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A ferrite layer is formed between the outer intermetallics layers and the martensite substrate during the hot stamping process of Al-Si coated press-hardened steel (PHS). Unexpectedly, it is found here that the ferrite layer does not effectively prevent brittle cracks propagating continuously from the intermetallics to the martensite substrate. This leads to a high stress intensity factor (SIF) at the crack tip, therefore initiating highly localized shear deformation in the martensite substrate, degrading the bendability of the steel. A thinner Al-Si coating produces thinner brittle intermetallics and ferrite layers, and therefore shorter coating cracks and smaller SIF at the crack tip, thus improving the bendability. In addition, a thinner Al-Si coating also has a lower material cost while keeping similar paintability and corrosion resistance. The thin Al-Si coating could potentially change the current practice of Al-Si coating, impacting the global automotive industry.

Nanostructured metals and alloys are generally strong but lack strain hardening due to enhanced grain boundary diffusion and sliding. Here, equal nanograined (ENG) and gradient nanograined (GNG) layered Cu/Ta architectures were acquired by introducing a hard and stable “artificial” interphase boundary zone (IBZ). The ENG architecture produced uniform plastic strain reaching 70% and high yield strength exceeding 1 GPa, which is attributed to the constraint effect of the tough IBZ on dislocation slip mediated co-deformation. The GNG architecture exhibited a remarkable linear strain hardening with hardening exponent of 1 due to the strong strain partitioning between soft and hard layers. The dominant deformation mechanism of the GNG nanocomposite evolves from partial dislocation emission to dislocation accumulation at interface with increasing strain based on the results from experiments and molecular dynamics simulations. This finding demonstrates that heterostructure with “artificial” IBZ may offer an alternative approach to design strong and tough materials.

In this work, we have synthesized dual-phase high entropy alloys (DP-HEAs) multilayers with a tunable layer thickness through a bottom-up approach. The DP-HEAs' multilayers reach the maximum hardness of 13.3 GPa at a critical thickness of 10 nm. The main strengthening mechanism is that dislocation movement is locked by both interior columnar grain boundaries and heterogeneous interfaces. A crystalline-to-amorphous transition occurs when layer thickness is below the critical thickness. The formation of amorphous DP-HEAs deformed by shear transformation zones with collective atomic rearrangement is responsible for the softening behavior.

Nanostructured metal thin films have high strength and high electrical conductivity, but their fatigue resistance is relative low due to the weak plasticity and ductility deformed by localization at grain boundary or interface. Here we report a carbon nanotube/copper laminate composite film with ultrahigh fatigue resistance, which exhibits history-independent cyclic bending response. The high dense wrinkling of composite film becomes the dominant cyclic deformation behavior. The cracks initiation and formation are voided by the wrinkling process releasing the bending stress. The residual stress and resistivity of composite film no longer increase even if the number of cycle reaches as high as105. This result provides us a novel insight to design nanoscale metal films with ultrahigh fatigue resistance and superior service ability.

The carbon nanotube fiber has not shown its advantage and been widely used in many applications due to low conductivity. Depositing a metallic layer on the carbon nanotube fiber surface became an effective method. However, the strength of the carbon nanotube fiber decreases with increasing conductivity and coating thickness. Here, we deposited nanocrystalline metal coating on carbon nanotube fibers by magnetron sputtering. The coating consists of an ultrathin Al transitional layer and a Cu layer. After annealing, the Al atoms and Cu atoms diffused into the carbon nanotube fiber to form diffuse interface and chemical reaction interface, leading to the sliding resistance increase and contact resistance decrease between carbon nanotubes. As a result, the new composite fiber with a 2 μm thick Cu layer can exhibit a superhigh effective strength of 996 MPa and electrical conductivity of 2.6 × 107 S/m. This structure achieves both conductivity and strength increasing simultaneously.

