In situ TEM straining of single crystal Au films on polyimide: Change of deformation mechanisms at the nanoscale

In situ transmission electron microscopy straining experiments were performed on 40, 60, 80 and 160 nm thick single crystalline Au films on polyimide substrates. A transition in deformation mechanisms was observed with decreasing film thickness: the 160 nm thick film deforms predominantly by perfect dislocations while thinner films deform mainly by partial dislocations separated by stacking faults. In contrast to the 160 nm thick film, interfacial dislocation segments are rarely laid down by threading dislocations for the thinner films. At the late stages of deformation in the thicker Au films prior to fracture, dislocations start to glide on the (0 0 1) planes (cube-glide) near the interface with the polymer substrate. The impact of size-dependent dislocation mechanisms on thin film plasticity is addressed.     

Intrinsic and extrinsic size effects on deformation in nanolayered Cu/Zr micropillars: From bulk-like to small-volume materials behavior

By using microcompression methodology, deformation of nanolayered Cu/Zr micropillars was systematically investigated within wide ranges of intrinsic layer thickness (5–100 nm) and extrinsic sample size (300–1200 nm). The intrinsic size effect, extrinsic size effect and their interplay were respectively revealed. Competition between the intrinsic and extrinsic size effects leads to experimental observation of a critical layer thickness of ～20 nm, above which the deformation is predominantly intrinsic-size-related and insensitive to sample size, while below which the two size effects are comparable. The underlying deformation mechanisms were proposed to transform from bulk-like to small-volume materials behavior. Deformation mode is correspondingly transited from homogeneous extrusion/barreling to inhomogeneous shear banding, but the two competing modes coexist in the layer thickness range from ～50 to 20 nm. In the regime of shear deformation, the extrinsic size dependence is displayed in that the deformation was controlled by shear bands nucleation in larger pillars while controlled by shear bands propagation in smaller pillars. A deformation mode map is developed to clearly elucidate the coupling intrinsic and extrinsic size effects on the deformation mode of nanolayered pillars.     

Atomistic investigation of the effects of temperature and surface roughness on diffusion bonding between Cu and Al

Molecular dynamics (MD) simulations are carried out to analyze the diffusion bonding at Cu/Al interfaces. The results indicate that the thickness of the interfacial layer is temperature-dependent, with higher temperatures yielding larger thicknesses. At temperatures below 750 K, the interface thickness is found to increase in a stepwise manner as a function of time. At temperatures above 750 K, the thickness increases rapidly and smoothly. When surface roughness is present, the bonding process consists of three stages. In the first stage, surfaces deform under stress, resulting in increased contact areas. The second stage involves significant plastic deformation at the interface as temperature increases, resulting in the disappearance of interstices and full contact of the surface pair. The last stage entails the diffusion of atoms under constant temperature. The bonded specimens show tensile strengths reaching 88% of the ideal Cu/Al contact strength.     

Transition from homogeneous-like to shear-band deformation in nanolayered crystalline Cu/amorphous Cu–Zr micropillars: Intrinsic vs. extrinsic size effect

The microcompression method was used to investigate the compressive plastic flow behavior of nanolayered crystalline/amorphous (C/A) Cu/Cu–Zr micropillars within wide ranges of intrinsic layer thicknesses (h ～ 5–150 nm) and extrinsic sample sizes (350–1425 nm) with the goal of revealing the intrinsic size effect, extrinsic size effect and their interplay on the plastic deformation behavior. The nanolayered C/A micropillars exhibited deformation behaviors of strain-hardening followed by strain-softening that were dependent on the thickness of the layers. At h ? 10 nm, the strain-softening is related to shear deformation that is caused by fractures in the amorphous layers. At h > 10 nm, however, the strain-softening is related to the reduction in dislocation density caused by dislocation absorption. Correspondingly, the deformation mode of the C/A micropillars transitioned from homogeneous-like to shear band type as h decreased to the critical value of ～10 nm, which is indicative of a significant intrinsic size effect. The extrinsic size effect on the plastic deformation also became remarkable when h was less than ～10 nm, and the interplay between the intrinsic and extrinsic size effects leads to an ultrahigh strength of ～4.8 GPa in the C/A micropillars, which is close to the ideal strength of Cu and considerably greater than the ideal strength of the amorphous phase. The underlying strengthening mechanism was discussed, and the transition in deformation mode was quantitatively described by considering the strength discrepancy between the two constituent crystalline and amorphous layers at different length scales.     

