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Interfaces often dominate plastic response of nanostructured metallic materials, e.g., nanolaminates (NLs), and thus engineering NLs to introduce controllable interfacial properties is a grand challenge. In this work, we comparatively studied the mechanical properties of both crystalline/crystalline NLs (C/CNLs) and crystalline/amorphous NLs (C/ANLs) in terms of the Hall-Petch slope ( k ) and the peak hardness ( H peak ). To characterize the interfacial properties of Cu-based NLs, we propose an energy factor (χ) in light of the interfacial residual dislocation energy determined by the coupling effects of lattice and moduli mismatch and the increased system energy caused by transmission slip determined by the mixing enthalpy (Δ H mix ). It is found that this energy factor χ can quantitatively capture well the variation of Hall-Petch slope k and peak hardness H peak of NLs with different types of interfaces. There are linear relationships for both k – χ and H peak – χ plots. For the C/CNLs with positive Δ H mix , both k and H peak decrease with increasing χ, whereas for both the C/CNLs with negative Δ H mix and the C/ANLs, k decreases with decreasing χ while H peak decrease with increasing χ. The underlying mechanisms for the different mechanical properties of these NLs are elucidated from the perspective of diatomic interactions characterized by the Δ H mix of a given NL system. These experimental findings provide deep insights into designing tunable interfacial structures in metallic materials to realize their high performance. [ABSTRACT FROM AUTHOR]

The magnetron sputtering technique was employed to prepare Zr-alloyed Cu thin films with Zr addition ranging from 0 to 7.0 at.%. Microstructure evolution with Zr doping was characterized by using the transmission electron microscope and atom probe tomography. Plastic deformation characteristics (hardness, strain rate sensitivity and activation volume) of the Cu-Zr alloyed films were examined by using nanoindentation testing. Significant Zr doping effects on the microstructure were clearly uncovered that (i) the Zr segregated at grain boundaries and tuned the grain boundary structure. With increasing the Zr addition, CuZr amorphous particles at grain boundaries (3.0 at.% Zr) and even three dimensional CuZr amorphous grain boundary network (7.0 at.% Zr) were formed; (ii) the grains (size and morphology) and nanotwins were notably influenced by the Zr doping. In particular, the nanotwin thickness was reduced from ∼25 nm in the pure Cu film down to ∼5 nm in the Cu-Zr alloyed films. Accompanied with the microstructure evolution, the Cu-Zr thin films displayed hardness and strain rate sensitivity highly sensitive to the Zr doping, i.e. , hardness increasing while strain rate sensitivity decreasing with raising the Zr addition. The strengthening and deformation mechanisms were discussed in terms of the microstructure-property relationship. Three regimes were divided within the studied Zr doping range: grain/nanotwin boundary-dominated strengthening mechanism in the regime I of pure Cu film, dislocation nucleation-controlled nanotwin softening mechanism in the regime II of Zr addition ≤3.0 at.%, and intergranular amorphous layer-mediated strengthening mechanism in the regime III of Zr addition up to 7.0 at.%. In addition, the reduction in strain rate sensitivity with Zr doping was quantitatively described by adopting a model that involves the thermally activated partials depinning mechanism based on the effective dislocation segment length. [ABSTRACT FROM AUTHOR]

In this work, hcp-to-bcc Ti phase transformation was uncovered in Ti/Ta nanostructured multilayers. The Ti nanolayers were fully transformed to bcc phase when the layer thickness ( h ) smaller than ~7.5 nm, accompanied with the formation of fully coherent interfaces. While in the greater layer thickness, Ti layers were partially phase-transformed, resulting in the simultaneous existence of bcc/bcc Ti/Ta coherent interfaces and bcc/hcp Ti/Ti phase boundaries. A thermodynamic model was proposed to account for the layer thickness h -dependent Ti phase transformation. The phase transformation caused an unusual size-dependent hardness, i.e., the smaller the layer thickness and the lower the hardness, which was quantitatively addressed by an ad hoc strengthening model. [ABSTRACT FROM AUTHOR]

