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.     

Isomechanical groups in bulk metallic glasses

Isomechanical groups are, as defined by Frost and Ashby [H.J. Frost and M.F. Ashby, Deformation–Mechanism Maps, Pergamon Press, Oxford, 1982], separate classes of materials that exhibit similar deformation and transport properties when normalised by an appropriate parameter. Fundamentally, this separation results from significant differences in material structure and bonding. Here, such an analysis is applied to 40 bulk metallic glasses (BMGs) grouped into three classes according to their Poisson's ratio, which is known to be an indicator of intrinsic toughness. Through rigorous statistical analysis, it is found that isomechanical groups are present and that they may result from (1) variation in the tendency for directional bonding, and (2) how liquid-like the structure is, which may be characterised by a quantification of local volumetric strain. These results suggest that, although experimentally observed properties from BMGs in different isomechanical groups are all typically considered within the same framework, differences in atomic packing and inter-atomic bonding mean that they should in fact be treated separately. These fundamental differences in bonding and structure may explain the known large variation in the tendency for toughness and plasticity in BMGs.     

Role of Ag microalloying on glass forming ability and crystallization kinetics of ZrCoAgAlNi amorphous alloy

Bulk metallic glasses(BMGs) with new chemical compositions(ZrCoAgAlNi) were fabricated and the effects of Ag minor addition on the glass forming ability(GFA) and crystallization kinetics were studied. The x-ray diffraction(XRD) test was applied to identify the amorphousness of BMGs or possible crystalline phases. Using differential scanning calorimeter(DSC), the thermal stability and crystallization kinetics under a non-isothermal condition at the different heating rates were studied. Considering the heating rate dependency of glass transition and crystallization kinetics, the activation energy was evaluated and measured for the mentioned processes. It was revealed that the rise in Ag content led to the decrease in activation energy for glass transition, while the activation energy for crystallization increased. The thermal stability and GFA were also studied and it was found that the Ag addition strongly affected the inherent features of BMGs. With the increase in Ag content, the atomic mobility and structural rearrangement changed in the material and consequently, the GFA and thermal stability were significantly improved.     

具有优良玻璃形成能力添加Al的CuZr基大块金属玻璃

在CuZr二元大块金属玻璃的基础上，利用铜模吸铸方法制备出了添加Al组元的CuZr 基大块 金属玻璃.CuZr基大块金属玻璃在很宽的成分范围内有很强的玻璃形成能力，在Al含量从4％ 到8％之间，CuZr基大块金属玻璃都可以做出直径至少5 mm的非晶样品.通过实验分析，解释 了CuZr基大块金属玻璃具有良好玻璃形成能力的物理机理.CuZr基金属玻璃组分简单、成本 低廉，有潜在的应用价值；同时，制备CuZr基金属玻璃的方法为开发新的大块金属玻璃体系 提供了一条切实有效的途经. 关键词： 大块金属玻璃 玻璃形成能力 CuZr基金属玻璃     

Strength,plasticity and brittleness of bulk metallic glasses under compression: statistical and geometric effects

To investigate the flaw sensitivity and reliability of bulk metallic glasses (BMGs), compressive testing was performed on a statistically significant number of specimens. Despite the fact that BMGs exhibit little or no macroscopic plasticity before failure (similar to other brittle materials), we observe surprisingly high uniformity in compressive strength. Weibull analysis was employed to study the statistical dispersion in strength, giving very high Weibull moduli of about 25 for an intrinsically brittle glass, and near 75 for an intrinsically malleable one. This high uniformity is encouraging for the use of BMGs in structural applications. Furthermore, we illustrate that subtle imperfections in the test geometry (i.e. miscut or deviations from orthogonality) dramatically affect the compression response. In brittle glasses these act as failure-critical flaws, whereas in malleable glasses they constrain shear bands, lead to tilting and bending during testing, and give rise to misleading macroscopic measurements of plastic deformation.     

Radiation of Dressed Excitons in the Semiconductor Microcavity

This paper reports a molecular dynamics simulation for atomic structure of copper nanocrystals,in which the grain size and lattice distortion were computed by X-ray diffraction simulation,and the distribution of energy and elastic modulus was calculated.The results show that the lattice distortion was stored not only at grain boundaries but also in the grain.The increase of atomic radius leads to the reduction of the elastic modulus of the nanocrystals.     

