Interfacial Dislocation Arrays Research Articles

Sliding friction of a pillar array interface: part I.

Biology is replete with examples, at length scales ranging from the molecular (ligand-receptor binding) to the mesoscopic scale (wing arresting structures on dragonflies) where shape-complementary surfaces are used to control interfacial mechanical properties such as adhesion, friction, and contact compliance. Related bio-inspired and biomimetic structures have been used to achieve unique interfacial properties such as friction and adhesion enhancement, directional and switchable properties. The ability to tune friction by altering surface structures offers advantages in various fields, such as soft robotics and tire manufacturing. Here, we present a study of friction between polydimethylsiloxane (PDMS) samples with surfaces patterned with pillar-arrays. When brought in contact with each other the two samples spontaneously produce a Moiré pattern that can also be represented as an array of interfacial dislocations that depends on interfacial misorientation and lattice spacing. Misorientation alone produces an array of screw dislocations, while lattice mismatch alone produces an array of edge dislocations. Relative sliding motion is accompanied by interfacial glide of these patterns. The frictional force resisting dislocation glide arises from periodic single pillar-pillar contact and sliding. We study the behavior of pillar-pillar contact with larger (millimeter scale) pillar samples. Inter-pillar interaction measurements are combined with a geometric model for relative sliding to calculate frictional stress that is in good agreement with experiments.

Sustainable fabrication of Cu/Nb composites with continuous laminated structure to achieve ultrahigh strength and excellent electrical conductivity

It is a challenge in practice to cost-effectively fabricate copper matrix composites having high mechanical properties and outstanding electrical conductivity for electric power applications. This report describes a new and simple mass production strategy that was developed to construct Cu/Nb multilayer composites having an ultra-thin continuous laminated structure processed by accumulative roll bonding (ARB). Benefiting from the synergistic contributions from alternating heterogeneous lamellar structures, regular arrays of interfacial dislocations as well as efficient electron transport channels and a low electron scattering structure, the fabricated Cu/Nb multilayer composites exhibit ultrahigh tensile strengths of ~1.2 GPa while retaining their electrical conductivity, thereby negating the general trade-off between strength and electrical conductivity. This study illustrates that interface strengthening is broadly important to improve the strength of multilayer composites. The excellent performance of the Cu/Nb multilayer composites indicates that these composites, having alternating heterogeneous lamellar structures, are promising for future energy and power applications. More significantly, the proposed method provides a potentially novel process for the high-yield production of nanolaminated composites and thereby gives a robust strategy for the development of structural and multifunctional materials.

Lattice mismatch and twist partitioning at commensurate dichromatic pattern of two-phase interfaces

Misfit dislocations are formed at a two-phase interface to reduce and even diminish coherency stress in the region far from the interface. Such semi-coherent interfaces are key structural features in a wide range of engineering materials. Burgers vectors of misfit dislocations are defined with the reference lattice named as commensurate dichromatic pattern (CDP) in the topological model. The CDP is not a geometrical average of boundary units as historically used in many examples. In this work, based on the Green’s function and theory of dislocations, both the mechanical effects of interfacial dislocation arrays and coherency strains due to geometry match at the interface are considered to get the CDP. We demonstrated that the mismatch and twist partitioning at a CDP are unequally partitioned in the two adjacent crystals, which only depends on elastic properties of the two crystals regardless of characters of misfit dislocations. Correspondingly, the method to determine the CDP of a two-phase interface in bi-crystal is developed.

Step flow growth of Mn5Ge3 films on Ge(111) at room temperature

The very first stages of the non-diffusive growth of Mn5Ge3 thin films on Ge(111) substrates are characterized by several techniques. Mn5Ge3 films are grown by molecular beam epitaxy using the co-deposition of Mn and Ge atoms at room temperature. XRD measurements demonstrate that the thin films are monocrystalline. The evolution of the RHEED intensity during the deposition and the AFM images show a step-flow growth mode. RHEED patterns, combined with TEM images, prove that the lattice mismatch of 3.7% is accommodated by the formation of an array of interfacial dislocations and by the presence of a residual strain in the thin films. These observations are supported by the numerical calculations of the critical nucleation volumes exhibiting very similar values, in the case of a pseudomorphic growth or in the case of an accommodation of the lattice mismatch by interfacial dislocations. Furthermore, the effect Ge/Mn stoichiometric and Mn-rich fluxes on the surface morphology is examined.

