Isoconversional <Emphasis Type="Italic">vs.</Emphasis> Model fitting methods

The crystallization of metallic glasses has been studied quite extensively using differential scanning calorimetry (DSC) technique. Most methods rely on isokinetic hypothesis for the kinetic analysis of crystallization for which the choice of a reliable model is very important. Due to inherent uncertainty in the determination of kinetic parameters, the model-free isoconversional analytical techniques were proposed. However, these isoconversional methods are scarcely used for metallic glasses. In the present work, the crystallization kinetics of Fe₆₇Co₁₈B₁₄Si₁ metallic glass through both isoconversional and isokinetic methods has been investigated and attention has been focused on the relative applicability of the two methods.     

Physical aging in Zr46.75Ti8.25Cu7.5Ni10Be27.5 typical bulk metallic glass manifested as enthalpy relaxation

Enthalpy recovery is not only an important characteristic of physical aging of glass, but also a good tool to investigate the physical aging. Using differential scanning calorimeter (DSC), the enthalpy recovery of Zr_46.75Ti_8.25Cu_7.5Ni₁₀Be_27.5 bulk metallic glass (BMG) was studied. The typical characteristics of enthalpy recovery of glass including the sub-T _g peak and ‘overshot’ were found in BMG. The evolution of the sub-T _g peak and ‘overshot’ were described by the free volume theory and Hodge’s model, respectively. It was found that the former failed to describe the enthalpy recovery in the BMG, while the latter could give a qualitative explanation. In combination with the dynamics in the BMG, the origin of the enthalpy recovery in the BMG was discussed. The results show that BMGs are an ideal material to investigate the physical aging. The further understanding of physical aging of BMGs is useful to clarify the nature of glass and improve the application and device of new types of BMGs. Supported by the National Natural Science Foundation of China (Grant No. 50671118)     

Atomic structures of Zr-based metallic glasses

The atomic structures of Zr-Ni and Zr-Ti-Al-Cu-Ni metallic glasses were investigated by using classical molecular dynamic (MD), reverse Monte Carlo (RMC), ab initio MD (AIMD) simulations and high resolution transmission electron microscopy (HRTEM) techniques. We focused on the short-range order (SRO) and medium-range order (MRO) in the glassy structure. It is shown that there are icosahedral, FCC-and BCC-type SROs in the Zr-based metallic glasses. A structural model, characterized by imperfect ordered packing (IOP), was proposed based on the MD simulation and confirmed by the HRTEM observation. Furthermore, the evolution from IOP to nanocrystal during the crystallization of metallic glasses was also explored. It is found that the growth from IOP to nanocrystal proceeds through three distinct stages: the formation of quasi-ordered structure with one-dimensional (1D) periodicity, then 2D periodicity, and finally the formation of 3D nanocrystals. It is also noted that these three growth steps are crosslinked. Supported by the National Natural Science Foundation of China (Grant Nos. 50431030 and 50471097), the National Basic Research Program of China (Grant No. 2007CB613901), and the Programme of Introducing Talents of Discipline to Universities (Grant No. B07003)     

金属材料力学性能的辐照硬化效应

开展金属材料力学性能的辐照硬化研究对抗辐照材料的设计及工程应用具有重要意义. 材料辐照损伤效应主要包括材料原子移位产生的辐照缺陷以及由核反应产生的氢、氦等气体杂质对材料性能的影响. 金属材料的辐照效应主要包括辐照硬化、辐照脆化和辐照蠕变等. 该文主要综述在低温(T < 0.3 T_m, T_m 是材料的熔点温度) 和低辐照剂量下, 由原子移位损伤产生的辐照缺陷所导致的辐照硬化行为, 即受辐照缺陷的影响, 材料的强度会升高. 材料的晶粒尺寸、晶界以及温度等因素对多晶材料的辐照硬化具有重要影响. 金属材料力学性能的辐照硬化研究是个多尺度问题, 其宏观力学性能既取决于微观尺度上辐照缺陷导致晶粒内部结构的变化, 也取决于细观尺度上晶粒间的相互作用. 该文从实验结果、数值模拟和理论模型三方面综述金属材料力学性能的辐照硬化研究进展. 在此基础上, 展望了该领域中存在的主要科学问题.      

双光子微纳加工技术结合化学镀工艺制备三维金属微弹簧结构

利用飞秒激光双光子微纳加工技术与化学镀工艺制备了三维金属微弹簧结构.采用扫描电子显微镜(SEM)及选区电子能谱(EDS)对镀层进行了表征,当化学镀时间为15 min时,所得到的镀层厚度约为130 nm.对不同电镀时间下获得的镀层电阻率进行了测定,实验结果表明,当电镀时间为35 min时得到的镀层电阻率约为80×10^-9 Ω·m,仅为银块体材料电阻率16×10^-9 Ω·m的5倍.利用这种方法,我们制备了总长度为28.75 μm、周期为2.93 μm的悬空金属弹簧结构,其中弹簧圈数为9圈,直径为6 μm,弹簧线分辨率为1.17 μm.文中所述的将双光子微纳加工技术与化学镀技术相结合的方法可以实现任意三维微金属结构与器件的制备,在微光学器件、微机电系统(MEMS)及微传感器等领域有着广泛的应用前景.     

