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Zr/Mo and Zr/Ti nanostructured multilayers manifest a peak hardness at the critical layer thickness of ~10 and ~20 nm, respectively. The non-monotonical thickness-dependent hardness is understood in terms of the back stress, which is closely correlated with the dislocation absorption probability by the interfaces. It appears that strength softening occurs as gliding dislocations can be absorbed by interfaces at smaller layer thickness. Furthermore, incoherent interface in the Zr/Mo with discontinuous slip systems is more effective to block dislocation motion than the Zr/Ti with continuous ones, resulting in a higher dislocation storage ability.

Nanostructured crystalline Al/amorphous AlN multilayer films with a wide layer thickness (h) range from ∼10 nm up to ∼200 nm were prepared by using magnetron sputtering. Nanotwins and 9R phase were substantially observed in the Al layers, showing a strong thickness dependence. The 9R phase predominantly penetrated through the Al layer in the II regime of h ≤ ∼20 nm, while mainly terminated within the layer interior in the I regime of h > ∼20 nm. On the contrary, the coherent nanotwins were boosted when h > ∼20 nm and the percentage of twinned Al grains was greatly increased. The formation mechanisms of 9R phase and coherent nanotwins were discussed in terms of the interfacial chemistry/physics modulated by the amorphous AlN layers, which displayed gradient characteristics and hence was sensitive to the layer thickness. A significant thickness dependence of hardness was also evident that the hardness monotonically increased with reducing h in the I regime, while reached a peak value and hold almost unchanged in the II regime. The hardness in the II regime is about 1 GPa greater than the predictions from an interfacial barrier crossing model. This discrepancy is mainly contributed by the layer-penetrating 9R phase rather than the nanotwins. This study provides a new perspective on fabricating nanotwinned Al by utilizing heterophase interfaces.

This study reveals two-stage wrinkling behavior of Al films deposited on polydimethylsiloxane (PDMS) substrates by magnetron sputtering at a high temperature. The hierarchical structures with nano-micro dual scale wrinkles were created in Al films. The micro-scale wrinkles originated from thermal contraction, with the wavelength increasing with increasing film thickness, while nano-scale wrinkles, with an almost unchanged wavelength, are induced by the expansion of the mixed layer on the PDMS surface. The results provide a facile method to produce the hierarchical wrinkled surfaces for functionality.

We report the formation of wrinkles with tunable wavelength in Ag films and Mo films deposited on polydimethylsiloxane (PDMS) and PDMS/Si substrates by magnetron sputtering at 50 °C. The PDMS-supported Ag films have the wrinkles with μm-sized wavelength, while the sub-μm-sized wrinkles are engendered in the Ag films on PDMS/Si substrates due to the constraint from the substrates. By contrary, Mo films with a similar thickness range exhibit wrinkles with μm-sized wavelength on the PDMS/Si substrates, much larger than that of Ag films on the same substrates. This indicates that the wrinkle wavelength in metal films depends closely on the substrates and the characteristics of metal films. The possible reasons for the discrepancy in wavelength between experiments and theoretical predictions are discussed. The wettability of the wrinkled surfaces is predominantly controlled by the length scale of the wrinkles. In the Ag films with sub-μm-sized wrinkles, droplet contact angle is highly sensitive to the surface roughness, i.e., increasing quickly with the surface roughness. However, in the PDMS-supported Ag and PDMS/Si-supported Mo films both with μm-sized wrinkles, the contact angle raises slowly with the surface roughness. It is thus evident that a wettability spectrum derived from the wrinkling pattern can be artificially developed by tuning the wavelength in different length scales through controlling of substrates, metal films, and film thicknesses.

The mechanical behavior of (Fe40Mn40Co10Cr10)100−xCx (x = 0, 2.2, 3.3 at.%) high entropy alloys (HEA) at various temperatures (298 K, 159 K and 77 K) was investigated. For Fe40Mn40Co10Cr10 HEA, the jointly activated twinning and martensitic phase transformation concurrently increase the strength and ductility at 77 K. In contrast, twinning and phase transformation gradually vanish as temperature decreases for carbon alloyed HEAs, resulting in ductile-brittle transition. Our findings indicate that the additional interstitial carbon atoms increase the f.c.c. phase stability, and inhibit the deformability of Fe40Mn40Co10Cr10 HEA at low temperature.

The buckling and post-buckling behavior of the Cu–Zr amorphous films is highly dependent on film thickness (h) and applied strain (e). A buckling map has been developed with h and e as axes, where four buckling modes are detected, including uncracked triangular buckle, cracked rectangular buckle, cracked triangular buckle, and patches. Formation of the four buckling modes will be discussed in terms of the film thickness and size-dependent deformation.

In this work, cohesive and adhesive properties of Ti thin films on polyimide substrates were studied systematically as a function of grain size by in situ optical microscopy, resistance measurements and synchrotron X-ray diffraction under uniaxial tensile testing. The characteristic of cohesive and adhesive behavior can be well reflected in the stress-strain curves determined by synchrotron X-ray diffraction. Generally, the yield strength decreases while the cracking strain increases with increasing grain size, due to the constraint effect of grain size on dislocation activity. Different from the yield strength, the fracture strength is almost unchanged with grain size. The fracture toughness is found to increase as the grain size increases, which is attributed to the enhanced plasticity during fracture process. Additionally, the interfacial adhesion energy is determined to be 3.8 J m−2. The buckling strain increases with increasing grain size as a result of enhanced plastic dissipation. The formation of patches, instead of buckles, in Ti film with smaller grain size is ascribed to the low deformation capability of the film.

