Salient shear bands and second-phase addition interactions of bulk metallic glass matrix composite

This article discusses Charpy impact testing and fracture morphology of the Zr_41.25Ti_13.75Cu_12.5Ni₁₀Be_22.5 bulk metallic glass matrix composite with long tungsten fibers. Energy to failure was measured via the impact test as well as by integrating the compressive stress-strain curves, and compared for various fiber fractions. Failure energy increased with fiber volume fraction by both measures. Observation of fracture surfaces was made by using scanning electron microscopy. The results show that the fracture surface of the unreinforced bulk metallic glass (BMG) exhibits three different regions, i.e., the impact zone, the transition zone, and the ridged zone, which have different morphology. The composites present uneven or jagged morphology on macroscopic scale, while the microstructure exhibits salient shear bands and second-phase addition interactions. Bridge formation between tungsten fibers is interpreted as evidence that the shear band propagation in the matrix is suppressed by the fibers. Furthermore, shear lips were observed for the composites containing over 50 pct, fiber volume fraction, showing a great improvement in toughness.     

Deformation and failure of Zr<Subscript>57</Subscript>Nb<Subscript>5</Subscript>Al<Subscript>10</Subscript>Cu<Subscript>15.4</Subscript>Ni<Subscript>12.6</Subscript>/W particle composites under quasi-static and dynamic compression

We have investigated the mechanical behavior of a composite material consisting of a Zr₅₇Nb₅Al₁₀Cu_15.4Ni_12.6 metallic glass matrix with 60 vol pct tungsten particles under uniaxial compression over a range of strain rates from 10⁻⁴ to 10⁴ s⁻¹. In contrast to the behavior of single-phase metallic glasses, the failure strength of the composite increases with increasing strain rate. The composite shows substantially greater plastic deformation than the unreinforced glass under both quasi-static and dynamic loading. Under quasi-static loading, the composite specimens do not fail even at nominal plastic strains in excess of 30 pct. Under dynamic loading, fracture of the composite specimens is induced by shear bands at plastic strains of approximately 20 to 30 pct. We observed evidence of shear localization in the composite on two distinct length scales. Multiple shear bands with thicknesses less than 1 μm form under both quasi-static and dynamic loading. The large plastic deformation developed in the composite specimens is due to the ability of the tungsten particles both to initiate these shear bands and to restrict their propagation. In addition, the dynamic specimens also show shear bands with thicknesses on the order of 50 μm; the tungsten particles inside these shear bands are extensively deformed. We propose that thermal softening of the tungsten particles results in a lowered constraint for shear band development, leading to earlier failure under dynamic loading.     

Effect of Strain Rate on Plastic Flow in Zr-Based Metallic-Glass-Reinforced Porous Tungsten Matrix Composites

The rate-dependent deformation of Zr₃₈Ti₁₇Cu_10.5Co₁₂Be_22.5 bulk metallic-glass-reinforced porous tungsten matrix composites was investigated over a wide range of strain rates. The composites were examined in two forms: the as-cast composite and the as-extruded composite by extrusion. In addition to showing greater strain hardening, the as-cast composite also shows much more obvious strain rate dependence of flow stress than the as-extruded composite. Microhardness tests were performed on the tungsten and the metallic glass phase in both composites, respectively. The results from the microhardness measurements indicate that the strain rate sensitivity of the as-extruded composite is primarily a result of strain rate sensitivity of the tungsten phase.     

Ductile Ti-Based Bulk Metallic Glasses with High Specific Strength

Ti-based bulk metallic glasses (BMGs) with large compressive plasticity were developed in the Ti-rich part of Vitreloy series BMGs (Ti_65–x Zr _x Cu₉Ni₈Be₁₈ alloys with x = 0, 5, 10, 15, and 20). The current materials exhibit high fracture strength reaching ~2.3 GPa and plastic strains up to ~8.3 pct after partial substitution of Zr by Ti. The plasticity of the investigated alloys strongly depends on the Zr content, which affects the elastic constants, such as Poisson’s ratio and shear modulus. This, in turn, has an impact on the shear transformation zone (STZ) volume and, hence, on the shear banding of the glasses.     