In this work, we present a tunable alloying strategy to prepare high entropy alloy (HEA) thin films with FCC/BCC dual-phase structure. The dual-phase HEA films consist of uniformly equiaxed grains with average size about 40 nm. Comparing with single-phase FCC HEA, the dual-phase HEA has higher hardness up to 10.4 GPa. The relative large atomic radius of Al leads to severe lattice distortion, resulting in a stronger solid solution strengthening behavior in the dual-phase HEA. Moreover, high dense heterogeneous phase interfaces effectively blocking dislocation motion enhance the strain hardening ability further.

Nanolayered metallic composites usually deform via a transition from homogeneous deformation to major shear banding with decreasing layer thickness, and thus the improvement of strength often sacrifices the plasticity of materials. Here, we show two methods to promote brittle-to-ductile transition in nanolayered Ag/Nb pillars. Intrinsically, while keeping the pillar diameter constant, the reduction of layer thickness can increase the strength of multilayers and suppress shear induced failure. Extrinsically, for a constant layer thickness, decreasing the diameter of pillar suppresses shear bands and promotes more uniform plastic deformation. Furthermore, the critical layer thickness at peak strength of multilayers increases monotonically with decreasing pillar diameter. Interface structures evolve from amorphous layer to coherent interface with reduction of layer thickness. Homogeneous co-deformation mediated by heterogeneous interfaces and columnar grain boundaries promotes a unique work hardening behavior. This study indicates that a combination of intrinsic and extrinsic size effect may enable the accomplishment of high strength and uniform deformation simultaneously.

The thermal stability of nanoscale Cu/Ru metallic multilayers with different modulation ratio (a fixed Ru layer thickness of 3 nm and varied Cu layer thickness (h) from 10 nm to 200 nm) was investigated by transmission electron microscope (TEM) analyses and nanoindentation tests. All of the Cu/Ru multilayers retained the layered structures up to 400 °C annealing. The Cu grain growth slowed with Cu layer thickness decreasing, illustrating an unusual size dependence, that is, “smaller is more stable.” Especially the average grain size was maintained as small as 25 nm for the Cu/Ru multilayers with h = 10 nm and the hardness only decreased by 6.5% after 400 °C annealing, which exhibits an excellent stability. The enhanced thermal stability of Cu/Ru multilayers originates from the textured columnar grains and semicoherent heterogeneous interfaces, which have a low interface energy. The amount of textured columnar grains and semicoherent heterogeneous interfaces increase with the Cu layer thickness decreasing is responsible for the enhanced thermal stability of Cu/Ru multilayers.

In this work, the single-layered amorphous NbMoTaW and multilayered amorphous NbMoTaW/TiVCr high entropy alloys were prepared to investigate the diffusion barrier performance. The microstructure and thermal stability of these samples were analyzed by X-ray diffraction, transmission electron microscope, and four-point probe method after annealing at different temperatures. In the single-layered structure, Cu silicide formed after annealing at 600 °C. The reason is that the amorphous NbMoTaW layer crystallizes during annealing, which causes the interdiffusion between Cu and Si through grain boundaries (GBs). However, the multilayered structure still maintains effective barrier performance as the annealing temperature is below 800 °C. Amorphous structure without GBs diffusion path, severe lattice distortion, and the high dense layer interface are responsible for the improved barrier property in the multilayered system.

Introducing parallel coherent twin boundaries (p-CTBs) is a high effective approach to material strengthening. However, it has been widely verified that the strengthening effect of p-CTBs reaches saturation at a critical twin lamellar thickness. In this work, we demonstrate by experiments and molecular dynamic simulations that the novel intersectant coherent twin boundaries (i-CTBs) involving Lomer-Cottrell (L-C) dislocation locks trigger off a strong strain hardening on nanostructured metal, which exceeds the strength induced by p-CTBs strengthening. A transition from strain burst to dislocation multiplication occurs as the p-CTBs turn into i-CTBs in nanoscaled single crystals. The i-CTBs with different orientations can significantly promote the formation of L-C dislocation locks inside a nanocrystal, reflecting an intrinsic correlation between the i-CTBs and the L-C dislocation locks. The unique strain hardening primarily originates from the synergistic strengthening effect of the i-CTBs blocking and the L-C locks pinning on the glissile dislocation slip. These findings provide the impetus for a new strategy to design high strength and high plasticity crystalline materials.

