基于分数阶流变模型的铁基块体非晶合金黏弹性行为研究

采用分数阶黏弹单元替代经典模型中的黏壶, 结合非晶合金在外加载荷作用下的微观结构演化, 建立了以分数阶微积分表示的非晶合金黏弹性本构模型. 并根据Hertz弹性理论及分数阶黏弹性本构模型, 推导了块体非晶合金在纳米压痕球形压头下的位移与载荷及时间关系式. 基于推导的解析式, 对铁基块体非晶合金在表观弹性区的纳米压痕位移与载荷及时间曲线进行了非线性拟合分析. 相较于整数阶模型, 分数阶模型不仅具有较高的拟合精度, 其拟合参数能敏锐地反应加载速率对块体非晶合金黏弹性行为的影响, 且参数的变化规律与载荷作用下非晶合金微观结构演化呈现出较强的相关性.     

不同力学激励形式探索La基非晶合金微观结构非均匀性

非晶合金力学行为与其微观结构非均匀性之间本征关联,是固体力学研究领域至今未能很好解决的重要科学问题之一.单一力学激励形式并不能有效地描述非晶合金微观结构非均匀性,特别是结构与动力学的关联.如何探索非晶合金结构信息,须将诸多因素综合,在不同力学激励下研究非晶合金微观结构非均匀性与变形机理.本研究以La₆₂Cu₁₂Ni₁₂Al₁₄非晶合金为模型体系,利用动态力学分析仪研究非晶合金动态弛豫行为.基于准点缺陷模型,对模型合金体系α弛豫和β弛豫进行了分离.借助于拉伸应变率跳实验,探索非晶合金高温流变行为.确定非晶合金塑性流变过程中弹性、滞弹性以及塑性变形的贡献.本研究从非晶合金动态力学弛豫行为和宏观塑性流变行为出发,尝试揭示微观非均匀性对非晶合金在不同激励形式中缺陷的激活、扩展和融合的物理本质.     

非晶力学流变的自组织临界行为

非晶材料是由液体快冷冻结而成的结构无序的亚稳态固体.在受力条件下,非晶材料表现出独特和复杂的流变行为,具有跨尺度的高度时空不均匀特征,并在一定条件下表现出自组织临界行为,和自然界以及物理系统中许多复杂体系的动力学行为相似.本文结合作者近年来在非晶合金流变行为方面的研究结果,对非晶材料流变的研究进展和物理机制的认识进行介绍,包括非晶材料流变的跨尺度特征、表征和微观结构机制,以及近年来发现的非晶力学流变的自组织临界行为、物理机制等.最后,对非晶材料流变行为研究中亟需解决的问题进行了总结和展望.     

非晶合金中的流变单元

非晶合金是一类具有诸多优异性能的先进金属材料,同时也是研究非晶态物质的模型体系.最近,大量的实验和模拟证据显示,在非晶合金中可能存在类似晶体中缺陷的"流变单元",这些动力学单元和非晶合金的的流变、物理、力学性能密切关联.本文主要综述了流变单元提出的背景、实验证据、流变单元的特征、激活与演化过程、相互作用以及相关的理论.文中提供了大量实验证据证明流变单元模型不仅可以帮助理解非晶态物质中如形变、玻璃转变、弛豫动力学以及非晶结构和性能的关系等重要的基本物理问题,而且可以指导非晶合金性能的调控和设计,获得性能优异的非晶合金材料.     

锆基非晶合金的动态弛豫和应力松弛

非晶合金动态弛豫、变形和微观结构非均匀性之间的关联是非晶态物理领域重要研究内容之一.本文选取玻璃形成能力优良,热稳定性好的Zr₄₈(Cu_5/6Ag_1/6)₄₄Al₈块体非晶合金作为研究载体,借助于动态力学分析及应力松弛实验探究了温度和结构弛豫对非晶合金动态弛豫和变形行为的影响.研究结果表明非晶合金动态弛豫行为对温度极其敏感,随温度升高表现为等构型、老化、玻璃转变和晶化4个阶段.基于玻璃转变温度之下等温测试,结构弛豫行为导致非平衡高能量状态非晶合金向低能量状态迁移,内耗随时间演化规律可用Kohlrausch-Williams-Watts (KWW)扩展指数方程描述.此外,基于KWW方程和激活能谱等方法分析了模型合金应力松弛过程中非均匀结构激活,该过程涉及弹性变形向非弹性变形的转变.由于非晶合金存在微观尺度的结构非均匀性与能量起伏,变形单元的激活并非为单一特征时间,而是服从一定分布.通过考虑对数时间尺度和变形单元激活的特征时间分布分别为对称Gauss分布和非对称Gumbel分布,可...     