Fatigue and fracture behavior of bulk metallic glass

Tensile, compressive, cyclic tension-tension, and cyclic compression-compression tests at room temperature were systematically applied to a Zr_52.5Cu_17.9Al₁₀Ni_14.6Ti₅ bulk metallic glass for comprehensive understanding of its damage and fracture mechanisms. Under tensile loading, the metallic glass only displays elastic deformation followed by brittle shear fracture. Under compressive loading, after elastic deformation, obvious plasticity (0.5 to 0.8 pct) can be observed before the final shear fracture. The fracture strength under compression is slightly higher than that under tension. The shear fracture under compression and tension does not occur along the maximum shear stress plane. This indicates that the fracture behavior of the metallic glass does not follow the Tresca criterion. The fracture surfaces show remarkably different features, i.e., a uniform vein structure (compressive fracture) and round cores coexisting with the radiating veins (tensile fracture). Under cyclic tension-tension loading, fatigue cracks are first initiated along localized shear bands on the specimen surface, then propagated along a plane basically perpendicular to the stress axis. A surface damage layer exists under cyclic compression-compression loading. However, the final failure also exhibits a pure shear fracture feature as under uniaxial compression. The cyclic compression-compression fatigue life of the metallic glass is about a factor of 10 higher than the cyclic tension-tension fatigue life at the same stress ratio. Based on these results, the damage and fracture mechanisms of the metallic glass induced by uniaxial and cyclic loading are elucidated.     

Effect of Nb Content on Mechanical Behavior and Structural Properties of W/(Zr55Cu30Al10Ni5)100−x Nb x Composite

In the present study, (Zr₅₅Cu₃₀Al₁₀Ni₅)_100−x Nb_(x=0,1,2,3) bulk metallic glass matrix/tungsten wire composites were fabricated by infiltration process. Structural studies were investigated by scanning electron microscopy and X-ray diffraction method. Also, mechanical behaviors of the materials were analyzed using quasi-static compressive tests. Results indicated that the best mechanical properties i.e., 2105 MPa compressive ultimate strength and 28 pct plastic strain before failure, were achieved in the composite sample with X = 2. It was also found that adding Nb to the matrix modified interface structure in W fiber/(Zr₅₅Cu₃₀Al₁₀Ni₅)₉₈Nb₂ since the stable diffusion band formation acts as a functionally graded layer. Finally, the observation of multiple shear bands formation in the matrix could confirm the excellent plastic deformation behavior of the composite.

     

Direct Observation on the Evolution of Shear Banding and Buckling in Tungsten Fiber Reinforced Zr-Based Bulk Metallic Glass Composite

The evolution of micro-damage and deformation of each phase in the composite plays a pivotal role in the clarification of deformation mechanism of composite. However, limited model and mechanical experiments were conducted to reveal the evolution of the deformation of the two phases in the tungsten fiber reinforced Zr-based bulk metallic glass composite. In this study, quasi-static compressive tests were performed on this composite. For the first time, the evolution of micro-damage and deformation of the two phases in this composite, i.e., shear banding of the metallic glass matrix and buckling deformation of the tungsten fiber, were investigated systematically by controlling the loading process at different degrees of deformation. It is found that under uniaxial compression, buckling of the tungsten fiber occurs first, while the metallic glass matrix deforms homogeneously. Upon further loading, shear bands initiate from the fiber/matrix interface and propagate in the metallic glass matrix. Finally, the composite fractures in a mixed mode, with splitting in the tungsten fiber, along with shear fracture in the metallic glass matrix. Through the analysis on the stress state in the composite and resistance to shear banding of the two phases during compressive deformation, the possible deformation mechanism of the composite is unveiled. The deformation map of the composite, which covers from elastic deformation to final fracture, is obtained as well.     

