Planar-to-wavy transition of Cu–Ag nanolayered metals: a precursor mechanism to twinning

The interface-mediated plastic deformation mechanisms of a semi-coherent Cu–Ag bimetal nanolayered structure subjected to out-of-plane tension are characterized by molecular dynamics simulations. Results show that the initially planar Cu–Ag nanolayers abruptly become wavy at a critical tensile strain. This planar-to-wavy interlayer transition is facilitated by the low shear resistance of the Cu–Ag interlayer interface, which slides to accommodate the out-of-plane deformation. The process redistributes misfit dislocations along the interface to reduce the bending energy of the wavy structure. High stress concentrations subsequently develop at the summits and valleys of the wavy Cu–Ag interlayer interfaces, from which micro-twinning partials are emitted. These results demonstrate that the wavelength of the wavy Cu–Ag nanolayer structure forms a critical length scale for the localization of spatially periodic defect sources for twin nucleation. This planar-to-wavy interlayer transition mechanism is only activated in nanolayered metals with interfaces that are amenable to sliding prior to twin or dislocation emissions.     

Microstructures and strengthening mechanisms of Cu/Ni/W nanolayered composites

Cu/Ni/W nanolayered composites with individual layer thickness ranging from 5?nm to 300?nm were prepared by a magnetron sputtering system. Microstructures and strength of the nanolayered composites were investigated by using the nanoindentation method combined with theoretical analysis. Microstructure characterization revealed that the Cu/Ni/W composite consists of a typical Cu/Ni coherent interface and Cu/W and Ni/W incoherent interfaces. Cu/Ni/W composites have an ultrahigh strength and a large strengthening ability compared with bi-constituent Cu–X (X?=?Ni, W, Au, Ag, Cr, Nb, etc.) nanolayered composites. Summarizing the present results and those reported in the literature, we systematically analyze the origin of the ultrahigh strength and its length scale dependence by taking into account the constituent layer properties, layer scales and heterogeneous layer/layer interface characteristics, including lattice and modulus mismatch as well as interface structure.     

Subsurface deformation studies of aluminium during wear and its theoretical understanding using molecular dynamics

Adopting the bonded interface technique for wear experiments under vacuum, this paper reports the nature of the localised shear bands that appear at the different deformation zones of the subsurface of aluminium under different sliding conditions. The plastic deformations are mapped under both low load/low sliding velocities as well as high load and high sliding velocities. A monotonic change in local plastic strain as a function of depth at low sliding velocities give way to a discontinuity separating two different zones with differing plastic behaviour for high sliding speed wear test. Besides shear bands, bonded interface also reveals the presence of kinks particularly in the samples subjected to wear test with high sliding velocities. A molecular dynamic simulation of the wear process successfully replicated the experimental observation, thus allowing us to discuss the mechanism of subsurface deformation during the wear process in the absence of any significant oxide layer for aluminium under sliding condition.     

Effect of interface structure on deformation behavior of crystalline Cu/amorphous CuZr sandwich structures

The effect of interface types (namely, sharp interface and graded interface) and its thickness on the deformation behavior of crystalline/amorphous/crystalline sandwich structures (CACSSs) under tensile loading are studied using molecular dynamics simulation. Compared with the CACSSs with sharp interface, the CACSSs with gradient interface consistently exhibit good plasticity when the interface thickness is larger than 6 nm, due to the coupling effects among crystalline layer, amorphous layer and crystalline–amorphous interface. With the increase of interface thickness, the plastic deformation mechanism of CACSSs with gradient interface changes from the local plastic deformation in amorphous layer to the homogeneous plastic deformation.     

Finite-element analysis of the mechanical behavior of Au/Cu and Cu/Au multilayers on silicon substrate under nanoindentation

Finite-element analysis of the nanoindentation into Au/Cu and Cu/Au multilayers was performed to deduce their mechanical characteristics from nanoindentation response. Different bilayer thicknesses, numbers, and sequences were studied using the load–displacement curve, hardness, indentation, and the residual surface profile as well as the von Mises equivalent stress. The characteristics of the multilayers were found to be dispersed between the Au and Cu. Nevertheless, if the indentation depth is smaller than the uppermost individual layer thickness of the multilayers, the intrinsic properties can be obtained. Using the von Mises equivalent stress as a failure criterion, the results showed that thinner multilayers would induce a greater potential of shear banding deformation. PACS 61.43.Bn; 62.20.-x; 68.03.Hj; 68.05.Cf; 68.08.De     