Analysis of plastic deformation in nanoscale metallic multilayers with coherent and incoherent interfaces

Nanoscale metallic multilayered (NMM) composites possess ultra high strength (order of GPa) and high ductility, and exhibit high fatigue resistance. Their mechanical behavior is governed mainly by interface properties (coherent and/or incoherent interfaces), dislocation mechanisms in small volume, and dislocation–interface interaction. In this work, we investigate these effects within a dislocation dynamics (DD) framework and analyze the mechanical behavior of two systems: (1) a bi-material system (CuNi) with coherent interface and (2) a newly developed tri-material system (CuNiNb) composed of both coherent and incoherent interfaces. For the bi-material case we analyze the influence of networks of interfacial dislocations whose nature and distribution are commensurate with the level of relaxation and loading of the structure. Misfit and pre-deposited interfacial dislocation arrays, as well as combinations of both, are studied and the dependence of strength on layer thickness is reported, along with observed dislocation mechanisms. It is shown that interfacial defect configurations significantly alter the strength and mechanical behavior of the material. Furthermore, it is shown that the implementation of penetrable interfaces in DD captures the strength dependence at layer thicknesses on the order of 3–7 nm. For the tri-material case we analyze the effects of coherent and incoherent interfaces in large-scale simulations. The results show that these materials have strong strength-size dependence but are limited by the strength of the incoherent (CuNb) interface which is weak in shear. The weak interface acts as a dislocation sink. This in turn induces an internal shear stress field that activates cross-slip in the adjacent CuNi interlace and thus causing softening. Moreover, it is shown that the yield stress of the CuNiNb system is controlled by the volume fraction of the Nb. Because Nb is the most compliant of the three materials, an increase in volume fraction of Nb decreases the overall yield strength of the material.

Interphase boundary structure and accommodation mechanism of lenticular martensite in Fe–Ni alloys

This study investigates the interphase boundary structures of lenticular martensites in Fe–33Ni and Fe–31Ni, and discusses the accommodation mechanism for transformation strain. Despite the large difference in the macroscopic morphology of the interphase boundary, the nature of interfacial dislocations in both alloys is almost the same; there are two sets of screw dislocations with a/2 [ 1 ¯ 1 ¯ 1] M and a/2 [1 1 ¯ 1] M Burgers vectors on the interphase boundary. On the basis of the array of interfacial dislocations, the shear planes of dislocations with a/2 [ 1 ¯ 1 ¯ 1] M and a/2 [1 1 ¯ 1] M Burgers vectors were determined to be (1 1 2) M (=(1 0 1) A) and ( 1 ¯ 1 2) M (=(0 1 1) A). Two sets of interfacial dislocations accommodate the atomistic misfit strain due to the change in the stacking sequence and the shape of parallel close packed planes. The macroscopic transformation strain and elastic strain energy of lenticular martensite are also reduced by the presence of the observed lattice invariant deformations.

The Early Stages of Quantum Dot Self-Assembly: A Kinetic Monte Carlo Simulation

The influence of a periodic strain field in a silicon substrate on the nucleation of surface atomic clusters is studied using a Kinetic Monte Carlo (KMC) model for Ge quantum dot formation. The effect of strain on island diffusion is determined by calculating the binding energy of an island with the Modified Embedded Atom potential. KMC simulations have been carried out on the basis of diffusion pathways on reconstructed Si surfaces and effective island diffusion kinetics. It is found that island diffusion coupled with surface stress fields play a dominant role in precisely forming atomic clusters during the early stages of evolution under an extremely weak inhomogeneous stain field. The results explain experimental observations on the ordering of Ge self-assembled quantum dos along underlying buried interfacial dislocation arrays in a relaxed 800 A Si0 85Ge0 15 buffer layer.