The mechanical property spectrum of steels can be expanded to new limits through combining the characteristic microstructures of different steel grades in a single steel. Here, we demonstrate this microstructural grading concept in a single steel sheet that possesses microstructures representative of two steel grades, i.e., the duplex medium Mn steel and the dual-phase steel, in the form of a multi-layered structure. This graded steel was produced by applying the surface mechanical attrition treatment to a duplex medium Mn steel with metastable retained austenite. The resulting graded steel collects the respective advantages of the two steels: high ductility of the former, and the Luders-band-free plastic flow of the latter. By carrying out systematic experiments and finite element simulations, we reveal that the suppression of Luders band formation in this graded steel is due to the high early-stage strain hardening rate of the dual-phase steel microstructure, which offsets the post-yield strain softening tendency of the medium Mn steel microstructure. The ductility, on the other hand, results from the transformation-induced plasticity effect of the medium Mn steel microstructure, which enhances the overall strain hardening rate and suppresses the necking of the dual-phase steel microstructure at large strain levels.

In this work, we have systematically investigated the effect of cyclic deformation on the strength and rate sensitivity of Cu/Ru multilayers with different individual layer thickness ( h ) by nanoindentation tests. It was found that cyclic deformation remarkably enhances the hardness of Cu/Ru multilayers comparing with the specimens by monotonic loading. The rate sensitivity ( m ) of multilayer exhibits an anomalous size dependence after nanoscale cyclic deformation. When h > 10 nm, the m linearly increases with increasing cycle number of loading-unloading ( s ). However, the m sharply decreases with increasing s when h m . An obvious Bauschinger effect is observed during cyclic loading, where the evolution of effective stress is consistent with the m . Cyclic deformation induced dislocation accumulation and arrays at the heterogeneous interface are the intrinsic plastic mechanism for the enhanced rate sensitivity. The formation of amorphous layers at the critical h is mainly responsible for the inverse size m .

The annealing induced grain growth (GG) and heterogeneous interface evolution of Cu/Ag multilayers with individual layer thickness ( h ) varying from 5 to 50 nm were investigated by transmission electron microscopy (TEM). The results demonstrate that the thermal stability of Cu/Ag multilayers exhibits strong length scale dependence. For samples with h h ≥20 nm, w h ere the interfaces remain remarkably intact. The existence of a large number of grain boundaries (GBs) decrease the stability of multilayers, while more heterogeneous interfaces contribute to resisting atomic diffusion, inhibiting GG. The equilibrium is achieved by a competitive process between GBs diffusion and heterogeneous interfaces resistance. Moreover, the formation of annealing twins in multilayer also significantly improve the microstructural stability.

This work presents a length scale dependent strain rate sensitivity (SRS) transition from positive to negative in Ti/Ni multilayers with unequal thickness of individual layer. The multilayers exhibit distinct interfacial structure and no Ti-Ni amorphous intermixed layer between Ni and Ti layers. It can be found that the hardness first increases with strain rate at large modulation period (λ), showing a positive SRS. Subsequently, a tunable (positive to negative) size dependent SRS phenomenon occurs with further decrease of λ. The present negative SRS is attributed to deterioration of the plasticity with decreasing λ and at larger strain rate, which induces microcracks formation.

The compression behavior of a Ti-8Al-1Mo-1 V alloy with bimodal microstructure has been studied in the strain rate range of 2 × 10-3 s−1-2 × 10-1 s−1. The strain rate sensitivity shows a positive–negative-positive changing tendency with increasing strain rate. Moreover, serrated flow behavior was found in the first two changing stages of the strain rate sensitivity. Dynamic strain aging caused by solute atom-dislocation interaction within ultrafine α/α' lamellae is proved to be responsible for the appearance of serrations as well as the negative strain rate sensitivity. The reasons for the other two stages of positive strain rate sensitivity have also been analyzed.

