Thermodynamic properties and mixing thermodynamic parameters of binary homogeneous metallic melts

After the investigation on the thermodynamic properties and mixing thermodynamic parameters of binary homogeneous metallic melts involving compound, peritectic as well as solid solution, it was found that the equations of mixing free energy △Gm and excess free energy △Cxs of them can be expressed by the following equations: and △GxS =△Gm -RT(alna + blnb), respectively.     

Modeling size effects on fatigue life of a zirconium-based bulk metallic glass under bending

A size effect on the fatigue-life cycles of a Zr₅₀Cu₃₀Al₁₀Ni₁₀ (at.%) bulk metallic glass has been observed in the four-point-bending fatigue experiment. Under the same bending-stress condition, large-sized samples tend to exhibit longer fatigue lives than small-sized samples. This size effect on the fatigue life cannot be satisfactorily explained by the flaw-based Weibull theories. Based on the experimental results, this study explores possible approaches to modeling the size effects on the bending-fatigue life of bulk metallic glasses, and proposes two fatigue-life models based on the Weibull distribution. The first model assumes, empirically, log-linear effects of the sample thickness on the Weibull parameters. The second model incorporates the mechanistic knowledge of the fatigue behavior of metallic glasses, and assumes that the shear-band density, instead of the flaw density, has significant influence on the bending fatigue-life cycles. Promising predictive results provide evidence of the potential validity of the models and their assumptions.     

Intrinsic and extrinsic size effects in the deformation of metallic glass nanopillars

Nanosized pillars with diameters ranging from 90 to 600 nm of four amorphous alloys, Cu₄₇Ti₃₃Zr₁₁Ni₆Sn₂Si₁, Zr₅₀Ti_16.5Cu₁₅Ni_18.5, Zr_61.8Cu₁₈Ni_10.2Al₁₀ and Al₈₆Ni₉Y_5, were fabricated and tested in situ in a transmission electron microscope. The major consideration when varying the composition was the change in bulk modulus and Poisson’s ratio, which may affect the deformation mode and ductility of metallic glasses (MGs) at the nanoscale. Differences between the deformation behavior of tapered (1.5-3°) and taper-free systems were also investigated. The yield stress of all the MGs measured through the in situ experiments is found to be essentially size independent, irrespective of tapering. With increasing size, all the MGs examined show a ductile-to-brittle transition under compression; the transition point, however, depends on the chemical composition of the specific MG investigated. The lower the μ/B ratio, the larger the pillar diameter above which more brittle behavior occurs. Al₈₆Ni₉Y₅ taper-free MG showed a transition threshold to brittle behavior at the largest pillar diameter of 300 nm. A micromechanical model is presented to explain the various dependencies.     

Understanding the properties and structure of metallic glasses at the atomic level

Liquids and glasses have been well known to human kind for millennia. And yet major mysteries remain in the behavior of glasses and liquids at the atomic level, and identifying the microscopic mechanisms that control the properties of glasses is one of the most challenging unsolved problems in physical sciences. For this reason, applying simplistic approaches to explain the behavior of metallic glasses can lead to serious errors. On the other hand because metallic glasses are atomic glasses with relatively simple structure, they may offer better opportunities to advance our fundamental understanding on the nature of the glass. The difficulties inherent to the problem and some recent advances are reviewed here.     

Indentation size effect and shear transformation zone size in a bulk metallic glass in two different structural states

The existence of an indentation size effect (ISE) in the onset of yield in a Zr-based bulk metallic glass (BMG) is investigated by employing spherical-tip nanoindentation experiments. Statistically significant data on the load at which the first pop-in in the displacement occurs were obtained for three different tip radii and in two different structural states (as-cast and structurally relaxed) of the BMG. Hertzian contact mechanics were employed to convert the pop-in loads to the maximum shear stress underneath the indenter. Results establish the existence of an ISE in the BMG of both structural states, with shear yield stress increasing with decreasing tip radius. Structural relaxation was found to increase the yield stress and decrease the variability in the data, indicating “structural homogenization” with annealing. Statistical analysis of the data was employed to estimate the shear transformation zone (STZ) size. Results of this analysis indicate an STZ size of ～25 atoms, which increases to ～34 atoms upon annealing. These observations are discussed in terms of internal structure changes that occur during structural relaxation and their interaction with the stressed volumes in spherical indentation of a metallic glass.     