Spinodal decomposition in a film with periodically distributed interfacial dislocations

Phase field approach is applied to modeling the spinodal decomposition process in a thin film with periodically distributed arrays of interfacial dislocations. The elastic stress field in the simultaneous presence of interfacial dislocations, substrate constraint, and compositional strains is obtained by solving the mechanical equilibrium equations using an iteration method. It is shown that the periodic stress field associated with the array of interfacial dislocations leads to a directional phase separation and the formation of ordered mesoscale microstructures. It is demonstrated that when the periodicity of the dislocation is small, the wavelength of the ordered microstructure tends to be the same periodicity as the dislocation array. The results have important practical implications that an ordered nanostructures could be produced by controlling the interfacial dislocation distribution.

Interfacial dislocations: Symposium in Honor of J.H. van der Merwe on the 50 th anniversary of his discovery Foreword

In 1949 Frank and van der Merwe published the seminal series of papers: “One Dimensional Dislocations” in Proc. Royal Soc. (1949) that began a 50 year effort in understanding the science of heteroepitaxial boundaries. The pioneering work of J.H. van der Merwe in providing analytical expressions for the energy of arrays of interfacial dislocations has found its way into many aspects of epitaxy and descriptions of interphase boundaries. The three sessions in this symposium of all invited talks started with a 50 year overview by van der Merwe of the development of interfacial energy of dislocation arrays and explored the application of these ideas primarily to epitaxy. Additional talks addressed various aspects of dislocation arrays, such as nucleation and obstacles to misfit dislocation generation and their kinetics. This special symposium was organized in St. Louis, Missouri on October 11 and 12, 2000 at the Fall TMS Meeting. The sessions were entitled: Interfacial Dislocations: Symposium in Honor of J. H. van der Merwe on the 50 Anniversary of His Discovery. Contributors included, in order of presentations: F.R.N. Nabarro, Doris Kuhlmann-Wilsdorf, Terence Mitchell, L.B. Freund, Gary J. Shiflet, H.I. Aaronson, J.P. Hirth, R.A. Rapp, Eric Kvam, Max W.H. Braun, W.A. Jesser, Robert Hull and David Jesson. The organizers are grateful for all the people who joined in as authors and participants to make the event a success.

Creep deformation of fully lamellar TiAl controlled by the viscous glide of interfacial dislocations

Creep mechanisms of fully lamellar TiAl with a refined microstructure ( γ lamellae: 100–300 nm thick, α 2 lamellae: 10–50 nm thick) have been investigated. A nearly linear creep behavior (i.e. the steady-state creep rate is nearly proportional to the applied stress) was observed when the alloy was creep deformed at low applied stresses (<400 MPa) and intermediate temperatures (650–810°C). Since the operation and multiplication of lattice dislocations within both γ and α 2 lamellae are very limited in a low stress level as a result of the refined lamellar microstructure, creep mechanisms based upon glide and/or climb of lattice dislocations become insignificant. Instead, the motion of interfacial dislocation arrays on γ/ α 2 and γ/ γ interfaces (i.e. interface sliding) has found to be a predominant deformation mechanism. According to the observed interfacial substructure caused by interface sliding and the measured activation energy for creep, it is proposed that creep deformation of the refined lamellar TiAl in the intermediate-temperature and low-stress regime is primarily controlled by the viscous glide of interfacial dislocations.

Structure of Crystalline Interfaces

The structure of crystalline interfaces, as observed by transmission electron microscopy, is reviewed with emphasis on the similarity of grain and interphase boundaries of the dislocation type. Small-angle grain boundaries and low misfit interphase boundaries between similar crystal structures largely condense their mismatch into arrays of interfacial dislocations having Burgers vectors in common with dislocations located in the bulk crystals. Large-angle grain boundaries near certain misorientations corresponding to good fit between the abutting grains contain dislocations with Burgers vectors which are not found in the bulk crystal. Partially coherent interphase boundaries between quite dissimilar crystals, for example, f.c.c. and b.c.c., may also contain such dislocations. Principally, because of the difficulties involved in the acquisition of interfacial dislocations, dislocation interphase boundaries, in particular, usually do not have the minimum energy structure.

