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Synergetic strengthening by designing of gradient nanostructures is a novel method to enhance strength in metals and alloys. Herein, we realize synergetic strengthening in strain-softening Zn alloy with gradient nanostructures. The results show that synergetic strengthening accounts for at least 25.5 pct of the yield strength of the strain-softening Zn alloy. The synergetic strengthening is induced by the microstructural gradient and the resultant strength gradient regardless of specific hardening or softening of the refined microstructures.

With aim to enhance the understanding of the relationship between the microstructure and mechanical properties of severe plastic-deformed Zn alloys, this work investigates the microstructural evolution and underlying deformation mechanisms of Zamak 3 alloy subjected to sliding friction treatment (SFT). With an increase in the strain from the matrix to the surface of the SFT Zn alloy, three types of microstructural characteristics are observed: (i) Gradual refinement of the Zn grains, (ii) First increase and then decrease in twin density, and (iii) Gradually elongated/broken eutectic phases. The results of mechanical tests reveal that work hardening plays a dominant role until strong softening resulting from microstructural refinement. The competition between hardening and softening related to the microstructural evolution determines the specific mechanical properties of Zn alloys.

The strength and corrosion resistance of 7000 series (Al-Zn-Mg-Cu) alloys seldom co-exist and are often trade-off with each other. In present study, an alternative surface nanocrystallization technique called sliding friction treatment (SFT) was adopted to successfully generate a gradient nanostructured layer on coarse-grained (CG) peak-aged AA 7075-T6 alloy sheet. The grain size on the topmost surface of SFTed sample was extremely reduced to nano-scale (average grain size: ∼56.2 nm). Excitingly, the SFTed 7075 possessed simultaneously improved mechanical strength and inter-granular corrosion (IGC) resistance relative to the CG counterpart. Specifically, both ultimate tensile strength (UTS) and yield strength (YS) of SFTed sample are even ∼ 40 MPa higher than those of untreated peak-aged CG sample. Meanwhile, the IGC test results demonstrate that the average maximum corrosion depth is considerably reduced from ∼179 μm to ∼40 μm after SFT treatment. The obviously refined grains account for the enhanced strength, while the improved IGC resistance should benefit from SFT-induced grain refinement, elimination of both grain boundary precipitates (GBPs) and precipitates free zones (PFZs) along the grain boundaries of SFT-induced nanocrystalline. Our results demonstrate that SFT processing offers a design pathway to fabricating 7000 alloys with modified strength-IGC resistance synergy.

The creep behavior of nanocrystalline Cu with an average grain size of 25 nm was investigated by nanoindentation test at room temperature. Using the creep strain rate versus creep stress data obtained at different loading rates, the activation volume and strain rate sensitivity were determined obtained by cooperating the continuous stiffness measurement (CSM) technique. The results showed that the activation volume first increases and then decreases, and the strain rate sensitivity first decreases and then increases with increasing the creep stress. The experimental activation volume and strain rate sensitivity versus the creep stress data exhibit very good agreements with the theoretical values calculated by the previous models, respectively. The analysis based on the data of the activation volume and strain rate sensitivity revealed that at lower stress, the grain boundary activities dominate and lead to the lower creep strain rates; at higher stress, the dislocation activities dominate and lead to the higher creep strain rates. The analysis based on the data of the nanoindentation test also revealed that the use of the CSM technique can lead to the continuous creep strain rate versus creep stress data, which allows us to uncover the creep mechanisms over a wide range of the creep stress from the initial to steady stage.

The finite element simulations show that non-equibiaxial residual stresses (RS) can shift the load-depth curve from the unstressed curve and cause elliptical remnant indentation in spherical indentation. Thus the relative load change between stressed and unstressed samples and the asymmetry of elliptical remnant indentation were employed as characteristic parameters to evaluate the magnitude and directionality of RS. Through theoretical and numerical analysis, the effects of RS on indentation load and remnant impression as well as the affect mechanism were systematically discussed. Finally, two equations which could provide foundations for establishing spherical indentation method to evaluate non-equibiaxial RS were obtained.

