纳晶金属材料氢脆的微观力学模型

针对纳晶金属材料的氢脆问题,该文提出一个新的理论模型。在该模型的理论框架内,认为堆积在距裂纹尖端最近晶界上的氢原子会阻止位错从裂纹尖端的发射,从而抑制裂纹的钝化,同时促进纳晶金属材料的脆性断裂。该文在有氢和无氢两种条件下,对纳晶镍的临界应力强度因子与晶粒尺寸之间的相互关系进行了对比。结果表明:由于氢原子的脆化作用,纳晶镍的临界应力强度因子下降30％之多,这种氢致纳晶金属材料脆化的现象随其晶粒尺寸的变小而愈加显著。     

Structure,composition, and mechanical properties of thin films of transition metals diborides

The effect of the deposition conditions on the structure, composition, and mechanical properties of thin films of diborides of transition metals that have been produced by high frequency magnetron sputtering. It has been shown that depending on the applied bias voltage and substrate temperature coatings of various structures are formed: from amorphous-like to nanocrystalline. Under the optimal energy conditions (bias voltage 50 V and substrate temperature 500°C) superstoichiometric thin films of transition metals diborides of grain sizes 20–30 nm, hardness 44 GPa, and anomalously high recovering of the imprint depth have been produced.     

Nanocrystalline metal/carbon composites produced by supersonic cluster beam deposition

In this work we show that supersonic cluster beam deposition is a viable method for the synthesis of nanocrystalline metal/carbon composites. By assembling carbon and metallic clusters seeded in a supersonic beam, we have grown films consisting of metal nanoparticles embedded in a nano-structured carbon matrix. Samples containing 3d transition metals (Ti, Ni) and noble metals (Au, Pd, Pt) with different metal abundances, particle size and dilution have been characterized by transmission electron microscopy. The influence of different metals on the structure of the carbon matrix has been investigated. Spatially resolved ultraviolet photoemission electron spectroscopy showed substantial surface oxidation of 3d transition metal clusters. On a micrometric scale, the spatial distribution of the metallic nanoparticles appeared to be homogeneous.     

Flexible cobalt phosphide network electrocatalyst for hydrogen evolution at all pH values

High-performance electrocatalysts for water splitting at all pH values have attracted considerable interest in the field of sustainable hydrogen evolution.Herein,we report an efficient electrocatalyst with a nanocrystalline cobalt phosphide (CoP) network for water splitting in the pH range of 0-14.The novel flexible electrocatalyst is derived from a desirable nanocrystalline CoP network grown on a conductive Hastelloy belt.This kind of self-supported CoP network is directly used as an electrocatalytic cathode for hydrogen evolution.The nanocrystalline network structure results in superior performance with a low onset overpotential of ～45 mV over a broad pH range of 0 to 14 and affords a catalytic current density of 100 mA·cm-2 even in neutral media.The CoP network exhibits excellent catalytic properties not only at extreme pH values (0 and 14) but also in neutral media (pH =7),which is comparable to the behavior of state-of-the-art platinum-based metals.The system exhibits an excellent flexible property and maintains remarkable catalytic stability during continuous 100-h-long electrolysis even after 100 cycles of bending/extending from 100° to 250°.     

On the nanocrystalline to glass transition during ball milling

Mechanical alloying performed by ball milling metallic powders leads to a nanocrystalline state and metastable phases such as supersaturated solid solutions and amorphous phases. The nanocrystalline state may act as a transition state for the crystal to glass transition. Assuming polymorphic (or partitionless) melting of a nanocrystalline supersaturated solid solution, it is found that a critical nanograin size for amorphization may be defined. This critical size depends on the concentration of the supersaturated solid solution.Application to the Zr based hexagonal solid solution Zr-Ni allows a quantitative evaluation of this effect. It is shown that for nanocrystalline size the classical T₀ curve is significantly lowered in temperature, yielding a polymorphous crystal to glass transition for smaller nickel concentration than for conventional crystalline sizes. Therefore, both supersaturation and grain refining to nanocrystalline dimensions work towards an easier amorphization by ball milling.     

