| Metal materials are the material basis of national economy,and they are widely used in industrial fields such as construction engineering,civil engineering and concrete.In practical engineering,most metal engineering components are in the role of constant stress for a long time,and creep deformation occurs.Therefore,a deep understanding and research on creep problems can help us to reveal the mechanical properties and failure laws of metal materials from the level of physical mechanism to ensure the safety of practical engineering.There are many microscopic defects inside metal materials,such as dislocations,vacancies,voids,grain boundaries,cracks,etc.During loading,the creep deformation is affected by above microscopic defects.Dislocations is critical for the creep process,and many creep models are proposed based on the basic framework of dislocation pile-up and climbing.Dislocations within the material slip and proliferate during loading,and the accumulation occurs after encountering obstacles,leading to the increase of the dislocation density and the formation of processing hardening phenomenon.However,the accumulated dislocation annihilate with other dislocations through climbing,resulting in reduced dislocation density and forming the phenomenon of recovery softening.These two phenomena constitute the basic framework of the creep process.Based on the recovery creep theory of pure metal,Weertman,Ivanov and Yanushkevich,Blum et al.have established different recovery creep models with 3-power based on the movement,structural characteristics of the edge dislocation,and the substructure inside the material in the steady-state creep stage.Later,Weertman considered the role of screw dislocation in the creep process to establish the intrinsic creep model with 5-power completely consistent with the experimental results.In recent years,there are a lot of exploration and research on microscopic defects at the levels of nano,micro and macrostructure.Now,the researchers are starting to study unconventional defect structures,such as triple junction of grain boundaries and nanodisturbance,and to explore the impact of these defect structures on the creep process.However,with the existing experimental conditions,it is difficult to observe the evolution of the above microscopic defects at the atomic scale.With the development of computer simulation technology,it is possible to make up for the experimental deficiency with simulation means,and more and more computational simulation methods are applied in the study of creep process.Despite the achievements on the creep mechanism of the triple junction and nanodisturbance,these studies still remain at the theoretical modeling level of local region,and lack the analysis and discussion on the overall creep process.On the other hand,in computational simulation studies on creep processes,there are still imprecision on spatial or time scale,which leads to a gap between the simulation results and experimental results.Recently,Phase field crystal(PFC)model developed based on density functional theory has become an effective method to study the microscopic defects such as dislocations,grain boundaries,voids,cracks and their evolution processes.The PFC model is applied in this paper to study the evolution,creep mechanism of micro defects,and conduct research in the following aspects:(1)In view of the primary stage of creep,different constant strains are applied to a perfect FCC crystals with the same size voids,the different situations where nanodisturbances are formed and expanded into dislocation dipoles is studied,and the creep constitutive equation relying on the extension of nanodisturbances is established on this basis.(2)For the steady stage of creep,different constant stresses are applied to the FCC polycrystalline system,the creep process of the FCC nano-polycrystalline system is studied,and the creep constitutive equation based on the triple junction is established.(3)For the acceleration stage of creep,the growing process of void within the FCC perfect lattice in the form of dislocations emission is studied,and the creep constitutive equation related to the growth velocity of the void is established.After the systematic research and analysis,the main results and conclusions obtained are as follows:(1)The formation and expansion process of the nanodisturbances within a perfect lattice with a void is simulated by PFC model.It is found that the number of nanodisturbances produced within the system is related to the constant strain.The larger the constant strain,the more the number of nanodisturbances produced and the larger the scale of the nanodisturbances.For this new phenomenon,an extended model of the nanodisturbance based on the non-crystallographic Burgers vector and the length of the nanodisturbance is established from an energy perspective.Based on this model,it is found that the nanodisturbance under constant strain exhibits a linear extension case,where the Burgers vector is linearly associated with the length of nanodisturbance.Finally,based on the above studies,the creep constitutive equation associated to the nanodisturbance extension is established.The creep equation shows that the length of nanodisturbance is inversely proportional to the creep strain rate.(2)A theoretical model of the decomposition mode transformation mechanism of the dislocation pile-up at the triple junction of grain boundaries is developed.By comparing the dislocation critical moving distance when the new head dislocation decomposed in different modes,it can be find that when the moving distance of the dislocations decomposed from the original head dislocation is greater than a certain critical value,the decomposition mode of the new head dislocation will change.At the same time,the transformation of the decomposition mode of the dislocation pile-up must be accompanied by the change of the angle of the triple junction.Based on the above decomposition mode transformation mechanism of the dislocation pile-up at triple junction,the creep constitutive equation associated to the critical transformation angle during the steady stage of creep is proposed.In this creep equation,the creep strain rate is proportional to the angle of triple junction when the decomposition mode of the dislocation pile-up is changed,which is the effect of the triple junction on the creep process.Meanwhile,the stress index of this creep equation is 4,and the dislocation motion speed in this model is basically consistent with the one in intrinsic creep process,indicating that the presence of triple junction in this case increases the dislocation density inside the material.(3)The creep process of FCC nano-polycrystalline is simulated by PFC model.In the creep process,the dislocation movement is mainly in the grain boundaries,but less inside the grain.The FCC nanopolycrystalline system was subjected under different stresses for creep simulation.By the logarithmic relationship between stress and strain rate,it is confirmed that the stress index is3,and the creep process is intrinsic creep.Due to the high content of triple junction in nanocrystalline materials,which has an important influence on the mechanical properties of materials,a creep constitutive equation based on triple junction was established on the basis of the creep simulation of FCC nanocrystalline materials.At the same time,the dislocation transport is mainly concentrated on the grian boundaries;it is the characteristic of Coble diffusion creep.Due to the high content of the triple junctions in the nanocrystalline material,it has an important influence on the mechanical properties of the material.Therefore,the creep constitutive equation based on the triple junction is established based on the creep simulation of FCC nano-polycrystalline.Therefore,the abundant structure of grain boundaries and triple junctions make the creep process of nanocrystalline materials have the characteristics of both intrinsic creep and Coble creep.(4)The creep process of a complete lattice with one void is simulated by PFC model.It is found that the void first is changed from a circle to a square during the creep process,and the four angles of this square void are distributed in the upper,lower,left and right directions.Then a perfect dislocation is emitted from the left and right tips of the void respectively,and the two perfect dislocations are decomposed into two partial dislocations respectively and the void continue to expand.In this paper,the rationality of the void growth during the creep process is analyzed according to the existing void growth model with dislocation emission.And it can be concluded that the above phenomenon can occur.According to the equation of void growth rate in the acceleration stage of creep,the strain rate in the acceleration stage of creep is deduced,and the relationship between void growth rate and strain rate is established,and the creep constitutive equation based on void growth rate is derived.In this creep equation,the strain rate is proportional to the length of a side of the square void,which is consistent with the relationship between the strain rate and the void size in the acceleration stage of creep.In conclusion,from the perspective of the internal defect structure inside nanocrystalline materials,the creep equations of strain rates at each stage of the creep process were established respectively.The characteristics of these equations show that,nanodisturbances can replace the traditional dislocation sources and become a new form to produce dislocations in nanocrystalline materials at the initial stage of creep.In the steady stage of creep,the effect of triple junction on the transport process of accumulated dislocations can directly affect the creep mechanism.In the acceleration stage of creep,the growth mechanism and speed of void can directly affect the creep strain rate.The above research results have certain theoretical guiding significance for the design and engineering application of creep materials. |
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