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The creep behavior of free-standing Cu films with bimodal grain size distribution was investigated at room temperature. It is found that the higher loading rate, the greater the primary creep strain rate and the smaller the steady-state creep strain rate. This unique creep behavior was explained by considering the competition between dislocation activities and dislocation stress fields. Given the grain size distribution, a modified phase-mixture model is proposed to predict the scaling behavior of creep rate and applied stress.

For small-scale metallic single crystals, systematic studies have established that external sample size ( ϕ ) has an obvious influence on the apparent strength, following a “smaller is stronger” fashion. In the present nanocrystalline (NC) Zr micropillars, deformation crossover leads to the strongest external size emerging at a critical ϕ ≈ 400 nm , similar to that of bulk-scaled NC metals. Above the critical external size the NC Zr pillars failed via highly inhomogeneous single-shear offset, below which they deformed via relative homogeneous shear.

The initial plasma and its spread speed have obvious influence on the dynamic characteristics of triggered vacuum switch (TVS). A special sensor is designed for measuring the initial charge of the trigger device of TVS in this paper, and the initial plasma diffusion characteristics are researched. The experimental results show that the diffusion velocity of the initial plasma charge increases with the increase of the steepness of the trigger current. When trigger current steepness increases 2 kA/μs, the maximum diffusion velocity of initial plasma increases reached to 150 cm/μs about.

Lightning can cause severe damage to aircraft, lightning direct effect test is very important for research of spacecraft material, design of structure of aircraft, but the limitation of the measuring technology and special test apparatus has become the technical bottleneck. Lightning current components for lightning direct effect test consist of component A (initial strode), component B (Intermediate current), component C (Continuing current) and component D (Restrike). Lightning current components A and D can be produced by the type of RLC circuit and crowbar circuit. The efficiency of the conventional RLC circuit in generating long duration time impulse current wave is relative low. To solve this problem, a replaceable electrodes triggered vacuum switch is designed and researched and then a high efficiency crowbar circuit which produces lightning current component A is designed in this paper.

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.

We compare the indentation hardness of crystal/crystal (C/C) Cu/Zr multilayers and the crystal/amorphous (C/A) Cu/Cu–Zr multilayers with equal individual layer thicknesses spanning from 1 to 200 nm. Both systems show similar layer thickness dependent evolution of hardness. At smaller layer thickness the C/C multilayers show softening, while no softening is observed in the C/A multilayers. The layer thickness dependent hardness is quantitatively estimated by using dislocation models. The significance of crystalline (Zr) layer and amorphous (Cu–Zr) layer on plastic deformation mechanisms is discussed.

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 5 nm to 250 nm. H is gradually increased with reducing λ down to ∼50 nm, 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 ∼50 nm. 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.

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 300 nm, and was analyzed by using a thermodynamic model. As λ decreases, the strength of multilayers increase and approach saturation of ∼2.88 GPa 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.

The strain hardening behavior of nanolayered Cu/Zr micropillars was studied using the microcompression technique, which revealed that both the strain hardening exponent and the strain hardening rate are controlled by the intrinsic rather than extrinsic size. The maximum strain hardening rate is observed at a layer thickness of ∼20 nm, which can be explained physically in terms of reduced dislocation storage rates. A dislocation model is modified to elucidate the dislocation storage in the nanolayered materials and predicts a critical intrinsic size below which the strain hardening rate drops off.

The mechanical behavior of incoherent Cu/Zr multilayers was studied in uniaxial compression experiments using micropillars with individual layer thicknesses (h) ranging from 5 to 100 nm. The deformation behavior of these micropillars are size dependent, transiting from dislocation dominated symmetrical slip at large h to shear localization induced by asymmetric slip and grain boundary mediated deformation at small h. During compression studies the multilayer micropillars exhibit a transition from strain hardening to shear softening at small h, and work softening at greater h. A maximum strain hardening rate is observed at a critical h of 20 nm, which was explained in terms of a transition from dislocation interactions to cross-slip of dislocations. The mechanical strength of the micropillars is also dependent on h, which was quantitatively analyzed using the confined layer slip model. In addition, the influence of pillar diameter on the mechanical behavior is also investigated. The effect of extrinsic size on the deformation mechanisms is discussed with respect to the intrinsic size effect with variation in h.

