Al/U固相界面反应的原位高温X射线衍射研究

采用磁控溅射沉积技术在贫铀上镀铝膜,利用X射线衍射原位研究了铝镀层与贫铀基体的界面反应,分析了界面反应产物随温度和时间的变化规律。实验结果显示,镀膜过程Al/U界面没有化合物形成,而在较高温度下(≥335℃)两相界面会发生反应并形成金属间化合物UAl_3相,反应动力学符合形核-生长机制。形成的UAl_3相为疏松的层状结构,且相应的衍射峰强度随温度的升高而逐渐降低,直至消失,文中对该现象的原因进行了分析和讨论。     

Mechanical failure and glass transition in metallic glasses

The current majority view on the phenomenon of mechanical failure in metallic glasses appears to be that it is caused by the activity of some structural defects, such as free-volumes or shear transformation zones, and the concentration of such defects is small, only of the order of 1%. However, the recent results compel us to revise this view. Through molecular dynamics simulation it has been shown that mechanical failure is the stress-induced glass transition. According to our theory the concentration of the liquid-like sites (defects) is well over 20% at the glass transition. We suggest that the defect concentration in metallic glasses is actually very high, and percolation of such defects causes atomic avalanche and mechanical failure. In this article we discuss the glass transition, mechanical failure and viscosity from such a point of view.     

Glass forming ability and in-situ composite formation in Pd-based bulk metallic glasses

Pd_60−xNi₁₀Cu₃₀P_x alloys with x=10–20 were prepared by chill-casting, Bridgman solidification and melt spinning, giving a cooling rate range from 0.63 to ∼10⁶ K/s. The effects of P content on the glass forming ability and in-situ bulk metallic glass (BMG) matrix composite formation in these alloys have been studied. Glass forming ability of the Pd–Ni–Cu–P alloys improves with increasing P content. This is interpreted in terms of the reduced glass transition temperature T_rg given by T_g/T_L and the crystallization rate constant K_T. F.c.c α-Pd crystalline phase reinforced BMG matrix composites have been synthesized in a wide range of compositions. The formation of the in-situ BMG matrix composite is explained qualitatively in terms of competition between eutectic growth and amorphous phase formation.     

Estimation of Gibbs free energy difference in Pd-based bulk metallic glasses

A new thermodynamic expression for Gibbs free energy difference AG between the under-cooled liquid and the corresponding crystals of bulk metallic glasses was derived. The newly proposed expression always gives results in fairly good agreement with experimental values over entire temperature range between the fusion temperature Tm and the glass transition temperature Tg of Pd40Ni40P20, Pd40Cu30Ni10P20 and Pd43Cu27Ni10P20, which possess different heat capacities. However, the TS and KN expressions cannot always provide results in good agreement with the experimental values. In addition, the deviations between the experimental values and the AG calculated by the proposed expression at Tg are smaller than those given by other expressions for all the bulk metallic glasses studied.     

A constitutive theory for metallic glasses at high homologous temperatures

The elastic–viscoplastic constitutive theory of Anand and Su [Anand L, Su C. J Mech Phys Solids 2005;53:1362] for metallic glasses has been extended to the high homologous temperature regime. The constitutive equations appearing in the theory have been specialized to model the response of metallic glasses in the temperature range 0.7ϑ_g ⪅ ϑ ⪅ ϑ_g and strain rate range [10⁻⁵, 10⁻²] s⁻¹. The material parameters appearing in the theory have been estimated for the metallic glass Pd₄₀Ni₄₀P₂₀ from the experimental data of De Hey et al. [De Hey P, Sietsma J, Van Den Beukel A. Acta Mater 1998;46:5873]. The model is shown to capture the major features of the stress–strain response, and the evolution of an order-parameter for this metallic glass. In particular, the phenomena of stress overshoot and strain softening in monotonic experiments at a given strain rate and temperature, as well as strain rate history effects in experiments involving strain rate increments and decrements, are shown to be nicely reproduced by the model.     

New nickel-based bulk metallic glasses with extremely high nickel content

The effect of boron addition on glass formation in the Ni₇₀Pd₁₀P₂₀ alloy was investigated. The composition containing 4 at% boron showed an improved glass-forming ability. A glassy Ni₇₀Pd₁₀P₁₆B₄ alloy rod with a diameter of 2.5 mm was prepared by a copper mold casting technique. This is the first time that a Ni-based bulk metallic glass with such an extremely high Ni content of 70 at% has been produced. The obtained glassy Ni₇₀Pd₁₀P₁₆B₄ alloy exhibited rather good mechanical properties and corrosion resistance.     