Epitaxial layer growth of Ag(111)-films on Si(100)

Deposition of Ag at 130 K results in epitaxial layer growth of a continuous Ag(1 1 1)-film on Si(100). The azimuthal orientation of the Ag(1 1 1)-crystallites is determined by the dimer rows of the Si(100) terraces and alternates by 90° from step to step of the Si substrate. In one direction the highly mismatched (100)- and (1 1 1)-lattices fit nearly exactly with a ratio of 4 Ag atoms to 3 Si atoms. In the other direction the remaining lattice mismatch of 2.2% is adjusted by a parallel array of interfacial dislocations which are observed with LEED (via spot splitting) and STM.

Interfacial dislocation arrays in twin boundaries of deformed titanium

Mechanical deformation of hexagonal close-packed (hcp) metals presents great interest. At low temperatures, the dominant deformation mechanism in these metals is mechanical twinning. The crystallography and core structure of twinning dislocations for the principal twin systems of hcp metals have been analyzed by Serra, Bacon and Pond. The properties of these twinning dislocations have been investigated by atomic scale computer simulation. These models can be used as a reference in order to investigate the properties of hcp metals during deformation. From a geometric point of view, different behavior is expected among distinct hcp metals. This could be attributed to the deviation of the individual c/a ratios from the ideal close-packed ratio (c/a = (8/3)[sup 1/2]), which also results in the different shear moduli of hcp metals. A study of these metals is performed in order to clarify the dominance of mechanical twinning in the mechanical deformation of hcp metals, at low temperatures and various deformation rates. In this work the deformation behavior of Ti is studied, by means of Transmission Electron Microscopy (TEM). The observations are focused in the range of 20% to 40% deformation, because under 20% mechanical twinning is negligible, whereas over 40% the vast quantity of dislocationmore » prohibits any clear observation. For 25% deformation, a mechanism of slip transmission of dislocations through the twin is observed.« less

Defects in Ni and Co silicide: Si interfaces

Silicide formation has been studied by deposition of Ni and Co on (001) and (111) Si substrates, followed by annealing. NiSi2 and CoSi2, exhibiting both the A (parallel lattices) and B (twin related lattices) type epitaxial relationships, and also CoSi (lattices rotated by 30° about a coincident [111] direction), have been investigated. Observations, using transmission electron microscopy, have been interpreted in the framework of a topological theory of interfacial discontinuities, and extensive agreement has been found between theory and experiment. In NiSi2Si A-type specimens, crystal dislocations were observed to accomodate misfit, although this was not completely relieved in general. In addition, irregular arrangements of dislocations with b=1/4<111> were observed on (001) interfaces and at the intersection of obtuse pairs of {111} facets. (001) interfaces were frequently found to facet on {111} planes, sometimes on a fine scale with facet spacings as small as 6 nm. NiSi2Si and CoSi2Si (111) type B specimens exhibit arrays of interfacial dislocations with b=1/6<211>, partially accommodating misfit. On theoretical grounds, these dislocations are thought to exhibit interfacial steps at their cores. Direct confirmation of this was obtained in plan view ultra-thin specimens by using a contrast technique based on a method for revealing steps on crystal surfaces. Arrays of interfacial dislocations were also observed in CoSiSi (111) specimens. The components of their Burgers vector parallel to the interface were determined to be close to 1/3<110>, in agreement with theory. Possible evidence of disclination formation was also found in these samples.

Strain interactions between dislocation arrays in epitaxial Si x Ge1-x /Si heterostructures

Lattice disregistry that exists in epitaxial overgrowths is often accommodated by interfacial dislocation arrays. The transition between strain accommodation by uniform interfacial shear and by interfacial dislocations is fairly sharp and thought to be controlled by energy minimization considerations. In this paper we demonstrate an extension of the Frank-van der Merwe approach by incorporating the continuum methodology of Eshelby to the analysis of strain interactions between arrays of interfacial dislocations in a bi-layered epitaxial film. Numerical examples are given for a Si/Si x Ge1-x /Si heterostructure. The importance of such an analysis to the study of defect propagation through strained layer superlattices is briefly discussed.