Modelling the strongest grain size in nanocrystalline FCC metals

A physical model is proposed to predict the critical grain size at which nanocrystalline FCC metals reach a maximum steady state flow stress. The model considers that nanocrystalline metals are composed of two phases. One is the grain boundary phase and the other is the grain interior phase. The grain boundary phase has specific deformation mechanism different to the grain interior phase. The critical grain size with the maximum steady state flow stress is predicted to decrease with deformation temperature and to increase with strain rate. Both normal and abnormal Hall–Petch relations can be described simultaneously by the model.     

Using Powder Metallurgy and Oxide Reduction to Produce Eco-Friendly Bulk Nanoporous Nickel

A low cost and scalable powder-based method for producing nanoporous Ni is demonstrated, and the only processing by-product is water vapor. This is achieved by creating a metal–matrix composite of Ni and NiO that is then reduced under dilute hydrogen to create pure Ni with nanocrystalline structure and nanoscale porosity. The technique is demonstrated in loose powder and in thin and bulk compacts with centimeter dimensions. These compacts can withstand extensive deformation without disintegrating, performing similarly to wrought Ni in specific strength. The unique processing strategy and resulting material can be readily extended to other metals and alloys.     

In-situ X-ray-diffraction studies of hydrogenated nanocrystalline gadolinium films

There is a great deal of interest in ultra-fine grained and nanocrystalline microstructure as a means of achieving enhanced strengths and interesting combinations of properties. Thin Pd-capped rare-earth metallic films switch reversibly from their initial reflecting state (metal phase) to visually transparent state (insulator or semiconductor phase) when exposed to gaseous hydrogen. Reversion to the reflecting state is achieved by exposure to air. Palladium-capped nanocrystalline gadolinium films with different grain sizes were prepared by rf-sputtering technique. Exposure of these metallic films to hydrogen resulted in formation of hydrides and increased disorder. The microstructure of the nc-Gd films were characterized by in-situ X-ray-diffraction studies during hydrogen loading. The grain size, the microstrain, and the lattice parameters were determined.     

A model of hydrogen absorption by metals

The influence of the electron spectrum of transition metals on the hydrogen absorption process is considered. An absorption model is proposed where electrons, followed by protons, from adsorbed molecules or atoms of hydrogen transit into the metal. In terms of this model, the driving force of the absorption process ΔX is equal to the difference of the total electron energies in a hydrogen atom and in a metal at the Fermi level: ΔX = E _H ? (E _F + A _work). The equilibrium state in the absorption process corresponds to the equation 13.53 = E _F + A _work, which makes it possible to find the Fermi energy of transition metals and the valence of hydrogen absorbing metals, to define the molecular formula of hydrides, and to calculate the maximum hydrogen content in hydride phases. The hydrogen content in the composition of hydrides is shown to decrease in approaching the middle of the transition-metal rows. In groups, on the contrary, the hydrogen content grows as the atomic weight of metals increases. A good agreement of calculated and experimental compositions of hydride phases is found for the transition metals whose d band is less than half-filled with electrons.     

Effect of a crystalline structure on the ion-electron emission of the Al + 6% Mg alloy

The ion-electron emission of nanocrystalline aluminum alloy Al + 6% Mg is investigated. The nanocrystalline structure in the alloy was manufactured by torsional deformation under quasi-hydrostatic pressure. The ion-electron emission coefficient for the nanocrystalline alloy sample is higher than that of the coarsely crystalline sample. The increase in the coefficient is caused by the changes in the electron structure of the metals due to their deformation. In particular, it is demonstrated that the electron work function for the alloy Al + 6% Mg with a crystalline structure is 0.4 eV lower than that for the coarsely crystalline structure.     

