Nanoscale Metallic Multilayers Research Articles

Combined size and texture-dependent deformation and strengthening mechanisms in Zr/Nb nano-multilayers

A combination of transmission electron microscopy analyses and nanomechanical measurements was performed in this study to reveal deformation and strengthening mechanisms occurring in sputtered Zr/Nb nanoscale metallic multilayers (NMMs) with a periodicity (L) in the range 6–167 nm. Electron diffraction analyses revealed a change in the crystallographic orientation of α-Zr when L≤27 nm, while Nb structure retained the same orientations regardless of L. For L > 60 nm, the strengthening mechanism is well described by the Hall-Petch model, while for 27 < L < 60 nm the refined CLS model comes into picture. A decrease in strength is found for L < 27 nm, which could not be simply explained by considering only misfit and Koehler stresses. For L≤27 nm, plastic strain measured across compressed NMMs revealed a change in the plastic behaviour of α-Zr, which experienced a hard-to-soft transition. At these length scales, the combination of two structural factors was found to affect the strength. These relate to the formation of weaker interfaces which extend the effective distance between strong barriers against dislocation transmission, thus producing a softening effect. The second effect relates to the crystallographic orientation change exhibited by α-Zr for L<27 nm with a consequent change of the dominant slip system. The actual strength at these smaller length scales was effectively quantified by taking these structural aspects into account in the interface barrier strength model.

Selective oxidation-induced strengthening of Zr/Nb nanoscale multilayers

The paper presents a new approach, based on controlled oxidation of nanoscale metallic multilayers, to produce strong and hard oxide/metal nanocomposite coatings with high strength and good thermal stability. The approach is demonstrated by performing long term annealing on sputtered Zr/Nb nanoscale metallic multilayers and investigating the evolution of their microstructure and mechanical properties by combining analytical transmission electron microscopy, nano-mechanical tests and finite element models. As-deposited multilayers were annealed at 350 °C in air for times ranging between 1 and 336 h. The elastic modulus increased by ∼20% and the hardness by ∼42% after 15 h of annealing. Longer annealing times did not lead to changes in hardness, although the elastic modulus increased up to 35% after 336 h. The hcp Zr layers were rapidly transformed into monoclinic ZrO2 (in the first 15 h), while the Nb layers were progressively oxidised, from top surface down towards the substrate, to form an amorphous oxide phase at a much lower rate. The sequential oxidation of Zr and Nb layers was key for the oxidation to take place without rupture of the multi-layered structure and without coating spallation, as the plastic deformation of the metallic Nb layers allowed for the partial relieve of the residual stresses developed as a result of the volumetric expansion of the Zr layers upon oxidation. Moreover, the development of residual stresses induced further changes in mechanical properties in relation to the annealing time, as revealed by finite element simulations.

Interfacial effect on strengthening nanoscale metallic multilayers - a combined Hall-Petch relation and atomistic simulation study

Besides the modulus difference, coherent stress and structural barrier mechanisms, the critical shear stress (CSS), which represents the resistance to the transmission of dislocation cross the interface, is another key factor influencing the hardness of nanoscale metallic multilayers (NMMs). However, it is very difficult to measure the CSS experimentally. In this study, we estimate the CSS based on the Hall-Petch relation via the available experimental hardness versus layer-thickness data. The obtained values are then verified by molecular dynamics simulations. With the proposed approach, 20 different NMMs are investigated systematically. The relative hardening contribution from the layers of the two constituent elements are quantified and these 20 NMMS grouped into two types except for one NMM. One group includes 12 NMMs whose interface hardening is contributed from both constituent element layers with different percentage. The other group includes 7 NMMs whose hardening contribution is mainly from the soft layer. We find that the peak hardness is not very sensitive to the CSS between the two constituent elements, rather is mainly determined by the average hardness of constituent layers. Moreover, it is more beneficial if the hard layer is about twice as hard as the soft layer for achieving high peak hardness.