A general, approximate solution for the configuration force (CF) between crack and an inclusion of arbitrary shape is developed. The solution is obtained by treating an inhomogeneous inclusion as a homogeneous one with transformation strain according to Eshelby equivalent inclusion theory. Then the CF associated with the transformation is computed from the work done during the transformation, which is taken as driving force for crack deflection and interfacial debonding in composite materials. The present solution provides a novel approach to interpret the important phenomenology of the crack deflection and interfacial debonding observed in composite materials.

The length scale dependent mechanical fatigue behaviors of nanocrystalline (NC) Cu thin films were systematically investigated in terms of the yield strength related deformation mechanisms. Maxima are observed for both the yield strength and fatigue lifetime. The variation of strength as well as the changes of fatigue damage suggests the deformation mechanism transition from dislocation-based to grain boundary-mediated as the characteristic dimensions reduce down to below a critical value. The optimized fatigue properties can be achieved by best combination strength and ductility.

Intermittently plastic compression tests of the Zr 52.5 Cu 17.9 Ni 14.6 Al 10 Ti 5 bulk metallic glass were performed under a geometrically constrained condition at different cross-head displacement rates. A flow stress-overshoot phenomenon was observed during the macroscopically plastic deformation. It was found that there is a close correlation between the stress-overshoot intensity and the spatial density of shear bands. The stress-overshoot intensity can be used as a parameter to characterize the ability of plastic flow initiation caused by the spatial nucleation of shear bands in the bulk metallic glass. The micromechanisms of shear band multiplication were discussed.

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 10 nm to 250 nm) 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.

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 = 25 nm is much sharper than that for λ = 50 nm, 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.

In Cu/Cr multilayers with modulation period (λ) ranging from 10 to 250 nm, maxima are observed for both tensile ductility and fracture at a critical λ ∼ 50 nm, 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.

The influence of the semi-solid forging temperature between 1000 and 1100 °C on the microstructure and mechanical properties of a new α+Ti 2 Cu alloy was investigated. The results indicated that more Ti 2 Cu tended to precipitate on grain boundaries at a higher forging temperature, and finally formed a network structure after forging at 1100 °C. The precipitation was found to be controlled by both peritectic and eutectoid reactions. The elevated temperatures resulted in more liquid along the prior grain boundaries, which increased the peritectic precipitation in this region and formation of precipitation zones during re-solidification. In addition, the liquid served to relax the stress concentrations caused by dislocation pile-ups within the grains and provided more nucleation sites for eutectoid Ti 2 Cu precipitates on grain boundaries. Hardness and room temperature tensile properties decreased as the forging temperature increased, and intergranular fractures were observed after semi-solid forging at 1050 and 1100 °C, which is also attributed to the grain boundary network obtained by semi-solid forging.

We perform systematic calculations to study the competition between the dislocation nucleation and void formation as stress relaxation mechanism in Cu interconnects under thermal stress, which is related to the aspect ratio (ratio of the film thickness to width) of the Cu lines. It is quantitatively found from both elastic-perfectly plastic model and kinematic strain hardening model that there exists a critical aspect ratio, below and above which the stress relaxation is dominated by the dislocation nucleation and void formation, respectively. The critical aspect ratio is revealed to modulate by both the length scale of the interconnects and the interfacial strength between the Cu lines and surroundings, suggesting potential application to achieve artificial controlling on stress relaxation mechanism in Cu lines. Calculations are in good agreement with available experiments.

