Abnormal ductility increase of commercial purity Al during accumulative roll bonding

In this paper, sheets of commercial purity Al were fabricated by the accumulative roll-bonding (ARB) method up to six cycles. To increase the shear deformation, no lubricant was used during the ARB processing and the samples were carried out for ARB processing without any preheat treatment. One interesting finding is that the ductility and strength both increased during the first several cycles of ARB processing. It is proposed that the initial rolling texture might play an important part in the subsequent ARB processing since the original Al sheets for ARB processing have not been subjected to any annealing. The microstructures of the specimens after each ARB cycle were investigated by transmission electron microscopy and correlated with the mechanical properties

Response of the al Σ5 〈001〉 left {310} symmetric tilt grain boundary to the shear deformation simulated by molecular dynamics

In the present study, shear response of the Al [001] symmetrical tilting Σ5 (310) grain boundary (GB) was investigated by a three dimensional bicrystal at 500~750 K. It was found that the GB gradually rotated around the [001] tilt axis during the shear deformation due to the combination of surface strain, GB sliding and GB coupled motion. These rotated grain boundaries were Σ5 asymmetrical or symmetrical tilt grain boundaries and led to the normal stress σxx in the bicrystal system. It was also found that the response of the grain boundary to the shear deformation was closely related to the temperatures. At lower temperature (500~650 K), further shear deformation was mediated by crack initiation or dislocation release which is closely related to the local stress condition and temperature etc. The lattice dislocations emitted from GB were identified as pure edge dislocations with Burgers vectors of 〈110〉/2. Interestingly, they have the [001] line direction and glide on the left curly bracket110right curly bracket planes. The reaction between grain boundary and lattice dislocations has been carefully discussed with its role in the shear deformation. At higher temperatures (above 700 K), after a short while of perfect coupling at the early stage the grain boundary quickly rotated and the two grains smoothly slid away from each other in the way of viscous grain boundary sliding under the shear deformation. 2014 by American Scientific Publishers

Molecular dynamics simulation of the grain boundary deformation behaviours in Al

Grain boundary (GB), a special solid-solid interface in materials, exists in all polycrystalline materials, thereby being closely related to physical, chemical and mechanical properties (conductivity, catalysis, segregation, corrosion, ductility and strength etc) of the polycrystalline materials, especially in ultra fine and nano crystalline materials. In the present study, the molecular dynamics method has been used to simulate the grain boundary deformation behaviours, which include grain boundary sliding (GBS) behaviour, grain boundary coupling behaviour and the interaction between grain boundaries and dislocations. The following results have been obtained. First, grain boundary sliding behaviour was strongly sensitive to the temperature. At low temperature, the GBS was very sensitive to the imposed ways of external applied forces. However, at high temperature, the two grains smoothly slid away from each other in the way of viscous grain boundary sliding under the shear deformation. Regardless of the type of driving force and misorientation, the bicrystal system tended to resist the applied force by GB rotation. Moreover, the grain boundary rotated among some of the Σ5 asymmetrical and symmetrical tilt GB boundaries under shear deformation and led to the normal stress σxx in the bicrystal system. Under particular circumstances, reaction of the grain boundary dislocations during shear deformation could release uncommon edge lattice dislocations from the grain boundary. The uncommon edge lattice dislocations with/2 Burgers vectors have the [001] line direction and glide on the {110} planes

Microstructural evolution and mechanical property of AA5050 alloy deformed by accumulative roll bonding

In this study, ultrafine-grained AA5050 sheets were fabricated by the accumulative roll bonding (ARB) process. Transmission electron microscope observations showed that at the early stage of ARB, the grain size was reduced in the normal direction and became elongated along the rolling direction. The elongated grains were cut out by dense dislocations, which then tangled and condensed, resulting in the formation of dislocation cells. As the deformation proceeded, the dislocation cells evolved to sub-grain boundaries and then grain boundaries. The ultrafinegrained microstructure was obtained via four ARB cycles. The tensile tests at 473 K and 523 K (200 degrees C and 250 degrees C) showed large elongations for strain rates of 1 × 103 s-1 and 1 × 104 s-

Coupled grain boundary motion in aluminium: The effect of structural multiplicity

The shear-induced coupled grain boundary motion plays an important role in the deformation of nanocrystalline (NC) materials. It has been known that the atomic structure of the grain boundary (GB) is not necessarily unique for a given set of misorientation and inclination of the boundary plane. However, the effect of the structural multiplicity of the GB on its coupled motion has not been reported. In the present study we investigated the structural multiplicity of the symmetric tilt Σ 5(310) boundary in aluminium and its influence on the GB behaviour at a temperature range of 300 K-600 K using molecular dynamic simulations. Two starting atomic configurations were adopted in the simulations which resulted in three different GB structures at different temperatures. Under the applied shear deformation each GB structure exhibited its unique GB behaviour. A dual GB behaviour, namely the transformation of one GB behaviour to another during deformation, was observed for the second starting configuration at a temperature of 500 K. The atomistic mechanisms responsible for these behaviour were analysed in detail. The result of this study implicates a strong relationship between GB structures and their behaviour, and provides a further information of the grain boundary mediated plasticity in nanocrystalline materials