Defects and Plastic-Deformation Modes of Bulk-Metallic Glasses

Results of the investigation of the point-defect manifestation in the recovery kinetics of Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 and Zr_52.5Ti₅Cu_17.9Ni_14.6Al₁₀ bulk-metallic glasses (BMGs) irradiated with 2.5 MeV electrons at 80 K (–193.15 °C) are presented. An observation of the pronounced annealing stages at 155 K and 130 K (–118.15 °C and –143.15 °C), and 225 K (–48.15 °C), shows that irradiation generates stable point defects in BMGs. The ultrasonic vibrations (USVs) of different amplitudes were used to investigate their effects on the cluster boundaries. The Kaiser effect is chosen as a tool for examining the boundary-slip initiation and impact of vibrations on the intercluster-boundary structure. Both the acoustic-emission activity and strength decrease due to the specimen pretreatment by USV. This effect is interpreted as a result of boundary softening under the USV. The inherent tensile strength of a Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 BMG (in atomic percent) in the as-cast state was determined by means of high-field mechanical loading using the field-ion microscopy. It was revealed that the strength is characterized by a strong size effect in a nanometer-scale range as a result of the manifestation of the structural nanoheterogeneities and, in part, the existence of the cluster boundaries.     

Thermal Stability, Glass-Formation Ability, and Mechanical Properties of (Zr41.2Ti13.8Cu12.5Ni10Be22.5)100?xNbx Amorphous Alloys

Bulk amorphous alloys of (Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5)_100−x Nb _x with x = 0, 5, 11, and 13 were prepared by water quenching. Differential scanning calorimeter (DSC) analysis revealed that the addition of Nb enhances the thermal stability but appreciably decreases the glass-forming ability (GFA) of the alloys. Scanning electron microscope (SEM) and compression tests indicated that the Nb addition effectively improves the strength and plasticity of a Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 amorphous alloy, which benefits from multiple shear bands induced by ductile crystalline phase dispersing in the amorphous matrix. The bulk amorphous alloy with x = 5 exhibits a fracture stress of 2070 MPa and total strain to fracture of 25.8 pct, respectively. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007, during the TMS Annual Meeting in Orlando, FL, under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.

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Tension-Tension-Fatigue Behaviors of a Zr-Based Bulk-Metallic-Glass-Matrix Composite

An improved tension-fatigue limit of 473 MPa is gained for Zr_58.5Ti_14.3Nb_5.2Cu_6.1Ni_4.9Be_11.0 bulk-metallic-glass-matrix composites (BMGMCs). High volume-fraction dendrites within the glass matrix induce increased effectiveness on the blunting and propagating resistance of the fatigue-crack tip. Each fine striation is created during one stress cycle on the basis of analysis of experiments and calculations. A distinguishingly decreased crack-growth rate for the BMGMCs, compared to the monolithic BMGs, prevails, due to the plastic deformation of the dendrites, evidenced by the synchrotron X-ray results.     

Tough Hypoeutectic Zr-Based Bulk Metallic Glasses

Significant softening of Zr-based bulk metallic glasses (BMGs) can be seen in a hypoeutectic Zr-enriched composition, which brings about very high toughness (for the Zr₅₅Cu₃₅Al₁₀ and Zr₆₀Cu₃₀Al₁₀ BMGs) and tensile plasticity at room temperature (for the Zr₇₀Ni₁₆Cu₆Al₈ BMG). The unique features of such BMGs include the formation of multiple shear bands and harmonic alternating movements that can immediately accommodate concentrated stresses and avoid accidental catastrophic fracture.     

Rheological analogs for the flow of hyperplastic and viscous-plastic materials

This paper describes original experimental results obtained in a study of the rheological behavior of amorphous bulk Vitralloy 1 (Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5), which shows linear viscous (Newtonian) flow over a certain range of deformation rates. These results were compared against results obtained for other superplastic alloys, and against results obtained for metal melts during the crystallization process. __________ Translated from Metallurg, No. 6, pp. 30–35, June, 2007. Based on materials from a paper presented at the AKT-2006 International Conference, Voronezh, September 2006.     