Role of interfaces in shock-induced plasticity in Cu/Nb nanolaminates

We investigate deformation of pure Cu, pure Nb and 30?nm Cu/30?nm Nb nanolaminates induced by high strain rate shock loading. Abundant dislocation activities are observed in shocked pure Cu and Nb. In addition, a few deformation twins are found in the shocked pure Cu. In contrast, in shocked Cu/Nb nanolaminates, abundant deformation twins are found in the Cu layers, but only dislocations in the Nb layers. High resolution transmission electron microscopy reveals that the deformation twins in the Cu layers preferentially nucleate from the Cu(112)//Nb(112) interface habit planes rather than the predominant Cu(111)//Nb(110) interface planes. Our comparative study on the shock-induced plastic deformation of the pure metals (Cu and Nb) and the Cu/Nb nanolaminates underscores the critical role of heterogeneous phase interfaces in the dynamic deformation of multilayer materials.     

Molecular dynamics simulation of structural change at metal/semiconductor interface induced by nanoindenter

The structures of the Si/Cu heterogenous interface impacted by a nanoindenter with different incident angles and depths are investigated in detail using molecular dynamics simulation.The simulation results suggest that for certain incident angles,the nanoindenter with increasing depth can firstly increase the stress of each atom at the interface and it then introduces more serious structural deformation of the Si/Cu heterogenous interface.A nanoindenter with increasing incident angle(absolute value) can increase the length of the Si or Cu extended atom layer.It is worth mentioning that when the incident angle of the nanoindenter is between-45° and 45°,these Si or Cu atoms near the nanoindenter reach a stable state,which has a lower stress and a shorter length of the Si or Cu extended atom layer than those of the other incident angles.This may give a direction to the planarizing process of very large scale integration circuits manufacture.     

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.     

Molecular dynamics study of plastic deformation mechanism in Cu/Ag multilayers

The plastic deformation mechanism of Cu/Ag multilayers is investigated by molecular dynamics(MD) simulation in a nanoindentation process. The result shows that due to the interface barrier, the dislocations pile-up at the interface and then the plastic deformation of the Ag matrix occurs due to the nucleation and emission of dislocations from the interface and the dislocation propagation through the interface. In addition, it is found that the incipient plastic deformation of Cu/Ag multilayers is postponed, compared with that of bulk single-crystal Cu. The plastic deformation of Cu/Ag multilayers is affected by the lattice mismatch more than by the difference in stacking fault energy(SFE) between Cu and Ag. The dislocation pile-up at the interface is determined by the obstruction of the mismatch dislocation network and the attraction of the image force. Furthermore, this work provides a basis for further understanding and tailoring metal multilayers with good mechanical properties, which may facilitate the design and development of multilayer materials with low cost production strategies.     

On high velocity impact of micro-sized metallic particles in cold spraying

In this study, a systematic examination of particle deformation behaviour in cold spraying was conducted for Cu particle using both the Lagrangian and Arbitrary Lagrangian Eulerian (ALE) methods. It is found that the meshing size in modelling by Largrangian method influences significantly the localized shear instability at interface areas. With refining the meshing size the onset velocity for interface shear instability decreases. The extrapolation of these results yields a reasonable critical velocity comparable to the actual one in cold spray practice. The results indicate that both the flattening ratio and compression ratio of the deformed particles increase with the increase in particle velocity, which are in good agreement with the experiment results. The ALE method provides a suitable way to examine the particle deformation in cold spraying. Moreover, the numerical results also show that there exists the similarity for the deformation of particles of different diameters.     

Cyclic deformation and fatigue cracking behaviors of Cu–28wt%Ag binary alloy

The cyclic deformation and fatigue cracking behaviors of coarse-grained Cu–28wt%Ag binary alloy were investigated under axial plastic strain amplitudes ranging from 10^?4 to 7.5 × 10^?4. It was found that the cyclic stress of the Cu–Ag alloy increased rapidly in the initial tens of cycles and became saturation with further cyclic deformation. The cyclic saturation stress increased with increasing the plastic strain amplitude. The interfaces are classified into two categories based on the orientations of the eutectic and the dendrites, i.e. type I and type II interfaces. The surface damage morphologies show that fatigue cracks normally nucleated either along the type I interfaces or along the slip bands (SBs), while no cracking occurred along the type II interface. Fatigue striations with different spacings appeared on the fracture surface, and secondary cracks along the striations were also observed. Based on the experimental results, the cyclic deformation and fatigue cracking behavior of the Cu–Ag binary alloy were discussed in detail.     

