Atomistic Simulation On The Evolution Of Superdislocation Dipoles In Several Intermetallic Compounds

Due to their many excellent properties including high strength,high hardness,high temperature and corrosion resistance,intermetallic compounds can be served as versatile structure materials in aerospace and other fields.Among different microstructure defects,dislocation is one of the main factors to determine the mechanical properties of intermetallic compounds.During many mechanical processes,for example fatigue and creep,the effective dislocation density of structrural material is highly dependent on the propagation and annihilation of dislocations as well as their interactions with other defects,thus significantly influencing the mechanical properties.In this case,the formation and evolution of the dipole type dislocations(hereafter referred to as dislocation dipoles)play a critical role in varying the mechanical properties of structural materials.Unfortunately,limited by the resolution of electron microscope,it is very difficult to observe and study the dislocation dipoles(especially the superdislocation dipoles)at the atomic scale,therefore,very limited experimental investigation has been reported on this respect.For this reason,theoretical simulations are very helpful and usually performed to explore these microstructure defects.However,most simulation models and parameters are far from those in a real deformation process and normally give rise to unreasonable dislocation dipole configurations,resulting in a lack of connection between dislocation dipole and subsequent point defect diffusion.Moreover,so far,the stable morphology,annihilation condition and product of superdislocation dipoles as well as their possible role in subsequent deformation are far from fully understood.Therefore,the impact mechanism of superdislocation dipoles on plastic deformation of materials has been comprehensively investigated by atomistic simulation in this dissertation.Firstly,molecular dynamics(MD)simulations were carried out to study the evolution of superdislocation dipoles at different temperatures,since the practical application of TiAl alloy normally involves reciprocating motion under high stress and at high temperature.Based on the MD results,the structures of evolution products of superdislocation dipoles were further analyzed,and provided a solid theoretical foundation for future researches on creep and fatigue involved in the plastic deformation.The simulation results clearly indicate that non-screw superdislocation dipoles can transform easily to locally stable dislocation dipoles or reconstructed cores at low temperature,while evolve to be isolated or interconnected point defect clusters or stacking fault tetrahedra at high temperature via short-range diffusion.The MD results show that non-screw superdislocation dipoles in γ-TiAl and α2-Ti3Al exhibit similar structure features as those in fcc and hcp metals,respectively.As for the longterm annealing with significant diffusion,60° superdislocation dipoles in y-TiAl are stable,whereas the stability of superdislocation dipoles in α2-Ti3Al increases with the increase in dipole height and orientation angle.Secondly,in order to fully uncover the detailed structures evolved from the superdislocation dipoles,MD simulations were performed to explore the selfinteractions between superdislocation dipoles in several Al-based intermetallic compounds including Ni3Al,Fe3Al and Cu3Al.The MD results indicate that the evolution products depend strongly on the height and orientations of superdislocation dipoles at low temperatures,forming complex structures such as hollows,reconstructed dipoles,faulted dipoles and so on.As for the cases of superdislocation dipoles with relatively large-height,the classical linear structures can be obtained.At high temperature,the superdislocation dipoles with the height of 1d in Cu3Al tend to evolve into point defects,while they are relatively stable and keep unchanged in Ni3Al and Fe3Al for all possible heights.For three considered intermetallic compounds(Ni3Al,Fe3Al and Cu3Al),the structure evolution behavior and results of non-screw superdislocations are very similar both at high and low temperatures.The superdislocation dipoles in Ni3Al and Fe3Al show higher stabilities compared to those in Cu3Al.Interestingly,the stability of superdislocation dipoles in all three systems increases with increasing the dipole height.Finally,atomistic simulation based on the Activation-Relaxation Technique was employed to systematically investigate the evolution of super-dipoles in γ-TiAl,α2Ti3Al,Ni3Al,Fe3Al and Cu3Al under the atomic-scale space resolution and macro-scale time resolution.It was found that there exist not only a large number of pathways with relatively small activation energies but also a small number of novel pathways with extremely low activation energies for the evolution from superdislocation dipoles to point defects,which results in the unique morphology of point defect clusters in these systems and further influence on mechanical processes.The results of superdislocation dipole evolution obtained by atomistic simulations can be integrated into mesoscale or constitutive models to evaluate the effects of superdislocation dipole evolution on the mechanical properties during deformation and fatigue.