Modelling and simulation of twin-roll casting of bulk metallic glasses

As an economic and direct route to continuous thin strip production from the melt, twin roll casting (TRC) has been established as an effective process for aluminium alloys. Its adaptation to casting of bulk amorphous alloy strip necessitates matching of the thermal and mechanical behaviour of the cooling multi-component melt to the requirements (especially cooling rate, and strip exit temperature and thermal gradient) of vitrification. Using a dedicated control volume numerical model of TRC, simulation of the casting of 2 mm thick Vit 1 (Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5) alloy strip shows that the acceptable casting speeds are in the range 2.5 to 3.5 cm/s. The effects of varying strip thickness and strip-roll heat transfer coefficient (HTC) on this casting window are assessed. The differences between modelling of conventional alloy solidification and metallic glass formation are presented.     

Mechanisms for fracture and fatigue-crack propagation in a bulk metallic glass

The fracture and fatigue properties of a newly developed bulk metallic glass alloy, Zr_41.2Ti_13.8Cu_12.5 Ni₁₀Be_22.5 (at. pct), have been examined. Experimental measurements using conventional fatigue precracked compact-tension C(T) specimens (∼7-mm thick) indicated that the fully amorphous alloy has a plane-strain fracture toughness comparable to polycrystalline aluminum alloys. However, significant variability was observed and possible sources are identified. The fracture surfaces exhibited a vein morphology typical of metallic glasses, and, in some cases, evidence for local melting was observed. Attempts were made to rationalize the fracture toughness in terms of a previously developed micromechanical model based on the Taylor instability, as well as on the observation of extensive crack branching and deflection. Upon partial or complete crystallization, however, the alloy was severely embrittled, with toughnesses dropping to ∼1 MPa . Commensurate with this drop in toughness was a marginal increase in hardness and a reduction in ductility (as measured via depthsensing indentation experiments). Under cyclic loading, crack-propagation behavior in the amorphous structure was similar to that observed in polycrystalline steel and aluminum alloys. Moreover, the crack-advance mechanism was associated with alternating blunting and resharpening of the crack tip. This was evidenced by striations on fatigue fracture surfaces. Conversely, the (unnotched) stress/life (S/N) properties were markedly different. Crack initiation and subsequent growth occurred quite readily, due to the lack of microstructural barriers that would normally provide local crack-arrest points. This resulted in a low fatigue limit of ∼4 pct of ultimate tensile strength.     

Deformation and Fracture Behavior of Metallic Glassy Alloys and Glassy-Crystal Composites

The present work demonstrates the deformation behavior of Zr-Cu-Ni-Al bulk glassy alloys and Zr-Ni-Cu-Al-Pd glassy foils as well as Ni-Cu-Ti-Zr bulk crystal-glassy composites. Fracture of Zr₆₀Cu₁₆Ni₁₄Al₁₀ and Zr_64.13Ni_10.12Cu_15.75Al₁₀ bulk glassy alloys is featured by nearly equal fraction areas of cleavage-like and vein-type relief. The observed pattern of alternating cleavage-like and vein-type patterns illustrates a result of dynamically self-organizing shear propagation at the final catastrophic stage. The deformation behavior of Zr_64.13Ni_10.12Cu_15.75Al₁₀ alloy has also been tested at LN₂ temperature. The strength of the sample decreases with temperature, and no clear serrated flow typical for bulk glassy samples tested at room temperature is observed in the case of the samples tested at LN₂ temperature. We also studied the deformation behavior of Zr-Ni-Cu-Al-Pd glassy foils thinned to electron transparency in situ in tension in a transmission electron microscope. We also present a Ni-Cu-Ti-Zr crystal-glassy composite material having a superior strength paired with a considerable ductility exceeding 10 pct. The metastable cP2 crystalline phase promotes a strain-induced martensitic transformation leading to pseudoelastic behavior as well as enhanced plasticity at room temperature. Underlying mechanisms of plastic deformation are discussed in terms of the interplay between the dislocation slip in the crystalline phase and the shear deformation in the glassy matrix.     