Deformation-mechanism map for nanocrystalline metals by molecular-dynamics simulation

Molecular-dynamics simulations have recently been used to elucidate the transition with decreasing grain size from a dislocation-based to a grain-boundary-based deformation mechanism in nanocrystalline f.c.c. metals. This transition in the deformation mechanism results in a maximum yield strength at a grain size (the 'strongest size') that depends strongly on the stacking-fault energy, the elastic properties of the metal, and the magnitude of the applied stress. Here, by exploring the role of the stacking-fault energy in this crossover, we elucidate how the size of the extended dislocations nucleated from the grain boundaries affects the mechanical behaviour. Building on the fundamental physics of deformation as exposed by these simulations, we propose a two-dimensional stress-grain size deformation-mechanism map for the mechanical behaviour of nanocrystalline f.c.c. metals at low temperature. The map captures this transition in both the deformation mechanism and the related mechanical behaviour with decreasing grain size, as well as its dependence on the stacking-fault energy, the elastic properties of the material, and the applied stress level.     

Growth kinetics of nc-Si:H deposited at 200 °C by hot-wire chemical vapour deposition

We report on the growth kinetics of hydrogenated nanocrystalline silicon, with specific focus on the effects of the deposition time and hydrogen dilution on the nano-structural properties. The growth in the crystallite size, attributed to the agglomeration of smaller nano-crystallites, is accompanied by a reduction in the compressive strain within the crystalline region and an improved ordering and reduction in the tensile stress in the amorphous network. These changes are intimately related to the absorption characteristics of the material. Surface diffusion determines the growth in the amorphous regime, whereas competing reactions between silicon etching by atomic hydrogen and precursor deposition govern the film growth at the high-dilution regime. The diffusion of hydrogen within the film controls the growth during the transition from amorphous to nanocrystalline silicon.     

The nanocrystalline structure formation in germanium subjected to severe plastic deformation

The peculiarities of nanocrystalline (NC) structure formation in germanium subjected to severe plastic deformation (SPD) are examined in this paper. Transmission electron microscopy (TEM), X-ray analysis and differential scanning calorimetry were employed in the structural study of germanium specimens. The crystal-to-amorphous transition induced by SPD in germanium is observed. The NC structure formation is the result of annealing at 850°C. Crystallites in the NC state have non-equilibrium grain boundaries (GB) and a particular “spread” diffraction contrast, observed by TEM, testifies to this.     

Microcrystalline to nanocrystalline silicon phase transition in hydrogenated silicon-carbon alloy films

Different classes of interesting materials (such as protocrystalline, microcrystalline and nanocrystalline) have been grown under conditions very near to those for the microcrystalline phase. In spite of the importance of these materials, a clear picture regarding their phase transitions is missing. A smooth transition from the microcrystalline to the nanocrystalline silicon phase, distinctly different from an abrupt order-disorder phase transition, has been demonstrated, for the first time, in hydrogenated silicon-carbon alloy films, prepared from a silane-methane gas mixture highly diluted in hydrogen, by varying the rf power in a plasma enhanced chemical vapour deposition system. The study has also provided the signature of medium range order in hydrogenated silicon-carbon alloy films.     

Revealing the atomistic deformation mechanisms of face-centered cubic nanocrystalline metals with atomic-scale mechanical microscopy: A review

Nanocrystalline metals have many functional and structural applications due to their excellent mechanical properties compared to their coarse-grained counterparts. The atomic-scale understanding of the deformation mechanisms of nanocrystalline metals is important for designing new materials, novel structures and applications. The review presents recent developments in the methods and techniques for in situ deformation mechanism investigations on face-centered-cubic nanocrystalline metals. In the first part, we will briefly introduce some important techniques that have been used for investigating the deformation behaviors of nanomaterials. Then, the size effects and the plasticity behaviors in nanocrystalline metals are discussed as a basis for comparison with the plasticity in bulk materials. In the last part, we show the atomic-scale and time-resolved dynamic deformation processes of nanocrystalline metals using our in-lab developed deformation device.     

