Angelina Khalitovna Akhunova

The molecular dynamics simulation is used to analyze the features of the deformation behavior and the process of fracture of graphene with dislocation dipoles with different arm. Moreover, the wrinkling of graphene during deformation is taken into account, which greatly reduces the strength of graphene. It has been established that an increase in temperature slightly affects the mechanical properties of graphene with dislocation dipoles, in contrast to defect-free graphene and graphene with a Stone–Wales defect. It is shown that a change in the distance between dislocations in a dipole does not significantly affect the elastic modulus and graphene strength. However, the presence of dislocation dipoles can affect graphene wrinkling during stretching.

The search for new materials for electronic or nanomechanical devices is of great importance today. Such 2D nanostructures as graphene have attracted much attention due to their unique physical and mechanical properties. Graphynes are a class of new nanostructures with sp- and sp2-hybridized carbon atoms arranged in a crystal lattice, which exhibit high strength and attractive electronic properties. In the present work, molecular dynamics is used to study the mechanical behavior of graphynes under tension at 0 K. Graphene, four graphynes β₁-, β₃-, γ₁-, γ₂- graphynes, and γ₁‑graphdiyne were considered. Tensile deformation is applied in such a way to allow wrinkling of the 2D structure. Two deformation mechanisms were found for all the considered structures: bond elongation and rotation combined with wrinkling/flattening of the 2D structure. It was found, that the higher the density of the structure, the higher the ultimate tensile strength under tension. Initially, weaker bonds between carbon atoms contributed to the tensile deformation, while at high strain, acetylenic bonds began to contribute. Changes in the valent angles also contribute significantly to the tensile behavior at different deformation stages, becoming critical at the final deformation stage. Tensile deformation is accompanied by the continuous wrinkling and then flattening of the 2D structure. Understanding the deformation behavior of such a novel structures as graphynes is of great importance for their future application in new electronic devices.

Effect of the Dislocation Dipoles with Different Arms on the Graphene Deformation Behavior: molecular dynamics

This research paper studies the fracture and mechanical properties of rippled graphene containing dislocation dipoles. The atomistic simulation is performed to study the deformation behavior of pristine and defective wrinkled graphene. Graphene wrinkling considerably decreases the ultimate tensile strength of graphene with and without defects but increases the fracture strain. For graphene with the dislocation dipoles, temperature increase slightly affects mechanical properties, in contrast to graphene and graphene with Stone–Wales defect. The extremely similar slopes of the stress-strain curves for graphene with the dislocation dipoles with different arms imply that the distance between dislocations in the dipole does not have noticeable effects on the elastic modulus and strength of graphene. Defects in graphene can also affect its wrinkling; for example, preventing wrinkle formation.

Computer simulation of pressure welding of cylindrical workpieces through an interlayer was carried out using the DEFORM-2D software package in a two-dimensional formulation (axisymmetric deformation).

Finite element modeling of the stress-strain state of disk-shaft type parts of gas turbine engines during pressure welding has been made. Pressure welding was carried out by three schemes of deformation of a disk and/or a shaft. In Scheme 1, the shaft was inserted into the disk by setting the displacement of the upper die, in Scheme 2 simultaneous action of a die on the upper end surface of the disk and shaft was set, and in Scheme 3 the shaft was inserted into the disk by setting the displacement of the die, then the action of the die on the upper end surface of the disk was set. Computer simulation is performed using the DEFORM-2D software package. The deformable nickel base alloys EP975 and EK79 were taken as materials for a disk and a shaft, respectively. The results of computer simulation have shown that for increasing quality of the welded joint between the disk and the shaft it is preferable to use pressure welding which is carried out with a combination of sequence actions on the shaft and disk, since in this case an effective summarized compressive stresses and radial strain were achieved which leads to an increase of the joint quality.

In the present work, three-dimensional finite element modeling of the friction stir processing of a 3-mm-thick 2024 aluminum alloy plates using DEFORM-3D software product has been carried out. The optimal pin length was determined based on the calculation of the distribution of effective deformation, temperature, and displacement of material points. Numerical modeling was done on pins in the form of a cylinder with a diameter of 2 mm and a length of l = 1.6, 1.8, 2.0, and 2.2 mm. Friction stir processing was carried out at a traversing rate of 2 mm/s, a tool rotation rate of 1000 rpm, and an axial force of 20 kN. The material behavior of the sheet was described using the Johnson-Cook model included in the standard DEFORM-3D library. The results showed that the most preferable under the considered processing conditions is a pin with a length of 2.0 mm because it allows one to obtain the widest and most symmetrical regions of intense heating and deformation.

Pressure welding (PW) is an advanced method for obtaining solid phase joints (SPJs). Using physical and finite element modelling (FEM) it is shown that PW under superplasticity conditions provides SPJs between dissimilar cast single-crystal intermetallic and deformed polycrystalline heat-resistant Ni-based superalloys. Special attention is paid to exclude the stagnant zones in the solid phase joining region using surface relief. The results of physical modelling and FEM are in qualitative agreement. It is revealed that the surface relief can decrease the stagnant zone and increase localisation of deformation in the joining region, which helps to improve the joining quality. A monotonous change in microhardness in the joint zone is shown.

A material plastic flow finite-element modeling during pressure welding (PW) of two-element and three-element samples was carried out for dissimilar wrought (EP975) and cast intermetallic (VKNA-type) alloys at 1125°C and 10-4 s-1 initial strain rate. The finite-element modeling was performed in a two-dimensional set-up (plane strain). In the two-element samples case, the top element was ultrafine grained EP975 and the bottom one was VKNA-25. The top and bottom elements of the three-element samples were made from VKNA-25 while the intermediate element was ultrafine-grained EP975. The geometry in the X and Y directions were 40.5 and 5 mm for the EP975 samples and 40.5 and 3 mm for the VKNA-25 samples respectively. Experiments on PW intermetallic Ni3Al alloy and heat-resistant nickel alloy EP975 with ultrafine-grained (UFG) structure were carried out. Samples were parallelepipeds of size 10x40x3 mm3 and 10x40x5 mm3 respectively. PW was carried out at 1125 ˚C. Several combinations of welded sample surfaces with flat and grooved relief were considered. The relief made on both welded surfaces was equivalent to increase of friction between them. This results in a shear deformation reduction near the contact surfaces and degrades the welding quality as compared to the flat surfaces conventional welding. Relief applied only to one of the elements being welded results in a more efficient welding process since active deformation zones are created in the central zone, where in the other samples stagnant zones are observed. There are no deformation peculiarities in PW of the three-element samples, compared to two-element sample.