In this work, the hardness and elastic modulus were investigated in Ag/Nb multilayers with the individual layer thickness ( h ) ranging from 1 to 50 nm. The hardness increases with decreasing h , and the magnitudes of hardness of multilayers with h ≤5 nm are higher than that of either constituent. This ultra-high strengthening results from the coherency stresses and high density stacking faults in Ag layers. Meanwhile, the enhancement of elastic modulus is achieved due to the lattice contraction when h ≤20 nm, where the modulus values are higher than that of either constituent, showing a supermodulus effect.

In this work, the hardness, electrical resistivity and thermal stability of annealed Ag/Nb multilayers were investigated with individual layer thickness (h) ranging from 3 to 100 nm after annealing at 200–400 °C. For all scales multilayers, the hardness improves 6–16% whereas the electrical resistivity decreases 6–12% after annealing at 200 °C compared with those of as deposited state. At 300 °C, the enhancement shows layer thickness dependent due to the different interfacial structures. The hardness improves about 4–10% and the electrical resistivity decreases 11–28% for multilayers with h ≥ 10 nm. A good combination of high strength and high conductivity has been obtained in annealed Ag/Cu multilayer with h ≥ 10 nm when annealing temperature is not more than 300 °C. Both hardness and electrical conductivity are enhanced by annealing process. The improvement of electrical conductivity mainly comes from the reduction of grain boundaries due to the grain growth. The enhancement of hardness relates with the increment of twin boundaries, the compressive stress and the lack of dislocations after annealing process.

In this paper, we prepared alloying Ti/Ni multilayers through changing modulation periods ( λ ) and subsequent annealing. Strain rate sensitivity of the multilayers was investigated by nanoindentation. It was found that the multilayer became full alloying with the highest value of hardness at λ =5.4 nm. Both the alloying degree and the corresponding hardness are remarkable dependent on λ . Only the multilayers accompanied by obvious stress-induced pop-in events exhibit negative strain rate sensitivity resulted from stress-induced martensitic transformation, while hardness of the other multilayers does not change with strain rate. In addition, the critical load for the first pop-in increases with strain rate, meaning that larger stress is needed to induce martensitic transformation at higher strain rate.

The formation mechanisms and grain size dependence of annealing coherent multiple-fold twins, such as twofold and fivefold twins, were investigated in nanocrystalline Cu with zero applied stress by a combination of transmission electron microscopy and molecular dynamics (MD) simulation. It was found that the formation frequency of twofold and fivefold twins with coherent twin boundaries (CTB) increases with decreasing grain size (d), reaching a maximum frequency at the critical size of 35 nm, followed by a reduction at d

Phase transformation and mechanical properties of two distinct Ni-rich Ni–Ti–Al thin films deposited at various temperatures were investigated in this paper. It is found that phase transformation temperatures and hardness of the films obviously increase with growth of Ni-rich precipitates, and their phase transformation hysteresis decreases. Especially, martensitic transformation start temperature of the Ni 49.7 Ti 45.3 Al 5 films exceeds room temperature and increases by 20 °C, and the Ni 44 Ti 32 Al 24 Heusler alloy films show near-zero transformation hysteresis. Besides, hardness of the two films increases by 12.8% and 22%, respectively. The evolutions are caused by the growth of Ni-rich precipitates which strengthens the matrix. The precipitation would sharply consume Ni content of the matrix that changes phase transformation behavior from the Ni-rich to Ti-rich films.

We utilize alloying of Ti/Ni multilayers to prepare Ti−Ni alloy films and investigate effects of modulation period ( λ ) on the alloying and mechanical properties. It is found that the annealed multilayers with the λ of 2.7–54 nm exhibit ultra-high hardness, far exceeding co-sputtering Ti−Ni alloy films, and the maximum hardness is found at λ =5.4 nm. Attractively, another anomalous hardness softening occurs at λ of 13.5 and 27 nm with increase of indentation depth besides the softening at λ =5.4 nm, which is simultaneously accompanied by the pop-in events including displacement jumps and sudden drop of load. The pop-in events are attributed to stress-induced martensitic phase transformation and the critical load for martensitic transformation increases with the decrease of λ .