Pd基非晶合金动态弛豫机制和应力松弛行为

作为潜在的功能及结构材料,高熵非晶合金在凝聚态物理和力学领域引起广泛的研究兴趣.高熵非晶合金宏观力学性能与微观结构非均匀性之间的关联是当前重要的科学问题之一.本文选取非晶形成能力良好的Pd_42.5Cu₃₀Ni_7.5P₂₀非晶合金和Pd₂₀Pt₂₀Cu₂₀Ni₂₀P₂₀高熵非晶合金作为模型体系,借助于动态弛豫行为及应力松弛实验建立了温度和物理时效对非晶合金高温变形机制与微观结构非均匀性之间的关联.研究结果表明Pd基非晶合金表现出“肩膀峰”β弛豫形式.玻璃转变温度以下物理时效非晶合金体系原子移动性导致β弛豫肩膀峰往更高的温度迁移.在应力松弛过程中,由于高构型熵的引入降低吉布斯自由能,这是高熵非晶合金具有较高激活能的原因.高熵非晶合金更难被激活,需要突破更高的能量势垒.物理时效时间增加,高熵非晶合金流变单元更小,这也得益于多主元高熵非晶合金慢扩散效应.高熵非晶合金激活体积的改变在物理时效下应力松弛...     

小角X射线散射表征非晶合金纳米尺度结构非均匀

表征纳米尺度结构非均匀对于理解非晶合金的变形、弛豫等动力学行为至关重要.受时空尺度限制,非晶合金纳米尺度结构非均匀的实验表征具有很大的挑战性.本文针对一种典型的锆基非晶合金,开展了同步辐射小角X射线散射原位拉伸实验.通过对散射曲线的定量分析,揭示了非晶合金在纳米尺度的非均匀结构图像.首先,Porod散射曲线呈现正偏离行为,表明非晶合金属于非理想两相散射体系,两相界面弥散且任一相内都存在电子密度涨落.基于散射曲线的Guinier定律分析,进一步揭示非晶合金中散射体形状远偏离球形,其特征尺度主要分布在0.8—1.6 nm之间,且在弹性变形阶段几乎不变.最后,通过Debye相关函数分析,发现这些纳米尺度散射体仅在1 nm之内存在强关联,符合非晶合金短程有序、长程无序的结构特征.研究结果表明非晶合金中存在具有复杂空间分布的纳米尺度非均匀结构.     

分数阶Willis环脑动脉瘤系统的混沌动力学分析与控制

整数阶Willis环脑动脉瘤系统在描述表现出黏弹性的血液在具有繁杂弹性的血管系统内的复杂血流动力学机理上有一定局限性;鉴于此,本文利用分数阶Caputo微分及其理论,提出分数阶Willis环脑动脉瘤模型(FWAS):证明FWAS解的存在惟一性;利用相图和Poincaré截面证明FWAS具有混沌特性,是其整数阶形式的合理推广;结合分岔图和倍周期分岔讨论脉冲压、系统阶次对FWAS的影响;采用通过非自治非线性系统的稳定性条件设计合理的控制器,以药物激励项函数作为脉冲函数进行脉冲控制这两种方法,对FWAS进行有效的控制.本文对FWAS的探讨将对脑动脉瘤的研究具有一定的理论指导意义.     

退火时间对Fe80Si9B10Cu1非晶合金纳米尺度结构不均匀性和磁性能的影响

研究了经历不同时间退火后, Fe₈₀Si₉B₁₀Cu₁非晶合金结构弛豫过程中纳米尺度结构不均匀性的演变及其对合金磁性能的影响.基于小角X射线散射和原子力显微镜分析,随着弛豫的进行,合金的纳米尺度结构不均匀性逐渐衰减.结合穆斯堡尔谱分析结果,弛豫态合金综合软磁性能的提高可归因于纳米尺度结构不均匀性的减弱.从流变单元模型来看,随着弛豫程度的加深,流变单元的体积分数显著降低,部分流变单元湮灭并转化为理想弹性基体.一方面,弛豫态样品的原子结构排列更加紧密,磁交换相互作用更强,饱和磁感应强度也更高;另一方面,准位错偶极子的数量密度随着流变单元在弛豫过程中的湮灭而逐渐减小,磁畴壁的钉扎效应减弱,合金的磁各向异性下降,矫顽力降低.本文从结构不均匀性的角度研究了Fe₈₀Si₉B₁₀Cu₁非晶合金弛豫过程中磁性能变化的结构机制,有助于建立铁基非晶合金结构和磁性能之间的关联性.     