Nanoindentation methodology was used to investigate the plastic deformation characteristics, including the hardness ( H ), strain rate sensitivity (SRS, m ) and activation volume ( V * ), of Cu/Mo nanostructured metallic multilayers (NMMs) with equal layer thickness ( h ) spanning from 10 to 200 nm before and after He-implantation at room temperature. Compared with the as-deposited Cu/Mo NMMs, the irradiated Cu/Mo samples exhibited the enhanced hardness particularly at great h , which is caused by the bubble-hardening effect. Unlike the as-deposited Cu/Mo NMMs displayed a monotonic increase in SRS (or a monotonic decrease in activation volume) with reducing h , the irradiated Cu/Mo samples manifested an unexpected non-monotonic variation in SRS as well as in activation volume. It was clearly unveiled that the SRS of irradiated Cu/Mo firstly decreased with reducing h down to a critical size of ~50 nm and subsequently increased with further reducing h , leaving a minimum value at the critical h . These phenomena are rationalized by considering a competition between dislocation-boundary and dislocation-bubble interactions. A thermally activated model based on the depinning process of bowed-out partial dislocations was employed to quantitatively account for the size-dependent SRS of Cu/Mo NMMs before and after irradiation. Our findings not only provide fundamental understanding of the effects of radiation-induced defects on plastic characteristics of NMMs, but also offer guidance for their microstructure sensitive design for performance optimization at extremes. [ABSTRACT FROM AUTHOR]

Nanoindentation methodology was used to measure the hardness of Cu/Ta crystalline/amorphous nanostructured multilayers (CANMs) before and after He ion-implantation at room temperature. These CANMs have a constant modulation period ( λ =25 nm) but different modulation ratios ( η = h Ta / h Cu ) spanning from 0.11 to 1.0. It is found that in sharp contrast to previously reported Cu/Cu-Zr CANMs by Zhang et al. [9] , the He-irradiated Cu/Ta samples exhibit much greater microstructure stability without radiation-induced devitrification (RID) of glassy Ta nanolayers at smaller η (except the sample at η =1.0). Both the as-deposited and irradiated Cu/Ta CANMs manifest the monotonically increased hardness with decreasing η , however, there is an unexpected transition from size-dependent irradiation hardening at η <1.0 to softening at η ≥1.0 caused by a competition between dislocation-bubble interactions in crystalline nanolayers and RID in glassy nanolayers. These findings not only provide fundamental understanding of the radiation-induced defect effect on plastic characteristics of CANMs, but also offer guidance for their microstructure sensitive design for performance optimization at a critical modulation ratio under extreme conditions. [ABSTRACT FROM AUTHOR]

Cu/Cr nanostructured multilayer films (NMFs) with modulation periods ( λ ) ranging from 250 nm to 10 nm and modulation ratios ( η ) ranging from 0.1 to 2.0 were prepared on flexible polyimide substrates by using magnetron sputtering. Upon uniaxial tensile testing, the critical cracking strain ( ε c ), critical buckling strain ( ε b ), and fracture toughness ( K IC ) of the NMFs were experimentally measured and all of the mechanical properties showed remarkable λ - and η -dependences. The cracking and buckling behaviors of the Cu/Cr NMFs were systematically investigated and both were found to depend strongly on the length scale. Based on an energy balance model, the interfacial adhesion energies ( Γ ) were determined using the measured buckle dimensions. Cracking maps and buckling maps were constructed from the experimental data to summarize the effects of λ and η on the cracking and buckling modes, respectively. In the cracking map, two regimes can be identified: one is brittle fracture with straight cracks and the other is ductile fracture with zigzag cracks. The ductile fracture regime is located in the region where λ ∼ 40 ± 20 nm and simultaneously η < ∼0.3, and the brittle-to-ductile transition is characterized by a fracture toughness criterion of K IC ∼ 12.5 MPa m 1/2 . In the buckling map, four regimes are distinguished: cracked rectangular buckles, uncracked rectangular buckles, cracked triangular buckles, and uncracked triangular buckles. The effects of the length-scale-dependent deformation and adhesion energies on the buckling behaviors were discussed. A combined dimensionless parameter of K IC / E f Γ ( E f : elastic modulus of the NMF) was proposed to assess the buckling behaviors, and the K IC / E f Γ contours coincided well with the boundaries dividing the four buckling regimes. [ABSTRACT FROM AUTHOR]