Domains and domain walls are among the key factors that determine the performance of ferroelectric materials. In recent years, a unique type of domain walls, i.e., the sawtooth-shaped domain walls, has been observed in ${\mathrm{BiFeO}}_{3}$ and ${\mathrm{PbTiO}}_{3}$. Here, we build a minimal model to reveal the origin of these sawtooth-shaped domain walls. Incorporating this model into Monte Carlo simulations shows that (i) the competition between the long-range Coulomb interaction (due to bound charges) and short-range interaction (due to opposite dipoles) is responsible for the formation of these peculiar domain walls and (ii) their relative strength is critical in determining the periodicity of these sawtooth-shaped domain walls. Necessary conditions to form such domain walls are also discussed.

Nanostructured face-center-cubic/body-center-cubic Cu/(NbMoTaW) multilayers were prepared with equal layer thickness (h) spanning from 5 to 100 nm. The hardness was found to increase with reducing h to ~50 nm, following the Hall–Petch relation. Below this critical thickness, a hardness plateau emerged indicative of an interface barrier strengthening mechanism. The high density of misfit dislocations at interfaces facilitates the yielding of high entropy alloy NbMoTaW layers at stresses far less than the strength of Cu/(NbMoTaW) estimated by rule-of-mixture. Compared with reported multilayers, the present Cu/(NbMoTaW) samples show the size-independent hardness at a larger size-range h

Microcompressive tests were performed to investigate the intrinsic modulation effect and extrinsic diameter effect on strength and deformation behaviors of nanolaminated Cu/Cu-Zr crystalline/amorphous micropillars within a diameter D range from 300 to 1500 nm as well as a modulation ratio η range from 0.1 to 3.0. Experimental results revealed a transition of D-dependent deformation mode from homogeneous-like to shear localization at large η, while D-independent homogeneous extrusion of Cu at small η. Furthermore, the flow strength increased with raising D at small η, but decreased at large η, exhibiting a D-insensitive strength at a critical η* ~ 0.5. These unusual results were rationalized in terms of a coupling effect of dislocation actions and shear transformation zone mediated operations.

Alloying effect on tensile ductility of nanostructured Cu-X (X = Zr and W) thin films was studied in comparison. Both Zr and W atoms segregated at grain boundaries (GBs) and increased the GB cohesion energy, leading to similar increase of ductility in as-deposited Cu-X films. After annealing treatment, however, changes in ductility showed different alloying effect: the Cu-Zr one increased while the Cu-W one decreased when compared with their as-deposited counterparts. This discrepancy was rationalized by different microstructural evolution that intergranular CuZr amorphous layer was produced in the Cu-Zr film while intergranular W grains were formed in the Cu-W one.

Coarse-grained (CG), fine-grained (FG), and ultrafine-grained (UFG) Al-2.5 wt% Cu (Al-Cu) alloys were respectively prepared, with and without 0.3 wt% Sc addition, for comparison. The influences of minor Sc addition on the microstructural evolution, tensile mechanical properties and pitting corrosion resistance were systematically studied at different grain scales. A significant Sc microalloying effect on the precipitation was observed that the minor Sc addition promoted the dispersion of finer θ′-Al2Cu precipitates in the CG alloy and favored the intragranular θ′-Al2Cu precipitation in the FG and UFG alloys, with the smaller grain size leading to a stronger Sc microalloying effect. The Sc addition induced convincing increases in the yield strength at all the three grain scales, and improved (in the CG and UFG Al-Cu alloys) or retained (in the FG Al-Cu alloy) the pitting corrosion resistance at the same time. This indicates that the inverse strength-pitting corrosion correlation as usually observed can be broken by minor Sc addition. The strengthening mechanisms were discussed and the grain size-dependent pitting corrosion resistance mediated by the Sc addition was rationalized in terms of a competition between the positive influence derived from the interfacial Sc segregation and the negative influence come from the deformation-induced dislocations. The present findings provide a possible approach to break the inverse strength-pitting resistance correlation in heat-treatable Al alloys by modifying the precipitate/matrix interfaces through the suitable microalloying atom segregation.

The buckling strain e B of Cr films and Cu films exhibits opposite film thickness h dependence: the e B of Cr films decreases monotonically, while the e 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 e 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 e B of Cu/Cr NMMs with λ .

