Ductile bulk metallic glass

We report on experimental evidence of pronounced global plasticity measured in monolithic Pt57.5Cu14.7Ni5.3P22.5 bulk metallic glass under both bending and unconfined compression loading conditions. A plastic strain of 20% is measured, never before seen in metallic glasses. Also, permanent deformation and a strain exceeding 3% before failure is observed during bending of 4 mm thick samples. To date, no monolithic metallic material has exhibited such a combination of high strength, extensive ductility, and high elastic limit. The large plasticity is reflected in a high Poisson ratio of 0.42, which causes the tip of a shear band to extend rather than initiate a crack. This results in the formation of multiple shear bands and is the origin of the observed large global ductility and very high fracture toughness, approximately 80 MPa m(-1/2).     

Pd-Si binary bulk metallic glass

Pd_80+x Si_20−x (x = 0, 1, and 2) binary metallic glasses with the diameter ranging from 7 to 8 mm were prepared by a combination of fluxing and water quenching or air cooling. Thermal analysis results show that with increasing Si content, the glass transition temperature T _g, the initial crystallization temperature T _x and the onset crystallization temperature T _p of Pd-Si binary glassy alloys increase. Moreover, the supercooled liquid region reaches 61 K. It indicates that Pd-Si binary alloys possess large glass forming ability, which can be greatly improved by fluxing treatment. Supported by the National Basic Research Program of China (Grant No. 2007CB613905) and the National Natural Science Foundation of China (Grant Nos. 50671050 and 50431030)     

Irreversible structural relaxation in a bulk Pd-Cu-Ni-P metallic glass

The evolution of the shear modulus and the damping decrement during irreversible structural relaxation in a bulk Pd₄₀Cu₃₀Ni₁₀P₂₀ metallic glass in a temperature range below the glass transition temperature has been studied with an inverse torsion pendulum at a frequency of ～25 Hz. It is shown that the irreversible relaxation can be recovered via quenching from temperatures above the glass transition temperature. The spectrum shape, the characteristic activation energies, and the attempt frequencies of the irreversible structural relaxation are estimated.     

New class of plastic bulk metallic glass

An intrinsic plastic Cu(45)Zr(46)Al(7)Ti(2) bulk metallic glass (BMG) with high strength and superior compressive plastic strain of up to 32.5% was successfully fabricated by copper mold casting. The superior compressive plastic strain was attributed to a large amount of randomly distributed free volume induced by Ti minor alloying, which results in extensive shear band formation, branching, interaction and self-healing of minor cracks. The mechanism of plasticity presented here suggests that the creation of a large amount of free volume in BMGs by minor alloying or other methods might be a promising new way to enhance the plasticity of BMGs.     

"Work-Hardenable" ductile bulk metallic glass

Usually, monolithic bulk metallic glasses undergo inhomogeneous plastic deformation and exhibit poor ductility (< 1%) at room temperature. We present a new class of bulk metallic glass, which exhibits high strength of up to 2265 MPa together with extensive "work hardening" and large ductility of 18%. Significant increase in the flow stress was observed during deformation. The "work-hardening" capability and ductility of this class of metallic glass is attributed to a unique structure correlated with atomic-scale inhomogeneity, leading to an inherent capability of extensive shear band formation, interactions, and multiplication of shear bands.     

Deformation behavior of Zr-based bulk metallic glass and composite in the supercooled liquid region

A Zr-based bulk metallic glass (BMG) with a composition of (Zr₇₅Cu₂₅)_78.5Ta₄Ni₁₀Al_7.5 and a bulk metallic glass matrix composite (BMGC) with a composition of (Zr₇₅Cu₂₅)_74.5Ta₈Ni₁₀Al_7.5 have been prepared by copper-mold casting. The compressive deformation behavior of the BMG and BMGC was investigated in the supercooled region at different temperatures and various strain rates ranging from 8×10⁻⁴ s⁻¹ to 8×10⁻² s⁻¹. It was found that both the strain rate and test temperature significantly affect the deformation behavior of the two alloys. The deformation follows Newtonian flow at low strain rates but non-Newtonian flow at high strain rates. The deformation mechanism for the two kinds of alloys was discussed in terms of the transition state theory. Supported by the National Natural Science Foundation of China (grant Nos. 50471060 and 50635020)     

Compressive fracture characteristics of Zr-based bulk metallic glass

The compressive fracture characteristics of Zr-based bulk metallic glass under uniaxial compression tests are studied.The zigzag rheological behavior is observed in the compression stress-strain curves of amorphous alloys.At room temperature the uniaxial compression fracture takes place along the plane which is at a 45-degree angle to the direction of the compressive stress.The microstructure of a typical fracture pattern is the vein network.A unique,finger-like vein pattern is found to exist at the fractur...     