Abstract: Two sets of Cu/Nb (face-centered cubic (fcc)/body-centered cubic) and Cu/Zr (fcc/hexagonal close-packed) nanostructured multilayer films (NMFs) have been prepared on a flexible polyimide substrate, with a wide range modulation period (λ) from 250 down to 5nm. The mechanical properties of the two NMFs have been measured upon uniaxial tensile testing and the buckling behaviors have been systematically investigated as a function of λ. A significant difference in the bucking behaviors was found between the two NMFs, with the buckles in the Cu/Nb NMF being mostly cracked, while the buckles were nearly crack-free in the Cu/Zr NMF. The different buckling behaviors, dependent on the constituent phases, are rationalized in the light of the disparity in mechanical properties. The criteria to characterize buckle cracking have been discussed with respect to the mechanical properties (e.g. yield strength, ductility and fracture toughness) of the NMFs. A modified energy balance model has been employed to estimate the adhesion energy of the NMFs on the polyimide substrate. Within the λ regime below a critical size (λcrit ) of ∼50nm a λ-independent adhesion energy of about 1.1 and 1.2Jm−2 has been determined for the Cu/Nb and Cu/Zr NMFs, respectively, which agrees well with previous reports on the metal film/polymer substrate systems. Within the λ regime greater than λcrit , however, the measured adhesion energy exhibit a strong size effect, i.e. increasing with increasing λ. The λ dependence of the evaluated adhesion energy is discussed in terms of the size-dependent deformation mechanism in NMFs. A micromechanics model has been utilized to quantify the critical modulation period of ∼50nm, where the deformation mechanism changes from dislocation pile-up to confined layer slip. [Copyright &y& Elsevier]

Abstract: The columnar-grained nanotwinned (cg-nt) Cu films exhibit high strength but low ductility. By introducing the nanocrystalline Cr layers into cg-nt-Cu films forming Cu/Cr nanostructured multilayers, it is found that the number of twins per grain in Cu as well as the twins density decreases with reducing Cu layer thickness, which are analyzed by “growth accident” related twins formation models. The suppression of twin formation via multilayer scheme significantly enhanced the ductility of cg-nt-Cu films without sacrificing their strength. [Copyright &y& Elsevier]

Abstract: The phase transition of Nb from body-centered cubic (bcc) to face-centered cubic (fcc) has been found in Cu/Nb nanostructured multilayers with modulation period (λ) spanning from 5 to 300nm, and was analyzed by using a thermodynamic model. As λ decreases, the strength of multilayers increase and approach saturation of ∼2.88GPa at a few nm layer thickness. Size dependent strengthening in present fcc/fcc Cu/Nb multilayers is explained in terms of dislocation based strengthening mechanisms. The enhanced modulus of fcc/fcc Cu/Nb is also observed. [Copyright &y& Elsevier]

Abstract: Hardness, activation volume and strain-rate sensitivity of Cu/Cr (face-centered cubic (fcc)/body-centered cubic) and Cu/Zr (fcc/hexagonal close-packed) nanostructured multilayer films have been systematically measured as a function of modulation period (L) and modulation ratio (η), respectively. Significant size effects were found for all the three plastic deformation characteristics, i.e. enhanced hardness and activation volume but reduced strain-rate sensitivity with decreasing the dimension length L. Microstructure evolution was statistically examined to rationalize these size dependences. It was crucially observed that abundant nanotwins existed in the Cu grains, though nanotwin formation was depressed with smaller L. This inverse size-dependent nanotwin formation is responsible for the reduction in strain-rate sensitivity, because the negative effect induced by the decreased nanotwins predominates over the positive effect coming from the raised interfaces/boundaries. A mechanistic model is modified to account for the interface effect as well as the nanotwin effect, which yields calculations of strain-rate sensitivity in broad agreement with the experimental results when L is larger than about 20nm. Below this critical length size, there are discrepancies between the calculations and the experimental results, due to the change in deformation mechanism from dislocation nucleation/slip in confined layers to dislocation crossing interfaces. A confined layer slip model is also modified by considering the nanotwin strengthening to quantitatively describe the L-dependent hardness. In addition, the effects of constituent phases and their relative content on the activation volume and strain-rate sensitivity of NMFs are discussed with regard to variation in η. [Copyright &y& Elsevier]

Abstract: Length-scale dependent hardness (H) and indentation modulus (E) are systematically investigated in nanostructured Cu/Cr multilayer films with a wide modulation period (λ) range from 5nm to 250nm. H is gradually increased with reducing λ down to ∼50nm, whereafter a λ-independent effect is shown. The theoretical analysis of confined dislocation gliding and interface barrier strength quantitatively assess the variation of H. The indentation modulus E, however, exhibits a non-monotonic evolution with λ and attains a maximum at the same critical λ of ∼50nm. This unusual variation in E is related to the formation of interfacial amorphous layers, and can be reasonably explained in terms of a competition between the enhancement effect induced by compressed out-of-plane interplanar spacing of the constituent layers and the reduction effect associated with free volume in the amorphous intermixing layer. This result offers great benefits in engineering ductile nanolaminates with high strength and low modulus by tailoring interfacial structure and/or introducing amorphous layers. [Copyright &y& Elsevier]