Al-XCu (X = 1.0, 1.5, and 2.5 wt%) alloys with and without 0.3 wt% Sc addition were prepared respectively. The effects of composition and heat treatment processes on the microstructural evolution were systematically investigated by using transmission electron microscope and atom probe tomography (APT). Both Al3Sc dispersoids and θ′-Al2Cu precipitates coexisted after artificial aging, and a strong Sc segregation at θ′-Al2Cu/matrix interfaces was detected and quantified through APT examinations. A Sc solute partitioning was demonstrated between in the Al3Sc dispersoids and at the θ′-Al2Cu/matrix interfaces, mediated by the Cu content. Increasing the Cu content, both the size and volume fraction of the Al3Sc decreased after solid solution treatment. As a result, the interfacial Sc segregation was accordingly intensified during subsequent aging treatment. A parameter of reduction in interfacial energy (Δγ), derived from APT analyses, was proposed to characterize the degree of interfacial Sc segregation. The coupling effect of Al3Sc dispersoids and θ′-Al2Cu precipitates on ductile fracture was discussed, where a micromechanical model was developed to describe a quantitative relationship between the fracture strain with Δγ as well as parameters of the Al3Sc dispersoids.

The effects of carbon content on the microstructure and room-temperature mechanical properties of Fe40Mn40Co10Cr10 high-entropy alloy (HEA) were systematically investigated. The results showed that heavy carbon alloyed HEA could possess supreme combination of high tensile strength (935 MPa) and high ductility (~ 74%). The excellent mechanical properties were ascribed to as follows: the high content interstitial carbon atoms strengthens the matrix greatly through suppressing dislocation motion and promoting the deformation-induced twinning at room temperature, which enhance the strength and ductility. Simultaneously, the ductility is further secured for single FCC structure maintained due to appropriate carbon alloying. Our findings provide a novel strategy for developing HEAs with excellent mechanical properties.

In this article, we propose a method to realize the pure shear deformation mode of dielectric elastomer (DE) membranes by tuning two in-plane prestresses. With utilization of carbon grease electrodes, VHB 4905 membranes are prestretched and attached into a retractable device, forming a pure-shear deformation controller. Experimental results demonstrate that, accurate pure shear deformation mode of DEs can be realized by tuning the mechanical loads in the two directions of the deformation controller. Furthermore, large deformation in the direction of free state can be achieved without electromechanical instabilities. The designed deformation controller accurately realizes the specific pure shear deformation mode of DEs and can be utilized to help design the practical soft actuators.

Experimental evidence demonstrated that equal channel angular pressing (ECAP) at cryogenic temperature, in comparison with ECAP at room temperature, led to promoted strength-ductility synergy in an Al-2.5wt.%Cu alloy. The simultaneous improvement is related to microstructural hierarchy of multimodal grains, low angle grain boundaries, and inter/intragranular precipitates, which was tuned by aging treatment in match with the low-temperature ECAP. The artificial aging could maintain multimodal grain size distribution, introduce a large number of low angle grain boundaries and produce intragranular precipitates to improve strength/ductility. A minor 0.3wt.% Sc addition was effective in optimizing the precipitations and further boosting the strength/ductility combination. The underlying mechanisms for higher strength and greater ductility were rationalized in terms of the low-temperature ECAP.

Dual precipitation in a microalloyed Al-2.4 wt% Cu-0.1 wt% Sc alloy was investigated in comparison with single precipitation in binary Al-2.4 wt% Cu and Al-0.1 wt% Sc alloys, respectively. Although Al3Sc nanoparticles are more readily produced at higher aging temperatures in the binary Al–Sc alloys, the Al3Sc precipitation in the Cu-added ternary alloy was completely suppressed at 723 K while unexpectedly promoted at 573 K-aging. Meanwhile, a significant Sc-dependence of θ ′ precipitation was also evidenced in the 573 K-aged Al–Cu-Sc alloy that the thickness of θ ′ precipitates kept almost unchanged even to prolonged time, resulting in a high aspect ratio of >200 for the θ ′ precipitates. The complex precipitation behaviors are rationalized in terms of strong interactions between the Sc and Cu solutes, which could be tailored to optimize the Al3Sc + θ ′ dual precipitation and hence to improve mechanical properties. Moreover, the Sc atoms fully stabilized in Al solid solution by Cu solute at 723 K was discussed, and the high strength in Al–Cu-Sc alloy with dual precipitation was evaluated by quantifying the strengthen contribution from different strengthening mechanism.

Pure molybdenum (PM) and La2O3 dispersion strengthened Mo alloy (ODS-Mo) were prepared by powder metallurgy through two different mixing method (solid-solid or SS mixing and solid-liquid or SL mixing). After annealed at 1050 °C for 1 h, the PM, SS-Mo and SL-Mo were compared in microstructure and mechanical properties. The microstructural examinations showed that the PM was mostly recrystallized with rather coarse grains, but the ODS-Mo alloys remained fine elongated grains. This discrepancy is due to a higher recrystallization temperature held in the ODS-Mo alloys. The uniaxial testing results showed that the ODS-Mo had higher tensile mechanical properties compared to the PM, which are attributed to the remarkable strengthening and ductilizing effect induced by the La2O3 particles. The low cycle fatigue (LCF) testing results revealed that the PM experienced cyclic hardening behaviors, while the ODS-Mo alloys exhibited cyclic softening behaviors. The Basquin-Manson-Coffin analyses results demonstrated that the ODS-Mo possessed a higher fatigue ductility and longer fatigue life than the PM. Two kinds of cracks, i.e., cleavage cracks and intergranular cracks, were experimentally observed to coexist on the fracture surface. In particular in the SL-Mo alloy, the two crack interacted and propagated forward until final fracture, leading to a step-like crack growth path and concomitantly enhanced fatigue ductility. In addition, different cycling dislocation structures were revealed between the PM and ODS-Mo alloys. Cycle fatigue mechanisms responsible for the fatigue behaviors and microstructure evolution during LCF testing were discussed. The best LCF resistance combined with superior uniaxial tensile mechanical properties found in the SL-Mo alloy were rationalized.