Molecular dynamics study of coupled layer thickness and strain rate effect on tensile behaviors of Ti/Ni multilayered nanowires

Novel properties and applications of multilayered nanowires(MNWs) urge researchers to understand their mechanical behaviors comprehensively.Using the molecular dynamic simulation,tensile behaviors of Ti/Ni MNWs are investigated under a series of layer thickness values(1.31,2.34,and 7.17 nm) and strain rates(1.0 × 10~8 s~(-1) ≤ε≤5.0 × 10~(10) s~(-1)).The results demonstrate that deformation mechanisms of isopachous Ti/Ni MNWs are determined by the layer thickness and strain rate.Four distinct strain rate regions in the tensile process can be discovered,which are small,intermediate,critical,and large strain rate regions.As the strain rate increases,the initial plastic behaviors transform from interface shear(the shortest sample) and grain reorientation(the longest sample) in small strain rate region to amorphization of crystalline structures(all samples) in large strain rate region.Microstructure evolutions reveal that the disparate tensile behaviors are ascribed to the atomic fractions of different structures in small strain rate region,and only related to collapse of crystalline atoms in high strain rate region.A layer thickness-strain rate-dependent mechanism diagram is given to illustrate the couple effect on the plastic deformation mechanisms of the isopachous nanowires.The results also indicate that the modulation ratio significantly affects the tensile properties of unequal Ti/Ni MNWs,but barely affect the plastic deformation mechanisms of the materials.The observations from this work will promote theoretical researches and practical applications of Ti/Ni MNWs.     

Al纳米线不同晶向力学行为和变形机制的模拟

采用分子动力学模拟计算方法,考察具有较高层错能的Al纳米线沿不同晶向的力学行为和变形机制。在相同计算条件下与具有较低层错能的Ni、Cu、Au和Ag等FCC金属纳米线进行比较。结果表明：在力学行为方面,Al纳米线的弹性模量呈现明显的结构各向异性,满足E_[111] > E_[110] > E_[100]的关系,这一关系在FCC金属纳米线中普遍成立;Al纳米线的屈服应力随晶向呈现σ_y[100] > σ_y[111] > σ_y[110]的关系,这一关系在具有较低层错能的FCC金属纳米线中不具有普遍性,这与体系中位错形成机制密切相关。根据拉伸变形过程微观结构的演变规律,阐明Al纳米线不同晶向的变形机制,并与具有较低层错能的Ni、Cu、Au和Ag等FCC金属纳米线的变形机制进行比较。结果表明,对于尺度较小的高层错能Al纳米线,Schmid因子和广义层错能均难以准确预测其变形机制。     

Universality and scaling of unstable plastic flow

The statistics of the jumplike plastic deformation of a Cu–Be alloy under the conditions of a low-temperature unstable plastic flow is studied experimentally. At a high strain rate, the parameters of the load jumps are found to be related by power laws, which corresponds to a scale-invariant behavior. A comparison with the data obtained for another mechanism of plastic instability, namely, the Portevin–Le Chatelier effect, points to the existence of universal laws governing the dynamics of a dislocation ensemble in the conditions of plastic instability.     

Thermal stability of Cu–Nb nanolamellar composites fabricated via accumulative roll bonding

In situ annealing within a neutron beam line and ex situ annealing followed by transmission electron microscopy were used to study the thermal stability of the texture, microstructure, and bi-metal interface in bulk nanolamellar Cu/Nb composites (h?=?18?nm individual layer thickness) fabricated via accumulative roll bonding, a severe plastic deformation technique. Compared to the bulk single-phase constituent materials, the nanocomposite is two orders of magnitude higher in hardness and significantly more thermally stable, e.g., no observed recrystallization in Cu at temperatures as high as 85% of the melting temperature. The nanoscale h?=?18?nm individual layer thickness is maintained up to 500°C, the lamellar structure thickens but is maintained up to 700°C, and recrystallization is suppressed even up to 900°C. With increasing temperature, the texture sharpens, and among the interfaces found in the starting material, the {112}_Cu?||?{112}_Nb interface with a Kurdjumov-Sachs orientation relationship shows the greatest thermal stability. Our results suggest that thickening of the individual layers under heat treatment coincides with thermally driven removal of energetically unfavorable bi-metal interfaces. Thus, we uncover a temperature regime that maintains the lamellar structure but alters the interface distribution such that a single, low energy, thermally stable interface prevails.     