Transmission electron microscopy investigation of interfaces in a two-phase TiAl alloy

The atomic structures of the γ/α2 and γ/γT interfaces in a TiAl alloy were investigated using conventional and high-resolution transmission electron microscopy (TEM) in order to understand the growth mechanisms and deformation behavior of the two-phase alloy. The results show that the α2 plates grow from the γ phase by the migration of a/6〈112〉 partial dislocation ledges across the faces and that the γ/α2 interface usually contains closely spaced arrays of interfacial dislocations. Deformation twins cut through both γ twin boundaries and α2 plates during deformation, although slip of twinning c slocations through α2 appears to be a difficult process. Both the γ/α2 and γ/γT interfaces can be imaged and modeled at the atomic level, although slight crystal and/or beam tilt can complicate image interpretation.

Structure of hexagonal and cubic CdS heteroepitaxial layers on GaAs studied by transmission electron microscopy

The initial heteroepitaxial growth and structure of thin CdS layers on GaAs has been studied by conventional and high-resolution transmission electron microscopy. The work highlights the dependence of CdS crystal type on GaAs substrate orientation. Wurtzite-structure CdS is formed on (111)A GaAs and it is found to relieve misfit stresses by the introduction of interfacial defects, often associated with steps at the interface. Sphalerite-structure CdS is produced by initial growth on (001)GaAs and, in this case, misfit stresses are more slowly relieved, first with the formation of an asymmetrical array of interfacial dislocations and inclined stacking faults.

Growth of single-crystal TiN/VN strained-layer superlattices with extremely high mechanical hardness

Single-crystal TiN/VN strained-layer superlattices (SLS’s) with layer thicknesses lTiN =lVN =λ/2 (where λ is the period of the superlattice) ranging from 0.75 to 16 nm have been grown on MgO(100 ) substrates by reactive magnetron sputtering. Cross-sectional transmission electron microscopy (TEM) and x-ray diffraction examinations showed that the films were single crystals exhibiting coherent interfaces and several orders of superlattice reflections. There was no evidence in either plan-view or cross-sectional TEM analyses of misfit interfacial dislocation arrays. The primary defects observed were dislocation loops with a diameter of 8–10 nm extending through several layers and small defects with a diameter of 1–2 nm that were confined within single layers. Microindentation hardness values H, measured as a function of λ in films with a total thickness of 2.5 μm, increased from 2035±280 kg mm−2 for Ti0.5V0.5N alloys (i.e., λ=0) to reach a maximum of 5560±1000 kg mm−2 at λ=5.2 nm and then decreased rapidly to 3950±550 kg mm−2 at λ=7.5 nm. Further increases in λ resulted in a slower decrease in H to 3640±550 kg mm−2 at λ=32 nm. The large error bars in the H values for the SLS samples were due to the difficulty in measuring such extremely high hardnesses in thin films. (H for pure single-crystal TiN and VN films were 2200±300 and 1620±200 kg mm−2, respectively.) SLS samples grown with constant λ=6.5 nm, but different lTiN /λ ratios, exhibited a maximum hardness at lTiN /λ≂0.3.

TEM observations of the structure of metal multilayers

Potential explanations for the anomalous properties, such as the “supermodulus effect”, of metal multilayers include arguments based on the consequences of high coherency strains and/or the relief of these strains by interfacial dislocation arrays as well as others based on Fermi surface interactions. We have been investigating the Cu/NiPd system with the aim of distinguishing between such explanations since both the misfit and e/a ratio can be changed by altering the Ni:Pd ratio in the alloy layers. Our work has required the development of a range of TEM methods for the structural analysis of metal multilayers, and we are in general concerned that the x-ray methods normally used provide data averaged over too large a region to be unambiguous given the inhomogeneities that necessarily occur in multilayer systems.

Energy-minimizing structures for interfacial dislocation arrays: Non-planar configuration in small-angle grain boundaries

Electron microscope observations of small-angle grain boundaries in zinc reveal that the boundaries are generally not planar but rather take on a puckered form. A linear isotropic elastic analysis of the interactions between the individual dislocation segments making up the boundaries reveals that the interaction forces are consistent with the observed behavior which is therefore expected to be general to dislocation-containing boundaries in all materials.