Dislocation Controlled Nanograin Boundary Evolution: Phase-field-crystal Method

Nano-grain boundary has attracted tremendous attention for many years,motivated by their structural characteristics,deformation mechanism,and singular properties.The key to regulating the plastic deformation of nanocrystals is the grain boundary-dislocation interaction,determining the yield behavior and elongation of materials.Hence,the in-depth study of the dynamics of nanocrystalline boundaries and the atomic mechanism of dislocation motion helps regulate the microstructure and optimize material properties.However,due to the limitations of the small size effect of nanocrystals and the time-space scale,it is difficult to dynamically detect nanoscale microstructures,such as grain boundary and dislocation.The phase-field-crystal method based on density functional theory can capture single discrete dislocation or display the coupling migration of nanograin boundary and dislocation over a diffusion time scale.In this paper,the internal relationship between microscopic defects and mechanical properties is construed under the action of stress,temperature,and alloy composition in the process of heat treatment,either alone or coupled,providing some atomic-scale theoretical reference information for the analysis,regulation,and design of new materials.The main research framework and conclusions are as follows:(1)Based on the single-component phase-field-crystal model,the nanograin boundary migration and its interaction with dislocations under uniaxial and biaxial stress loading are revealed.The results show that stress-induced grain boundary migration,including the slip incubation period,dislocation reaction period,and periodic fluctuation period,is the internal cause of material hardening.Dislocation pair decomposition and grain boundary emission dislocation are the key links of grain boundary bending and fracture during the slip incubation period.Dislocation pair annihilation or grain boundary absorption dislocation is the primary way to reduce dislocation density in the dislocation reaction period and periodic fluctuation period.Specifically,the initiation of new dislocations,mediated by inherent dislocation pairs decomposition and foreign dislocation pairs merging,runs through the entire stage.The nucleation of sub-grains or the decomposition of dislocation pairs are the results of a single slip or single climb of atoms,respectively.The annihilation of dislocation pairs involves multiple modes of motion,i.e.,slipping after climbing,climbing after slipping,or climbing-sliding cooperative motion.Compared with uniaxial stress deformation,biaxial stress loading presents the characteristics that synergistic deformation,no dislocation locks(limiting dislocation slip),and low dislocation density,providing a certain buffer space for plastic deformation.(2)Temperature is a necessary factor to induce the softening(pre-melting)of lattice atoms at grain boundary defects(dislocation).The microscopic mechanism of temperature-induced grain boundary pre-melting and its correlation with the average atomic density is explored by analyzing the relationship between the free energy,atomic density,and dislocation configuration of the system.The early evolution process of grain boundary liquid pool under a stress-free state mainly involves four morphological characteristics,namely solid state→small droplet state→large liquid pool→uniform liquid film.The essence of the enlargement of liquid pools is the complete fracture of the solid bridge in two melting regions.Simultaneously,it is clarified that the decrease in average atomic density inhibits the crystalline phase characteristics of atoms in the dislocation aggregation region from the perspective of thermodynamics,which contributes to the formation of liquid pools.These characteristics provide a prerequisite for the application of high-temperature strain in the later stage to some extent.(3)The hardening-softening competition of thermal-mechanical coupling can promote polycrystalline grain refinement.The number of grains increased from 6 to 24.Three grain refinement mechanisms(sub-grain nucleation and growth mechanism,grain boundary bowing mechanism,and intragranular softening mechanism)are found for the first time.The sub-grain nucleation and growth mechanism is caused by the dislocation pair decomposition driving the fragmentation of the original symmetrical tilting grain boundaries.A large number of dislocation proliferation makes the material obtain a certain hardening ability.The grain boundary bowing mechanism is induced by dislocation pair decomposition in “tongue” and grain boundary self-emission dislocation.Both of them will lead to asymmetric tilting grain boundary folds,but the latter is characterized by no new dislocations and an overall multi-stage,multi-site direct bowing.For the intragranular softening mechanism,the increase in disorder degree is caused by the distortion of the local atomic array layer.The interaction between atoms in the softening zone and the surrounding environment produces new dislocation pairs,which evolve into the core of new grains.These softening zones can act as channels for dislocation movement to accelerate migration and achieve good plastic deformation compatibility.(4)Based on the two-component phase-field-crystal model,the chain reaction about spinodal decomposition mediated grain boundary segregation on misorientation(1°～60°)dependence is comprehensively deciphered,providing a new clue for microstructure-control led spinodal decomposition strengthening alloys.The component waves are initially controlled by solute segregation of grain boundary defects i.e.,dislocations(1° ≤ ≤ 15°)and fivemembered-units(16° ≤ ≤ 60°).A new verified sixth-order polynomial has been fitted.Coupling the spinodal decomposition strengthening theory,the grain orientation range that produces a more uniform segregation network is first refined,namely,28° ≤ ≤ 35°.