High temperature micropillar compression of Al/SiC nanolaminates

The effect of the temperature on the compressive stress–strain behavior of Al/SiC nanoscale multilayers was studied by means of micropillar compression tests at 23 °C and 100 °C. The multilayers (composed of alternating layers of 60 nm in thickness of nanocrystalline Al and amorphous SiC) showed a very large hardening rate at 23 °C, which led to a flow stress of 3.1 ± 0.2 GPa at 8% strain. However, the flow stress (and the hardening rate) was reduced by 50% at 100 °C. Plastic deformation of the Al layers was the dominant deformation mechanism at both temperatures, but the Al layers were extruded out of the micropillar at 100 °C, while Al plastic flow was constrained by the SiC elastic layers at 23 °C. Finite element simulations of the micropillar compression test indicated the role played by different factors (flow stress of Al, interface strength and friction coefficient) on the mechanical behavior and were able to rationalize the differences in the stress–strain curves between 23 °C and 100 °C.     

The influences of temperature and microstructure on the tensile properties of a CoCrFeMnNi high-entropy alloy

An equiatomic CoCrFeMnNi high-entropy alloy, which crystallizes in the face-centered cubic (fcc) crystal structure, was produced by arc melting and drop casting. The drop-cast ingots were homogenized, cold rolled and recrystallized to obtain single-phase microstructures with three different grain sizes in the range 4–160 μm. Quasi-static tensile tests at an engineering strain rate of 10^?3 s^?1 were then performed at temperatures between 77 and 1073 K. Yield strength, ultimate tensile strength and elongation to fracture all increased with decreasing temperature. During the initial stages of plasticity (up to ～2% strain), deformation occurs by planar dislocation glide on the normal fcc slip system, {1 1 1}〈1 1 0〉, at all the temperatures and grain sizes investigated. Undissociated 1/2〈1 1 0〉 dislocations were observed, as were numerous stacking faults, which imply the dissociation of several of these dislocations into 1/6〈1 1 2〉 Shockley partials. At later stages (～20% strain), nanoscale deformation twins were observed after interrupted tests at 77 K, but not in specimens tested at room temperature, where plasticity occurred exclusively by the aforementioned dislocations which organized into cells. Deformation twinning, by continually introducing new interfaces and decreasing the mean free path of dislocations during tensile testing (“dynamic Hall–Petch”), produces a high degree of work hardening and a significant increase in the ultimate tensile strength. This increased work hardening prevents the early onset of necking instability and is a reason for the enhanced ductility observed at 77 K. A second reason is that twinning can provide an additional deformation mode to accommodate plasticity. However, twinning cannot explain the increase in yield strength with decreasing temperature in our high-entropy alloy since it was not observed in the early stages of plastic deformation. Since strong temperature dependencies of yield strength are also seen in binary fcc solid solution alloys, it may be an inherent solute effect, which needs further study.     

Microstructure and strengthening mechanisms in Cu/Fe multilayers

Nanostructured Cu/Fe multilayers on Si (1 1 0) and Si (1 0 0) substrates were prepared by magnetron sputtering, with individual layer thicknesses h varying from 0.75 to 200 nm. The growth orientation relationships between Cu and Fe at the interfaces were determined to be of the Kurdjumov–Sachs and Nishiyama–Wasserman type. Nanoscale columnar grains in Fe, with an average grain size of 11–23 nm, played a dominant role in the strengthening mechanism when h ? 50 nm. At smaller h the hardness of Cu/Fe multilayers with (1 0 0) texture approached a peak value, followed by softening due to the formation of fully coherent interfaces. However, abundant twins were observed in Cu/Fe films with (1 1 1) texture when h = 0.75 nm, which led to the retention of high hardness in the multilayers.     

Size effects on yield strength and strain hardening for ultra-thin Cu films with and without passivation: A study by synchrotron and bulge test techniques

We present a systematic study of the mechanical properties of different Cu, Ta/Cu and Ta/Cu/Ta films systems. By using a novel synchrotron-based tensile testing technique isothermal stress–strain curves for films as thin as 20 nm were obtained for the first time. In addition, freestanding Cu films with a minimum thickness of 80 nm were tested by a bulge testing technique. The effects of different surface and interface conditions, film thickness and grain size were investigated over a range of film thickness up to 1 μm. It is found that the plastic response scales strongly with film thickness but the effect of the interfacial structure is smaller than expected. By considering the complete grain size distribution and a change in deformation mechanism from full to partial dislocations in the smallest grains, the scaling behavior of all film systems can be described correctly by a modified dislocation source model. The nucleation of dissociated dislocations at the grain boundaries also explains the strongly reduced strain hardening for these films.     