In this work, we investigate the plastic deformation characteristics (hardness, strain rate sensitivity and activation volume) of nanotwinned AlxCoCrFeNi (x = 0, 0.1, 0.3) high entropy alloy thin films (HEAFs). The extremely thin nanotwins with thickness of ~ 2 nm soften these HEAFs, but enhance their strain rate sensitivity. We combine the detwinning softening process with conventional strengthening mechanisms to elucidate the mechanical properties of AlxCoCrFeNi HEAFs. These findings provide deep insights into design strategies to explore the promising high entropy alloys, especially the nanotwinned HEAFs at micro- and nano-scale.

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.

The effect of pre-strain on the solute clustering, mechanical properties, and work-hardening of a naturally aged Al-Cu-Mg alloy was comprehensively investigated using a three-dimensional atom probe, electrical resistivity, and hardness measurements. The pre-strain promoted the rapid formation of solute clusters but suppressed the growth of solute clusters during prolonged aging. An increase in pre-strain caused a decrease in the saturated number density of solute clusters, leading to a reduction in cluster strengthening. By taking into account the coupling effect of solute clusters and pre-existing dislocations, models were proposed to address the yield strength and work-hardening behaviors of the Al-Cu-Mg alloy, respectively. It reveals that the solute-cluster strengthening is comparable to dislocation strengthening in the naturally aged alloy. However, the work hardening is not significantly affected by the presence of the solute clusters. The findings reported in this paper will be helpful for the development of a naturally aged Al-Cu-Mg alloy with improved performance by controlling the pre-strain.

The effects of the antiphase domain (APD) and the elastic energy on the morphology evolution of γ′ precipitates in Ni-Al binary alloy during aging process are studied with the phase-field method. The merging-splitting phenomenon between the two neighboring γ′ precipitates is observed. The changes of γ′ area fraction and average γ′ size are analyzed for different APDs and for different elastic energies, respectively. It is shown that the merging-splitting phenomenon may occur when the radii of neighboring γ′ precipitates with different APDs are smaller than 24 nm and the critical size for the merged γ′ precipitate splitting is about 108 nm. Besides, the coarsening of γ′ precipitates is significantly impeded by the APDs but accelerated by the elastic energy, and the change of average γ′ size can be divided into two stages with different mechanisms.

Due to their interface and nanoscale effects associated with structural peculiarities of nanostructured, face-centered-cubic (FCC) ultrafine-grained/nanocrystalline (UFG/NC) metals, in particular nanotwinned (NT) metals exhibit unexpected deformation behaviours fundamentally different from their coarse-grained (CG) counterparts. These internal boundaries, including grain boundaries and twin boundaries in UFG/NC metals, strongly interact with dislocations as deformation barriers to enhance the strength and strain rate sensitivity (SRS) of materials on the one hand, and play critical roles in their microstructural evolution as dislocation sources/sinks to sustain plastic deformation on the other. In this work, building on the findings of twin softening and (de)twinning-mediated grain growth/refinement in stretched free-standing NT–Ni foils, a constitutive model based on the thermally activated depinning process of dislocations residing in boundaries has been proposed to predict the steady-state grai...

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.

${\mathrm{CaCu}}_{3}{\mathrm{Ti}}_{4}{\mathrm{O}}_{12}$ (CCTO) has a large dielectric permittivity plateau near room temperature due to several dynamic processes. Here, we consider the combined effects of localized charge carriers (polarons) and conductive charge carriers using a recently proposed statistical model [Phys. Rev. B 96, 054115 (2017)] to fit and understand its permittivity measured at different frequencies over a broad temperature range. We found that, at the lowest temperature, the small permittivity is related to frozen polarons, and the increase at higher temperatures is associated with the thermal excitation of polarons that gives rise to the Maxwell-Wagner effect. The final rapid increase at the highest temperature is attributed to thermally activated conductivity. Such an analysis enables us to separate the contributions from localized polarons and conductive charge carriers and quantify their activation energies, which also explains the permittivity plateau near room temperature. In particular, we show that the subtle balance between the number of activated polarons and their polarizability causes CCTO to have a permittivity plateau with small dielectric loss.

Microstructural evolution and mechanical/electrical properties of aged Al-0.2 wt%Zr-0.05 wt%Sc alloys with different Si additions (0, 0.05, 0.10 and 0.15 wt%) were comparatively studied by using transmission electron microscopy, electrical conductivity measurements, and microhardness measurements. A significant Si addition effect was demonstrated that the Si addition accelerated the precipitation kinetics and enhanced the heterogeneous precipitation of Al3(Sc, Zr) nanoparticles. However, both the peak-aging temperature and peak microhardness during isochronal aging were insensitive to the Si addition. The underlying mechanism is related to a partitioning effect of Si among Sc, Zr and vacancy that produces a saturated concentration of Si-vacancy clusters to serve as heterogeneous nuclei. It was further manifested that dual-scale particles, i.e., Al3Zr dispersoids and Al3(Sc1-xZrx) nanoprecipitates in different length scales, could be produced by deliberately designing the heat treatment protocol. A coupling effect of the dual-scale particles led to an enhanced threshold stress in high-temperature creep testing, indicative of an improved creep resistance promoted by the minor Si addition. Moreover, the recrystallization resistance was seldom affected by Si addition, while slightly improved by introducing dual-scale particles.