Identifying Deformation and Strain Hardening Behaviors of Nanoscale Metallic Multilayers Through Nano-wear Testing

In complex loading conditions (e.g., sliding contact), mechanical properties, such as strain hardening and initial hardness, will dictate the long-term performance of materials systems. With this in mind, the strain hardening behaviors of Cu/Nb nanoscale metallic multilayer systems were examined by performing nanoindentation tests within nanoscratch wear boxes and undeformed regions (as-deposited). Both the architecture and substrate influence were examined by utilizing three different individual layer thicknesses (2, 20, and 100 nm) and two total film thicknesses (1 and 10 µm). After nano-wear deformation, multilayer systems with thinner layers showed less volume loss as measured by laser scanning microscopy. Additionally, the hardness of the deformed regions significantly rose with respect to the as-deposited measurements, which further increased with greater wear loads. Strain hardening exponents for multilayers with thinner layers (2 and 20 nm, n ≈ 0.018 and n ≈ 0.022, respectively) were less than that determined for 100 nm systems (n ≈ 0.041). These results suggest that single-dislocation-based deformation mechanisms observed for the thinner systems limit the extent of achievable strain hardening. This conclusion indicates that impacts of both architecture strengthening and strain hardening must be considered to accurately predict multilayer performance during sliding contact across varying length scales.

Fracture toughness determination by repetitive nano-impact testing in Cu/W nanomultilayers with length-scale-dependent films properties

Nanoscale metallic multilayers based on Cu/W have been considered as a potential material for structural applications in nuclear reactors and for the cladding of storage tanks for advanced fuels kept at high temperatures. The understanding of how mechanical properties change in relation to periodicity, λ, is required in order to use Cu/W nano-multilayers as a protective coating against radiation damage. The aim of this work is to demonstrate the feasibility of using the repetitive-nano-impact technique to obtain quantitative fracture toughness, KC, values in nano-multilayers and assess its variation as a function of λ.

Residual stress within nanoscale metallic multilayer systems during thermal cycling

Projected applications for nanoscale metallic multilayers will include wide temperature ranges. Since film residual stress has been known to alter system reliability, stress development within new film structures with high interfacial densities should be characterized to identify potential long-term performance barriers. To understand factors contributing to thermal stress evolution within nanoscale metallic multilayers, stress in Cu/Nb systems adhered to Si substrates was calculated from curvature measurements collected during cycling between 25°C and 400°C. Additionally, stress within each type of component layers was calculated from shifts in the primary peak position from in-situ heated X-ray diffraction. The effects of both film architecture (layer thickness) and layer order in metallic multilayers were tracked and compared with monolithic Cu and Nb films (1µm total thickness). Analysis indicated that the thermoelastic slope of nanoscale metallic multilayer films (with 20nm and 100nm individual layer thicknesses) depends on thermal expansion mismatch, elastic modulus of the components, and also interfacial density. The layer thickness (i.e. interfacial density) affected thermoelastic slope magnitude (−1.23±0.09MPa/°C for 20nmCu/Nb vs. −0.89±0.03MPa/°C for 100nmCu/Nb) while layer order had minimal impact on stress responses after the initial thermal cycle (−0.82±0.07MPa/°C for 100nm Cu/Nb). When comparing stress responses of monolithic Cu and Nb films to those of the Cu/Nb systems, the nanoscale metallic multilayers show a similar increase in stress above 200°C to the Nb monolithic films, indicating that Nb components play a larger role in stress development than Cu. Phase specific stress calculations (Cu vs. Nb) from X-ray diffraction peak shifts in 20nm Cu/Nb collected during heating reveal that the component layers within a multilayer film respond similarly to their monolithic counterparts.

