Anelasticity of nanocrystalline metals

Nanocrystalline (n-) materials posses a characteristic ultrafine structure where small crystallites less than 100 nm in diameter are connected by highly disordered grain boundaries (GBs). Although the recent mechanical tests on the high-density n-metals suggest that the dynamic modulus is close to the polycrystalline metals, we have found a large anelastic strain comparable with the elastic strain in the quasi-static tests. The estimated activation energy as low as 0.2 eV suggests that certain cooperative motions of many atoms in the GBs are responsible for this large anelastic strain. The strain amplitude dependence (SAMD) in the resonant frequency, f, was measured by the vibrating reed technique, where f decreased by about 1% with increasing strain, , up to 10⁻⁴ and then turned to increase showing the saturation at =10⁻⁴–5×10⁻³. This strange SAMD in f showed no change by the low-temperature irradiation, in contrast, f showed a large increase due to the accumulation of the point defects probably in the GBs during the irradiation. We surmise that the (an)elastic properties of n-metals are governed by the several processes not only in the GBs but also in the crystallites.     

Ni-Al-Re/Ru合金的层错能

使用置换原子计算层错能的热力学模型计算了Ni-Al-Re(Ru)合金的层错能,研究了合金元素和温度对层错能的影响.结果表明:随着温度的提高,Ni-Al-Re(Ru)合金的层错能增加.Ni-6?-4%Re合金的层错能随着温度的提高线性增加.Ni-6%Al-4%Ru合金的层错能,在温度低于500℃时随着温度的提高呈抛物线规律增加,高于500℃时随着温度的提高呈线性规律增加.Al原子可明显降低Ni-6%Al-4%Re(Ru)合金的层错能.随着Al含量的提高,原子偏聚自由能(ΔGγ→εs)降低,使Al原子自发偏聚,并促进γ'有序相的形成和数量的增加,是合金层错能降低的主要原因.而Ru原子降低合金的偏聚自由能,可提高γ'有序相的稳定性.随着温度的升高,原子偏聚引起的自由能(ΔGγ→εs)增加可抑制原子偏聚;当温度高于500℃时,含Ru合金有比Ni-Al-Re合金高的原子偏聚自由能(ΔGγ→εs),故元素Ru能抑制原子的偏聚和TCP相的析出.     

Stacking fault directed growth of thin ZnO nanobelt

Thin ZnO nanobelts with an average width of 7.5 nm have been synthesized using vapor-phase transport method. It was found that stacking faults directed the growth of the thin nanobelts along the <01 1¯0> direction with {2¯ 1¯10} top/bottom surfaces and {0001} side surfaces. The {0002} stacking fault with translation of 1/3<01 1¯0> extends throughout the entire length of the ZnO nanobelts. The growth steps at the {01 1¯0} growth fronts resulted from the {0002} stacking fault are believed to direct fast axial growth of the thin ZnO nanobelts. The thin ZnO nanobelts are expected to be promising candidates for highly sensitive chemical and biological sensor applications.     

Nonlinear microstructural constitutive equation of nanocrystalline metals

Summary. Based on experimental observations, nanocrystalline materials are modeled as composite systems in which the amorphous interfacial phase is treated as the matrix, whereas the nano-scale single crystals are modeled as inclusions. Generally speaking, the elastic moduli of nanoscale crystals are higher than those of the amorphous matrix phase, and the deformation mechanism of nanocrystalline materials depends heavily on the size of the crystals. For conventional macro size crystal materials, such as coarse-grained polycrystalline materials, the deformation mechanism due to dislocation movement is dominant. When the crystal size is reduced to a certain critical value, plastic deformation is caused by shear banding in the amorphous matrix. In order to model such a deformation mechanism in nanocrystalline materials, constitutive equations are established based on internal variable theory. The proposed model reveals the relation between the yield strength and the grain size of the material.     

Embrittlement of nanocrystalline nickel by liquid metals

Metallographic and fractographic tests of liquid metal embrittlement are performed for nanocrystalline Ni-Hg systems. It is shown that the behaviour of nanocrystalline nickel under these conditions is close to that of ordinary polycrystalline materials. The presence of a stage of subcritical crack growth is demonstrated. As nanocrystalline grains have none of their own intrinsic dislocations, it is assumed that subcritical crack growth in liquid metal environment can be realized through the mechanism of dissolution of atoms from the crack tip. This dissolution-condensation model of liquid metal embrittlement, developed for polycrystals, can also be applied to nanocrystals.     

Improving the ductility of nanocrystalline bcc metals

Nanocrystalline metals present extremely high yield strengths but limited ductility. Using atomistic simulations, we show that the fracture resistance of bcc nanocrystalline materials increases with decreasing grain size below a critical grain size. There appears to be a "most brittle" grain size corresponding to the "strongest size" that has been postulated. Impurities that strengthen the grain boundaries can improve ductility significantly for the relatively larger grain sizes, whereas ductility decreases for the smallest grain sizes.     

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.     

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.     