As is known, the ambient environment, such as temperature and humidity, may affect the electromechanical properties of dielectric elastomer (DE). In this research, we carry out an analysis to explore the humidity effect on electromechanical performance of a viscoelastic DE, by prescribing the shear moduli, the viscous damping, and the relative dielectric constant of the DE are humidity-dependent values. The dependency relationships between these parameters and the relative humidity are obtained by fitting the presupposed models to experimental data. The humidity effect on static and dynamic electromechanical performances is explored by exhibiting the creeping deformation of both elastic and non-elastic stretch of the viscoelastic DE. Phase paths and Poincare maps are utilized to detect the humidity effect on the stability and periodicity of the nonlinear vibration. Finally, the variation tendency of resonant frequency of the viscoelastic DE system is investigated by continuously varying the relative humidity.

For multicomponent Al-based alloys, one of the most valuable approaches of microstructural design is to find a path to maximize the multiple microalloying effects while overcoming their negative counterparts. In this paper, triple Sc-Fe-Si microalloying was performed in an Al–Cu alloy to assemble the co-existence of θ′-Al2Cu and Al3Sc precipitates. Besides, the mutual interactions among triple microalloying elements were utilized to maximize the positive effects on tailoring the dual precipitates, as illustrated in two goals of microstructural design: (i). As to θ′-Al2Cu plates, the multiple Sc-Fe-Si segregation at θ′-Al2Cu/matrix interface was preferentially created in the alloy before creep (as-aged condition). During subsequent high-temperature creep, such interfacial warden will be rapidly reinforced by solute repositioning, as a process of accumulating solutes diffusing from both inner θ′-Al2Cu precipitate and outer matrix to limit the interfacial migration. (ii). For Al3Sc precipitates, the beneficial Si microalloying effect on encouraging the nucleation of Sc-rich entities (precursor of Al3Sc) was successfully acquired during aging, while the detrimental Si effect on accelerating Al3Sc coarsening is generally prohibited by tuning Fe–Si synergy within Al3Sc interior. The establishments of (i) and (ii) enable the satisfactory dispersion as well as the outstanding thermal stability of dual precipitates in the current Al-Cu-Sc-Fe-Si system, leading to a good creep resistance at a high homologous temperature of 0.61Tm ~ 300 °C (where Tm is the melting temperature of the α-Al matrix).

Creep tests were performed on the high stacking fault energy (SFE) nanotwinned (NT) Ni free-standing foils with nearly the same twin thickness at room temperature (RT) to investigate the effects of grain size and loading rate on their microstructural stability and creep behaviour. The grain growth mediated by the twinning/detwinning mechanism at low applied stresses ( 800 MPa) were uncovered in the present NT-Ni foils during RT creep, both of which are attributed to the interactions between dislocations and boundaries. It appears that a higher initial dislocation density leads to a faster primary creep strain rate and a slower steady-state creep strain rate. Unlike the non-twinned metals in which grain growth often enhances the creep strain rate, the twinning/detwinning-mediated grain growth process unexpectedly lowers the steady-state creep strain rate, whereas the detwinning-mediated grain refinement process ac...

The purpose of this study is to investigate the evolution of residual stress during designing a 89-pass narrow gap welding of nuclear rotor pipes. A two-dimensional finite element model is employed to calculate the residual stress in welding process and heat treatment process, and then the evolution of residual stress during the welding process was discussed in detail. The investigated results show that the overall trend of hoop stress on the inner surface decreases gradually before pass 30, while the trend remains unchanged after pass 30. The through wall axial residual stress at the weld centreline demonstrates a distribution of bending type before pass 38, while this bending type distribution turns into a clearly self-equilibrating type after pass 38.

In this article, by utilizing the dielectric elastomer composite actuators (DECAs), we design and fabricate a stiffness-tunable robotic gripper. Firstly, the voltage-induced electromechanical deformation of the DECA is investigated by applying a ramping voltage. Subsequently, effects of different factors, including the jamming pressure, the roughening process of fibers, the ratio of overlapping area of fibers, and the strain-stiffening of DEs, on the output force performance of the DECA are considered, respectively. Afterwards, the grasping and gripping performance is explored by exhibiting a comparison of the gripper with and without stiffness-tunable effect. Finally, the output gripping force of the stiffness-tunable gripper under different jamming pressures and object sizes is investigated. The maximum size limitation of the gripped object and the output force of the robotic gripper (induced by stiffness tunability) are found to be 12 cm and 5 N, respectively.

