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.
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•Nanoindentation creep behavior and mechanisms of nanocrystalline Ni and Ni- wt.% Fe alloy were investigated.•A loading rate-controlled mode in cooperating with continuous stiffness ...measurement technique was used.•Continuous creep strain rate versus creep stress data were obtained on each single sample.•Continuous apparent activation volume data were obtained on each single sample.•Nanocrystalline Ni and Ni- wt.% Fe alloy show different creep properties in transient and steady-state regimes.
Nanoindentation creep behavior and underlying mechanisms of nanocrystalline (NC) Ni and NC Ni-20 wt% Fe (Ni-Fe) alloy were investigated at room temperature by a new testing method. The continuous creep strain rate versus creep stress data and hence the continuous apparent activation volume versus creep stress data in the entire holding stage on each single sample were achieved under a loading rate-controlled mode by a cooperative use of the continuous stiffness measurement (CSM) technique. Furthermore, the dependence of the apparent activation volume on the creep stress was interpreted by performing a theoretical analysis. Based on the above experimental and theoretical results and the high-resolution transmission electron microscope observation of the microstructures, the effects of the loading rate and stacking fault energy on the nanoindentation creep behavior of NC Ni and NC Ni-Fe alloy and the underlying mechanisms were analyzed. It was demonstrated that NC Ni-Fe alloy shows higher creep resistance in the transient regime, but lower creep resistance in the steady-state regime compared to NC Ni. The contact stiffness data obtained by the CSM technique in a strain rate-controlled mode can be used to determine the creep stress data from the creep displacement data obtained in a loading rate-controlled mode.
Tensile properties of an electric brush-plated nanocrystalline Cu with an average grain size of 59nm were investigated at different strain rates. This nanocrystalline Cu exhibits an excellent ...combination of strength and ductility with its ultimate tensile strength increasing from 635MPa to 1000MPa and total elongation decreasing from 15.8% to 9.9% as strain rate increases from 10−4s−1 to 1s−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.
The effect of Zn addition on the microstructures and mechanical behaviors of as-cast Mg-2.5Y-1Ce-0.5Mn alloy was investigated. Microstructure observation demonstrated that with the addition of 1wt%, ...3wt% and 5wt% Zn, the ternary phases of LPSO phase, LPSO phase +W-phase and W-phase +T-phase (Mg-Zn-Ce) are precipitated orderly, and the volume fraction of eutectic phases increases. The results of tensile tests demonstrated that with increasing Zn addition, the yield strength Y S of as-cast alloys increases continuously while the ultimate tensile strength UTS and elongation δ increase nonlinearly. Based on the analysis of microstructure, nanoindentation results and deformation surfaces, it found that the Y S is increased by the increased volume fraction of hard ternary phases. The largest δ in 1wt% Zn alloy is contributed from the LPSO phase with an excellent plastic accommodation while the insufficient accommodated role of LPSO phase and the easily broken W-phase deteriorate the ductility of 3wt% Zn alloy. The hard and brittle T-phase also damages the ductility, while the fine grains contribute to the moderate elongation in 5wt% Zn alloy. The UTS mainly arises from a sustainable increase of strain hardening ability after yielding that associated with both the high yielding point and excellent ductility.
The influences of thermomechanical treatment on the microstructural evolution and nano-mechanical behaviors of CrCoNi medium-entropy alloy (MEA) were systematically studied through a series of ...nanoindentation experiments. Cold-rolling induced substantial grain refinement and during subsequent annealing, the CrCoNi MEA exhibited a highly temperature-dependent recrystallized microstructure characterized by significant variations in grain size, dislocation density, and twin fraction with increasing annealing temperature. The hardness of the cold-rolled sample increased with annealing temperature, reaching its highest value at 600 °C and then decreasing gradually. Annealing was also found to influence the creep behaviors of the CrCoNi MEAs. A cold-rolled sample annealed at 700 °C exhibited the highest resistance to creep. The results of a detailed microstructure examination, calculations, and theoretical analysis revealed that annealing twins play a key role in increasing both the strength and the creep resistance of the CrCoNi MEAs. Short-range order clusters also contributed to the strength and creep resistance.