Nanoindentation tests were performed at room temperature, to study the coupling effects of grain size and strain rate on the indentation size effect (ISE), pop-in effect and creep behavior of refine-grained AZ31 alloys. The relatively inconspicuous ISE of the refine-grained AZ31 alloys compared with its coarse-grained (CG) counterpart is the result of more density of statistically stored dislocations involved in the plastic deformation. The gradually disappeared pop-in effects in the nanocrystalline (NC)/ultrafine-grained (UFG) AZ31 are associated with no/few deformation twinning involved in the plastic deformation. For the NC/UFG AZ31, the highly unstable dislocation absorption and interactions between dislocation and high-angle GBs are responsible for the primary and latter nanoindentation creep behaviors, respectively. While for the CG+twinning/CG AZ31, the relevant mechanisms are the relatively stable dislocation motion and interactions between dislocation and low-angle GBs.

Fully dense core-shell (CS) structured Ti-6Al-4V (Ti64) compact was synthesized by spark plasma sintering (SPS) from oxidized Ti64 powder. Microstructure characterization and compositional analyses show that the shell is with near single α phase and the core is composed of α lamellae packed by β phase with a continuous β phase in between. Phase transformation from (α+β) phase to a near single α phase in shell is attributed to oxygen penetration, thereby forming such a novel CS structure. The CS Ti64 compact exhibits a yield strength of about 1260 MPa and a fracture elongation of 13%. Such excellent mechanical properties are attributed to solid strengthening of oxygen and the unique core-shell structure. The mechanical properties can be further enhanced by manipulating the microstructure in core and shell via heat treatment.

An ultrafine grained Cu with a wide distribution of grain size and an average grain size of d = 110 nm was prepared by electric brush-plating technique. Tensile properties, microstructure and deformation surface morphologies of this ultrafine grained Cu were compared with two results on an electric brush-plated nanocrystalline Cu ( d = 59 nm) and an electrodeposited ultrafine grained Cu ( d = 200 nm) previously prepared by our group. Based on these comparisons, the strain rate dependences of strength and ductility of these three materials and underlying mechanisms were studied. It was revealed that the pronounced strain rate sensitivity (larger m value) of these three materials is a result of the competition between the dislocation and grain boundary mediated deformations. The grain boundary mediated deformation plays a very important role in controlling the tensile ductility of these three materials, which affects the deformation accommodation by promoting different modes of strain localization and hence leads to the completely different strain rate dependences of tensile ductility (the elongations decrease, remains unchangeable and increase with increasing strain rate).

The effect of microstructural characteristic on mechanical properties of Ti-1300 alloy is investigated to reveal the strengthening mechanism of precipitated α phase, and its corrosion behavior is emphatically examined in 2 M HCl solution using potentiodynamic polarization and immersion tests. The results shows that the strength property of Ti-1300 alloy is sharply increased with the precipitation of acicular secondary α phase (α s ), and many α s phases intersect each other and keep the Burgers orientation relationship with β phase in the local colony, which prevent the dislocation motion. The lamellar microstructure exhibits the highest strengthening effect, but the ductility worsens. The corrosion resistance of the alloy with different microstructure is also significantly different. Ti-1300 alloy with bimodal/lamellar microstructure is presented a high corrosion rate from corrosion current density and weight loss, indicating the inferior corrosion resistance and passive behavior. SEM morphology analysis of Ti-1300 alloy after immersion tests revealed α phase and α+β interphase boundary act as the preferential dissolution locations, and the formed microgalvanic cells between the precipitated α s phase and β matrix as well as the accumulative of β stabilizing element in β phase are identified as the main factor affecting the corrosion performance of Ti-1300 alloy.

A nanocrystalline surface layer of AZ31 Mg alloy was fabricated successfully by sliding friction treatment. The microstructure was characterized by a transmission electron microscopy (TEM). In the process of plastic strain-induced grain refinement, the nucleation and growth of new grains as well as grain split are all observed. These experimental phenomena indicate that discontinuous dynamic recrystallization (dDRX) and continuous dynamic recrystallization (cDRX) are responsible for grain refinement of Mg nanocrystallites. Further analysis reveals that stored strain energy (SSE) plays a critical role in the grain refinement mechanisms (dDRX or cDRX): high SSE facilitates dDRX and low SSE conduces to cDRX.

