Molecular dynamics simulation of the interaction between a mixed dislocation and a stacking fault tetrahedron

A high number-density of nanometer-sized stacking fault tetrahedra are commonly found during irradiation of low stacking fault energy metals. The stacking fault tetrahedra act as obstacles to dislocation motion leading to increased yield strength and decreased ductility. Thus, an improved understanding of the interaction between gliding dislocations and stacking fault tetrahedra are critical to reliably predict the mechanical properties of irradiated materials. Many studies have investigated the interaction of a screw or edge dislocation with a stacking fault tetrahedron (SFT). However, atomistic studies of a mixed dislocation interaction with an SFT are not available, even though mixed dislocations are the most common. In this paper, molecular dynamics simulation results of the interaction between a mixed dislocation and an SFT in face-centered cubic copper are presented. The interaction results in shearing, partial absorption, destabilization or simple bypass of the SFT, depending on the interaction geometry. However, the SFT was not completely annihilated, absorbed or collapsed during a single interaction with a mixed dislocation. These observations, combined with simulation results of edge or screw dislocations, suggest that defect-free channel formation in irradiated copper is not likely by a single dislocation sweeping or destruction process, but rather by a complex mix of multiple shearing, partial absorption and defect cluster transportation that ultimately reduces the size of stacking fault tetrahedra within a localized region.     

Atomistic simulation of the stacking fault energy and grain shape on strain hardening behaviours of FCC nanocrystalline metals

ABSTRACT

Ultra-fine grained copper with nanotwins is found to be both strong and ductile. It is expected that nanocrystalline metals with lamella grains will have strain hardening behaviour. The main unsolved issues on strain hardening behaviour of nanocrystalline metals include the effect of stacking fault energy, grain shape, temperature, strain rate, second phase particles, alloy elements, etc. Strain hardening makes strong nanocrystalline metals ductile. The stacking fault energy effects on the strain hardening behaviour are studied by molecular dynamics simulation to investigate the uniaxial tensile deformation of the layer-grained and equiaxed models for metallic materials at 300?K. The results show that the strain hardening is observed during the plastic deformation of the layer-grained models, while strain softening is found in the equiaxed models. The strain hardening index values of the layer-grained models decrease with the decrease of stacking fault energy, which is attributed to the distinct stacking fault width and dislocation density. Forest dislocations are observed in the layer-grained models due to the high dislocation density. The formation of sessile dislocations, such as Lomer–Cottrell dislocation locks and stair-rod dislocations, causes the strain hardening behaviour. The dislocation density in layer-grained models is higher than that in the equiaxed models. Grain morphology affects dislocation density by influencing the dislocation motion distance in grain interior.     

Mechanisms of stacking fault tetrahedra destruction by gliding dislocations in quenched gold

The destruction processes of stacking fault tetrahedra (SFTs) induced by gliding dislocations were examined by transmission electron microscopy (TEM) in situ straining experiments for SFTs with edge lengths ranging from 10 to 50 nm. At least four distinct SFT destruction processes were identified: (1) consistent with a Kimura–Maddin model for both screw and 60° dislocations, (2) stress-induced SFT collapse into a triangular Frank loop, (3) partial annihilation leaving an apex portion and (4) complete annihilation. Process (4) was observed at room temperature only for small SFTs (～10 nm); however, this process was also frequently observed for larger SFTs (～30 nm) at higher temperature (～853 K). When this process was induced, the dislocation always cross-slipped, indicating only screw dislocations can induce this process.     

Explicit incorporation of cross-slip in a dislocation density-based crystal plasticity model

We present a crystal plasticity model that incorporates cross-slip of screw dislocations explicitly based on dislocation densities. The residence plane of screw dislocations is determined based on a probability function defined by activation energy and activation volume of cross-slip. This enables the redistribution of screw-dislocations and dislocation density patterning due to the effect of stacking fault energy. The formulation is employed for explaining the cross-slip phenomenon in aluminium during uniaxial tensile deformation of ?100? single crystal and a single slip orientation of single crystal, and compare the results with experimental observations. The effect of cross-slip on the stress–strain evolution is seen using this explicit treatment of cross-slip.     