Softening caused by profuse shear banding in a bulk metallic glass

By controlling the specimen aspect ratio and strain rate, compressive strains as high as 80% were obtained in an otherwise brittle metallic glass. Physical and mechanical properties were measured after deformation, and a systematic strain-induced softening was observed which contrasts sharply with the hardening typically observed in crystalline metals. If the deformed glass is treated as a composite of hard amorphous grains surrounded by soft shear-band boundaries, analogous to nanocrystalline materials that exhibit inverse Hall-Petch behavior, the correct functional form for the dependence of hardness on shear-band spacing is obtained. Deformation-induced softening leads naturally to shear localization and brittle fracture.     

新型Pr基大块非晶及其特性研究

用铜模吸铸法获得了直径为5mm的一种新的Pr基大块非晶.与以往其他稀土-过渡金属(RE-TM)大块非晶不同的是，这种新的Pr基大块非晶具有明显的玻璃转变和稳定的过冷液相区，且其玻璃转变温度在目前已知的大块非晶中是最低的，T_g=409K.研究了该大块非晶的玻璃转变动力学，并给出了Kauzmann温度T_k、Vogel-Fulcher温度T⁰_g及脆性参数m等重要参数. 关键词： 大块非晶 玻璃转变 脆性参数m     

Deformation behavior of Fe-based bulk metallic glass during nanoindentation

Fe-based bulk metallic glasses (BMGs) normally exhibit super high strength but significant brittleness at ambient temperature. Therefore, it is difficult to investigate the plastic deformation behavior and mechanism in these alloys through conventional tensile and compressive tests due to lack of distinct macroscopic plastic strain. In this work, the deformation behavior of Fe₅₂Cr₁₅Mo₉Er₃C₁₅B₆ BMG was investigated through instrumented nanoindentation and uniaxial compressive tests. The results show that serrated flow, the typical plastic deformation feature of BMGs, could not be found in as-cast and partially crystallized samples during nanoindentation. In addition, the deformation behavior and mechanical properties of the alloy are insensitive to the applied loading rate. The mechanism for the appearance of the peculiar deformation behavior in the Fe-based BMG is discussed in terms of the temporal and spatial characteristics of shear banding during nanoindentation. Supported by the National Natural Science Foundation of China (Grant Nos 50571109, 10572142 and 56771102) and the National Basic Research Program of China (973 Program)(Grant No 2007CB613900)     

Shear band formation and ductility in bulk metallic glass

The variations in the chemical compositions of the metallic glasses reported in the literature, as well as the overall lack of experimental data concerning the inhomogeneous deformation behaviour of metallic glass, make the evaluation of the effects of shear band/fracture behaviour on the mechanical properties of metallic glasses difficult. Isolating the effect of local shear band formation on bulk inhomogeneous flow would appear to be a first step in approaching this problem. The mechanical behaviour of Vitreloy metallic glass at room temperature and at various strain rates in tension and compression was investigated. The formation of multiple shear bands was observed at high strain rates. An increase in strain rate leads to enhanced ductility in tension and compression. Some aspects of the deformation processes in tension and compression are discussed.     

Microstructure and mechanical properties of a partially crystallized La-based bulk metallic glass

The evolution of microstructure upon partial crystallization and its influence on the mechanical properties such as hardness, elastic modulus and viscosity in a La 55 Al 25 Cu 10 Ni 5 Co 5 bulk metallic glass alloy are studied. Specimens with various volume fractions of crystalline phases were obtained by annealing the as-cast amorphous alloy above its glass transition temperature and were characterized by transmission electron microscopy. Microscopic examination of the heat-treated samples shows short-range-ordered domains prior to nanocrystallization within the amorphous matrix, followed by the growth and impingement of the crystallites. Whereas the hardness of the annealed samples increases linearly with increasing crystallinity, the elastic modulus and the viscosity both increase abruptly when the crystalline volume fraction is about 40 vol.%, with a only minor variation on either side of this range. The sudden rises in the modulus and viscosity are similar to those in the literature data on the fracture strength of partially crystallized bulk amorphous alloys that shows a steep drop in strength at 30-50 vol.% crystallinity. On the basis of the microscopic observations, it is suggested that the interaction and formation of rigid networks of crystalline phases upon the attainment of a critical second-phase volume fraction may be the possible reason for the sudden change in mechanical properties. Percolation theory is utilized in further substantiating this hypothesis.     