Ultrafine-grained (UFG) pure magnesium and AZ31 alloy were prepared by sliding friction treatment (SFT). Nanoindentation tests of UFG Mg and AZ31 alloy were performed under a wide range of loading strain rates from 0.004/s to 0.4/s with a holding time of 500 s at room temperature to study the grain size and alloy element influence on the creep behavior and the corresponding deformation mechanisms. It is shown that the refined grain size could promote creep behavior, while the alloy elements would enhance the creep resistance. Creep stress exponent deduced in the ending of the holding regime were used to illustrate the creep diversity in these materials. The UFG samples exhibit a reduced creep stress exponent because of significantly increased grain boundary activities. Besides that, the solute atoms of Al and Zn endow the material with an improved creep resistance by limiting interactions between dislocations and boundaries.

Tensile tests over a wide range of strain rates (1.04 × 10 −6 to 1.04 s −1 ) were performed on electrodeposited nanocrystalline (NC) Ni and Ni-Co alloys with different grain sizes to systematically investigate the coupling effects of strain rate and grain size on the mechanical behaviors. It was found that the grain size significantly affected the dependency of mechanical responses to the imposed strain rate. In particular, peculiar fluctuations of flow stress and elongation-to-failure with variation of strain rate were observed in the Ni-8.6 wt%Co alloy with the finest grain size. By carefully analyzing the changes in strain rate sensitivity exponent and apparent activation volume, such unique phenomenon was rationalized in terms of the synergy of grain size and strain rate on the transition of deformation mechanisms including thermal-activated and mechanical-driven grain boundary (GB) activities, interactions of dislocations with GBs and diffusional creep, which act alone or in concert to dominate the plasticity. Building on classifying the characteristics of plastic deformation on the nano-scale, a two dimensional deformation mechanism map was proposed to comprehensively elucidate the interactive effects of grain size and strain rate on the deformation mechanisms and the related mechanical behaviors in face-centered-cubic NC Ni and Ni-based alloys.

Oxidized Ti powder was consolidated successfully by using spark plasma sintering into a fully dense bulk core-shell (CS) structured Ti alloy with isolated soft coarse-grained Ti cores and hard Ti-O solid solution shells. The temperature effect on the mechanical responses of CS Ti was assessed experimentally through tensile testing during 500–700 °C at a constant strain rate of 0.0005 s −1 and compared with those for its annealed counterpart and a pure Ti compact. All samples exhibit a decreased strength with the temperature. CS Ti exhibits a significantly enhanced tensile strength at all the testing temperatures compared with CP Ti. Contrary to the substantial brittleness of the annealed sample at 500 °C and 600 °C, CS Ti presents a much enhanced ductility at elevated temperatures, being 12% at 500 °C and 77% at 600 °C. At 700 °C, all the samples show comparable elongation. The superior mechanical performances of CS Ti at elevated temperatures can be attributed to its novel heterogeneous microstructure along with high O content.

A novel Ni foam-Ni3S2@Ni(OH)2-graphene sandwich-structured electrode (NF-NN-G) with high areal mass loading (8.33 mg cm−2) has been developed by sulfidation and hydrolysis reactions. The conductivity of Ni3S2 and Ni(OH)2 were both improved. The upper layer of Ni(OH)2, covered with a thin graphene film, is formed in situ from the surface of the lower layer of Ni3S2, whereas the Ni3S2 grown on Ni foam substrate mainly acts as a rough support bridging the Ni(OH)2 and Ni foam. The graphene stabilized the Ni(OH)2 and the electrochemical properties were effectively enhanced. The as-synthesized NF-NN-G-5mg electrode shows a high specific capacitance (2258 F g−1 at 1 A g−1 or 18.81 F cm−2 at 8.33 mA cm−2) and an outstanding rate property (1010 F g−1 at 20 Ag−1 or 8.413 F cm−2 at 166.6 mA cm−2). This result is around double the capacitance achieved in previous research on Ni3S2@Ni(OH)2/3DGN composites (3DGN=three-dimensional graphene network). In addition, the as-fabricated NF-NN-G-5mg composite electrode has an excellent cycle life with no capacitance loss after 3000 cycles, indicating a potential application as an efficient electrode.

