMGMCs) exhibit improved toughness. However, most of in-situ BMGMCs are developed through extremely rapid cooling, during which the solidification process cannot be controlled effectively. As a consequence, the microstructures of in situ two-phase composites, even with the same composition, vary from one to another, highly depend on the cooling rate. In the present investigation, the Ti-Zr-V-Cu-Be BMGMCs was prepared by using copper mould suction process, and the evolution of microstructures at semi-solid stage was investigated. The results indicated that microstructures of billets, produced by copper mould suction and water quenching after isothermal holding at semi-solid stage, mainly contained β-Ti phase and glass matrix. Isothermal holding temperature and time interval determined the final morphology of β-Ti phase. Compression tests showed that semi-solid processing could further improve ductility of BMGMCs effectively.

Mechanical, deformation and fracture behaviors of bulk metallic glass composites reinforced by spherical B2 particles

Ti45Cu40Ni7Zr5Sn2.5Si0.5 alloys were prepared under various cooling rate conditions during solidification. The alloys exhibited different volume fractions of B2 particles with 0 ~ 40 vol% in an amorphous matrix. Monolithic bulk metallic glass of 1 mm diameter showed no macroscopic plasticity and exhibited the typical vein patterns in a maximum shear stress plane on the fracture surface. However, a bulk metallic glass composite containing the B2 particles revealed obvious plasticity (~ 5.6%) with a remarkable work-hardening behavior that resulted from a stress-induced martensitic transformation of the B2 particles. Moreover, the composite displayed the complicated fracture morphologies containing of three types of fracture features. Through detailed investigations on the microstructural evolution, mechanical, deformation and fracture characteristics, the influence of B2 particle on overall behavior of the TiCu-based bulk metallic glass composites was elucidated.

Origin of large plasticity and multiscale effects in iron-based metallic glasses

The large plasticity observed in newly developed monolithic bulk metallic glasses under quasi-static compression raises a question about the contribution of atomic scale effects. Here, nanocrystals on the order of 1–1.5 nm in size are observed within an Fe-based bulk metallic glass using aberration-corrected high-resolution transmission electron microscopy (HRTEM). The accumulation of nanocrystals is linked to the presence of hard and soft zones, which is connected to the micro-scale hardness and elastic modulus confirmed by nanoindentation. Furthermore, we performed systematic simulations of HRTEM images at varying sample thicknesses, and established a theoretical model for the estimation of the shear transformation zone size. The findings suggest that the main mechanism behind the formation of softer regions are the homogenously dispersed nanocrystals, which are responsible for the start and stop mechanism of shear transformation zones and hence, play a key role in the enhancement of mechanical properties.

Development of High Strength Ni–Cu–Zr–Ti–Si–Sn In-Situ Bulk Metallic Glass Composites Reinforced by Hard B2 Phase

In the present study, the influence of atomic ratio of Zr to Ti on the microstructure and mechanical properties of Ni–Cu–Zr–Ti–Si–Sn alloys is investigated. The alloys were designed by fine replacement of Ti for Zr from Ni39Cu20Zr36−xTixSi2Sn3. The increase of Ti content enhances glass forming ability of the alloy by suppression of formation of (Ni, Cu)10(Zr, Ti)7 phase during solidification. With further increasing Ti content up to 24 at.%, the B2 phase is introduced in the amorphous matrix with a small amount of B19′ phase from alloy melt. The bulk metallic glass composite containing B2 phase with a volume fraction of ~ 10 vol% exhibits higher fracture strength (~ 2.5 GPa) than that of monolithic bulk metallic glass (~ 2.3 GPa). This improvement is associated to the individual mechanical characteristics of the B2 phase and amorphous matrix. The B2 phase exhibits higher hardness and modulus than those of amorphous matrix as well as effective stress accommodation up to the higher stress level than the yield strength of amorphous matrix. The large stress accommodation capacity of the hard B2 phase plays an important factor to improve the mechanical properties of in situ Ni-based bulk metallic glass composites.

In-situ synthesis and mechanical properties of Zr-based bulk metallic glass matrix composites manipulated by nitrogen additions

Our study investigates in-situ synthesis and mechanical properties of Zr-based bulk metallic glass (BMG) matrix composites via arc plasma-induced accelerated displacement reaction (APADR) process. The aluminum nitride precursor under arc plasma-induced ultra-high temperature results in higher contents of dissolved nitrogen as well as precipitation of zirconium nitride (ZrN) particles in a Zr-based amorphous matrix. The nitrogen in the matrix results in a decrease of crystallization resistance (lower Tx and reduced glass-forming ability), but an increase of mechanical stability (a decrease of strain burst sizes). In particular, in-situ formed ZrN, which exhibits a homogeneous distribution and strong interfacial bonding with the matrix, causes an increase in compressive fracture strength and significant plastic deformation in the composite compared with the monolithic BMG. The formation of multiple shear bands and the enhancement of shear band interactions by the dissolved nitrogen as well as the in-situ formed ZrN particles were carefully confirmed by a statistical analysis on serrated flows. These results give us a guideline on how to manipulate nitrogen contents and fabricate in-situ BMG matrix composites with improved mechanical properties via APADR process.