Abstract: The plastic deformation and fracture behavior of two different types of Cu/X (X =Nb, Zr) nanostructured multilayered films (NMFs) were systematically investigated over wide ranges of modulation period (λ) and modulation ratio (η, the ratio of X layer thickness to Cu layer thickness). It was found that both the ductility and fracture mode of the NMFs were predominantly related to the constraining effect of ductile Cu layers on microcrack-initiating X layers, which showed a significant length-scale dependence on λ and η. Experimental observations and theoretical analyses also revealed a transition in strengthening mechanism, from single dislocation slip in confined layers to a load-bearing effect, when the Cu layer thickness was reduced to below ∼15nm by either decreasing λ or increasing η. This is due to the intense suppression of dislocation activities in the thin Cu layers, which causes a remarkable reduction in the deformability of the Cu layers. Concomitantly, the constraining effect of Cu layers on microcrack propagation is weakened, which can be used to explain the experimentally observed λ and η-dependent fracture mode transition from shear mode to an opening mode. Furthermore, the fracture toughness of the NMFs is also found to be sensitive to both λ and η. A fracture mechanism-based micromechanical model is developed to quantitatively assess the length-scale-dependent fracture toughness, and these calculations are in good agreement with experimental findings. [Copyright &y& Elsevier]

Abstract: It was recently suggested that the fracture mode in nanostructured metallic multilayer films (NMFs) is related to the strengthening mechanism. Here, based on extensive experimental examinations on the nanoscale damage of Cu/Cr NMFs with wide ranges of modulation period (from 10nm to 250nm) and modulation ratio (from 0.11 to 3.0), we conclude that the dominant factor controlling the fracture mode in NMFs is the constraint effect from the ductile layer on the brittle layer, rather than the strengthening mechanism. This constraint effect is quantitatively assessed using a fracture mechanism-based micromechanical model, which yields predictions in broad agreement with the experimental observations. [Copyright &y& Elsevier]

At a constant modulation period (λ), nanostructured Cu/Cr multilayer films exhibit ductility scaling linearly with yield strength that varies with modulation ratio. The films with different λ have their own scaling relationship. The scaling slope for λ =25nm is much sharper than that for λ =50nm, indicating that a stronger interface constraint causes a larger reduction in ductility. These scaling relationships can be understood by referring to macroscopic fracture models based on a critical stress criterion. [Copyright &y& Elsevier]

In Cu/Cr multilayers with modulation period (λ) ranging from 10 to 250nm, maxima are observed for both tensile ductility and fracture at a critical λ ∼50nm, different from the monotonic λ dependence known for monolithic films. This unusual behavior is explained, via quantitative assessments based on a micromechanical model, by considering the competing thickness effects on the size of the microcracks initiated in the Cr layers and on the role of the ductile Cu layer in blocking crack propagation. [Copyright &y& Elsevier]

Abstract: Intragranular nano-sized α phase with lamellar morphology was induced in the multifunctional Ti–24Nb–4Zr–7.9Sn (wt.%) alloy by short-time thermal treatment. This peculiar α+β dual microstructure resulted in a promising combination of mechanical properties including low modulus (47GPa), high strength (850MPa) and reasonable ductility (∼10%). This nano-sized network of lamellar alpha phase has been obtained by short-time ageing at 573K on solution-treated (ST) samples and at 973K on cold-rolled (CR) samples, respectively. Suppression of the stress-induced martensitic transformation, keeping the intrinsic modulus at a low level, has been clearly noticed advantageously in both cases. In the treated specimens, a stable strain recovery of about 2.7% was obtained until tensile fracture during cycling loading. [Copyright &y& Elsevier]

Abstract: ZnO nanowires have been successfully synthesized on Si(100) substrate by a simple physical vapor deposition method. Thin film ZnO layer used as the nucleation site can avoid the contamination from the metal catalysts, and it can also control the growth direction of ZnO nanowires. Well-aligned ZnO nanowire arrays along the normal direction of the substrate can be obtained by controlling different growth temperature, which was demonstrated by XRD and FESEM analysis. A strong ultraviolet emission at room temperature was observed in all ZnO nanostructures. In addition, the growth mechanisms of the ZnO nanowires with different growth temperature is discussed in details. [Copyright &y& Elsevier]