Indentation size effect (ISE) on the hardness in the nanocrystalline (NC) (~24 nm) Ta and coarse-grained (CG) counterpart has been investigated by the nanoindentation tests at room temperature. The experimental results show that at shallow indentation depth ranging from 50 nm to 200 nm, the measured hardness is obviously smaller than the values predicted by the Nix-Gao model in the CG Ta, which is in contrast to the tendency in the NC Ta. At indentation depth less than 600 nm, obvious ISE can be observed in the both of CG and NC Ta. When the indentation depth is larger than 600 nm, the observed ISE in the CG Ta would gradually disappear in the NC Ta. Based on the deformation mechanism at different indentation depth, the density of geometrically necessary dislocations (GNDs) ( ρ G − N C ) and statistically stored dislocations (SSDs) ( ρ S − N C ) constructed by the Nix-Gao model are used to explain this phenomena.

Hu et al. [1] reported the strains measured during unloading of nanocrystalline Cu in the course of deformation at room temperature. The inelastic strain rates during unloading are found to be larger than the quasi-stationary rates. This remarkable result is interpreted in terms of recovery strain acting as a second deformation mechanism in addition to the primary mechanism of glide of free dislocations.

Plastic flow behavior of high nitrogen nickel-free austenitic stainless steel with solution, hot rolling and cold rolling treatments were examined and analyzed by using the Ludwigson equation. It was shown that this stainless steel exhibit a two-stage strain hardening behavior, which can be well described by the Ludwigson equation. Based on the Ludwigson equation and the Considere instability criterion, two equations of the uniform plastic strain and the stress ratio (true ultimate tensile strength/true yield strength) to the strain hardening exponents were derived. Based on these two equations, the dependence of the strength and ductility on the true yield strength and strain hardening behavior was discussed and connected with the observation results of microstructures.

An efficient method is demonstrated to synthesize reduced graphene oxide/CoO nanowires (rGO/CoO) mutually-supporting porous structure, which is in situ prepared by annealing self-assembled graphene oxide (GO)/Co(CO 3 ) 0.5 (OH)·0.11H 2 O nanowires composite in N 2 atmosphere. The CoO nanowires are uniformly and tightly anchored on the rGO sheet surface, preventing the aggregation of the CoO nanowires and the restacking of rGO sheets. As an anode material for lithium ion batteries (LIBs), rGO/CoO electrode displays excellent electrochemical performance including a high reversible capacity of 994 mA h g −1 even after 100 cycles with 96.3% capacity retention of the initial discharge capacity, super rate capability (718, 577, 504, 388 and 262 mA h g −1 at 0.8, 3.2, 5, 7 and 10C, respectively, 1C = 1 A g −1 ) and extremely excellent cycling performance at high rates (621 mA h g −1 at 1C after 620 cycles; 520 mA h g −1 at 3C even after 750 cycles).

Zamak 3 alloy treatment by sliding-friction treatment (SFT) was investigated by nanoindentation to explore the influence of microstructure and strain rate on nanoscale deformation at room temperature. The results show that obvious material softening occurs in the ultrafine-grained (UFG) Zn alloy and strain-hardening happens in the twinning-deformed layer, respectively. It can be concluded that almost constant values of V in the UFG Zn alloy contribute to the dislocations moving along the grain boundary (GB) not cross the grain interior. In the twinning-deformed layer, the highly frequent dislocation&ndash, twinning boundary (TB) interactions are responsible for subsequent inverse Cottrell&ndash, Stokes at lower stress, which is quite different from dislocation&ndash, dislocation reaction inside the grain in their coarse-grained (CG) counterpart.