Characteristics of clad aluminum hollow billet prepared by horizontal continuous casting

The 3003/4045 clad hollow billets are prepared in the present study. Microstructures, solute distribution and bonding strength of the interfacial regions were investigated. The effects of plastic deformation on the evolution of microstructure and microhardness of the interfaces were also studied. The results show that metallurgical bonding between the solid and liquid Al alloys can be obtained with optimal parameters. Si and Mn atoms diffuse across the interface to form a diffusion layer with the thickness about 30 μm on average. The mean tensile-shear strength of as-cast clad hollow billet is 85.3 ± 9.2 MPa, and the strength of the interface is higher than that of 3003 alloy. Incompatible deformation between 3003 and 4045 layers occurs during rolling processes, and the needle-like Si phase transforms to the dispersive particles. The gradient distribution of microhardness across the interface is retained after the deformation.     

Temperature dependent tensile and fracture behaviors of Mo–10 wt.% Cu composite

Uniaxial tension tests are carried out for the Mo–10 wt.% Cu (Mo–10Cu) composite under a scanning electron microscope (SEM) at a temperature range from 25 °C to 725 °C. The stress–strain curves are obtained with both the tensile strength and the fracture strain peaked at 500 °C. Further raise of temperature would reduce the tensile strength and the fracture strain. In-situ SEM observations reveal the microstructure characteristics for Mo–10Cu composite at different temperatures. The fracture is of brittle inter-granular type when uni-axially tensioned at room temperature. As the temperature increases, formation of slip bands and linkage of micro-voids via plastic shear are observed. The fracture is characterized by mixed inter-granular fracture and plastic shear. The fracture is of predominantly plastic shear when uni-axially tensioned at 500 °C. Under uniaxial tension at temperatures higher than 650 °C, Mo–10Cu composite embrittles due to the insolubility of molybdenum and copper, and the activated grain boundary diffusion of Cu. These results are of importance for the basic understanding of the microstructure–mechanical properties relationship, as well as for the evaluation of Mo–Cu composites in practical applications.     

Mechanical behavior of nanoscale Cu/PdSi multilayers

Berkovich nanoindentation and uniaxial microcompression tests have been performed on sputter-deposited crystalline Cu/amorphous Pd_0.77Si_0.23 multilayered films with individual layer thicknesses ranging from 10 to 120 nm. Elastic moduli, strengths and deformation morphologies have been compared for all samples to identify trends with layer thicknesses and volume fractions. The multilayer films have strengths on the order of 2 GPa, from which Cu layer strengths on the order of 2 GPa can be inferred. The high strength is attributed to extraordinarily high strain hardening in the polycrystalline Cu layers through the inhibition of dislocation annihilation or transmission at the crystalline/amorphous interfaces. Cross-sectional microscopy shows uniform deformation within the layers, the absence of delamination at the interfaces, and folding and rotation of layers to form interlayer shear bands. Shear bands form where shear stresses are present parallel to the interfaces and involve tensile plastic strains as large as 85% without rupture of the layers. The homogeneous deformation and high strains to failure are attributed to load sharing between the amorphous and polycrystalline layers and the inhibition of strain localization within the layers.     

Controlling Al/Cu composite diffusion layer during hydrostatic extrusion by using colloidal Ag

In this study, intermetallic compound formation at the interface between aluminum and copper during hydrostatic extrusion was simulated by performing a solid state diffusion bonding experiment with various processing parameters, including bonding temperature and pressure and holding time, and by inserting an Ag colloid layer between the aluminum and copper. Regression equations were developed to predict thickness of diffusion layer and interface hardness.An intermetallic compound formed at the interface between the Al and Cu during diffusion bonding at 420 °C and 240 MPa for 60 min, and it was effectively controlled by inserting an Ag colloid. These experimental data will be useful for setting up processing parameters to prepare Al/Cu matrix composite materials by using hydrostatic extrusion.     

Microstructure and mechanical properties of Laves phase-reinforced Fe–Zr–Cr alloys

Multi-phase Fe_90?xZr₁₀Cr_x alloys with 0 ≤ x ≤ 10 containing cubic C15 and hexagonal C14/C36 Laves phases have been prepared by copper mold casting. The microstructure of the samples consists of micrometer-sized Laves phase particles embedded in an ultrafine eutectic matrix of alternating lamellae of α-Fe and Laves phases. Room temperature compression tests of the binary alloy reveal a high strength of 1900 MPa combined with a plastic strain of about 9%. The addition of Cr improves the plastic strain up to 17% while reducing the strength only by about 70 MPa. The increased plastic deformation is linked to the specific structural features of the Laves phases. For the binary alloy, shearing and crack formation within the C15 phase limits plastic deformation. In contrast, in the samples containing Cr no shearing occurs within the C14/C36 phases and crack formation, which is observed at the particle/ferrite interface, is retarded.     