This work presents a length-scale induced anomalous plastic deformation of Cu/Ta multilayers with individual layer thickness (h) varying from 1 nm to 100 nm. The hardness increases with decreasing h and reaches a maximum at 10 nm. A sharp softening has been observed when h is below 10 nm. Furthermore, the results show that the phases of Ta layers vary from the coexistence of α-Ta and β-Ta to single α-Ta as h decreases from 10 nm to 5 nm. The phase transition causes the variation of interface structures as well as the hardness of Ta layers and results in the anomalous decrease in hardness. The strengthening mechanisms at different length scales are discussed separately.

Indentation creep and stress relaxation tests were performed on rolled and annealed nanocrystalline (NC) Ni to study the influence of microstructure evolution on plastic deformation behavior. Dislocation density (ρ) increases with increasing rolling strain, reaching a maximum at 20% strain, followed by a decrease at larger strain. The ρ of Ni decreases significantly with increasing annealing temperature. Softening behavior is observed in NC Ni with grain size

Two bilayers of Ru/TaN with low N concentration and high N concentration (TaN L and TaN H ) were used to determine the effect of N effusion on the barrier property. The results show that Ru/TaN H bilayer exhibits a better barrier property, in which RuN existed even after annealing at 650 °C. The improved barrier property is attributed to the formation of RuN and N atoms stuffing in grain boundaries of Ru layer by sufficient effusion N atoms from TaN H during annealing.

The strain rate dependent stretchability and fracture behavior of the polymer-supported Cu films was studied by uniaxial tension test. The crack spacing of the films is dependent not only upon the strain rate but also the thickness of the films. At a high strain rate, cracks parallel to loading direction are found to occur more easily in thicker film than thinner one. Delamination and crack along loading direct facilitate each other, accelerating the failure of films. The calculated energy release rate decreases with increasing strain rate, which can be used to explain fracture behavior of film/substrate.

The ductility of nanocrystalline Cu films with different thickness ranging from 40 nm to 600 nm was tested by uniaxial tension. The ductility of the films was determined by the changes of the statistical crack density and the electrical resistance with tensile strain. It was found that the ductility decreases with increasing the film thickness, exhibiting an inverse size effect compared to the previous results. The 40-nm-thick Cu films sustain strains up to 20% without cracks, while the 600-nm-thick Cu films fracture at only 5% with channel cracks. The effective interfacial bond strength between the films and the substrate is suggested to be responsible for the ductility of Cu films.

Pseudoelasticity of nanocrystalline Ni 43 Ti 38 Al 19 thin films with thickness of 600 nm is investigated by nanoindentation. It is found that the films with grain size of 12–28 nm are composed of L2 1 –Ni 2 TiAl phases, Ni 3 Ti precipitates and few B2–NiTi phases. The films with the L2 1 phases exhibit prominent pseudoelasticity, which strongly depends on grain size and indentation depth at the nanoscale. The pseudoelasticity decreases gradually with increasing grain size and indentation depth; among them the highest pseudoelasticity recovery ratio reaches 92.7%. With decreasing the grain size from 28 to 12 nm, the hardness of the films increases initially and then decreases, showing an inverse Hall–Petch effect.

The rate sensitivity of hexagonal close-packed nanocrystalline Ru films was investigated by nanoindentation, which is found to be a function not only of grain size but also of indentation depth. The enhanced magnitude of the sensitivity is comparable to that of face-centered cubic nanocrystalline metals. Experimental results suggest that the interaction of dislocations with grain boundaries (GBs) and the diffusion along the tip–sample interface are the dominant mechanisms, whereas both GB sliding and Coble creep are ruled out as the rate-controlling deformations.