Grain size effect on deformation twinning and detwinning

This article systematically overviews the grain size effect on deformation twinning and detwinning in face-centered cubic (fcc) metals. With decreasing grain size, coarse-grained fcc metals become more difficult to deform by twinning, whereas nanocrystalline (nc) fcc metals first become easier to deform by twinning and then become more difficult, exhibiting an optimum grain size for twinning. The transition in twinning behavior from coarse-grained to nc fcc metals is caused by the change in deformation mechanisms. An analytical model based on observed deformation physics in nc metals, i.e., grain boundary emission of dislocations, provides an explanation of the observed optimum grain size for twinning in nc fcc metals. The detwinning process is caused by the interaction between dislocations and twin boundaries. Under a certain deformation condition, there exists a grain size range where the twinning process dominates over the detwinning process to produce the highest density of twins.     

高能球磨Mg-Zr-Ni合金组织演变

为了改善Mg-Ni合金的电化学性能,采用高能球磨技术合成了Mg-Zr-Ni储氢合金,通过改变球磨条件和添加合金元素Zr,利用XRD物相分析和电化学测量技术,研究了Mg-Ni合金的组织演变过程及其对电化学容量的影响.结果表明,高能球磨Mg-Ni和Mg-Zr-Ni合金都经历了非晶态向纳米晶态的转变过程,用少量Zr替代部分Mg后,促进了高能球磨Mg-Zr-Ni合金的非晶化和纳米晶化的过程.与非晶态Mg(Zr)Ni相比,纳米晶的Mg(Zr)Ni中氢更易放出,放电曲线主要呈现高电位放电特征,添加Zr后合金的放电容量有所下降.     

纳米金属块体材料力学性能研究进展

主要总结了纳米金属块材的力学性能的最新研究进展，包括弹性和滞弹性、强度和硬度、拉伸和压缩塑性力学性能、断裂韧度、蠕变和超塑性、疲劳和磨损性能，与粗晶多晶材料进行了比较；探讨了影响纳米金属力学行为的本征和非本征因素；并简要分析了纳米晶金属的形变机制。     

High-Field Galvanomagnetic Properties and Structure of Ni-Mn-Ga Nanocrystalline Alloys with Shape Memory

Ni-Mn-Ga based compounds are of great interest due to their magneto- and temperature operated shape memory. To obtain new results on their electronic structure, we studied the low-temperature kinetic properties and the structure of Ni-Mn-Ga alloys at temperatures T?T _M , T _C (T _M is the temperature of the martensitic transition and T _C the temperature of the magnetic (Curie) transition, which are both close to room temperature). Ordered (cast) and disordered samples, having a nanocrystalline substructure, were investigated. The galvanomagnetic and electrical properties were measured in the temperature interval from 2 to 80 K and in magnetic fields of up to 15 T. We find that the electrical and the high-field properties of these alloys strongly change due to the transition into the nanocrystalline state.     

Nanostructured Ti-catalyzed MgH2 for hydrogen storage

Nanocrystalline Ti-catalyzed MgH(2) can be prepared by a homogeneously catalyzed synthesis method. Comprehensive characterization of this sample and measurements of hydrogen storage properties are discussed and compared to a commercial MgH(2) sample. The catalyzed MgH(2) nanocrystalline sample consists of two MgH(2) phases-a tetrahedral β-MgH(2) phase and an orthorhombic high-pressure modification γ-MgH(2). Transmission electron microscopy was used for the observation of the morphology of the samples and to confirm the nanostructure. N(2) adsorption measurement shows a BET surface area of 108 m(2) g(-1) of the nanostructured material. This sample exhibits a hydrogen desorption temperature more than 130?°C lower compared to commercial MgH(2). After desorption, the catalyzed nanocrystalline sample absorbs hydrogen 40 times faster than commercial MgH(2) at 300?°C. Both the Ti catalyst and the nanocrystalline structure with correspondingly high surface area are thought to play important roles in the improvement of hydrogen storage properties. The desorption enthalpy and entropy values of the catalyzed MgH(2) nanocrystalline sample are 77.7 kJ mol(-1) H(2) and 138.3 J K(-1) mol(-1) H(2), respectively. Thermodynamic properties do not change with the nanostructure.