Sc microalloying has been regarded as one of the most effective methods for improve the high-temperature resistance of Al-based alloys via additional Al3Sc precipitation. However, synergetic precipitation of Al3Sc and some conventional precipitates (i.e. θ′-Al2Cu) is usually difficult in traditional series Al alloys due to the huge temperature gaps between their formation, which limits the Sc microalloying effect in multicomponent systems. Here, an optimized isochronal aging strategy was illustrated in an Al–Cu-Sc alloy to achieve better ambient- and high-temperature properties in comparison to the artificially aged counterparts. A series of microstructural characterizations reveal that, upon isochronal aging, the Sc-rich entities preferentially form in regions adjacent to θ′-Al2Cu precipitates at relatively low temperature stage (∼250 °C) and are responsible for refining the θ′-Al2Cu precipitation in Al–Cu-Sc alloy. At higher aging temperature (∼300 °C), the preferentially formed Sc-rich entities will be collected by adjacent pre-existing θ′-Al2Cu precipitates, accompanying with additional Al3Sc formation in matrix. The coexistence of θ′-Al2Cu and Al3Sc precipitates realized by isochronal aging contributes to the improved aging hardening behavior as well as better creep resistance at 300 °C in Al–Cu-Sc alloy compared with its Sc-free or isothermally aged counterparts.

Nanostructured crystalline Al/amorphous AlN multilayer films, with a wide layer thickness (h) range from 10 to 200 nm, were prepared and exposed to high-temperature annealing from 200 to 600 °C. Microstructure of all the multilayers was highly stable under the temperature up to 400 °C. Apparent grain coarsening happened in multilayers with h > ~50 nm under 500 °C. However, the Al grains in h ≤ ~50 nm multilayers were extremely stabilized under even 500 °C annealing, showing a high thermal resistance up to about 0.8Tm (Tm: melting point of bulk pure Al). The stabilization mechanisms mainly include the constraint effect by heterophase interface and the nanotwinning in Al layers, as demonstrated by statistical measurement results and ex-situ/in-situ transmission electron microscope observations. When the annealing temperature was raised up to 600 °C, the heterophase interfaces disappeared in h ≤ ~20 nm samples but were well preserved when h > ~20 nm, revealing that the thermal failure modes were quite sensitive to h. Based on the microstructural evolution, a thermal failure map was finally developed in variation with h and annealing temperature. Furthermore, hardness of the multilayers was measured and hardening mechanism was also discussed according to the microstructural evolution after annealing.

How to design ultra-strong face-centered cubic (FCC) metals is a grand challenge in materials community. In general, nanotwinned (NT) FCC metals manifest the maximum strength as the intrinsic size ratio of twin thickness–to–grain size is achieved, above which softening caused by detwinning occurs, whereas below this intrinsic size ratio strengthening caused by dislocation–twin boundary interactions occurs. In this work, we used the room temperature creep as an effective pathway to strengthen NT-Ni with the high stacking fault energy by controlling microstructure in terms of the intrinsic size ratio. We experimentally discovered in NT-Ni at the room temperature steady-state creep stage that there is a critical grain size of ∼230 nm for the intrinsic size ratio transition from 0.04 in the large grain size regime to 0.14 in the small grain size regime. In particular, this intrinsic size ratio can be taken as a signature to estimate the microstructural stability, which linearly scales with the twinnability of FCC metals. Compared with the as-deposited NT-Ni foils, their counterparts undergoing the steady-state creep stage manifest significantly enhanced strength without losing their elongation to failure. This work provides new insights into designing the strongest NT metals via the creep process.

The microstructure and mechanical properties of the twin-roll cast (TRC) and traditional direct-chill cast (DC) Al–Mn strip have been compared using electron backscattered diffraction, scanning electron microscope, transmission electron microscope, and atom-probe tomography. The TRC Al–Mn strip showed a heterogeneous microstructure and texture along the thickness direction of the strip, forming a 'hard shell-soft core' structure. This heterogeneous microstructure resulted in a mutual constraint between the surface region and the center region, enabling the TRC strip to be plastically deformed to a total elongation >30% prior to fracture. In comparison, the DC strip, with a relatively homogeneous microstructure and texture, exhibited a uniform elongation close to zero. The distinct tensile properties presented by the two strips have been rationalized in terms of the differences in their microstructural features, including grain size/orientation, dislocation density, coarse constituent particles and solute atoms. A micromechanics model has been employed to understand the superior ductility of the TRC strip.

High-entropy FeCoNiAl0.5 alloy was produced and subjected to heat treatment at different temperatures (400, 600, and 800 °C). The microstructure evolution, magnetic properties and mechanical properties of the alloy were found to be highly dependent on the annealing temperature. The as-cast alloy was composed of FeCoNi-riched matrix (face-centered cubic structure) and the Al-riched region (body-centered cubic structure). After annealed at 400 °C, Al–Ni-based ferromagnetic rod-like precipitates with body-centered cubic structure were abundantly produced within the matrix, improving saturation magnetization (Ms), coercivity (Hc) and hardness. Increasing the annealing temperature to 600 °C, not only gave rise to a higher number density of the rod-like precipitates, but also to the formation of some nanosized spherical precipitates. This microstructure feature is more effective in strengthening the material (achieving a peak hardness of ~304 HV) but at the same time results in slight declination of Ms and Hc. When the annealing temperature was up to 800 °C, the most strengthening nanoscale precipitates dissolved, and the hardness of the alloy was reduced accordingly. The main strengthening mechanisms are discussed, and a good combination of mechanical and magnetic properties under optimized heat treatment may be available.