Local atomic structure of Pd–Ni–P bulk metallic glass examined by high-resolution electron microscopy and electron diffraction

Structural fluctuation in a Pd₄₀Ni₄₀P₂₀ bulk metallic glass is investigated by transmission electron microscopy and electron diffraction. Local atomic ordered regions with a fcc-(Pd,Ni) type structure was sharply imaged by a high-resolution electron microscopy (HREM) attached with a Cs-corrector. Interference function for the glassy state was obtained from electron-diffraction intensity profiles using energy-filter and imaging-plate techniques. We used a reverse Monte Carlo (RMC) simulation method to develop a realistic structure model. The model consists of a dense-random-packing structure, in which an fcc ordered region with Pd, Ni, and P atoms was embedded. The structure model is consistent with the diffraction and HREM results. In Voronoi polyhedral analysis of the RMC simulated structure, P-centered (Pd,Ni)-P trigonal prisms are found primarily in the matrix structure embedding the fcc-cluster. Around Pd and Ni atoms deformed-fcc type polyhedra were frequently observed. From these local structural features, nanoscale phase separation was revealed to occur during the glass formation.     

The dependence of shear modulus on dynamic relaxation and evolution of local structural heterogeneity in a metallic glass

Starting from the nanoscale structural heterogeneities intrinsic to metallic glasses (MGs), here we show that there are two concurrent contributions to their microscale quasi-static shear modulus G_I: one (μ) is related to the atomic bonding strength of solid-like regions and the other (G_II) to the change in the possible configurations of liquid-like regions (dynamic relaxation). Through carefully designed high-rate nanoscale indentation tests, a simple constitutive relation (μ = G_I + G_II) is experimentally verified. On a fundamental level, our current work provides a structure–property correlation that may be applicable to a wide range of glassy materials.     

Computational analysis of the atomic size effect in bulk metallic glasses and their liquid precursors

The atomic size effect and its consequences for the ability of multicomponent liquid alloys to form bulk metallic glasses are analyzed in terms of the generalized Bernal’s model for liquids, following the hypothesis that maximum density in the liquid state improves the glass-forming ability. The maximum density that can be achieved in the liquid state is studied in the 2(N-1) dimensional parameter space of N-component systems. Computer simulations reveal that the size ratio of largest to smallest atoms are most relevant for achieving the maximum packing for N = 3–5, whereas the number of components plays a minor role. At small size ratio, the maximum packing density can be achieved by different atomic size distributions, whereas for medium size ratios the maximum density is always correlated to a concave size distribution. The relationship of the results to Miracle´s efficient cluster packing model is also discussed.     

Effects of ion implantation on surface structures and properties for bulk metallic glass

A plate-like Zr₅₅Cu₃₀Ni₅Al₁₀ bulk metallic glass (BMG) was prepared and Co ion implantation was carried out. The effects of ion implantation on surface structures and properties of the BMG have been studied. The results showed, after Co ion implantation, the BMG specimen still maintained an amorphous structure on the whole. Ion implantation changed the thermodynamic parameters and enhanced the thermal stability of the amorphous alloy. Along the depth direction, cobalt element implanted basically existed in a metal state. The maximum enhancement of surface micro-hardness was about 103 Hv or 18%. The average friction coefficients before and after ion implantation were 0.331 and 0.168, respectively, and the corresponding grinding trace widths were 80.23 and 60.17 μm, respectively, i.e., the Co ion implantation has reduced the average friction coefficient and corresponding grinding trace width by about 52.3% and 32.68%, respectively. The reasons of efficient improving the surface performances by ion implantation surface modification in BMG were analyzed and discussed.     

喷嘴结构和工艺参数对雾化锌粉粒度的影响

在自行设计的雾化制粉装置上,研究了限制式喷嘴的关键结构参数如输液管内径,喷嘴出长度等。以及雾化工艺参数如压力及液锌的过热度等对锌粉粒度的影响,将所制备的锌粉进行了筛分,对粒径的大小和分布进行了分析。实验发现,颗粒粒度的分布服从对数－正态分布规律。在一定的条件下,输液管内径越大,输液管突出长度越长,锌平均粒度dm,体积平均直径dvm和Sauter平均直径dVs越大,即颗粒越粗,雾化压力越大,液锌的过热度越高,dm,dVm和Sauter平均直径dVs越小,即粉末越细。     

Effects of minor Cu and Si additions on glass forming ability and mechanical properties of Co-Fe-Ta-B Bulk metallic glass

Effect of Cu and Si substitutions for Co and B on the glass forming ability (GFA) of Co_(43-x)Cu_xFe₂₀Ta_5.5B_(31.5-x)Si_y (x=0-1.5 and y=5-10) were systematically investigated by X-ray diffraction, optical microscopy, scanning electron microscopy, and differential scanning calorimetry. In order to evaluate the contribution of copper and silicon, appropriate amounts of copper and silicon were individually introduced to the base alloy composition. By using the effects of copper and silicon together, significant enhancement was obtained and the critical casting thickness (CCT) of the base alloy was increased three times from 2 mm to 6 mm. Moreover, mechanical properties of the alloys were examined by compression tests and Vickers hardness measurements. The compression test results revealed that the glassy alloys having enhanced GFA shows high strength of about 3500-4000 MPa. In addition, existence of (Co,Fe)₂B and (Co,Fe)_20.82Ta_2.18B₆ crystalline phases in glassy matrix influences the hardnesses of the alloys compared to monolitic glassy structure having hardness of about 1200 Hv.     