纳米晶体微观畸变与弹性模量的模拟研究

采用分子动力学方法模拟纳米晶体铜原子的结构,又对纳米晶体铜原子进行了X射线衍射模拟．计算了晶粒尺寸和点阵畸变,还计算了能量分布和弹性模量等．结果表明不但晶界产生很大的应力场,而且晶粒内部的畸变也起着与晶界相似的重要作用．由于原子半径的增加,导致弹性模量的减少. 关键词：     

Recent Advancements in Bulk Metallic Glasses and Their Applications: A Review

Bulk metallic glasses (BMGs), that display extraordinary properties of high strength, corrosion resistance, polymer-like formability, and excellent magnetic properties, are emerging as modern quintessential engineering materials. BMGs have garnered significant research enthusiasm owing to their tremendous technological and scientific standing. In this article, the recent advancements in the field of BMGs and their applications are put in a nutshell. Novel state-of-the-art production routes and nano/microimprinting strategies with salient features capable of circumventing the processing related complexities as well as accelerating modern developments, are briefly summarized. Heterogeneous BMG composite systems that lead to incredible combination of otherwise conflicting properties are highlighted. Biocorrosion studies and recent developments in the field of magnetic BMGs are presented owing to their significance for prospective biomedical and magnetic applications, respectively. In the last section, the current status of BMGs applications in the field of catalysis, biomedical materials, structural materials, functional materials, microelectromechanical systems (MEMS), and micro/macro devices are summed up.     

Ti-Zr-Be-Fe quaternary bulk metallic glasses designed by Fe alloying

A series of Ti-Zr-Be-Fe bulk metallic glasses(BMGs)with good glass-forming ability(GFA)and high specific strength have been developed.With different alloying routes and content of Fe,it is found that these alloys exhibit different GFA and mechanical properties.The effects of Fe addition on the GFA and mechanical properties of Ti-Zr-Be alloy are systemically investigated.The possible mechanisms for the improvement or damage to the GFA by addition of Fe can be interpreted in view of the mixing enthalpy,atomic size differences and electronegativity differences of the alloys,while the mechanical properties strongly depend on the Poisson’s ratio and free volume concentration.The experimental results also show that alloying technology is an effective method to improve the GFA and mechanical properties of Ti-Zr-Be glassy alloy.     

A Modified Free Volume Model for Characterizing of Rate Effect in Bulk Metallic Glasses

We investigate the plastic deformation and constitutive behaviour of bulk metallic glasses （BMGs）. A dimensionless Deborah number DeiD = tr/ti is proposed to characterize the rate effect in BMGs, where tr is the structural relaxing characteristic time of BMGs under shear load, ti is the macroscopic imposed characteristic time of applied stress or the characteristic time of macroscopic deformation. The results demonstrate that the modified free volume model can characterize the strain rate effect in BMGs effectively.     

Energy dissipation in fracture of bulk metallic glasses via inherent competition between local softening and quasi-cleavage

Compression, tension and high-velocity plate impact experiments were performed on a typical tough Zr_41.2Ti_13.8Cu₁₀Ni_12.5Be_22.5 (Vit 1) bulk metallic glass (BMG) over a wide range of strain rates from ～10^?4 to 10⁶ s^?1. Surprisingly, fine dimples and periodic corrugations on a nanoscale were also observed on dynamic mode I fracture surfaces of this tough Vit 1. Taking a broad overview of the fracture patterning of specimens, we proposed a criterion to assess whether the fracture of BMGs is essentially brittle or plastic. If the curvature radius of the crack tip is greater than the critical wavelength of meniscus instability [F. Spaepen, Acta Metall. 23 615 (1975); A.S. Argon and M. Salama, Mater. Sci. Eng. 23 219 (1976)], microscale vein patterns and nanoscale dimples appear on crack surfaces. However, in the opposite case, the local quasi-cleavage/separation through local atomic clusters with local softening in the background ahead of the crack tip dominates, producing nanoscale periodic corrugations. At the atomic cluster level, energy dissipation in fracture of BMGs is, therefore, determined by two competing elementary processes, viz. conventional shear transformation zones (STZs) and envisioned tension transformation zones (TTZs) ahead of the crack tip. Finally, the mechanism for the formation of nanoscale periodic corrugation is quantitatively discussed by applying the present energy dissipation mechanism.     

Zr-Cu-Ni-Al-Nb大块非晶合金的晶化行为、力学性能及电化学腐蚀行为的研究

利用水冷铜模铸造法成功制备了Zr_65-xCu_17.5Al_7.5Ni ₁₀Nb_x (x=0，2，5)大块非晶合金. X射线衍射、热分析研究结果表 明，Nb的添加显著改变了非晶合金的晶化行为，促进了二十面体准晶相的形成. 各合金的准 静态压缩实验表明，Nb的适量添加有利于提高大块非晶合金的强度和塑性. 其中x=5的大块 非晶合金的抗压强度σ_b和塑性应变量ε_p 关键词： Zr基大块非晶合金 晶化行为 力学性能 耐腐蚀性能     

Influence of free volume and medium-range order on the deformation response of rapidly solidified and bulk Zr-based (Zr52Ti6Al10Cu18Ni14) metallic glass