In-situ neutron scattering study of crystallization in a Zr-based bulk metallic glass

In this paper, we report the structure, optical, and gas-sensing properties of Co-doped ZnO nanocrystals prepared by a simple solvothermal route. The red-shift of the band-gap edge is attributed to a merging of donor and conduction bands due to Co doping. Photoluminescence (PL) spectra were used to study the dependence of doping on the deep-level emission, which show obvious enhanced blue-green emission after Co doping. Gas sensors were prepared and tested for the detection of C₂H₅OH in air. It is found that the Co-doped nanocrystals have a significantly better sensing performance than pure ZnO, which is comparable to the Au-functionalized ZnO sensors. We provide a possible explanation in terms of the sensing mechanism of the surface reaction process.     

Dynamic strength behavior of a Zr-based bulk metallic glass under shock loading

Dynamic strength behavior of Zr51Ti5Ni10Cu25Al9 bulk metallic glass(BMG) up to 66 GPa was investigated in a series of plate impact shock-release and shock-reload experiments.Particle velocity profiles measured at the sample/Li F window interface were used to estimate the shear stress,shear modulus,and yield stress in shocked BMG.Beyond confirming the previously reported strain-softening of shear stress during the shock loading process for BMGs,it is also shown that the softened Zr-BMG still has a high shear modulus and can support large yield stress when released or reloaded from the shocked state,and both the shear modulus and the yield stress appear as strain-hardening behaviors.The work provides a much clearer picture of the strength behavior of BMGs under shock loading,which is useful to comprehensively understand the plastic deformation mechanisms of BMGs.     

Zr-based bulk metallic glass as a cylinder material for high pressure apparatuses

Zirconium-based bulk metallic glass (Zr-based BMG) has outstanding properties as a cylinder material for piston-cylinder high pressure apparatuses and is especially useful for neutron scattering. The piston-cylinder consisting of a Zr-based BMG cylinder with outer/inner diameters of 8.8/2.5?mm sustains pressures up to 1.81?GPa and ruptured at 2.0?GPa, with pressure values determined by the superconducting temperature of lead. The neutron attenuation of Zr-based BMG is similar to that of TiZr null-scattering alloy and more transparent than that of CuBe alloy. No contamination of sharp Bragg reflections is observed in the neutron diffraction pattern for Zr-based BMG. The magnetic susceptibility of Zr-based BMG is similar to that of CuBe alloy; this leads to a potential application for measurements of magnetic properties under pressure.     

Fracture characteristics of bulk metallic glass under high speed impact

High speed impact experiments of rectangular plate-shaped Zr₄₁Ti₁₄Cu_12.5Ni₁₀Be_22.5 bulk metallic glass (BMG) were performed using a two-stage light gas gun. Under spherical shock waves with impact velocities ranging from 0.503 km/s to 4.917 km/s, obvious traces of laminated spallation at the back (free) surface and melting (liquid droplets) at the impact point were observed. The angles about 0?, 17?, 36?, and 90? to the shocking direction were shown in the internal samples because of the interaction between the compressive shock waves and the rarefaction waves. The compressive normal stress was found to induce the consequent temperature rise in the core of the shear band.     

Spin-glass behavior and magnetocaloric effect in Tb-based bulk metallic glass

A noncollinear-ferromagnetic spin-glass-like state was observed in Tb₅₅Co₂₀Al₂₅ bulk metallic glass due to the strong random magnetic anisotropy. Associated with this behavior, we observed a comparatively large magnetic entropy change (ΔS_m is 9.75 J K⁻¹ kg⁻¹) in a field change of 7 T and a correspondingly high value of the magnetic refrigeration capacity (RC is 540 J kg⁻¹) with almost no hysteresis loss in the vicinity of the so-called Curie temperature. This opens the possibility of using this material for magnetic cooling purposes.     

Tracer measurements of atomic diffusion inside shear bands of a bulk metallic glass

Atomic diffusion in deformed Pd(40)Ni(40)P(20) bulk metallic glass containing a single family of deformation-induced shear bands was measured by the radiotracer technique. The significant, by orders of magnitude, enhancement of the diffusion rate with respect to that in the untransformed matrix suggests that the shear bands represent short-circuit diffusion paths. Correlations between diffusivity, viscosity, and the excess free volume distribution inside of shear bands are discussed.     

Isoconfigurational elastic constants and liquid fragility of a bulk metallic glass forming alloy

Samples of Zr(46.25)Ti(8.25)Cu(7.5)Ni(10)Be(27.5) were isothermally annealed and quenched near the glass transition temperature and studied by the pulse-echo overlap technique. The shear modulus G of the samples shows a strong reversible dependence on annealing temperatures and, thus, on the specific configurational potential energy of the equilibrium liquid. The low-T dependence of G of the configurationally frozen glasses shows linear temperature dependence as expected by Debye-Grüneisen theory. The T dependence of G in the liquid state is directly related to the viscosity and fragility of the liquid.