Effect of special rotational deformation on the dislocation emission from a branched crack tip in deformed nanocrystalline materials

The problem of the special rotational deformation interacting with an internal crack is studied by developing a theoretical model of deformed nanocrystalline materials. Using the complex variable method of Muskhelishvili and the conformal mapping technique, the expressions of complex potentials and stress fields are obtained analytically. The stress intensity factors (SIFs) near the crack tips and the critical SIFs for the first lattice dislocation emission from the branched crack tip are calculated. The effects of important parameters such as grain size, the length of internal crack, and the angle between the main crack and its branched crack on the critical SIFs for dislocation emission are evaluated in detail. As a result, the special rotational deformation has great influence on the growth of internal crack and the emission of lattice dislocations from the branched crack tip. The disclination quadrupole produced by the special rotational deformation will shield the branched crack tip under a certain condition. Moreover, when the main crack approaches its branched crack, it will stop the emission of lattice dislocations from the branched crack tip

Effect of cooperative grain boundary sliding and migration on dislocation emission from a branched crack tip in deformed nanocrystalline solids

A theoretical model is established to describe the effect of cooperative grain boundary (GB) sliding and migration on dislocation emission from the tip of branched crack in deformed nanocrystalline solids. The explicit solutions of complex potentials are obtained by means of complex variable method and conformal mapping technique. The critical stress intensity factors (SIFs) for the first lattice dislocation emission from the tip of branched crack are calculated. The effects of the lengths of branched crack and main crack, and the angle between their planes on the critical SIFs for dislocation emission are evaluated in detail. The results indicate that the emission of lattice dislocations from the tip of branched crack is strongly influenced by cooperative GB sliding and migration. When main crack approaches the branched crack, dislocation emission from the tip of branched crack will be suppressed. The main crack tends to propagate while shorter branched crack is prone to be blunted by emitting lattice dislocations from its tip. As a special case, when the planes of main crack and the branched crack are flattened out into one, the present results are in good agreement with previously known results

Molecular dynamics simulation of primary irradiation damage in Ti-6Al-4V alloys

Displacement cascade behaviors of Ti-6Al-4V alloys are investigated using molecular dynamics (MD) simulation. The embedded atom method (EAM) potential including Ti, Al and V elements is modified by adding Ziegler-Biersack-Littmark (ZBL) potential to describe the short-range interaction among different atoms. The time evolution of displacement cascades at the atomic scale is quantitatively evaluated with the energy of primary knock-on atom (PKA) ranging from 0.5 keV to 15 keV, and that for pure Ti is also computed as a comparison. The effects of temperature and incident direction of PKA are studied in detail. The results show that the temperature reduces the number of surviving Frenkel pairs (FPs), and the incident direction of PKA shows little correlation with them. Furthermore, the increasing temperature promotes the point defects to form clusters but reduces the number of defects due to the accelerated recombination of vacancies and interstitial atoms at relatively high temperature. The cluster fractions of interstitials and vacancies both increase with the PKA energy, whereas the increase of interstitial cluster is slightly larger due to their higher mobility. Compared to pure Ti, the presence of Al and V is beneficial to the formation of interstitial clusters and indirectly hinders the production of vacancy clusters

Influence of grain boundary sliding near a nanovoid on crack growth in deformed nanocrystalline materials

A theoretical model is developed that examines the effect of grain boundary (GB) sliding near a nanovoid on crack growth in deformed nanocrystalline materials. By combining complex variable method of Muskhelishvili, superposition principle of elasticity and distributed dislocation technique, the singular integral equations are solved numerically and then the stress intensity factors (SIFs) near the left crack tip are obtained. The influences of the location of the disclination dipole, the dipole arm, the orientation and the relative crack length on the SIFs near the left crack tip are evaluated in detail. The results indicate that the wedge disclination dipole produced by the GB sliding shields mode I crack tip, but anti-shields mode II crack tip. At the same time, surface stresses of the nanovoid characterized by positive surface elasticity and negative surface elasticity both show shielding effect to mode I SIFs of the crack tip, yet have negligible effect on mode II SIFs of the crack tip. Meanwhile, the mode I crack tip is more significantly shielded by the negative residual surface stresses than that of the positive residual surface stresses