Dislocation configurations around a nanoindentation in the surface of a fcc metal

We report a scanning tunnelling microscopy investigation of the emission of dislocations around nanoindentations in the form of dislocation arrangements previously called hillocks , consisting of two pairs of Shockley partial dislocations, each encompassing a stacking fault. The spatial arrangement and size distribution of hillocks around the nanoindentation traces are studied. We show that standard dislocation theory for an isotropic continuum can be used to describe the stability of the hillocks, their size and spatial distribution and the broadening of the corresponding extended dislocations near the surface. A model is proposed in which hillocks originate from the split into dislocations partials of primary perfect dislocation loops punched into the crystal by the scanning tunnelling microscope tip. This model implies the operation of a novel dislocation mechanism involving long-range transport of matter across the surface.     

Modeling of Dislocation Generation and Interaction During High-Speed Deformation of Metals

Recent experiments by Kiritani et al. [1] have revealed a surprisingly high rate of vacancy production during high-speed deformation of thin foils of fcc metals. Virtually no dislocations are seen after the deformation. This is interpreted as evidence for a dislocation-free deformation mechanism at very high strain rates. We have used molecular-dynamics simulations to investigate high-speed deformation of copper crystals. Even though no pre-existing dislocation sources are present in the initial system, dislocations are quickly nucleated and a very high dislocation density is reached during the deformation. Due to the high density of dislocations, many inelastic interactions occur between dislocations, resulting in the generation of vacancies. After the deformation, a very high density of vacancies is observed, in agreement with the experimental observations. The processes responsible for the generation of vacancies are investigated. The main process is found to be incomplete annihilation of segments of edge dislocations on adjacent slip planes. The dislocations are also seen to be participating in complicated dislocation reactions, where sessile dislocation segments are constantly formed and destroyed.     

A dislocation dynamics study of the strength of stacking fault tetrahedra. Part I: interactions with screw dislocations

We present a comprehensive dislocation dynamics (DD) study of the strength of stacking fault tetrahedra (SFT) to screw dislocation glide in fcc Cu. Our methodology explicitly accounts for partial dislocation reactions in fcc crystals, which allows us to provide more detailed insights into the dislocation–SFT processes than previous DD studies. The resistance due to stacking fault surfaces to dislocation cutting has been computed using atomistic simulations and added in the form of a point stress to our DD methodology. We obtain a value of 1658.9 MPa, which translates into an extra force resolved on the glide plane that dislocations must overcome before they can penetrate SFTs. In fact, we see they do not, leading to two well differentiated regimes: (i) partial dislocation reactions, resulting in partial SFT damage, and (ii) impenetrable SFT resulting in the creation of Orowan loops. We obtain SFT strength maps as a function of dislocation glide plane-SFT intersection height, interaction orientation, and dislocation line length. In general SFTs are weaker obstacles the smaller the encountered triangular area is, which has allowed us to derive simple scaling laws with the slipped area as the only variable. These laws suffice to explain all strength curves and are used to derive a simple model of dislocation–SFT strength. The stresses required to break through obstacles in the 2.5–4.8-nm size range have been computed to be 100–300 MPa, in good agreement with some experimental estimations and molecular dynamics calculations.     