Mo– x La 2 O 3 ( x =0, 0.5, 1.0, 1.5, and 2.0 wt%) alloys were prepared by using the solid–solid mixing/doping method. Addition of La 2 O 3 particles effectively refines the grains and significantly elevates the recrystallization temperature of the Mo alloys. Both intragranular and intergranular La 2 O 3 particles are formed in the alloys, with size and volume fraction increasing with La 2 O 3 additions. Room temperature (RT) tensile testing results show that the Mo–La 2 O 3 alloys with 0.5–1.0 wt% La 2 O 3 additions have high strength (100 MPa above the pure Mo) and simultaneously great elongation (2 times of the pure Mo). Elongation will be reduced when La 2 O 3 addition beyond 1.5 wt%. The strengthening mechanisms of the Mo–La 2 O 3 alloys are mainly intragranular particle strengthening and grain boundary strengthening. The ductile fracture is predominantly controlled by the microcrack nucleation at intergranular particles. However, high temperature (HT) tensile testing results show that the Mo–La 2 O 3 alloys with 1.0–1.5 wt% La 2 O 3 additions have the superior strength–elongation combination. Microvoids (dimples) are formed after intragranular particle debonded. The HT deformation mechanisms involve the microvoid coalescence in the grain interior and the intergranular microcrack propagation. The ductile fracture depends on a competition between the intergranular and intragranular damage development. The present experimental results provide in-depth insight into the coupling effect of intergranular and intragranualr particles on the ductile fracture of Mo–La 2 O 3 alloys at RT and HT, respectively.

Both twinning and its reversible process, i.e., detwinning, play important roles in crystalline plasticity, especially for nanostructured metals and alloys. Contrary to the popular belief that the nanotwinned (NT) microstructure is quite stable, in this work, we reported the twinning/detwinning-mediated grain growth (GG) and dislocation nucleation-controlled strength softening in the high stacking fault energy (SFE) NT-Ni foils with different grain sizes but nearly identical twin lamellar thickness during tension at room temperature. We further proposed two possible dislocation–boundary interaction mechanisms that cause GG via twinning/detwinning processes, and elucidated strain rate effect on the double-inverse grain size dependence of twinning/detwinning in nanostructured Ni samples. Our findings provide in-depth insights into the manipulation of internal features of metals and alloys with high SFE to achieve optimum robust performance for practical applications and fundamental understanding of the twinning/detwinning behavior of NT-materials to predict their stability and evolution of microstructures.

This article describes the development of an analytical model to study the coupled nonlinear oscillation and stability evolution of viscoelastic dielectric elastomers (DEs) under non-equibiaxial tensile forces by utilizing the method of virtual work. Numerically calculated results are employed to predict this nonlinear dynamic behavior. The resonant frequency (where the amplitude-frequency response curve peaks) and the amplitude-frequency response of the deformation in both in-plane directions are tuned by varying the values of tensile force. The oscillation response in the two in-plane directions exhibits strong nonlinearity and coupling with each other, and is tuned by the changing tensile forces under a specific excitation frequency. By varying the values of tensile forces, the dynamic viscoelastic creep in a certain in-plane direction can be eliminated. Phase diagrams and Poincaré maps under several values of tensile forces are utilized to study the stability evolution of the DE system under non-equibiaxial tensile forces.

Mo–0.6 wt%La2O3–xZrB2 (x=0, 0.5, 1.0, 1.5, 2.0 wt%) alloys were prepared by using the solid–solid mixing/doping method. Microstructure, tensile strength, and ductility were experimentally examined to investigate the effect of ZrB2 addition. Microstructural observations showed that the addition of 0.5–1.5 wt% ZrB2 effectively reduces the grain size of the alloys and remarkably varies the second phase particles distributed at grain boundaries (GBs). The size of intergranular La2O3 particles was decreased with the increasing ZrB2 addition up to 1.5 wt%. ZrO2 particles were produced through reaction between ZrB2 and oxygen, which were predominantly distributed at GBs, increasing the volume fraction of intergranular second phase particles while decreasing the oxygen concentration at GBs. Tensile testing results displayed that the yield strength of alloys was relatively insensitive to the ZrB2 addition, which was quantitatively understood with respect to the strengthening mechanisms. The ductility, however, was strongly dependent on the additions. In particular, the 1.5 wt% ZrB2-doped alloy exhibited a striking ductility increased by over 40% when compared with the ZrB2-free Mo–La2O3 alloy, although the two alloys have the same level in yield strength. The ductilization is mainly related to the GB purifying effect derived from the reaction addition, which overwhelms the weakening effect of intergranular particles on ductility. The findings in the current work provide a possible approach to modify the GBs and hence to improve the ductility of BCC alloys such as in present Mo alloys.

Nanolayered materials exhibit unique plastic deformation behavior, i.e. , a bulk-like to small-scale plasticity transition and a scaling behavior of extremely small activation volume ( V * b 3 ) with intrinsic size. This unusual plastic transition phenomenon likely stems from the dislocations respectively emit from interfaces and Frank–Read-type bulk sources below and above a critical intrinsic size ∼20 nm. We quantitatively capture the scaling relationship between strain rate sensitivity and intrinsic size by a phenomenological model based on the Orowan–Frank–Read process.

