Strengthening at nanoscaled coherent twin boundary in f.c.c. metals

This paper analyses slip transfer at the boundary of nanoscaled growth twins in face-centred cubic (f.c.c.) metals for strengthening mechanism. The required stress for slip transfer, i.e. inter-twin flow stress, is obtained in a simple expression in terms of stacking fault energy and/or twin boundary (TB) energy, constriction energy and activation volume. For nanotwinned Al, Cu and Ni, inter-twin flow stress versus twin thickness remarkably shows Hall–Petch relationship. The Hall–Petch slope is rationalized for various reactions of screw and non-screw dislocations at the TB. Additionally, strengthening at the boundary of nanoscaled deformation twins in f.c.c. metals is analysed by evaluating required twinning stress. At small nanograin size, the prediction of deformation twin growth stress shows inverse grain-size effect on twinning, in agreement with recent experimental finding.     

Brittle-ductile behavior of a nanocrack in nanocrystalline Ni: A quasicontinuum study

The effects of stacking fault energy, unstable stacking fault energy, and unstable twinning fault energy on the fracture behavior of nanocrystalline Ni are studied via quasicontinuum simulations. Two semi-empirical potentials for Ni are used to vary the values of these generalized planar fault energies. When the above three energies are reduced, a brittle-to-ductile transition of the fracture behavior is observed. In the model with higher generalized planar fault energies, a nanocrack proceeds along a grain boundary, while in the model with lower energies, the tip of the nanocrack becomes blunt. A greater twinning tendency is also observed in the more ductile model. These results indicate that the fracture toughness of nanocrystalline face-centered-cubic metals and alloys might be efficiently improved by controlling the generalized planar fault energies.     

Discovery,characterization and modelling of novel shape memory behaviour of fcc metal nanowires

Novel shape memory behaviour was discovered recently in single-crystalline fcc nanowires of Cu, Ni and Au with lateral dimensions below 5?nm. Under proper thermomechanical conditions, these wires can recover elongations up to 50%. This phenomenon only exists at the nanoscale and is associated with reversible lattice reorientations within the fcc lattice structure driven by surface stresses. Whereas the propagation of partial dislocations and twin planes specific to fcc metals are the required mechanism, only materials with higher propensities for twinning (e.g. Cu and Ni) show this behaviour and those with lower propensities for twinning (e.g. Al) do not. This paper provides an overview of this novel behaviour with a focus on the transformation mechanism, driving force, reversible strain, size and temperature effects and energy dissipation. A mechanism-based micromechanical continuum model for the tensile behaviour is developed. This model uses a decomposition of the lattice reorientation process into a reversible, smooth transition between a series of phase-equilibrium states and a superimposed irreversible, dissipative propagation of a twin boundary. The reversible part is associated with strain energy functions with multiple local minima and quantifies the energy conversion process between the twinning phases. The irreversible part is due to the ruggedness of the strain energy landscape, associated with dislocation nucleation, gliding and annihilation, and characterizes the dissipation during the transformation. This model captures all major characteristics of the behaviour, quantifies the size and temperature effects and yields results which are in excellent agreement with data from molecular dynamics simulations.     

Alloying effect on grain-size dependent deformation twinning in nanocrystalline Cu–Zn alloys

Grain-size dependency of deformation twinning has been previously reported in nanocrystalline face-centred-cubic metals, which results in an optimum grain-size range for twin formation. Here, we report, for the first time in experiments, the observed optimum grain sizes for deformation twins in nanocrystalline Cu–Zn alloys which slightly increase with increasing Zn content. This result agrees with the reported trend but is much weaker than predicted by stacking-fault-energy based models. Our results indicate that alloying changes the relationship between the stacking-fault and twin-fault energy and therefore affects the optimum grain size for deformation twinning. These observations should be also applicable to other alloy systems.     