Acoustic properties of metallic glasses in the mesoscopic regime by inelastic X-ray scattering

The mechanical response of bulk metallic glasses is ultimately controlled by interatomic forces between the randomly distributed atoms. The study of collective dynamics in amorphous systems, either liquid or glassy, opens a window to better understand this interaction. We present here results of recent IXS experiments performed at the ID16 beamline of the European Synchrotron Radiation Facility (ESRF) on a metallic glass. In particular, the high-frequency response of Pd₇₇Si_16.5Cu_6.5 alloys, produced as a ribbon with an ultra-fast quenching rate (～10⁶ K/s) and as a bulk with a slow one (～10² K/s), was determined. Previous experiments showed a discrepancy between the sound speed of metallic glasses measured by ultrasounds and by IXS. This was explained as a consequence of density fluctuations in the glass, although other explanations are possible. Here, the dispersion relation of the acoustic modes at room temperature and pressure are measured for both samples, allowing us to compute the sound speed and discuss the differences in the acoustic properties of both samples.     

Structural mechanisms of the microalloying-induced high glass-forming abilities in metallic glasses

The issue, doping-induced high glass-forming abilities (GFAs) in metallic glasses (MGs), has been investigated by systematic experimental measurements coupled with theoretical calculations in the ZrCuAgAl quaternary alloys. It is found that minor Ag addition leads to a high fraction of Ag-centered icosahedral like local structures, as well as enhanced atomic-packing efficiency and structural regularity in Zr- and Cu-centered major clusters. This can stabilize the amorphous structure and the GFA is accordingly enhanced. However, these structural factors have smaller values when excessive Ag atoms are added, leading to a deteriorated GFA. The present work provides an understanding of GFAs in multicomponent alloys and will shed light on the development of more MGs with high GFAs.     

Heavy rare earth based bulk metallic glasses with high thermal stability

We report a family of ternary Gd–Al–Co alloys based on the heavy gadolinium rare earth element can be readily cast into fully glassy rod by a conventional casting method. It is found that the bulk metallic glasses (BMGs) have much high thermal stability (i.e. high glass transition temperature and crystallization temperature) and high moduli compared with those of other known rare-earth based BMGs. It is confirmed that, in addition to the strong chemical interaction among the components, the high bulk modulus of the base component in the BMGs is dominantly responsible for the high thermal stability. The thermal stability is correlated with the bulk modulus of the base element in the rare earth based BMGs.     

Diffusion in metallic glasses: Analysis from the atomic bond defect perspective

Diffusion in metallic glasses and deeply undercooled liquids is analyzed using the concept of ‘atomic bond deficiency’ (BD). The process is considered to be thermally activated hopping of diffusing atoms between the first nearest-neighbor (NN) and the second NN positions at the BD defects. Cooperative movements of multiple adjacent atoms are required because of (1) the small extra volumes created by BD defects and (2) the short-range-only constraint. The activation energy Q depends on the bond strengths, size of diffusing atoms, the elasticity of the matrix and the effective number of the involved matrix atoms. Application to tracer diffusion in Zr–Ni and Ti–Ni shows that 10 or fewer matrix atoms may be involved, and the size effect of diffusing atoms on Q agrees with experimental measurements. Other observations, including the sharp slope of the Q versus pre-exponential factor, the isotope and pressure effects are explained.     

Magnetic properties and magnetocaloric effect of FeCrNbYB metallic glasses with high glass-forming ability

The influences of Cr addition on the Curie temperature (T_C), glass-forming ability (GFA), and magnetocaloric effect were investigated in FeCrNbYB metallic glasses. It was found that the addition of Cr element slightly decreases the GFA and saturation magnetization, whereas effectively modulates T_C. By the method of copper mold casting, bulk metallic glasses (BMGs) with critical diameters up to 5 mm can be obtained in Fe_68−xCr_xNb₄Y₆B₂₂ (x = 2–6) alloys. The resulting metallic glasses exhibit T_C of 271–367 K and excellent magnetocaloric properties, including magnetic entropy change of 0.76–1.05 J/kg K, and refrigerant capacity of 83–93 J/kg under a low field change of 1.5 T. In addition, they exhibit a wide supercooled liquid region of 116–135 K. The successful synthesis of the FeCrNbYB BMGs with near room-temperature magnetocaloric properties is encouraging for the future development of Fe-based BMGs as a new magnetic refrigerant in magnetic cooling system.     