The mechanical behavior of nanoscale metallic multilayers: A survey

The mechanical behavior of nanoscale metallic multilayers (NMMs) has attracted much attention from both scientific and practical views. Compared with their monolithic counterparts, the large number of interfaces existing in the NMMs dictates the unique behavior of this special class of structural composite materials. While there have been a number of reviews on the mechanical mechanism of microlaminates, the rapid development of nanotechnology brought a pressing need for an overview focusing exclusively on a property-based definition of the NMMs, especially their size-dependent microstructure and mechanical performance. This article attempts to provide a comprehensive and up-to-date review on the microstructure, mechanical property and plastic deformation physics of NMMs. We hope this review could accomplish two purposes: (1) introducing the basic concepts of scaling and dimensional analysis to scientists and engineers working on NMM systems, and (2) providing a better understanding of interface behavior and the exceptional qualities the interfaces in NMMs display at atomic scale.

Concurrent interface shearing and dislocation core change on the glide dislocation-interface interactions: a phase field approach

Strengthening in nanoscale metallic multilayers is closely related to the glide dislocation-interface interaction. The interface can be sheared by the stress of the approaching glide dislocation with its core changed. How the concurrent interface shearing and the dislocation core change influence such interaction dominated strength is studied using three dimensional phase field microelasticity modeling and simulation. The simulated results show that when the glide dislocation is close to or away from the interface, the width of its core changes abruptly in accompany with the interface shear zone broadening or shrinking, respectively. A wider interface shear zone is developed on the interface with a lower shear strength, and can trap the glide dislocation at the interface in a lower energy state, and thus leads a stronger barrier to dislocation transmission. The results further show that the continuum model of the dislocation without the core-width change underestimates the interfacial barrier strength especially for the glide dislocation transmission across weak interfaces.

Thermal stability of nanoscale metallic multilayers

Metallic nanolayered thin films/foils, in particular Ni/Al multilayers, have been used to promote joining. The objective of this work is to evaluate the thermal stability of nanoscale metallic multilayers with potential for joining applications. Multilayers thin films with low (Ti/Al and Ni/Ti), medium (Ni/Al) and high (Pd/Al) enthalpies of exothermic reaction were prepared by dual cathode magnetron sputtering. Their thermal stability was studied by: i) differential scanning calorimetry combined with X-ray diffraction (XRD), ii) in-situ XRD using cobalt radiation, and iii) in-situ transmission electron microscopy. It was possible to detect traces of intermetallic or amorphous phases in the as-deposited short period (bilayer thickness) multilayers, except for the Ti/Al films where no reaction products that might be formed during deposition were identified. For short periods (below 20nm) the equilibrium phases are directly achieved upon annealing, whereas for higher periods intermediate trialuminide phases are present for Ti/Al and Ni/Al multilayers. The formation of B2-NiTi from Ni/Ti multilayers occurs without the formation of intermediate phases. On the contrary, for the Pd–Al system the formation of intermediate phases was never avoided.The viability of nanoscale multilayers as “filler” materials for joining macro or microparts/devices was demonstrated.

Microstructure and mechanical properties of physical vapor deposited Cu/W nanoscale multilayers: Influence of layer thickness and temperature

Based on our previous knowledge on Cu/Nb nanoscale metallic multilayers (NMMs), Cu/W NMMs show a good potential for applications as heat skins in plasma experiments and armors, and it could be expected that the substitution of Nb by W would increase the strength, particularly at high temperatures. To check this hypothesis, Cu/W NMMs with individual layer thicknesses ranging between 5 and 30nm were deposited by physical vapor deposition, and their mechanical properties were measured by nanoindentation. The results showed that, contrary to Cu/Nb NMMs, the hardness was independent of the layer thickness and decreased rapidly with temperature, especially above 200°C. This behavior was attributed to the growth morphology of the W layers as well as the jagged Cu/W interface, both a consequence of the low W adatom mobility during deposition. Therefore, future efforts on the development of Cu/W multilayers should concentrate on optimization of the W deposition parameters via substrate heating and/or ion assisted deposition to increase the W adatom mobility during deposition.