Structure and stability of Σ3 grain boundaries in face centered cubic metals

Σ3 grain boundaries form as a result of either growth twinning or deformation twinning in face centered cubic (fcc) metals and play a crucial role in determining the mechanical and electrical properties and microstructural stability. We studied the structure and stability of Σ3 grain boundaries (GBs) in fcc metals by using topological analysis and atomistic simulations. Atomistic simulations were performed for Cu and Al with empirical interatomic potentials to reveal the influence of stacking fault energy on the morphology of the twinned grains. Three sets of tilt Σ3 GBs were studied with respect to the tilt axis parallel to ?111?, ?112?, and ?110?, respectively. We showed that Σ3{111} and Σ3{112} GBs are thermodynamically stable and the others will dissociate into terraced interfaces regardless of the stacking fault energy. The morphology of the nano-twinned grains in Cu is predicted from the above analysis and found to match with experiments.     

Effect of vacancies on incipient plasticity during contact loading

Atomistic simulations and experimental nanoindentation tests were used to examine the effect of vacancies on the inception of plastic deformation in Ni. Molecular dynamics have shown the effect of vacancy position on the yield load and demonstrate a variety of mechanisms which are responsible for the inception of plastic deformation during indentation. In cases where the vacancy position is close to regions of high shear stresses the nucleation of dislocations is related to the location of a vacancy; however, homogeneous nucleation of dislocations is also observed for vacancy-containing crystals. Complementary experiments have been used to demonstrate the effect of indenter size on the onset of yielding in the presence of vacancies. Both the simulations and experiments show that larger indenter tips increase the chance of weakening the material in the presence of vacancies.     

γ-TiAl金属间化合物面缺陷能的分子动力学研究

运用分子动力学方法，对γ-TiAl金属间化合物的面缺陷能(层错能和孪晶能)进行了研究. 计算得到γ-TiAl不同滑移系(或孪生系)的整体堆垛层错能曲线，结果表明，γ-TiAl较一般fcc晶体结构的金属可动滑移系(孪生系)的数量减少，在外界条件下呈脆性. 研究孪生系(1/6)〈112〉{111}的弛豫的整体堆垛层错(GSF)能和整体孪晶(GTF)能曲线，对不稳定层错能γ_usf、稳定层错能γ_sf和不稳定孪晶能γ_usf值进行分析，可以预知， γ-TiAl的主要变形机理为孪生系(1/6)〈112〉{111}的孪生和普通滑移系(1/6)〈110〉{111}的滑移，以及超滑移系(1/2)〈011〉{111}的滑移. 关键词： γ-TiAl')" href="#">γ-TiAl 堆垛层错能 孪晶能 分子动力学     

Atomistic Simulations of the Effect of Helium on the Dissociation of Screw Dislocations in Nickel

The interactions of He with dissociated screw dislocations in face-centered-cubic(fcc) Ni are investigated by using molecular dynamics simulations based on an embedded-atom method model.The binding and formation energies of interstitial He in and near Shockley partial cores are calculated.The results show that interstitial He atoms at tetrahedral sites in the perfect fcc lattice and atoms occupying sites one plane above or below one of the two Shockley partial cores exhibit the strongest binding energy.The attractive or repulsive nature of the interaction between interstitial He and the screw dislocation depends on the relative position of He to these strong binding sites.In addition,the effect of He on the dissociation of screw dislocations are investigated.It is found that He atoms homogeneously distributed in the glide plane can reduce the stacking fault width.     

Secondary particle emission from a ferromagnetic binary compound

Secondary ion emission from an ion-bombarded binary compound in the ferro- and paramagnetic states has been studied using experimental methods and molecular dynamics simulations. The experiments were performed with a widely used NiPd binary compound, which was bombarded by obliquely incident 10-keV Ar ions. It is established that the intensity of Ni⁺ and Pd⁺ ion emission from a polycrystalline NiPd sample decreases significantly when it passes from the ferromagnetic to paramagnetic state. This effect is explained by a change in the surface binding energy and density of surface states at the Fermi level and by a cumulative process related to sputtering. The energies and directions of emission of secondary particles from poly- and single-crystalline NiPd samples in the ferromagnetic state have been jointly studied. It is established that the maximum of the polar angular distribution of secondary particles deviates toward the normal to the irradiated surface with a decrease in their energy. A difference in the azimuthal distribution of emitted Ni and Pd particles has been observed during the bombardment of a (001)NiPd crystal face and explained by specific features of correlated collisions. The best agreement of experimental data with the results of molecular dynamics simulations is obtained if the calculations are performed with allowance for an experimentally established modified composition of the three uppermost surface layers [39].