Effects of Test Temperature and Loading Conditions on the Tensile Properties of a Zr-Based Bulk Metallic Glass

The effects of changes in test temperature (20 °C, 260 °C, 330 °C, and 380 °C), strain rate (10⁻⁵ to 10⁻¹ s⁻¹), and loading conditions (displacement control vs loading-rate control) on the tensile behavior of Zr_41.2Ti_13.8Cu_12.5Ni₁₀Be_22.5 (LiquidMetal 1 (LM1)), a bulk metallic glass (BMG), have been determined. Significant effects of the test temperature, strain rate, and loading condition were observed on the strength, ductility/elongation, and mechanisms of failure (shear, ductile rupture, etc.). This material exhibited extensive elongation (i.e., >100 pct) prior to failure when tested near the glass transition temperature (T _g  ≈ 375 °C) at sufficiently low strain rates, while higher strain rates or lower test temperatures produced shear fracture at low elongation. The flow and fracture behavior was also significantly affected by the loading condition (i.e., displacement vs loading-rate control). The effective strain rate necessary to cause failure in shear without significant global flow was several orders of magnitude lower in loading-rate control than in displacement control. Samples exhibiting high elongation tested in displacement control gently and convexly drew to a near point (i.e., ductile rupture). Samples tested at the same temperature exhibiting high elongation in loading-rate control rapidly and concavely necked, followed by drawing to a constant diameter “wire” (i.e., ductile drawing), eventually failing by nearly pure ductile rupture. All samples that displayed significant elongation did so inhomogeneously, and were characterized by non-Newtonian global flow. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.

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连续钨丝增强锆基块体金属玻璃变形过程中剪切带演化

研究体积分数为60%的连续钨丝/Zr41.2Ti13.8Cu12.5Ni10Be22.5块体金属玻璃复合材料准静态压缩变形过程中剪切带的演化过程。发现在弹性变形段不形成剪切带,剪切带是在塑性变形过程中产生并发展的,且剪切带的数量随着变形量的增加而增大,间距随着变形量的增加而减小;当间距减小到一定值时产生剪切裂纹并不断扩展,最终导致断裂破坏。     

Yield criteria and strain-rate behavior of Zr57.4Cu16.4Ni8.2Ta8Al10 metallic-glass-matrix composites

We have examined the yielding and fracture behavior of Zr_57.4Cu_16.4Ni_8.2Ta₈Al₁₀ metallic-glass-matrix composites with a small volume fraction (∼4 pct) of ductile crystalline particles under quasi-static uniaxial tension and compression and dynamic uniaxial compression. The yield stress of the composite is the same for quasi-static tension and compression, consistent with a von Mises yield criterion. The measured average angle between the shear bands and the loading axis in quasi-static compression is 47±2 deg, significantly larger than the value of ∼42 deg typically reported for single-phase metallic glasses. Finite element modeling (FEM) shows that the measured value is consistent with both the von Mises criterion (48±4 deg) and the Mohr-Coulomb criterion (46±5 deg). The fracture surface angles, however, are 41±1 deg (compression) and 54±2 deg (tension), in good agreement with observations of single-phase metallic glasses. At low strain rates (<10⁻¹ s⁻¹), the yield stress is independent of strain rate, while at higher strain rates (>10⁰ s⁻¹), the failure stress decreases with increasing strain rate, which again is similar to the behavior of single-phase glasses. These results indicate that while the presence of the particles has a significant effect on the yield behavior of the composites, the fracture behavior is largely governed by the properties and behavior of the amorphous matrix.     

Zirconium based bulk metallic glass / tungsten fibre composite — fabrication and characterization

The aim of the present work was to fabricate and characterize a composite consisting of Zr based bulk metallic glass as the matrix and W fibres as the reinforcement. This kind of composite because of its very high impact toughness has got widespread applications including in some strategic areas. The glass forming Zr₅₂Ti₆Al₁₀Cu₁₈Ni₁₄ (at%) alloy was selected for this purpose because of its high glass forming ability and tungsten was selected as the reinforcing medium because of its high melting point, non reactivity with the liquid phase and high strength. The composite was fabricated in a unique way where a preform of W wires was made and the glass forming alloy was vitrified with this preform as reinforcement using the copper mold casting technique The composite produced was characterized using optical microscopy and EPMA studies. Compression testing was done to evaluate the mechanical properties of the composite.