分子动力学模拟冷却速率对非晶合金结构与变形行为的影响

非晶合金因具有独特的无序结构、优异或独特的各种性能以及良好的应用前景,而受到专家学者的广泛关注.其中,制备过程中的冷却速率对非晶的结构与性能起着非常重要的调控作用.本文采用分子动力学的模拟方法,分别以4种冷却速率获得相同尺寸的Zr_(48)Cu_(45)Al_7三元非晶合金的制备态原子结构模型,并模拟了各制备态模型的压缩变形过程.在此基础上系统地研究冷却速率对非晶微观结构及其变形行为的影响.研究表明:在施加大冷却速率时,非晶合金保留更多高温液态的结构特征,如五次对称性低的团簇数量较多,原子堆积较为松散,自由体积含量更多,并存在更多的"类液区".上述大冷却速率所对应的结构特征导致了非晶发生变形时,屈服强度降低,表现出软化行为,同时降低了剪切带形成与发生局域化变形的概率,从而提高了非晶的塑性.     

Structural,Elastic, and Electronic Properties of a New Phase of Carbon

Based on density function theory with the ultrasoft pseudopotential scheme in the frame of the local density approximation and the generalized gradient approximation, the structural, elastic, and electronic properties of carbon with P222₁ phase have been systematically studied in this paper. The calculated results show that the P222₁ phase of carbon is mechanically stable and dynamically stable at ambient pressure. The anisotropy studies of Young's modulus, Poisson's ratio, shear anisotropic factor, the percentage of elastic anisotropy for bulk modulus, the percentage of elastic anisotropy for shear modulus and the universal anisotropic index show that P222₁ phase of carbon exhibits anisotropy. In addition, P222₁ phase is an indirect semiconductor with bandgap of 3.423 eV. But, the band gap of P222₁ phase for carbon increase with increasing pressure.     

Secondary relaxation and dynamic heterogeneity in metallic glasses:A brief review

Understanding mechanical relaxation, such as primary(α) and secondary(β) relaxation, is key to unravel the intertwined relation between the atomic dynamics and non-equilibrium thermodynamics in metallic glasses. At a fundamental level, relaxation, plastic deformation, glass transition, and crystallization of metallic glasses are intimately linked to each other, which can be related to atomic packing, inter-atomic diffusion, and cooperative atom movement. Conceptually, βrelaxation is usually associated with structural heterogeneities intrinsic to metallic glasses. However, the details of such structural heterogeneities, being masked by the meta-stable disordered long-range structure, are yet to be understood. In this paper, we briefly review the recent experimental and simulation results that were attempted to elucidate structural heterogeneities in metallic glasses within the framework of β relaxation. In particular, we will discuss the correlation amongβ relaxation, structural heterogeneity, and mechanical properties of metallic glasses.     

Kinetic Simulation of Fractal Aggregation on Liquid Surfaces

A modified fractal growth model based on the deposition, diffusion, and aggregation (DDA) with cluster rotation is presented to simulate two-dimensional fractal aggregation on liquid surfaces. The mobility (including diffusion and rotation) of clusters is related to its mass, which is given by D_m=D₀ s^-γ_D and θ_m=θ₀s^-γ_θ, respectively. We concentrate on revealing the details of the influence of deposition flux F, cluster diffusion factor γ_D and cluster rotation factor γ_θ on the dynamics of fractal aggregation on liquid surfaces. It is shown that the morphologies of clusters and values of cluster density and fractal dimension depend dramatically on the deposition flux and migration factors of clusters.     

金属玻璃的断裂机理与其断裂韧度的关系

本文选取了三种不同断裂韧度值的金属玻璃Zr41.25Ti13.75Ni10Cu12.5Be22.5,Ce68Al10Cu20Co2和Fe41Co7Cr15Mo14Y2C15B6,通过压缩实验测量了它们的应力-应变关系;样品断裂以后观察了其断口形貌,发现这三种金属玻璃具有不同的断裂模式.经过对这三种金属玻璃做缺口三点弯曲实验,利用数字散斑技术研究了缺口前端应变集中方向弹性应变场的演化过程.根据金属玻璃的屈服准则,阐述了不同断裂韧度值的金属玻璃的断裂机理.     