Experimental results revealed that the aging precipitation behaviors were significantly enhanced in an ultrafine grained (UFG) Al–Cu–Sc alloy when compared with its coarse grained (CG) counterpart. In the UFG Al–Cu–Sc alloy aged at 398 K for 20 h, a large number of θ ′-Al 2 Cu particles with an average radius of about 38 nm were precipitated within the grain interior. While in the CG alloy aged at the same condition, no precipitations were found. The ultrafast precipitation behaviors observed in the UFG alloy is rationalized by developing a precipitation kinetics model. This model is based on the classical N-model framework of Kampmann and Wagner (KWN), but is modified to capture some precipitation features in the UFG regime, such as highly enhanced diffusion and greatly reduced nucleation energy barriers. The modified model yields predictions in good agreement with experimental data, which are several orders of magnitude faster than those predicted by the classical model. This work indicates that the classical N-model, after some modifications, is still applicable for the quantitative description of precipitation behaviors in UFG Al alloys.

The microstructural evolution and hardness of He-irradiated Nb/Zr crystalline/crystalline multilayers with a wide range of layer thickness h from 5 to 100 nm were investigated under three different ion dose Φ of 1.0 × 1016, 1.0 × 1017 and 5.0 × 1017 ions·cm−2 at room temperature. Careful microstructural examinations found the occurrence of a radiation-induced amorphization phenomenon near the interface. Besides, He bubbles were distributed within the Nb layers and at the interface. Both the amorphization and He bubble distribution were highly dependent on h and Φ, which caused the mechanical properties sensitive to h and Φ. With reducing h, the hardness of as-deposited Nb/Zr multilayers showed a maximum at h = 10 nm. While the He-irradiated samples manifested a hardening behavior at Φ = 1.0 × 1016 ions·cm−2, a softening behavior at Φ = 5.0 × 1017 ions·cm−2 and a transition from hardening to softening at Φ = 1.0 × 1017 ions·cm−2 with reducing h. These strengthening behaviors were rationalized by a coupling effect of dislocation-amorphous and dislocation-bubble interactions. Modified mechanistic models based on interactions of dislocations with obstacles were employed to quantitatively describe the underlying strengthening mechanisms.

Ionic electroactive polymer (iEAP) actuators have attracted great attention in the fields of micro-electromechanical systems (MEMS) and biomedicine due to their remarkably large strain under low-voltage stimulation. As actuation performance is mainly dominated by the electrochemical and electromechanical processes, the electrode layer and its structure are increasingly crucial to the actuators. In this research, we introduced a nanoporous carbon nanotube film with superior conductivity (1000-2000 S/cm) as a cost-effective alternative electrode in fabricating iEAP actuators. The actuators were assembled by hot-pressing a Nafion/EMImBF4 electrolyte layer between two electrode films under vacuum conditions. Based on the developed porous hierarchal structure and superior electrical and mechanical properties of the electrode film, the actuators showed highly improved electrochemical and electromechanical properties and can operate controllably and durably under various voltage amplitudes and waveforms with a wide frequency range. The generated strain and stress reach up to 1.26% and 5.27 MPa, respectively, which are greater than the performances of the actuators based on polymer-supporting nano carbon electrodes. Once the voltage increases by 1 V, the strain and stress will increase by 0.336% and 1.407 MPa, respectively. The actuators can perform more than 20,000 cycles with an initial drop less than 20% in the operation stroke, showing a relatively durable and controllable cycle life. The newly developed actuators can be promising candidates for artificial muscles in academic interest and industrial applications.

As to the increasing demands for the light materials in weight-sensitive fields, scandium (Sc) is arguably known as one of the most attractive and effective microalloying elements to develop high-performance aluminum-based alloys applied at elevated temperature. This report exemplifies how the Sc microalloying effect can be fully utilized to seek for the synergetic stabilization between dual precipitates, more than simply forming the well-known Al3Sc nanoprecipitates. Two sets of precipitation protocols are introduced to create a coexistence of θ′-Al2Cu and Al3Sc precipitates in an Al–Cu–Sc alloy, and the thermal stabilization of the two precipitates is found to be closely correlated. In the case of θ′-Al2Cu precipitate decomposed because of an invalid protection from weak interfacial Sc segregation, the Al3Sc precipitates will coarsen much quickly, which induces a simultaneous loss in thermal stability and strengthening effect of the dual precipitates. In contrast, a sensible approach is proposed to induce a strong Sc segregation at the θ′-Al2Cu/matrix interface to greatly stabilizes the θ′-Al2Cu precipitates even crept at 300°C and concomitantly suppresses the coarsening of Al3Sc precipitates, leading to extremely high creep resistance. The acceleration effect of the Cu on the Al3Sc coarsening is illustrated and can be alternatively prohibited by modified Sc partitioning to stabilized θ′-Al2Cu precipitates as Cu storage and thus provides a much stronger coupling precipitate strengthening as well as creep resistance at elevated temperatures. This strategy is broadly applicable to other high-temperature alloy containing Sc together with primary elements such as Si, Mg, and Zn via the creation of precipitate co-stabilization.

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.