非晶态合金表面的水润湿动力学

采用分子动力学模拟方法研究了改进的Simple pointcharge模型SPC/E水滴在Cu_(50)Zr_(50)非晶薄膜上的润湿行为和铺展过程.通过与CuZr(110)和(100)晶面对比研究发现,水滴在Cu_(50)Zr_(50)非晶薄膜表面上表现出较高的铺展速度.水滴在非晶合金表面的铺展过程中形成了明显的吸附层;而在晶态表面,水滴铺展前沿呈脚状形态.分析结果表明非晶表面的水分子在吸附层内呈现完全无序的单层排列方式,而在晶态表面,特别是(100)晶面,吸附层水分子呈双层有序排列.这种吸附层结构的差异导致了吸附层内水分子方向的差异:非晶表面吸附层内水分子方向倾向平行于表面,而晶态基底上吸附层内的水分子倾向于指向液滴内部.由此造成了非晶表面吸附层中的水分子与液滴内部以相对较弱的氢键相互作用,这使得上层水分子能够较容易扩散至吸附层前沿,促进液滴铺展.     

非晶合金及合金液体的局域五次对称性

简要回顾了从20世纪30年代至今,有关非晶合金及合金液体的局域结构五次对称性的实验、理论和模拟研究.在简单液体的早期研究中,人们已经意识到五次对称性在简单液体的无序结构、过冷和晶化等起着重要作用,二十面体短程序作为五次对称性的典型代表受到了广泛关注.自从Frank提出简单液体中二十面体短程序的结构单元,大量的理论和实验研究已经明确在简单液体、合金液体和金属玻璃中存在局域五次对称性,并且建立了局域五次对称性与合金液体复杂动力学行为、玻璃转变、液体-液体相变以及非晶合金的形变等统一的定量描述和物理图像,表明了局域五次对称性作为结构参量的简单、普遍和有效性.     

块体非晶合金的韧塑化

块体非晶合金因其独特的原子结构而具有许多优异的力学性能,成为近年来材料领域的研究热点之一,但是由于其在变形过程中的室温脆性和应变软化等关键问题一直制约着其实际工程应用.为解决此问题,块体非晶合金领域的研究者们提出了多种方案,包括通过在非晶合金中调控其内禀特性如弹性常数、结构不均匀性,通过外加手段改变其应力及缺陷状态,通过外加和内生的方法在非晶基体中引入晶态增强相等方式,获得了一系列力学性能优异的块体非晶合金及其复合材料.特别是利用"相变诱导塑性"(transformation-induced plasticity,TRIP)概念研制出的块体非晶合金复合材料,同时具有大的拉伸塑性和加工硬化能力.本文围绕块体非晶合金的韧塑化这个关键科学问题,对单相非晶及非晶复合材料的韧塑化方案及机理进行了综述,着重介绍了TRIP韧塑化块体非晶合金复合材料的制备、性能、组织调控及韧塑化机理等,并对此领域的未来发展进行了展望.     

Highly reflective subtractive color filters capitalizing on a silicon metasurface integrated with nanostructured aluminum mirrors

Metasurface‐based color filters have been recently applied for the creation of imaging devices and color printing in a subwavelength scale. In this work, a highly reflective subtractive color filter featuring an excellent color contrast is conceived and demonstrated, by exploiting a crystalline‐silicon nanopillar (NP)‐based dielectric metasurface integrated with an aluminum disk mirror (DM) and holey mirror (HM) at the top and bottom, respectively. A deep suppression in reflection is acquired through a magnetic dipole (MD) resonance that is supported by the constituent NP, and can be effectively tailored via the control of the NP diameter. The cooperation of the nanostructured DM and HM plays a dual role of dramatically boosting the efficiency and reinforcing the confinement of the MD in the NP, which is primarily accountable for the reduction in the spectral bandwidth. For the manufactured filters, both a high reflection efficiency reaching ∼70% and relatively small bandwidth of ∼55 nm are attained, thus leading to a broad palette of vivid and bright colors. The proposed devices are supposed to exhibit a polarization‐insensitive operation and a relaxed angular tolerance, thereby facilitating the implementation of miniaturized display/imaging devices with a high resolution and excellent color fidelity.

     

Effect of icosahedral clusters on β-relaxations in metallic glasses

The most pronounced β-relaxation was found in the Y-based binary metallic glasses(MGs). The correlation betweenβ-relaxation and local atomic structure was studied. The dynamic mechanical measurements were performed for three chosen binary systems: Zr-, Ti-, and Y-based MGs. The experimental results show that, in each system,the larger negative enthalpy of mixing(?Hm) between the component elements makes β-relaxation become more pronounced. The less negative value of ?Hm facilitates the formation of icosahedral clusters, which have a pinning effect on the excitation of β-relaxations and correspondingly make the β-relaxation become less pronounced. These chemical effects on β-relaxations can only be compared in the same MG system, and it is not suitable for the comparison between different systems due to the different features of the major metallic elements.