Atomic Scale Simulation Of Superdislocation Dipoles Evolution In Several Intermetallic Compounds

Due to their many excellent properties,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,the effective dislocation density of structrure material is highly dependent on the propagation and the 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(hereinafter referred to as dislocation dipoles)play a critical role in varying the mechanical properties of structure 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,quite few experimental result has been reported on this respect.For this reason,theoretical simulations are 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 influence mechanism of superdislocation dipoles on plastic deformation of materials has been comprehensively investigated by atomistic simulation.Firstly,molecular dynamics(MD)simulations were carried out to study the evolution of superdislocation dipoles at different temperatures,since the practical application of y-TiAl alloy normally involves reciprocating motion under high stress and at high temperature.Based on the MD resultes,the structures of evolution products of superdislocation dipoles were further analyzed,which can provide 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 easily transform 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 ?-TiAl and ?2-Ti3Al exhibit similar structure features as those in fcc and hcp metals,respectively.As for the long-term annealing with significant diffusion,60° superdislocation dipoles in ?-TiAl are stable,whereas the stability of superdislocation dipoles in ?2-Ti3Al increases with increasing the 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 self-interactions between superdislocation dipoles in Al-based intermetallic compounds including Ni3Al,Fe₃Al and Cu3Al.The MD results indicate that the evolution products are closely related to the height and orientations of superdislocation dipoles at low temperatures,forming complex structures such as hollows,reconstructed dipoles,stacking fault dipoles and so on.As for the cases with relatively large height of superdislocation dipoles,the classically linear structures can be obtained after the evolution of this type of superdislocation dipoles.At high temperature,the superdislocation dipoles with the height of 1d in Cu3Al tend to evolve into point defects,while 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 relatively higher stabilities with respect to those in Cu3Al.Interestingly,the stability of superdislocation dipoles in all three systems increases with increasing the dipole height.Finally,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 in?-TiAl,?2-Ti3Al Ni3Al,Fe₃Al and Cu3Al,based on the activation relaxation technique.The results of superdislocation evolution can be integrated into mesoscale or constitutive models to evaluate the effects of superdislocation evolution on mechanical properties.