Size dependent deformation behaviour and dislocation mechanisms in 〈1 0 0〉 Cu nanowires

Abstract

Molecular dynamics simulations have been performed to understand the size-dependent tensile deformation behaviour of 〈1 0 0〉 Cu nanowires at 10 K. The influence of nanowire size has been examined by varying square cross-section width (d) from 0.723 to 43.38 nm using constant length of 21.69 nm. The results indicated that the yielding in all the nanowires occurs through nucleation of partial dislocations. Following yielding, the plastic deformation in small size nanowires occurs mainly by slip of partial dislocations at all strains, while in large size nanowires, slip of extended dislocations has been observed at high strains in addition to slip of partial dislocations. Further, the variations in dislocation density indicated that the nanowires with d > 3.615 nm exhibit dislocation exhaustion at small strains followed by dislocation starvation at high strains. On the other hand, small size nanowires with d < 3.615 nm displayed mainly dislocation starvation at all strains. The average length of dislocations has been found to be same and nearly constant in all the nanowires. Both the Young’s modulus and yield strength exhibited a rapid decrease at small size nanowires followed by gradual decrease to saturation at larger size. The observed linear increase in ductility with size has been correlated with the pre- and post-necking deformation. Finally, dislocation–dislocation interactions leading to the formation of various dislocation locks, the dislocation–stacking fault interactions resulting in the annihilation of stacking faults and the size dependence of dislocation–surface interactions have been discussed.     

层错四面体对单晶铜层裂行为影响的分子动力学研究

层错四面体是一种典型的三维空位型缺陷,广泛存在于受辐照后的面心立方金属材料中,对材料的力学性能有显著的影响.目前,关于层错四面体对辐照材料层裂行为的影响还缺乏深入系统的研究.本文使用分子动力学方法模拟了含有层错四面体的单晶铜在不同冲击速度下的层裂行为,对整个冲击过程中的自由表面速度及微结构演化等进行了深入的分析.研究发现,层错四面体在冲击波作用下会发生坍塌,并进一步诱导材料产生位错、层错等缺陷.在中低速度加载下,层错四面体坍塌引起的缺陷快速向周围扩展,为孔洞提供了更宽的形核区域,促进了孔洞的异质成核,造成材料层裂强度大幅度减小.当冲击速度较高时,层错四面体坍塌导致的局部缺陷对材料的层裂强度不再有明显影响.     

Molecular dynamic studies of the interaction of a/6⟨112⟩ Shockley dislocations with stacking fault tetrahedra in copper. Part II: Intersection of stacking fault tetrahedra by moving twin boundaries

The interactions of moving twin boundaries with stacking fault tetrahedra (SFTs) have been studied by molecular dynamics. The results reveal a spectrum of processes occurring during these interactions. In general, they lead to damage of the parent SFT and formation of new defects in the twin lattice. The character of these defects depends on the nature of the twinning front, the size of the SFT and its orientation with respect to incoming dislocations. Typical structures that may be produced in the twin include product-SFTs, free vacancies, planar stacking faults bounded by partial dislocations, mutually linked stacking faults on non-coplanar {111} _T planes, small {111} _T tetrahedra and their partial forms. Dislocation mechanisms involved in the formation of these defects are being analyzed.     

Characteristics of the evolution of defect structure during phase transformations in the Pd-H system

The pattern of evolution of the defect structure in the Pd-H system during phase transformations is formulated on the basis of x-ray data. It is shown that once the random dislocations formed during phase transformations in the a phase reach their critical density, they assemble into dislocation walls. This process results in the formation of a cellular dislocation substructure in the α phase. After the formation of the cellular substructure in the α phase the random dislocations created during phase transformations climb into the hydride phase, thereby curtailing the evolution of defect structure in the α phase. The subsequent influx of dislocations into the β phase maintains continued evolution of the defect structure (from cellular to block dislocation substructure). Not until that time is it possible for the evolution of the defect structure in the α phase to terminate, culminating in the formation of a block substructure. The nature of the observed phenomenon is discussed. Fiz. Tverd. Tela (St. Petersburg) 39, 1275–1281 (July 1997)     

Closed and open-ended stacking fault tetrahedra formation along the interfaces of Cu–Al nanolayered metals