Elevated temperature dependence of hardness in tri-metallic nano-scale metallic multilayer systems

A tri-layer nano-scale metallic multilayer (Cu/Ni/Nb) system with a mixture of incoherent and coherent interfaces was investigated to determine the effect of elevated temperature conditions on the strength at temperature and after annealing. Elevated temperature nanoindentation showed a reduction in the temperature sensitivity of hardness as individual layer thickness decreases (i.e. thinner layers retain strength better at elevated temperatures). This is explained using the confined layer slip model which suggests the drop in stress is due to both changes in the shear modulus of the film as well as dislocation/interface interactions. Molecular dynamic simulations of Cu/Nb bi-layers are presented in support of the concept that dislocation interactions at incoherent interfaces are less temperature sensitive than dislocation–dislocation interactions within the layers, supporting the experimental results.

Strain rate sensitivity and related plastic deformation mechanism transition in nanoscale Ag/W multilayers

Nanoscale metallic multilayers are a promising structural material for fabricating the next generation of nanoscale devices. Even though the mechanical properties of the multilayers have been extensively studied, the interpretation for the strain rate sensitivity of the multilayers is still lacking. In present study, the hardness and strain rate sensitivity of Ag/W multilayers with a wide range of modulation period Λ and modulation ratio η deposited by d.c. sputtering deposition were evaluated by nanoindentation testing. X-ray diffraction and transmission electron microscopy were carried out to investigate the texture and microstructure of the multilayers. Experimental results indicated that a mechanism transition occurred as the plastic deformation of the Ag/W multilayers was accommodated by the two constituent layers together at Λ≤50nm, while the plastic deformation localized mainly in Ag layers when Λ>50nm. A modified rule of mixture and plastic deformation localization model was proposed to describe the variation of strain rate sensitivity for the multilayers with smaller and larger Λ, respectively.

The effect of interfacial imperfections on plastic deformation in nanoscale metallic multilayer composites

Nanoscale metallic multilayer (NMM) composites represent a class of advanced engineering materials that are shown to exhibit high structural stability, mechanical strength, high ductility, toughness and resistance to fracture and fatigue. This paper addresses the question of the effect of the interface imperfections on the strengthening of NMMs with incoherent interfaces, by performing molecular dynamics simulations. Two types of interfaces are considered, a perfect one, and one with discontinuities (steps and ledges). Our simulations demonstrate that the result of the interaction between dislocations and interfaces is the creation of interfacial disconnections made of a dislocation that spreads within the interface, and a step entrapped at the interface. The energy calculations show that these steps increase the total energy of the system and enhance the strengthening effect of the interface by adding extra barriers to slip transmission, thus improving the mechanical properties of the structure.

The void nucleation strengths of the Cu–Ni–Nb- based nanoscale metallic multilayers under high strain rate tensile loadings

The mechanical behavior of Cu–Ni–Nb- based nanoscale metallic multilayers (NMM) under high strain rate loadings is investigated in this work using molecular dynamics simulations. The simulations of NMMs with various individual layer thicknesses under uniaxial tensile strains at two different controlled strain rates (109/s and 1010/s) are performed. This type of loading condition generates a stress state necessary for void nucleation commonly observed under shock loading. The mechanisms for void nucleation in the NMMs are examined and identified; the void nucleation strengths (VNS) of the NMMs and their variations with respect to increasing individual layer thickness as well as available nucleation sites (affected by addition of interfacial disconnections) are obtained and explained. It is discovered that the void always nucleate from within the Cu layers, where the partial dislocations intersect with each other or with existing stacking faults. The void nucleation strength of the NMMs is closely related to the density of available sites for void nucleation. By introducing interfacial steps into the incoherent interfaces of the NMMs the abundance of dislocation sources is changed, thus more (less) sites for void nucleation are produced which decrease (increase) the void nucleation strength of the NMMs.