Reducing grain size of materials down to the nanoscale regime usually leads to tremendous increase in strength yet at the expense of other key mechanical properties, including creep resistance. Here we demonstrate that nanostructuring endows a CrMnFeCoNi high-entropy alloy and a CrCoNi medium-entropy alloy with excellent combination of extraordinarily high hardness of 11–13 GPa, which is three times higher than those of their coarse-grained counterparts, and much enhanced creep resistance compared with conventional nanocrystalline metals. Grain boundary strengthening and stacking fault strengthening are the two major reasons responsible for the elevated hardness. The combined influence of low stacking fault energy and small grain sizes enables the prosperity of stacking faults, which also accounts for the higher strength of the ternary alloy than that of the quinary one. Theoretical analysis together with microstructural characterization suggest that the increasing interfacial chemical complexity suppresses grain boundary mediated processes, leading to much improved creep resistance with a dislocation-dominant creep mechanism in the nanocrystalline alloys. Our findings shed light on a new perspective for achieving simultaneous ultra-high strength and considerable structural robustness in structural materials.

Deformed coarse-grained polycrystalline metals always unload elastically where permanent dislocation network well-developed in the loading regime hinders the movement of dislocations and allows only the elastic relaxation of stress. Such elastic unloading behavior is, however, unexpected in nanocrystalline metals because the dislocation network cannot effectively form inside nanometer-scale grains. In this work, we report the experimental finding of significant plastic deformation that emerges in the unloading regime in the compressive cyclic test at room temperature of nanocrystalline Cu. The magnitude of plastic strain produced during unloading depends strongly on loading and unloading rates. This plastic unloading behavior arises from the rapid absorption of dislocations accumulated during loading, which was quantitatively interpreted by performing the incremental unloading test and developing a relationship between the dislocation density and the loading and unloading rates based on the models of the statistical absorption of dislocations by grain boundaries and the dislocation emission from grain boundary ledges. Concurrently, the evolution of deformation structures during the cyclic deformation was also analyzed in terms of the interactions of gliding dislocation–twin boundaries.

Creep behaviors of nanocrystalline (NC) Cu, Ni-20(wt.%)Fe and Ni with comparable grain sizes were examined by nanoindentation tests. It was showed that the creep behaviors depend on loading strain rate and stacking fault energy (SFE). At high loading strain rate, the initial creep rates of three NC materials are far higher than those predicted by the models of the Coble creep and the grain boundary sliding and are the highest and lowest for NC Cu and NC Ni, respectively, while that of NC Ni–20Fe lies between them due to their different SFE. At low loading strain rate, the creep behaviors do not exhibit significant difference among three NC materials. Our analysis revealed that grain boundary dislocation sources can be activated at high loading strain rate and the emitted dislocations from grain boundaries can be effectively stored in the loading regime, but cannot at low loading strain rate. The very high initial creep rates of three NC materials are attributed to the rapid absorptions of the stored dislocations in the loading regime and the newly nucleated dislocations in the holding regime. The higher creep rate of NC Cu as compared with those of NC Ni–20Fe and NC Ni arises from the higher density of the stored dislocations caused by the enhanced interactions of dislocations with twins and stacking faults due to its lower SFE.

To avoid aggregation in the production of the active electrode material, Ni–Zn system materials (NixZn1−xOH, NiO–ZnO and NixZn1−xS) were synthesized by using a belt reaction zone model, and then were characterized systematically in this work. Among these materials, NixZn1−xS porous spheroid nanoparticles with diameters ∼30 nm possess abundant interconnected micropores caused by the Kirkendall effect in the synthesis, leading to a high surface area of 148.4 m2 g−1 and special paths for ion diffusion. In the three-electrode system testing, NixZn1−xS porous spheroid nanoparticles show the highest specific capacitance of 1867 F g−1 at a current density of 1 A g−1, as well as excellent rate capability and cycling stability. Using NixZn1−xS as the positive electrode and active carbon as the negative electrode, the asymmetric supercapacitor device exhibits an excellent electrochemical performance. The results provide us with a modified method to synthesize metal hydroxides, oxides and sulfides, in order to obtain materials with high supercapacitor performance.