Fractal growth of the dense-packing phase in annealed metallic glass imaged by high-resolution atomic force microscopy

Unlike crystalline metals, which have a well-understood periodical structure, the amorphous structure of metallic glasses (MGs) is still poorly understood, particularly when such a structure rearranges itself at the nanoscale under external agitations. In this article, we provide compelling evidence obtained from a recently developed high-resolution atomic force microscopy (HRAFM) technique that reveals the nanoscale structural heterogeneity after thermal annealing in a Zr–Ni metallic glass. Through the HRAFM technique, we are able to uncover the annealing-induced fractal growth of the dense-packing phases in the binary MG thin film, which exhibits a fractal dimension of ～1.7, in line with a two-dimensional diffusion limited aggregation process. The current findings not only reveal the evolution process of atomic packing in the annealed MG thin film, but also shed light on the possible cooling rate effect on the atomic structure of MGs.     

Y元素添加对CuZrAl块体金属玻璃的结构和力学性能的影响(英文)

研究添加Y元素对CuZrAl块体金属玻璃的结构和力学性能的影响。结果表明,添加Y元素提高CuZrAl体系的玻璃形成能力,而且由于添加Y元素可以降低该体系的结合能,从而降低其断裂强度。Cu45Zr48Al7块体金属玻璃的断裂表面主要呈脉状,而Cu46Zr42Al7Y5块体金属玻璃的断裂表面则很平滑。TEM观察表明,Cu45Zr48Al7的微观结构为非晶基体中含有纳米相,然而Cu46Zr42Al7Y5块体金属玻璃为全非晶结构。     

Thermodynamic properties and mixing thermodynamic parameters of Ba-Al, Mg-Al, Sr-Al and Cu-Al metallic melts

Application of equations of mixing thermodynamic parameters formulated on the basis of the coexistence theory of metallic melts in Ba-Al, Mg-Al, Sr-Al and Cu-Al melts leads to fruitful results that not only the evaluated mass action concentrations agree well with the measured activities, but also the calculated mixing thermodynamic parameters are quite coincident with the experimental values. Moreover, the calculated mass action concentrations strictly obey the mass action law. The evaluated mixing thermodynamic parameters have very fine regularity: the mixing free energy is composed of standard free formation energies of all compounds and chemical potentials of all structural units at equilibrium; the mixing enthalpy consists of standard formation enthalpies of all compounds; the mixing entropy is composed of standard entropies of all compounds and configuration entropies of all structural units at equilibrium. As the equations of mixing thermodynamic parameters formulated are widely applicable to metallic melts involving compound formation, they can be used as the second practical criterion to determine whether thermodynamic models of metallic melts are correctly formulated.     

Effects of Ce and Mm additions on the glass forming ability of Al–Ni–Si metallic glass alloys

The effects of Ce and Mm contents on the glass forming ability (GFA) of melt-quenched Al_89−xNi₈Ce_xSi₃ and Al_89−xNi₈Mm_xSi₃ (x = 0, 1, 3, 5, 7 at.%) alloys have been systematically investigated by X-ray diffraction (XRD) and differential scanning calorimetry (DSC). According to the XRD and DSC results, both Ce and Mm elements can enhance the GFA and thermal stability of the Al–Ni–Si alloys. Moreover, only the x = 5 and x = 7 alloys are totally amorphous in both systems quenched at the wheel speed of 36.6 m/s. Compared with amorphous Al₈₄Ni₈Ce₅Si₃ alloy at different cooling rates, amorphous Al₈₄Ni₈Mm₅Si₃ alloy has higher GFA which is considered to have relation to the different atomic structure of the amorphous alloy.     

Dynamic mechanical behavior of a Zr-based bulk metallic glass during glass transition and crystallization

The dynamic mechanical behaviors of the Zr41Ti14Cu12.5Ni8Be22.5Fe2 bulk metallic glass (BMG) during continuous heating at a constant rate were investigated. The glass transition and crystallization of the Zr-based BMG were thus characterized by the measurements of storage modulus E and internal friction Q-1. It was found that the variations of these dynamic mechanical quantifies with temperature were interre-lated and were well in agreement with the DSC trace obtained at the same heating rate. The origin of the first peak in the internal friction curve was closely related to the dynamic glass transition and subsequent primary crystallization. Moreover, it can be well described by a physical model, which can characterize atomic mobility and mechanical response of disordered condense materials. In comparison with the DSC trace, the relative position of the first internal friction peak of the BMG was found to be dependent on its thermal stability against crys-tallization.     

Prediction of the annealing effect on room-temperature shear modulus of a metallic glass

It has been shown that the Interstitialcy theory allows precise prediction of shear modulus magnitude of a metallic glass after complicated high temperature heat treatment followed by quenching to room temperature.