Mechanical heterogeneity and its relation with glass-forming ability in Zr-Cu and Zr-Cu-Al metallic glasses

Molecular dynamics simulations of nanoindentation using a spherical indenter were adopted to quantitatively probe the local mechanical heterogeneity (MH) in Zr-Cu and Zr-Cu-Al metallic glasses. Distinct MH at the nanometer scale has been revealed by statistically analyzing the first pop-in stress of different regions in the metallic glasses with an indenter of 4 nm in diameter. More interestingly, it is found that the degree of MH has a close relation to glass-forming ability of alloys, i.e., the smaller MH, the better glass-forming ability. Our findings not only shed light on the intrinsic feature of atomic structures, but also have important implications in understanding the structure-property relationship of MGs.     

Fabrication of CuZr(Al) bulk metallic glasses by high pressure torsion

In this paper, pure Cu, Zr and Al metal sheets were alternatively stacked and, then, cold roll-bonded. The overall compositions of the stacks were Cu₇₁Zr_29, Cu₆₂Zr_38, Cu₅₃Zr₄₇ and Cu₅₂Zr₃₈Al₁₀. The bonded materials were subsequently sliced into disks and, then, used for high pressure torsion (HPT) process. After HPT process up to maximum equivalent strain of about 1200 at ambient temperature, bulk materials with 10 mm in diameter and 0.30 mm in thickness were obtained. The phase constitution and microstructural evolutions of the HPTed samples with various rotations ranging from 0.5 to 20 were studied by XRD, FESEM and TEM. Thermal properties of the HPTed samples were analyzed by DSC. It was found that CuZr(Al) BMGs could be fabricated with a wide composition range by the HPT process. Tensile tests were used to measure the mechanical properties of the HPT processed samples. The mechanism of the solid-state amorphization during the HPT process was put forward and discussed.     

Design of Bulk metallic glasses with high glass forming ability and enhancement of plasticity in metallic glass matrix composites: A review

To overcome some of the limits of existing metallic alloys, a new alloy design concept has been introduced recently in order to control the crystallinity, i.e. to utilize crystalline, quasicrystalline, and amorphous structures. In particular, bulk metallic glasses (BMGs) receive great attention because of their unique properties due to their different atomic configuration. Recently, significant progress in enhancing glass forming ability (GFA) has led to the fabrication of BMGs having potential for application as structural and functional materials. Moreover, successful design of BMG matrix composite microstructure suggests that the plasticity of BMGs can be controlled properly. In this review article, we introduce recent research results on the design of BMGs with high GFA and on the enhancement of plasticity in metallic glass matrix composites.     

Interpreting size effects of bulk metallic glasses based on a size-independent critical energy density

Sample-size effects on compressive deformation behavior have been carefully studied for several bulk amorphous alloy systems. The yielding strength is almost unchanged for all sized samples but the compressive plasticity is increased with the decrease of the sample sizes. It was found that there existed a critical plane energy density above which the catastrophic fracture would occur. This critical energy density is dependent on the composition of the glassy samples but independent of the sample sizes. Large samples reach the critical plane energy density within much smaller strain, thus resulting in the observed size-dependence of the macroscopic plasticity.     

In situ tensile deformation of Fe-rich metallic glass at elevated temperatures using hard X-ray diffraction

The present work focuses on the study of the mechanical properties of the Fe_72.5Cu₁Nb₂Mo₂Si_15.5B₇ metallic glass using in situ hard X-ray diffraction techniques. In situ tensile deformation tests provided detailed information about the mechanical properties of the investigated Fe-based metallic glass. Analyzing series of two-dimensional XRD patterns in reciprocal space yields strain tensor components of the amorphous alloy providing insight about its deformation mechanisms. Comparing tensile tests performed at different temperatures indicates that the deformation mechanism gradually changes from purely elastic to completely plastic regime. The Poisson ratio ν of the investigated alloy increases with increasing deformation temperature however the fracture strength σ_f shows opposite behavior.