Stacking fault tetrahedra (SFTs) are volume defects that typically form by the clustering of vacancies in face-centred cubic (FCC) metals. Here, we report a dislocation-based mechanism of SFT formation initiated from the semi-coherent interfaces of Cu–Al nanoscale multilayered metals subjected to out-of-plane tension. Our molecular dynamics simulations show that Shockley partials are first emitted into the Cu interlayers from the dissociated misfit dislocations along the Cu–Al interface and interact to form SFTs above the triangular intrinsic stacking faults along the interface. Under further deformation, Shockley partials are also emitted into the Al interlayers and interact to form SFTs above the triangular FCC planes along the interface. The resulting dislocation structure comprises closed SFTs within the Cu interlayers which are tied across the Cu–Al interfaces to open-ended SFTs within the Al interlayers. This unique plastic deformation mechanism results in considerable strain hardening of the Cu–Al nanolayered metal, which achieves its highest tensile strength at a critical interlayer thickness of ~4 nm corresponding to the highest possible density of complete SFTs within the nanolayer structure.     

Dislocation formation and twinning from the crack tip in Ni3Al: molecular dynamics simulations

The mechanism of low-temperature deformation in a fracture process of L12 Ni3Al is studied by molecular dynamic simulations.Owing to the unstable stacking energy,the [01ˉ1] superdislocation is dissociated into partial dislocations separated by a stacking fault.The simulation results show that when the crack speed is larger than a critical speed,the Shockley partial dislocations will break forth from both the crack tip and the vicinity of the crack tip;subsequently the super intrinsic stacking faults are formed in adjacent {111} planes,meanwhile the super extrinsic stacking faults and twinning also occur.Our simulation results suggest that at low temperatures the ductile fracture in L12 Ni3Al is accompanied by twinning,which is produced by super-intrinsic stacking faults formed in adjacent {111} planes.     

Shrinking stacking fault through glide of the Shockley partial dislocation in hard-sphere crystal under gravity

Disappearance of a stacking fault in the hard-sphere crystal under gravity, such as reported by Zhuet al. [Nature 387, 883 (1997)], has successfully been demonstrated by Monte Carlo simulations. We previously found that a less ordered (or defective) crystal formed above a bottom ordered crystal under stepwise controlled gravity [Moriet al. J. Chem. Phys. 124, 174507 (2006)]. A defect in the upper defective region has been identified with a stacking fault for the (001) growth. We have looked at the shrinking of a stacking fault mediated by the motion of the Shockley partial dislocation; the Shockley partial dislocation terminating the lower end of the stacking fault glides. In addition, the presence of crystal strain, which cooperates with gravity to reduce stacking faults, has been observed.     

强流脉冲电子束辐照下单晶铝中的堆垛层错四面体

利用强流脉冲电子束技术对单晶铝进行了辐照，并利用透射电镜对强流脉冲电子束诱发的空位簇缺陷进行分析.实验结果表明，强流脉冲电子束能够诱发位错圈、孔洞甚至堆垛层错四面体这种通常在高层错能金属中不能形成的空位簇缺陷，并且三种不同类型的空位簇缺陷的形核过程并不同时发生，三种空位簇缺陷存在着密切的关系.根据实验结果提出了堆垛层错四面体形成与生长机理.     

强流脉冲电子束辐照下单晶铝中的堆垛层错四面体

利用强流脉冲电子束技术对单晶铝进行了辐照,并利用透射电镜对强流脉冲电子束诱发的空位簇缺陷进行分析.实验结果表明,强流脉冲电子束能够诱发位错圈、孔洞甚至堆垛层错四面体这种通常在高层错能金属中不能形成的空位簇缺陷,并且三种不同类型的空位簇缺陷的形核过程并不同时发生,三种空位簇缺陷存在着密切的关系.根据实验结果提出了堆垛层错四面体形成与生长机理. 关键词： 强流脉冲电子束 堆垛层错四面体 单晶铝 空位簇缺陷     

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.