On the intrinsic ductility of electrodeposited nanocrystalline metals

While nanocrystalline materials hold promise for structural applications in which increased strength is beneficial, their adoption has been hindered by concerns over the achievable ductility, resulting largely from considerable data scatter in the literature. A statistically significant set of 147 electrodeposited nanocrystalline tensile specimens was used to investigate this topic, and it was found that while necking elongation obeys similar processing quality and geometrical dependencies as conventional engineering metals, the intrinsic ductility as measured by uniform plastic strain was unexpectedly independent of microstructure over the grain size range of 10–80 nm. This indicates that the underlying physical processes of grain boundary-mediated damage formation are strain-oriented phenomena that can be defined by a critical plastic strain regardless of the strength of the material as a whole.     

Stacking fault energy and planar defects on the basal plane in the beta-alumina system

Commercial forms of beta alumina have been examined using transmission electron microscopy. Measurements of the stacking fault energy on the basal plane have been made from the separation of partial dislocations and the equilibrium separation of partials at dislocation nodes, and estimated as 0.6 to 1.65×10^–3 Jm^–2. The structure of two- and three-block beta-alumina has been discussed and their relationship and transformation examined. It has been shown that transformation from two- to three-block beta-alumina cannot be accomplished by a simple shear. Structures generated by the passage of partial dislocations on the glide plane are discussed, and simple twining on the basal plane is examined and shown to be possible in the three-block, but not in the two-block material.     

Fatigue properties and crack behavior of ultra-fine and nanocrystalline pure metals

The fatigue behavior of metals is strongly governed by the grain size variation. As the tensile strength, the fatigue limit increases with decreasing grain size in the microcrystalline (mc) regime. A different trend in mechanical properties has been demonstrated in many papers for metals with ultra-fine (<1 μm) (ufg) and nanocrystalline (<100 nm) (nc) grain size in particular in the yield stress and fatigue crack initiation and growth. In the present paper the fatigue properties of pure metals (Al, Ti, Ni and Cu) produced via equa-channel-angular pressing (ECAP) is shown. The mechanical properties and in particular the fatigue behavior of electrodeposited nanocrystalline Ni (20 and 40 nm mean grain size) has been analyzed in the present paper by means of stress- and strain-controlled tests and the results compared with those of the ultra-fine grain counterpart (270 nm mean grain size). The fatigue crack initiation and growth of the described materials were studied. The high cycle fatigue and crack behavior of nanocrystalline electrodeposited cobalt has been analyzed in this paper by means of stress-controlled tests and the results compared with those of the microcrystalline counterpart. The fatigue crack initiation and growth of the described materials was studied over a broad range of stress levels.     

Phonon softening on the specific heat of nanocrystalline metals

Specific heat enhancement in several common nanocrystalline metals is established by comparison with their bulk counterparts. Measurements were carried out in Fe, Cu, Ni and binary alloy LaAl(2). The excess specific heat is evidenced as a low temperature peak below 65 K and a high temperature slope above 150 K. The experimental data are in good agreement with a model which considers contributions from the grain boundary and core atoms in the nanoparticles. This model is supported by Raman spectroscopy measurements, showing a softening of the frequency phonon modes associated with a size reduction and increase of the atomic disorder.     

Surface energies of hcp metals using equivalent crystal theory

The equivalent crystal theory method of Smith et al. [Phys. Rev. B 44 (1991) 6444] originally formulated for fcc and bcc metals, and semiconductors, is here extended to hcp metals and applied to calculate surface energies. The (0 0 1) surface energies obtained for 22 hcp metals are in good agreement with the results of both experiment and ab initio calculations.     

Dislocation junctions as indicators of elementary slip planes in body-centered cubic metals

In body-centered cubic (bcc) metals, an unambiguous determination of the elementary slip planes remains difficult owing to several possible interpretations of the glide activity, of slip steps on the specimen surface or features of the dislocation microstructure. In this article, a method is proposed to determine the elementary slip planes in bcc metals based on the line directions of sessile junctions resulting from the interaction of mobile dislocations with \({a}/2\langle 111\rangle \)  Burgers vector. The proposed method allows to determine slip activity inside a material and not at its surface, where other effects may play a role. It is in principle applicable to determining the elementary slip plane in any crystalline material. Particularly, it may help to resolve a long-standing debate of the nature of the elementary slip planes in bcc metals.     

Synthesis and properties of nanocrystalline metals and alloys prepared by mechanical attrition

The grain size in powder samples can be reduced to nanometer scales during heavy cyclic mechanical deformation as produced in a standard ball mill. For pure bcc and hcp metals, intermetallic compounds and solid solutions, nanocrystalline materials can be synthesized at temperatures close to room temperature with a grain size ranging from 5 to 15 nm. During this process three different stages have been observed with (i) the deformation being localized in shear bands, (ii) formation of small angle grain boundaries separating the individual grains and (iii) formation of large angle grain boundaries with a completely random orientation of the nanosized grains. Thermal analysis of these samples reveals excess energies of up 40% of the heat of fusion and excess heat capacities of up to 20% in comparison to the undeformed state thus exceeding by far any values determined for conventional deformation processes and the energy of grain boundaries in fully equilibrated polycrystalline samples. These thermophysical data are in agreement with a theoretical model adopting a free volume approach for the grain boundaries based on the universal equation of state at negative pressure.