Free volume and medium-range order (MRO) present in rapidly solidified ribbons (RSRs) and bulk metallic glasses (BMGs) of Zr₅₂Ti₆Al₁₀Cu₁₈Ni₁₄ have been probed by high resolution electron microscopy, fluctuation microscopy, positron annihilation and differential scanning calorimetry. In the as-solidified condition, RSRs showed higher free volume and lower MRO in comparison to BMGs. Within BMGs, the central regions showed higher MRO and lower free volume than the peripheral regions. Uniform deformation of BMGs and RSRs modified their structures, where in, free volume increased in the former and reduced in the latter. These changes in structures led to work hardening in RSRs and work softening in BMGs. Such behaviour could be explained by invoking a concept of critical free volume in the glass phase. For samples (in as-solidified condition) having free volume higher than the critical value, free volume decreased with deformation and showed work-hardening behaviour. In contrast, the work softening behaviour was noticed in samples having free volume lower than the critical free volume.     

Three-point bending fracture characteristics of bulk metallic glasses

This paper presents the SEM micrographs for the three-point bending fracture surfaces of Zr-based, Ce-based and Mg-based bulk metallic glasses (BMGs), which show the dimple structures in the three kinds of BMGs. The shapes of the giant plastic deformation domain on the fracture surface are similar but the sizes are different. The fracture toughness KC and the dimple structure size of the Zr-based BMG are both the largest, and those of the Mg-based BMG are the smallest. The fracture toughness KC and the dimple structure size of the Ce-based BMG are between those of the Zr-based and the Mg-based BMG. Through analyzing the data of different fracture toughnesses of the BMGs, we find that the plastic zone width follows w = (KC/σY)2/(6π).     

Nanomechanical analysis of Clostridium tyrobutyricum spores

In this work we report on the measurement of the Young modulus of the external surface of Clostridium tyrobutyricum spores in air with an atomic force microscope. The Young modulus can be reliably measured despite the strong tip-spore adhesion forces and the need to immobilize the spores due to their slipping on most substrates. Moreover, we investigate the disturbing factors and consider some practical aspects that influence the measurements of elastic properties of biological objects with the atomic force microscopy indentation techniques.     

由微观参量表示的金属单晶体杨氏模量的解析计算式

根据原子物理晶体模型 ,采用原子间相互作用的势函数分析方法 ,得到了由微观物理量表示的金属单晶体杨氏模量的解析计算式 .计算了 7种金属晶体的杨氏模量及金属晶体的断裂强度 ,计算结果与实际值吻合很好 .     

Theoretical strength and the onset of plasticity in bulk metallic glasses investigated by nanoindentation with a spherical indenter

The mechanical behavior of bulk metallic glasses (BMGs) was investigated by nanoindentation with a spherical indenter. The transition from perfectly elastic behavior to plastic deformation was clearly observed as a pop-in event (sudden displacement excursion) on the load-displacement curves. Hertzian stress analysis was used to describe fully the load-displacement behavior during elastic deformation and to determine the theoretical shear strengths of the BMGs.     

The structure of a metallic glass system using EXELFS and EXAFS as complementary probes

The short-range atomic order around all three constituent atoms in a prototypical bulk metallic glass (BMG) system was probed in a complementary way, using extended X-ray absorption fine structure for neighborhood of the higher atomic number elements, and extended energy loss fine structure (EXELFS) for the lower atomic number ones. The Pd_xNi_(80−x)P₍₂₀₎ system is a prototype for a whole class of BMG formers which are 80% transition metal and 20% metalloid. We find that the structure of these BMGs could be explained in terms of those of glasses at the end of the BMG range, namely, Pd₆₀Ni₂₀P₂₀ and Pd₃₀Ni₅₀P₂₀. The binary phosphide crystals near x=0 and 80 are found to be simulate very well the local atomic structure of Pd₃₀Ni₅₀P₂₀ and Pd₆₀Ni₂₀P₂₀ glasses, respectively. The best glass former in this series, Pd₄₀Ni₄₀P₂₀, is best described by a weighted average of Pd₃₀Ni₅₀P₂₀ and Pd₆₀Ni₂₀P₂₀ structures.     

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     

Size dependence of the mechanical properties of ZnO nanobelts

The Young's modulus of ZnO nanobelts was measured using an atomic force microscope following the modulated nanoindentation method. The nanobelts have a rectangular cross-section, with width-to-thickness ratios ranging 1–10 and lengths up to a few millimetres. The Young's modulus of two nanobelts with width-to-thickness ratio of 2.2 and 1.3 was measured at 55 and 108?GPa, respectively, indicating a size dependence of the elastic properties of the nanobelts.