Molecular dynamics study on the atomic mechanisms of coupling motion of [0 0 1] symmetric tilt grain boundaries in copper bicrystal

Recent research has revealed that some grain boundaries (GBs) can migrate coupled to applied shear stress. In this paper, molecular dynamics (MD) simulations were performed on sixteen [0 0 1] symmetric tilt GBs of bicrystal Cu to identify atomic-scale GB migration mechanisms and investigate their dependence on GB structure. The misorientation angles (θ) of the sixteen GBs cover the interval from 0° to 90° and a wide range of Σ values. A general method was proposed to explore the possible GB structures for each misorientation angle. Molecular statics simulation at a temperature of 0K were carried out first to determine the equilibrium and some possible metastable structures of the sixteen investigated [0 0 1] GBs. MD simulations were then conducted on the bicrystal models at equilibrium by applying a shear strain parallel to the GB plane. Shear deformation caused the tangential translation of the grain and induced normal motion of the GBs. This boundary coupling motion was present in the entire range of misorientation angles. Different mechanisms of coupled boundary motion at atomic scale were carefully examined in this work. The common feature of these mechanisms can be regarded as the displacement of local atoms and rotation of certain structure unit. Structure phase transformation of GB was found during the migration of Σ17(4 1 0) and Σ73 (8 3 0) GBs

Molecular dynamics study on the grain boundary dislocation source in nanocrystalline copper under tensile loading

Grain boundary (GB) is the interface between different oriented crystals of the same material, and it can have a significant effect on the many properties of materials. When the average or entire range of grain size is reduced to less than 100 nm, the conventional plastic deformation mechanisms dominated by dislocation processes become difficult and GBmediated deformation mechanisms become increasingly important. One of the mechanisms that can play a profound role in the strength and plasticity of metallic polycrystalline materials is the heterogeneous nucleation and emission of dislocations from GB. In this study, we conducted molecular dynamics simulations to study the dislocation nucleation from copper bicrystal with a number of 〈1 10〉 tilt GBs that covered a wide range of misorientation angles (θ).Wewill show from this analysis that the mechanic behavior of GBs and the energy barrier of dislocation nucleation from GBs are closely related to the lattice crystallographic orientation, GBenergy, and the intrinsic GBstructures. An atomistic analysis of the nucleation mechanisms provided details of this nucleation and emission process that can help us to better understand the dislocation source in GB

A molecular dynamics simulation of fracture in nanocrystalline copper

A large-scale molecular dynamics simulation was used to investigate the propagation of cracks in three dimensional samples of nanocrystalline copper, with average grain sizes ranging from 5.34 to 14.8 nm and temperatures ranging from 1K to 500 K. It was shown that intragranular fracture can proceed inside the grain at low temperature, and plastic deformation around the tip of the crack is accommodated by dislocation nucleation/emission; indeed, both fully extended dislocation and deformation twinning were visible around the tip of the crack during fracture. In addition, due to a higher concentration of stress in front of the crack at a relative lower temperature, it was found that twinning deformation is easier to nucleate from the tip of the crack. These results also showed that the decreasing grain size below a critical value exhibits a reverse Hall-Petch relationship due to the enhancing grain boundary mediation, and high temperature is better for propagating ductile cracks. © (2013) Trans Tech Publications, Switzerland

Brittle versus ductile behaviour of nanotwinned copper: A molecular dynamics study

Nanotwinned copper (Cu) exhibits an unusual combination of ultra-high yield strength and high ductility. A brittle-to-ductile transition was previously experimentally observed in nanotwinned Cu despite Cu being an intrinsically ductile metal. However, the atomic mechanisms responsible for brittle fracture and ductile fracture in nanotwinned Cu are still not clear. In this study, molecular dynamics (MD) simulations at different temperatures have been performed to investigate the fracture behaviour of a nanotwinned Cu specimen with a single-edge-notched crack whose surface coincides with a twin boundary. Three temperature ranges are identified, indicative of distinct fracture regimes, under tensile straining perpendicular to the twin boundary. Below 1.1 K, the crack propagates in a brittle fashion. Between 2 K and 30 K a dynamic brittle-to-ductile transition is observed. Above 40 K the crack propagates in a ductile mode. A detailed analysis has been carried out to understand the atomic fracture mechanism in each fracture regime