Room temperature interfacial reactions in electrodeposited Au/Sn couples

Two Au/Sn couples electrodeposited on metallized Si wafers, a Au/Sn couple (Au deposited first) and a Sn/Au couple (Sn deposited first), were employed to study the room temperature interfacial reactions between Au and Sn. The electroplating sequence has a significant effect on the phases and the microstructures of the reaction regions. AuSn₄ and AuSn are formed in the as-deposited Au/Sn couples, while Au₅Sn and AuSn are formed in the as-deposited Sn/Au couples. Upon ageing, AuSn₂ formation depends on the Sn (Au) content (i.e. the Sn (Au) layer thickness) in the Au/Sn (Sn/Au) couples. In aged Au/Sn couples, with 1.5 μm of Au and 3 μm of Sn, AuSn, AuSn₂ and AuSn₄ are sequentially distributed in the reaction region with fine Kirkendall voids at the AuSn/Au interface. In the aged Sn/Au couples, with 15 μm of Sn and 1.5 μm of Au, AuSn is distributed on either side of the original Sn/Au interface, while Au₅Sn only exists on the Au side. The presence of a thin interfacial SnO₂ film significantly affects phase formation and reaction kinetics.     

Effect of specimen thickness on the creep response of a Ni-based single-crystal superalloy

Creep tests on Ni-based single-crystal superalloy sheet specimens typically show greater creep strain rates and/or reduced strain or time to creep rupture for thinner specimens than predicted by current theories, which predict a size-independent creep strain rate and creep rupture strain. This size-dependent creep response is termed the thickness debit effect. To investigate the mechanism of the thickness debit effect, isothermal, constant nominal stress creep tests were performed on uncoated PWA1484 Ni-based single-crystal superalloy sheet specimens of thicknesses 3.18 and 0.51 mm under two test conditions: 760 °C/758 MPa and 982 °C/248 MPa. The specimens contained initial microvoids formed during the solidification and homogenization processes. The dependence of the creep response on specimen thickness differed under the two test conditions: at 760 °C/758 MPa there was a reduction in the creep strain and the time to rupture with decreasing section thickness, whereas at 982 °C/248 MPa a decreased thickness resulted in an increased creep rate even at low strain levels and a decreased time to rupture but with no systematic dependence of the creep strain to rupture on specimen thickness. For the specimens tested at 760 °C/758 MPa microscopic analyses revealed that the thick specimens exhibited a mixed failure mode of void growth and cleavage-like fracture while the predominant failure mode for the thin specimens was cleavage-like fracture. The creep specimens tested at 982 °C/248 MPa in air showed the development of surface oxides and a near-surface precipitate-free zone. Finite-element analysis revealed that the presence of the alumina layer at the free surface imposes a constraint that locally increases the stress triaxiality and changes the value of the Lode parameter (a measure of the third stress invariant). The surface cracks formed in the oxide scale were arrested by further oxidation; for a thickness of 3.18 mm the failure mode was void nucleation, growth and coalescence, whereas for a thickness of 0.51 mm there was a mixed mode of ductile and cleavage-like fracture.     

Comparisons between homogeneous boundaries and heterophase interfaces in plastic deformation: Nanostructured Cu micropillars vs. nanolayered Cu-based micropillars

Both the homogeneous boundaries and the heterophase interfaces play important roles in crystalline plasticity as they often serve as obstacles for dislocation motion, as well as dislocation sources/sinks. In this work, microcompression tests were carefully performed to explicitly identify the relevant plasticity mechanisms of nanostructured micropillars with five distinct nanostructures: (i) Cu nanotwinned multicrystalline micropillars; (ii) Cu nanocrystalline multicrystalline micropillars; (iii) Cu/X (X = Cr, Zr) nanotwinned nanolayered micropillars; (iv) Cu/X nanocrystalline nanolayered micropillars; and (v) Cu/Cu–Zr crystalline/amorphous nanolayered micropillars. By characterizing their stress–strain response and evolution of strain-rate sensitivity (SRS) with strain, our findings elucidate the effects of homogeneous boundaries, heterophase interfaces and their coupling effects on the plastic yield, and reveal the fundamentally different roles these perform in the rate-limiting process of nanomaterials. In sharp contrast to the normal strain-dependent SRS in nanostructured Cu micropillars with homogeneous boundaries that monotonically decreases with increasing strain, nanolayered Cu-based micropillars with heterophase interfaces exhibit inverse strain-dependent SRS that monotonically increases with increasing strain. These expected (normal) and unexpected (inverse) SRSs are quantitatively explained by a dislocation model in terms of the strain-related dislocation mean free path. These findings provide valuable insights into our understanding of the fundamental roles that homogeneous boundaries and heterophase interfaces play in plastic deformation.     