Multiscale modeling and simulation of deformation in nanoscale metallic multilayer systems

Nanoscale metallic multilayers (NMM) have very high strength approaching a fraction of the theoretical limit. Their increased strength is attributed to the high interface density and is limited by the interfacial strength. As the density of interfaces increases (due to smaller layer thicknesses) the strength of NMM structures becomes increasingly determined by the specific nature and properties of the interfaces and is most likely controlled by the nucleation of dislocations from the interfaces. With focus on material systems with incoherent interfaces, we performed MD simulations to determine the controlling deformation mechanisms at different length scales for Cu–Nb multilayers under biaxial tensile deformation conditions. The results of the simulations show that there is a transition in the operative deformation mechanism in NMMs from Hall–Petch strengthening for the length scales of sub microns to microns, to individual dislocations confined to glide in individual layers for few nm to few tens of nm, and dislocation-nucleation-controlled models for less than few nanometers. Based on these results, we develop a Molecular dynamics-based rate-sensitive model for viscoplastic flow which describes the anisotropic deformation behavior of NMMs at different length scales.

Direct Observations of Confined Layer Slip in Cu/Nb Multilayers

In situ nanoindentation of a 30 nm Cu/20 nm Nb multilayer film in a transmission electron microscope revealed confined layer slip as the dominant deformation mechanism. Dislocations were observed to nucleate from the Cu-Nb interfaces in both layers. Dislocation glide was confined by interfaces to occur within each layer, without transmission across interfaces. Cu and Nb layers co-deformed to large plastic strains without cracking. These microscopy observations provide insights in the unit mechanisms of deformation, work hardening, and recovery in nanoscale metallic multilayers.

Deformation mechanisms, size effects, and strain hardening in nanoscale metallic multilayers under nanoindentation

The strain hardening and the related surface pile-up phenomena in CuNi, CuNb and CuNiNb nanoscale multilayered metallic (NMM) composites are investigated using atomistic simulations of nanoindentation on such multilayers with varying individual layer thickness. Using empirical load-stress and displacement-strain relations, the obtained load-depth curves were converted to hardness-strain curves which was then fitted using power law. It is found that the extent of surface pile-up is inversely related to the hardening exponent of the NMMs. Two deformations mechanisms which control the surface pile phenomenon are discovered and discussed. Furthermore, from the stress-strain data, it is found that interfaces and their types play a major role in strain hardening; the strain hardening rate increases with strain when incoherent interfaces are present. The relationship between the hardening parameters and the interfacial dislocation density as well as the relationship between interfacial density and length scales, such as layer thickness and indentation depth, are analyzed, and it is found that the hardness in these NMM has strong inverse power law dependence on the layer thickness.

Direct Observation of Crack Propagation in Copper–Niobium Multilayers

The failure of a cross-sectional 65 nm-thick copper and 150 nm-thick niobium multilayer thin film was investigated via an in situ transmission electron microscopy straining experiment. The fracture of the free-standing multilayer films was associated with confined dislocation slip within layers containing and preceding the crack tip. Four crack hindrance mechanisms were observed to operate during crack propagation: microvoid formation, crack deviation, layer necking, and crack blunting. Failure was observed to occur across and through the copper and niobium layers but never within the interfaces or grain boundaries. These results are discussed relative to the length-scale-dependent deformation mechanisms of nanoscale metallic multilayers.

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.

Theoretical explanation of Ag/Cu and Cu/Ni nanoscale multilayers softening

Relationship between metallic multilayers hardness and monolayer thickness has been investigated and explained for electroplated Ag/Cu and Cu/Ni multilayers using a modified Thomas–Fermi–Dirac electron theory. Experiments reveal that the peak hardness of Ag/Cu multilayers occurs at the monolayer thickness of about 25 nm, while the peak hardness of Cu/Ni multilayers occurs at about 50 nm. Critical monolayer thickness corresponding to the peak hardness is approximated by the grain size limit of stable dislocations in Ag crystals for the Ag/Cu multilayers and in Cu crystals for Cu/Ni multilayers. Grains size limits are calculated based on a modified Thomas–Fermi–Dirac electron theory. Developed relationship between the critical monolayer thickness and the grains size limit helps understand nanoscale metallic multilayers softening.