The influence of prior cold deformation on precipitating of alpha phase as well as the variation of hardness during aging has been investigated in solution-treated Ti-10Mo-8V-1Fe-3.5Al alloys. The results show that alpha phase precipitation could be obviously accelerated by the prior cold deformation. In the predeformed samples, a network pattern structure was observed with an optical microscope after aging treatment. It could be attributed to the phenomenon that the plate-shape alpha precipitates prefer to nucleate and grow in the regions with a high density of dislocations, especially inside slip bands. The hardness of both the predeformed and undeformed TB3 specimens after different aging times was measured and further predicted by a proposed strengthening model based on the grain refinement mechanism of the beta phase. The predicted results are consistent with the experimental results, especially in the later aging stages.

Thermal fatigue of Cu interconnects 60 nm thick and 5-15 mu m wide was investigated by using alternating current to generate cycling temperature and strain/stress. The fatigue curves exhibit two regions, i.e. high- and low-cycle regions, which correspond to low and high thermal strains, respectively. The high-cycle region is found to be controlled by all unusual thermal Fatigue mechanism of damage hands, which is related to a unique structure comprising only a single layer of grains distributed along the thickness. (C) 2008 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

The compressive deformation and fracture behaviors of a Zr47Cu46Al7 bulk metallic glass (BMG) and its composites with different microstructures were investigated. For the fully amorphous alloy, due to the formation of coarse primary shear bands and multiple secondary shear bands, it exhibited a high plasticity up to 14.5% with a 'work softening' during compressive test. With the increase in the volume fraction of primary crystallizing phases, the BMG composites displayed low strength and poor plasticity even failing in a brittle mode. Furthermore, the effects of microstructures on the compressive deformation and fracture behaviors of the BMG composites were discussed. (c) 2007 Published by Elsevier B.V.

Cu46Zr47Al7 bulk metallic glass (BMG) and its composites in plate with different thicknesses up to 6 mm were prepared by copper mold casting. Primary crystallizing phases with different microstructures and volume fractions could be obtained under different cooling rates, forming some composites with different mechanical properties. Under compression tests, the 2-mm-thick monolithic BMG has a yield strength of 1894 MPa and a high fracture strength of up to 2250 MPa at plastic strain up to 6%, exhibiting apparent “work-hardening” behavior. The 4-mm-thick Cu46Zr47Al7 BMG composite containing martensite phase yields at 1733 MPa and finally fails at 1964 MPa with a plastic strain of 3.7%.

The ductile–brittle transition of pure Al sheet, sandwiched by 2024 Al alloy through explosive cladding, was investigated at decreasing thicknesses through experimental and numerical methods. The results showed that this transition resulted from confined plastic deformation and consequent high triaxial tensile stress in the Al interlayer.

The microstructural evolution process of previously deformed ZA27 alloy during partial remelting was studied. After being compressed by a ratio of 20% in length at 250 °C, the microstructure of ZA27 alloy changed from developed dendrites to coarsened short dendrites and near-equiaxed grains, then in turn to united pieces, big lumps, small pieces, polygon particulates, and finally to spherical grains that would grow and agglomerate and become larger in size and irregular in shape during following partial remelting at semi-solid temperature of 460 °C. The structure evolution resulted from the eutectics first melted between boundaries of the primary grains and then penetrated into polygon boundaries that formed due to recrystallization, resulting in the separation of the primary grains into small polygon grains, followed by spheroidizing. This process was mainly discussed through analyzing the phase transformations occurred during partial remelting.

Extended abstract of a paper presented at Microscopy and Microanalysis 2011 in Nashville, Tennessee, USA, August 7–August 11, 2011.

For polymer-supported metal thin films used in flexible electronics, the definition of the fatigue lifetime at microcrack nucleation (FLMN) should be more physically meaningful than all the previous definitions at structural instability. In this paper, the FLMN of Cu films (with thickness from 100 nm to 3.75 µm) as well as Al thin films (from 80 to 800 nm) was experimentally characterized at different strain ranges and different thicknesses by using a simple electrical resistance measurement (ERM). A significant thickness dependence was revealed for the FLMN and a similar Coffin–Manson fatigue relationship observed commonly in bulk materials was found to be still operative in both the films. Microstructural analyses were carried out to verify the feasibility of ERM correspondingly.