Molecular Dynamics Simulation of the Evolution of Interfacial Dislocation Network and Stress Distribution of a Ni-Based SingleCrystal Superalloy

The evolution of misfit dislocation network at γ/γ＇ phase interfaces and the stress distribution characteristics of Ni-based single-crystal superalloys under different temperatures of 0, 100 and 300 K are studied by molecular dynamics （MD） simulation. It was found that a closed three-dimensional misfit dislocation network appears on the γ/γ＇ phase interfaces, and the shape of the dislocation network is independent of the lattice mismatch. Under the influence of the temperature, the dislocation network gradually becomes irregular, a/2 [110] dislocations in the γ matrix phase emit and partly cut into the γ＇ phase with the increase in temperature. The dislocation evolution is related to the local stress field, a peak stress occurs at γ/γ＇ phase interface, and with the increase in temperature and relaxation times, the stress in the γ phase gradually increases, the number of dislocations in the γ phase increases and cuts into γ＇ phase from the interfaces where dislocation network is damaged. The results provide important information for understanding the temperature dependence of the dislocation evolution and mechanical properties of Ni-based single-crystal superalloys.

Quantitative study on interactions between interfacial misfit dislocation networks and matrix dislocations in Ni-based single crystal superalloys

The interactions between the moving dislocation within matrix channel and the interfacial misfit dislocation networks on the two-phase interfaces in Ni-based single crystal superalloys are studied carefully via atomic modeling, with special focus on the factors influ- encing the critical bowing stress of moving dislocations in the matrix channel. The results show that the moving matrix dislocation type and its position with respect to the interfacial misfit dislocation segments have considerable influences on the interactions. If the moving matrix dislocation is pure screw, it reacts with the interracial misfit dislocation segments toward dislocation linear energy reduction, which decreases the critical bowing stress of screw dislocation due to dislocation linear energy release during the dislocation reactions. If the moving matrix dislocation is of 60^-mixed type, it is obstructed by the interaction between the mixed matrix dislocations and the misfit interfacial dislocation segments. As a result, the critical bowing stress increases significantly because extra interactive energy needs to be overcome. These two different effects on the critical bowing stress become in- creasingly significant when the moving matrix dislocation is very close to the interracial misfit dislocation segments. In addition, the matrix channel width also has a significant influence on the critical bowing stress, i.e. the narrower the matrix channel is, the higher the critical bowing stress is. The classical Orowan formula is modified to predict these effects on the critical bowing stress of moving matrix dislocation, which is in good agreement with the computational results.

Orientation-Dependent Morphology and Evolution of Interfacial Dislocation Networks in Ni-Based Single-Crystal Superalloys:A Molecular Dynamics Simulation

In this paper,the morphology and evolution of interfacial dislocation networks of(100),(110)and(111)interphases of Ni-based single-crystal superalloys are studied by molecular dynamics(MD)simulations.Three-dimensional cubic-type and sandwich-type models are chosen to explore the orientation-dependent morphology of dislocation networks,and their respective advantages and disadvantages are compared.From the simulations,it is observed that various lattice orientations and model types lead to different morphologies of dislocation networks.Based on the analysis of average atomic energy and dislocation characteristics,the(100)orientation model has a more regular dislocation network,lower energy and better stability than the(110)and(111)orientation models after MD relaxation,which are supported by previous experimental and numerical simulations.Moreover,the cubic-type model has lower energy and better stability than the sandwich-type model.This will be helpful for choosing a more appropriate and reasonable model for simulating the interfacial dislocation networks of Ni-based single-crystal superalloys.

The criterion of anomalous slip at OK in body centered cubic metals

It is generally accepted that anomalous slip(AS) takes place by hexagonal dislocation networks(HDNs) in body centered cubic(BCC) metals,but the role of the HDN formation process in AS has rarely been investigated so far.In this work,the critical yield conditions of the HDNs and isolated dislocations were first calculated,respectively,by molecular statics simulations in two BCC metals.Based on these data,a novel mechanism,entitled as the "conjugated dislocation sources"(CDS),to analyze the formation of the HDNs was proposed for the first time and then incorporated into the criterion of the occurrence of AS.Our prediction is in agreement with experimental observations.Contrary to previous study,it has been revealed that the multiplication of isolated screw dislocations involved in AS has to be considered for correctly understanding the AS origin.

Inconsistent creep between dendrite core and interdendritic region under different degrees of elemental inhomogeneity in nickel-based single crystal superalloys

The creep inconsistency between dendrite core and interdendritic region is investigated in a nickel-based single crystal superalloy under 1373 K and 137 MPa.Two specimens with higher and lower degree of elemental inhomogeneity on dendritic structures are compared.For specimen with higher inhomogeneity,stronger segregation of refractory elements reinforces the local strength in dendrite core,but damages the strength in interdendritic region.Creep strain is accumulated faster in interdendritic region giving rise to promoted dislocation shearing inγphase,faster degradation of dislocation networks and facilitated topological inversion of rated structures.Although the segregation of refractory elements produces a high density of topologically close-packed(TCP)phase in dendrite core,faster accumulation of creep strain forms microcracks prior in interdendritic region that gives rise to final rupture of the specimen.In another specimen,increased solid solution time gives rise to overall reduced inhomogeneity.Creep inconsistency is relieved to show more uniform evolution of dislocation substructures and rafting between dendrite core and interdendritic region.The second specimen is ruptured by formation and extension of microcracks along TCP phase although the precipitation of TCP phase is relatively restricted under reduced inhomogeneity.Importantly,the balance of local strength between dendrite core and interdendritic region results in over 40%increase of creep rupture life of the second specimen.

Influence of Ta/Al ratio on the microstructure and creep property of a Ru-containing Ni-based single-crystal superalloy

Two alloys with different Ta and Al contents were applied to study the influence of Ta/Al ratio on the microstructural evolution and creep deformation under high temperature.The increase of Ta/Al ratio made theγ/γ'lattice misfit more negative and enhanced the volume fraction ofγ'phase,which produced cubic and smallγ'phase in the initial microstructures.These initial tinyγ'phases impeded the dislocations movement and delayed the course of complete raftedγ'phase during the origination of creep deformation,which prolonged the time of the primary creep stage.Moreover,the increase of Ta/Al ratio and addition of Ru produced the denser and stable dislocation networks,the high APB energy and better solution strengthening,which hindered the climbing and sliding of dislocations,and restrained the formation of superdislocations in theγ'precipitate.The second creep stage was extended,and the minimum creep rate was reduced.Hence,the increase of whole creep life of the alloy containing high Ta/Al ratio was attributed to the prolongation of the primary and second creep stages,and the low minimum creep rate.The appearance of the topological inversion phenomenon during the tertiary creep stage was the primary cause for the sudden increase of the creep strain rate of the alloy containing low Ta/Al ratio.However,the high creep strain rate of the alloy containing high Ta/Al ratio during the tertiary creep stage was related to the occurrence and extending of the cracks near the voids.Both alloys would lose efficacy within 20 h-30 h.