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•The microstructural evolution induced by cold-rolling and subsequent annealing in CrCoNi medium-entropy alloy was explored.•Nano-mechanical and creep behaviors of the CrCoNi alloys experienced thermomechanical treatment were systematically investigated.•Annealing twins play a key role not only in elevating the strength but also in enhancing the creep resistance of the CrCoNi alloys.
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.
•Nanocrystalline CrMnFeCoNi HEA and CrCoNi MEA were magnetron sputtered.•Nanostructuring endows both alloys with ultra-high strength of 11–13 GPa.•Grain boundary and stacking fault contribute predominantly to the elevated hardness.•Interfacial chemical complexity leads to much improved creep resistance in both alloys.•Dislocation-dominant creep mechanism prevails in nanocrystalline HEA and MEA.
The development of transition metal-based materials to replace precious metal electrocatalysts in a facile and efficient approach is of great importance for water splitting. The NiS/MoS2 complex ...grown in situ on carbon paper (NiS/MoS2/CP) by a one-step hydrothermal method was successfully constructed as an efficient electrocatalyst. The synergetic effects of the superhydrophilic/aerophobic surface, hierarchical nanostructure and strong mechanical adhesion to a highly conductive substrate give the NiS/MoS2/CP complex outstanding catalytic activity and durability. Specifically, it exhibits relatively low overpotentials of 119 mV (at a current density of 10 mA cm−2) and 314 mV (at a current density of 100 mA cm−2) for the hydrogen evolution reaction and oxygen evolution reaction, respectively. In addition, the NiS/MoS2/CP complex can also be used as an electrolyzer that reaches a current density of 10 mA cm−2 at a cell voltage of 1.48 V without decay after 30 h of durability testing, making it an ideal electrocatalyst for water splitting towards practical application.
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•The surface of carbon paper was transformed to be super-hydrophilic/aerophobic via calcination.•NiS/MoS2 composite is grown in-situ on carbon paper by a facile one-step hydrothermal method.•The NiS/MoS2/CP complex exhibits high catalytic activity and excellent durability.
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.
Creep behaviors of nanocrystalline (NC) Cu, Ni-20(wt.%)Fe and Ni with comparable grain sizes were examined by nanoindentation tests (the above graphs are just for NC Cu). It was showed that the creep behavior depends on loading strain rate and stacking fault energy (SFE). Comparing the initial creep rates of three NC materials with those predicted by the models of the Coble creep and the grain boundary (GB) sliding could help us determine the possible mechanisms responsible for the creep deformation of NC metals and alloys in the nanoindentation tests. Display omitted
•High creep strain rate can be induced by increasing loading strain rate.•High creep strain rate can be induced by reducing SFE.•Creep deformation mechanism can be identified by Coble and GB models.•Creep behavior are related with the dislocation structure in the loading regime.
Abstract A sizing agent mainly consisting of polyethersulfone and acidified multi‐walled carbon nanotubes (MWCNT) was developed for surface modification of carbon fibers (CF) to enhance the ...interfacial bond and to form carbon fiber‐carbon nanotube (CF‐MWCNT) multiscale reinforcements. To prepare the modified carbon fiber/epoxy resin composites (CFRP), a vacuum molding technique was used in this study. The effects of the MWCNT‐containing sizing agent on the morphology and chemical structure of the CF surface were observed by scanning electron microscopy and Fourier transform infrared spectroscopy. The influence of MWCNT length and content on the mechanical properties of CFRP were also studied. The experimental results showed that the strength of the CFRP decreased with the increase of MWCNT length, and the MWCNT with a content of 0.05 wt% had a good reinforced effect on the CFRP. After the sizing treatment, the interlaminar shear strength (ILSS) of the CFRP is increased. These enhancements are attributed to the improved mechanical bonding and chemical bonding between CF and resin matrix. The MWCNT provides nanosized rough surface to CF and these acidified MWCNTs attached to the surface of CF with polar oxygen‐containing functional groups improve the compatibility with reinforcement and resin matrix. Highlights Carbon fiber was modified by a sizing agent mainly consisting of polyethersulfone and acidified carbon nanotube (CNT). CNTs enhance the interfacial chemical bonding and mechanical bonding. The CNT length affects the mechanical properties of the carbon fiber/epoxy resin composites (CFRP). The modified carbon fiber has enhanced the mechanical properties of the CFRP.
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