Achieving maximum hardness in semi-coherent multilayer thin films with unequal layer thickness

Sources of plastic strengthening in [0 0 1] epitaxial Cu/Ni multilayer thin films are examined using measurements of in-plane lattice parameter and hardness (H) for films of different bilayer period (Λ) and Ni volume fraction (% Ni). Similar to other investigations, H for 50% Ni–50% Cu films increases with decreasing bilayer period down to Λ = 20 nm, where interfaces are coherent. A new finding is that H for semi-coherent films increases with % Ni. This strategy yields the largest reported H for this system (5.2 GPa for 60% Ni/40% Cu, Λ = 60 nm), showing that smaller is not always stronger. The rationale for the increased H is the development of a large interfacial dislocation density during the elasto-plastic transition to fully plastic yield. This strengthens Cu/Ni interfaces to slip propagation. The results are interpreted with a dislocation-based model that furnishes estimates of interfacial dislocation line energies, pinning strengths to confined layer slip, and interface barrier strengths to slip transmission.     

Fundamental differences in the plasticity of periodically twinned nanowires in Au,Ag, Al,Cu, Pb and Ni

The role played by nanoscale twins is becoming increasingly important in order to understand plasticity in nanowires synthesized from metals. In this paper, molecular dynamics simulations were performed to investigate the synergistic effects of stacking fault energy and twin boundary on the plasticity of a periodically twinned face-centered cubic (fcc) metal nanowire subjected to tensile deformation. Circular nanowires containing parallel (1 1 1) coherent twin boundaries (CTBs) with constant twin boundary spacing were simulated in Au, Ag, Al, Cu, Pb and Ni using different embedded-atom-method interatomic potentials. The simulations revealed a fundamental transition of plasticity in twinned metal nanowires from sharp yield and strain-softening to significant strain-hardening as the stacking fault energy of the metal decreases. This effect is shown to result from the relative change, as a function of the unstable stacking fault energy, between the stress required to nucleate new dislocations from the free surface and that to overcome the resistance of CTBs to the glide of partial dislocations. The relevance of our predictions to realistic nanowires in terms of microstructure, geometry and accuracy in predicting the generalized planar and stacking fault energy curves is also addressed. Our findings show clear evidence that the plastic flow of twinned nanowires under tension differs markedly between fcc metals, which may reconcile some conflicting observations made in the past.     

Nanoindentation of single-crystalline gold thin films: Correlating hardness and the onset of plasticity

Understanding the mechanical properties of materials with external dimensions on the nanometer scale is crucial for the design and fabrication of nanoelectronics and nanosystems. Metal thin films exhibit a size-dependent hardening effect that scales inversely with the film thickness down to 200 nm. The thickness range below 200 nm is mostly unexplored and initial experiments indicate a change in the scaling law. Here, the mechanical properties of single-crystalline Au films are investigated in the thickness range from 31 to 858 nm by nanoindentation. Maximum shear stresses at the onset of plasticity are determined by the finite element method. While the hardness increases with decreasing film thickness, as expected from macroscopic experiments, the onset of plasticity shifts to lower shear stresses for thinner films. These observations are interpreted with respect to detailed observations of the microstructures of the films investigated.     

Energy level diagrams of C60/pentacene/Au and pentacene/C60/Au

The electronic structures of pentacene and C₆₀ interfaces were investigated using ultraviolet photoelectron spectroscopy (UPS) and X-ray photoelectron spectroscopy (XPS). The magnitudes of measured interface dipole were 0.11 eV and 0.07 eV for the C₆₀ deposited on pentacene (C₆₀/pentacene) and the pentacene deposited on C₆₀ (pentacene/C₆₀), respectively. The obtained C 1s spectra on these samples show that no significant chemical bonds at the interface. The offsets of the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) at the C₆₀-pentacene interface were 1.29 eV and 0.89 eV for C₆₀/pentacene/Au, while for pentacene/C₆₀/Au they were 1.5 eV and 1.1 eV. In this paper we present the complete energy level diagrams of C₆₀/pentacene/Au and pentacene/C₆₀/Au.