The effects of barrier layers and annealing temperature on texture variation, grain growth and void forming of nanocrystalline Cu films were investigated by X-ray diffraction, transmission electron microscope and scanning electron microscope (SEM). The variation in texture and grain size of Cu films with annealing temperature is different for Cu/Ti and Cu/Ta. The activation energies of grain growth of Cu films on Ti and Ta, respectively, are 19.7 and 23.4 kJ mol −1 , which are much closer to that of grain boundary diffusion of Cu. The average diameter of about 400 nm for surface voids of Cu/Ti is larger than that of Cu/Ta structure. Furthermore, both the electrical resistivity measurement and SEM observation imply that Cu/Ti rather than Cu/Ta structure tend to fail easier as annealing temperature exceed 400 °C.

Nanoindentation creep tests were carried out at the maximum indentation load from 500 to 9000 μN to study the indentation size effect (ISE) on the creep behavior of amorphous, nanocrystalline (NC) bcc and NC tetragonal Ta films. For NC bcc and tetragonal Ta films, the creep strain rate e ˙ decreases and stress exponent n increases with enhanced peak loads or indent depth, and are therefore both indentation size dependent. However, an inverse ISE on e ˙ and n is found for amorphous Ta films. The difference of the ISE is attributed to the distinct creep deformation process. Several creep mechanisms including self-diffusion along the indenter/specimen interface, grain boundary diffusion and sliding, and dislocation climb have been introduced to interpret the ISE for NC Ta films. The inverse ISE on amorphous Ta films is explained by the shear transformation zone theory.

Nanoindentation creep tests were carried out at maximum indentation loads from 500 to 9000 μN to study the indentation size effects (ISEs) on the creep behavior of nanocrystalline tetragonal Ta films. The experimental results show that the hardness, creep strain rate and stress exponent are all indentation size-dependent. The ISE on the creep behavior is explained by grain boundary diffusion and sliding, and self-diffusion along the indenter/specimen interface and along the free surface of specimen.

Phyllostachys praecox is a favorite bamboo shoot species that has been widely planted in southern China. High rate of fertilization and heavy winter mulch have been a common practice to gain a good yield and better economic benefit. To get an insight into the effects of fertilization and winter mulch on soil labile organic carbon pools, a trial of different types and rates of fertilizers was conducted from May 2002 to April 2003. Soils in the mixed treatments with both mineral and organic fertilizers (treatments: 1-3) were generally more abundant in soil microbial biomass carbon (MBC) (P < 0.05) as compared with treatments of single mineral fertilizer (treatments: 4-6), with MBC for treatments 1 and 2 generally at maximal level and for treatments 4 and 5 at minimal level. The abundance of soil MBC increased with the rate of organic fertilizers applied. Soil MBC content was measured periodically during the year, with the highest in October and December 2002, moderate in August 2002 and February 2003, and the lowest in April 2003. Soil water-soluble organic carbon (WSOC) of all treatments was higher in the August and October, decreased in the December and February, and increased again in the April. It was found that the treatments with mixed mineral and organic fertilizers had much higher WSOC (P < 0.05), compared with the pure mineral fertilizer treatments. Soil WSOC increased with the amount of organic fertilizer applied. Winter mulch enhanced soil MBC and WSOC, and the ratios of MBC in the mulch treatments to non-mulched treatments were on average 1.60 and 1.52 in February and April 2003, respectively, while the corresponding ratios of WSOC were on average 1.39 and 1.73 in the February and April, respectively. The high rate of single mineral fertilizer application was not recommended in bamboo management. Both mineral and organic fertilizers would need to be applied for sustaining soil fertility and long-term bamboo production in subtropical China.

In the present paper, the effects of electric field annealing on interface diffusion of Cu/Ta/Si stacks were studied by means of XRD, XPS and TEM. The barrier property of Ta films was evaluated based on the diffusion of Cu atoms. It was found that the external electric field accelerates the diffusion of Cu atoms through Cu/Ta/Si interfaces during annealing. With the increment of annealing temperature, the effect of the electric field upon the atomic diffusion becomes more significant. The mechanism of accelerated interface diffusion is suggested and the failure of Ta barrier layer is discussed based on the mobility of vacancies and Cu atoms inside Cu/Ta/Si stacks caused by the electric field.