P213 BN: a novel large-cell boron nitride polymorph

A new boron nitride polymorph, P2₁3 BN (space group: P2₁3), is investigated by first-principles calculations, including its structural properties, stability, elastic properties, anisotropy and electronic properties. It is found that the new boron nitride polymorph P2₁3 BN is mechanically, dynamically and thermodynamically stable. The bulk modulus (B), shear modulus (G) and Young's modulus of P2₁3 BN are 91 GPa, 41 GPa and 107 GPa, respectively, all of which are larger than that of Y carbon and TY carbon. By comparing with c-BN, the Young's modulus, shear modulus and Poisson's ratio of P2₁3 BN show tiny anisotropy in the (001), (010), (100) and (111) planes. At the same time, in contrast with most boron nitride polymorphs, P2₁3 BN is a semiconductor material with a smaller band gap of 1.826 eV. The Debye temperature and the anisotropic sound velocities of P2₁3 BN are also investigated in this work.     

Temperature dependence of induced magnetic anisotropy in metallic glasses (Fe1 − xCox)78Si10B12

The variations of induced magnetic anisotropy with annealing and measuring temperatures in metallic glasses (Fe_{1 − x}Co_x)₇₈Si₁₀B₁₂ have been measured. It was found that K_u (T) was proportional to Mⁿ_s (T) for T below 200°C, and the index n varied with the cobalt content x and annealing conditions, not being smaller than 3. To the predictions of the existing pair-ordering and single-ion theories, the above results are anomalous. By considering the distributions of exchange integrals and activation energies in metallic glasses, this anomalous behaviour could be explained properly.     

一个新的自由能函数对介电高弹薄膜的多组等双轴预拉伸下力电耦合实验的预测

当介电高弹聚合物薄膜被施以面内等双轴预拉伸后, 受到厚度方向的电压作用时, 薄膜在力场和电场共同作用下产生大变形. 电场采用Maxwell应力分析, 力场采用橡胶弹性模型分析. 拟合这类变形的常用橡胶弹性模型主要有Neo-Hookean, Arruda-Boyce, Gent等模型. 这些模型对实验数据的定量拟合存在不同程度的偏差. 通过对实验数据的分析, 结合数学方法, 提出了一个新的自由能函数模型. 通过该模型对VHB4905介电高弹聚合物薄膜的多组等双轴预拉伸电力耦合实验进行拟合, 并以Neo-Hookean, Gent模型作为对照, 结果与实验数据拟合很好, 比对照模型的偏差明显缩小.     

Fast ion-conduction in CuBr---Cu2MoO4---CuPO3 glasses

Cuprous ion-conducting glasses were prepared in the pseudoternary system CuBr---Cu₂MoO₄---CuPO₃. These glasses showed high-ion conductivities in the range of 10⁰ to 10⁻² Sm⁻¹ at room temperature. The conductivities at room temperature, σ₂₉₈, of the glasses were increased with an increase in the CuBr content of the glasses and the activation energy for conduction, E_a, were decreased with an increase in the CuBr content. The σ₂₉₈ of the glasses containing CuBr were slightly larger than those of the glasses containing CuI.     

Electronic,optical, and mechanical properties of BN,AlN, and InN with zinc-blende structure under pressure

In this work, the electronic, optical, and mechanical properties of BN, AlN, and InN under the action of pressure are calculated. For each of these compounds, the energy band structure, band gaps(E~L_g, E~Γ_g, E~X_g), refractive index(n),dielectric constants(ε_∞, ε_0), elastic constants(C_11, C_12, C_44), and relevant parameters such as bulk(B_u), shear(S_h), and Young's(Y_0) moduli are studied, and other important parameters such as bond-stretching(α), bond-bending(β) force constant, internal-strain parameter(ζ), effective charges(e~*_T, Z~*), anisotropy factor(I_s), Poisson's ratio(P_o), Cauchy ratio(C_a), the ductility index(μ_D), and linear compressibility(C0_) are also calculated. The effects of pressure on all studied properties are investigated. Our results are in good agreement with the available experimental and theoretical data for BN,AlN, and InN.