Nanostructured Bi–Bi2O2CO3–ZnO composites were successfully prepared through a facile one-step hydrothermal method. The composite was constructed by ZnO nanorods with a diameter of 30–40 nm embedded in Bi2O2CO3 platelets with a small quantity of Bi particles dispersed on the platelets, which further assembled into micron clusters. The photocatalytic performance of the composite was evaluated by the degradation of methyl orange and phenol under UV and visible light irradiation, respectively. The results revealed that the composites showed enhanced photocatalytic activity in comparison with either ZnO or Bi2O2CO3, and the Bi–Bi2O2CO3–ZnO photocatalyst with 20 mol% of Bi2O2CO3 exhibited the best photocatalytic efficiency. The reusability test explored the reliable stability and reusability for industrial applications, which was endowed by the special feature of nanostructured micron clusters. The superior photocatalytic activity could be attributed to the heterojunction interfaces and compound band gap structure which facilitated the separation of the photo-generated electrons and holes at the interface and promoted an effective charge transfer path. Additionally, the existing metal Bi also acted as electron traps to manifest a positive effect. The present study might give an insight into the design of novel heterostructured materials for the light harvesting related environment management and energy conversion applications.

Biocompatible coatings, such as calcium phosphate (CaP), have been applied on biodegradable magnesium and its alloy to control their rapid degradation rates. This paper reports a further enhancement to the CaP coating performance by applying a simple fluorine post-treatment on a CaP coating to form an F–CaP composite coating. The CaP coating was first deposited on AZ60 magnesium alloy samples via a phosphating method, then subjected to different post-treatment solutions to obtain F–CaP composite coatings. The effects of pH and post-treatment time were analyzed and the optimal corrosion resistance was obtained at pH 12 for 2 h. The F–CaP coating consisted of mainly fluoridated hydroxyapatite, tricalcium phosphate and magnesium fluoride. An electrochemical corrosion measurement in simulated body fluid revealed that the polarization resistance of the F–CaP coating is about 3 and 26 times higher than those of the HA and CaP coatings, respectively, due to its denser coating structure and smoother surface. During immersion tests, the F–CaP coating also exhibited an improved corrosion resistance with more active biomineralization than the CaP and HA coatings. This fluorine post-treatment offers a novel practical approach in improving the biodegradation behavior of CaP coatings on biodegradable magnesium implants.

Nanoindentation morphologies of crystalline copper have been probed at the grain scale. Experimental tests have been conducted on nanocrystalline (NC), ultrafine-grained (UFG), and coarse-grained (CG) copper samples with a new Berkvoich indenter at the strain rate of 0.04/s without holding time at an indentation depth of 2000 nm at room temperature. As the grain size increases, the height of the pile-up around the residual indentation increases and then exhibits a slightly decrease in the CG Cu. The maximum of the pile-up in the CG Cu obviously deviates from the center of the indenter sides. Our analysis has revealed that the dislocation motion and GB activities in the NC Cu, some cross- and multiple-slip dislocations inside the larger grain in the UFG Cu, and forest dislocations from the intragranular Frank-Read sources in the CG Cu would directly induce this distinct pile-up effect.

Tensile properties of an electric brush-plated nanocrystalline Cu with an average grain size of 59 nm were investigated at different strain rates. This nanocrystalline Cu exhibits an excellent combination of strength and ductility with its ultimate tensile strength increasing from 635 MPa to 1000 MPa and total elongation decreasing from 15.8% to 9.9% as strain rate increases from 10 − 4 s − 1 to 1 s − 1 . Analysis based on the characterization results of transmission electron microscopy (TEM), scanning electron microscopy (SEM) and general area detector diffraction detection system (GADDS) on the as-brush-plated and deformed NC specimens revealed that the excellent combination of strength and ductility arises from the enhanced dislocation strain hardening ability and the improved deformation accommodation role played by GB sliding.

Discontinuous shear thickening observed in many dense suspensions is rather complicated but takes place as a transient. Its details after jamming are still not clear. By using a modified rheometer with an analog-to-digital converter, we show that there are three amplitude regimes of shear strain after jamming. First, the shear modulus and the force chain angle increase with strain; the jammed state gradually deepens. Second, the jammed state is stable against a further increase in shear stress; the force chain angle is constant and the force network is stable. Third, the jammed state cannot resist a further increase in shear stress; with the increase in the force chain angle, the force chain network is broken.