Thermal stability of self-supported nanolayered Cu/Nb films

We report the development of thermally stable nanoscale layered structures in sputter deposited Cu/Nb multilayered films with 75?nm individual layer thickness, vacuum annealed at temperatures of 800°C or lower. The continuity of the layered structure was maintained and layer thickness unchanged in the annealed films. The nanolayers were observed to be offset by shear at the triple-point junctions that had equilibrium groove angles and were aligned in a zigzag pattern. A mechanism is proposed for the evolution of this ‘anchored’ structure that may be resistant to further morphological instability.     

可压缩Kelvin-Helmholtz不稳定性低耗散锐界面方法直接模拟

采用低耗散WENO(weighted essential non-oscillatory)格式及锐界面方法模拟可压缩Kelvin-Helmholtz不稳定性问题.由于物质界面被描述成一种接触间断, 该方法可精确求解切向速度间断.基于优化模板对原始光滑指标进行正规化后, 得到一种低耗散WENO格式.修正后的方法显著降低了普通流动区域的过衰减问题, 保持了良好的激波捕捉性能, 并可获得与混合格式相当的求解精度.不同于以往求解单一流体或易混界面时, 通过初始设定有限宽度的剪切层或快速数值耗散以抑制高波数模态, 该方法允许高波数扰动的发展.计算结果表明, 高波数扰动展现出与以往理想Kelvin-Helmholtz不稳定性问题数值模拟或线化理论结果不同的特征, 但与有限厚度的剪切层结果相符.      

过渡元素掺杂对Mo力学性能的第一性原理研究

基于密度泛函理论,采用第一性原理方法计算了在Mo中掺杂摩尔百分比分别为2.08%和4.17%的过渡金属元素W,Ti,Cu和Fe后,体系在[111](110)滑移系统上的广义层错能以及解理能,并研究了掺杂元素对Mo的剪切形变以及脆性一韧性的影响,研究发现,掺杂W和Ti原子会使体系剪切形变的发生变得困难,并使Mo材料变脆;而掺杂Cu和Fe原子则会使体系剪切形变的发生变得相对容易,并使Mo材料的韧性增强,此外,随着掺杂浓度的增加,掺杂W会使体系剪切形变的发生变得更为困难,并使Mo材料脆性更强;而掺杂Fe则会使体系剪切形变的发生变得更为容易,并使Mo材料的韧性更强。     

Computer simulation of the matrix-inclusion interphase in bulk metallic glass based nanocomposites

Atomistic models for matrix-inclusion systems are generated. Analyses of the systems show that interphase layers of finite thickness appear interlinking the surface of the nanocrystalline inclusion and the embedding amorphous matrix. In a first approximation, the interphase is characterized as an amorphous structure with a density slightly reduced compared to that of the matrix. This result holds for both monatomic hard sphere systems and a Cu(47.5)Zr(47.5)Al(5) alloy simulated by molecular dynamics (MD). The elastic shear and bulk modulus of the interphase are calculated by simulated deformation of the MD systems. Both moduli diminish with decreasing density but the shear modulus is more sensitive against density reduction by one order of magnitude. This result explains recent observations of shear band initiation at the amorphous-crystalline interface during plastic deformation.     

纳米铜薄膜塑性变形中空位型缺陷形核与演化的分子动力学研究

本文采用分子动力学方法模拟了纳米单晶铜薄膜在单向拉伸载荷作用下的塑性变形过程, 重点分析了空位型缺陷的形核过程和演化机理. 在模拟过程中, 采用镶嵌原子势描述原子间的相互作用. 模拟结果表明纳米铜薄膜中塑性变形起源于位错的表面形核, 而空位型缺陷的形核及演化都与晶体内部的位错运动密切相关. 空位型缺陷通常从位错割阶及层错交截处开始形核, 以单空位、层错四面体和不规则空位团等形式存在. 关键词： 纳米薄膜 塑性变形 空位 层错四面体