Abstract: ZnO nanowires array has been synthesized via a simple physical vapor deposition method at low growth temperature without using any catalyst. By controlling different growth parameters, the nanowires exhibit different shapes: needle-like with sharp tips or rod-like with flat tips. All the nanowires are single crystal with a growth direction along c-axis. The rod-like nanowires are well vertical aligned to the substrate surface with c-axis preferred orientation. The morphology difference of ZnO nanowires is believed originated from different growth mechanisms. [Copyright &y& Elsevier]

Abstract Previous studies demonstrated that the He ion irradiation damage tolerance of nanostructured metallic multilayers (NMMs) was closely dependent on the layer thickness (h): smaller h led to lower irradiation hardening. Here in Mo/Zr NMMs, we uncovered that the h -dependent irradiation hardening and corresponding plastic deformation characteristics are also sensitive to the He+ irradiation fluences. At a low irradiation fluence of 1.0 × 1016 He+·cm−2, the irradiation hardening decreased monotonically with reducing h. While at a high irradiation fluence of 1.0 × 1017 He+·cm−2, a non-monotonic h -dependence was unexpectedly displayed with the minimum irradiation hardening at h of ∼25 nm. The irradiation fluence also affects the strain rate sensitivity (m) remarkably. Irradiation of 1.0 × 1016 He+·cm−2 induced a transition in SRS m from positive in the as-deposited Mo/Zr NMMs to negative SRS in their irradiated counterparts. This transition was rationalized in terms of dynamic strain aging that considered dislocation-bubble interactions. Irradiation of 1.0 × 1017 He+·cm−2, however, resulted in a non-monotonic h -dependence of m , with the bottom located at a turning point of h ∼50 nm. Coupling effect of the layer thickness and the applied fluence on irradiation hardening and plastic deformation characteristics was quantitatively elucidated by employing a strengthening model and a thermally activated model, respectively, where parameters of the characteristic microstructural features, i.e., the layer thickness and irradiation (He) defects, were both included. Graphical abstract Image 1 Highlights • The Mo/Zr NMMs with smaller h display the greater radiation resistance with a lower bubble density. • Under the high-fluence irradiation, a critical thickness h (∼15 nm) exists below which the irradiation hardening becomes more significant. • The low-fluence irradiated Mo/Zr NMMs present the negative SRS m , while the high-fluence irradiated Mo/Zr NMMs present the non-monotonic size-dependent positive SRS m. [ABSTRACT FROM AUTHOR]

The buckling strain ε B of Cr films and Cu films exhibits opposite film thickness h dependence: the ε B of Cr films decreases monotonically, while the ε B of Cu films increases monotonically with the increasing h . The delamination of Cr films follows the strain energy criterion while the Cu films are buckle-limited. The ε B of Cu/Cr nanostructured metallic multilayers (NMMs), much higher than that of Cr films, increases as the modulation period λ increases. A modified strain energy criterion involving interfacial adhesion and plastic deformation has been developed to describe the evolution of ε B of Cu/Cr NMMs with λ . [ABSTRACT FROM AUTHOR]

Abstract: Single-crystal Sb doped ZnO nanowires were fabricated on Si (100) substrate by a chemical vapor deposition method. The photoluminescence properties of ZnO nanowires were studied with the temperature ranging from 81 to 306K. At 81K, the recombination of the acceptor-bound exciton was predominant in PL spectrum, which was attributed to the transition of the (SbZn–2VZn) complex bound exciton. The activation of A0X and the acceptor binding energy had been calculated to be 16.8 and 168meV, respectively. [Copyright &y& Elsevier]

Abstract: A kinetic analysis was done in the nano-MgH2 and MgH2/Me x O y -nanocomposite (Me x O y =V2O5, Cr2O3 and Fe3O4) using a new kinetic model proposed recently in our group. The summarized data of hydriding rate are very well fitted using the new model, and it indicates that the rate-controlling step is the hydrogen diffusion through the hydride phase. Compared with the characteristic absorption time of these nano-MgH2 and MgH2/Me x O y -nanocomposite, the difference in the kinetic property was investigated in an explicit analytic form and the order of reaction rate and the optimal amount of oxide catalyst Cr2O3 can be determined exactly. These facts support that our model is reasonable in the nano-MgH2 and MgH2/Me x O y -nanocomposite system. [Copyright &y& Elsevier]

Nanocrystalline ZnO films have been grown by cathodic electrodeposition from dimethylsulfoxide solutions containing ZnCl2 on Si substrates at room temperature. The effects of ZnCl2 concentration on the structural and optical properties of the films were studied. The mean grain size of ZnO particles increases with the increase of ZnCl2 concentration. The exciton emission dominated in the photoluminescence spectra. [Copyright &y& Elsevier]