Molecular Dynamics (MD) Simulation for the study of Creep Deformation Sabila Kader Pinky

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1 Molecular Dynamics (MD) Simulation for the study of Creep Deformation Sabila Kader Pinky 1. Abstract The creep-deformation behavior of Ni-based single crystal superalloys under tensile loading at various temperatures and strain rates are studied by using molecular dynamics (MD) simulations. The simulation results show that the primary deformation mechanisms in Ni/Ni3Al superalloy follow dislocation theory. The studies on the effects of temperature showed that the yield strain and yield stress decreased as temperature increased. However, the influences of strain rate showed that the high strain rate led to the increase of yield stress. Moreover, based on MD simulations, the combined influences of temperature and load was also investigated. The simulated results are supported by related experimental findings. 2. Introduction Due to the scale of the interfaces is on the nanoscale, molecular dynamics (MD) has become very useful tool for studying the morphology and evolution of the interfacial dislocation networks under the load and temperature and exploring the relationship between the damage of dislocation networks and mechanical properties of alloys from an atomistic standpoint. Besides, molecular dynamics (MD) method has also been extensively employed to study the dislocation behaviors at the γ/γ phase interfaces due to the special nanoscale of the interfaces. Using MD to simulate the evolution of dislocation networks and dislocation motion at the γ/γ phase interface, the construction of an atomic model has a great influence on the microstructure evolution and mechanical properties, and the evolution of dislocation network and the relevant mechanical properties are strongly dependent on temperature and loading conditions of alloys. Figure 1 Microstructure of Ni3Al Ni-based superalloys are mainly strengthened by ϒ/ϒ precipitates and solution additions and are produced in various forms suitable for application at high temperature, due to their excellent creep and fatigue strength and good corrosion resistance. Owing to their excellent reliability and mechanical properties, Ni-based

2 superalloys have been used as materials for blades and vanes in gas turbines. It is well known that Ni-based superalloys are strengthened by fine and ordered ϒ/ϒ precipitates embedded in an fcc _ matrix. These alloys are far stronger than pure ϒ or ϒ single phase materials due to the presence of ϒ/ϒ interfaces which prevents dislocation motion at high temperatures. Investigating the creep-deformation mechanism can benefit the efforts to develop high-temperature alloys, to achieve more-resistant alloys, and to rationalize the creep behavior of these alloys. The purpose of the present work is to explore in detail the creep behavior under constant load at different temperatures and strain rate and to gain a deeper understanding of the hightemperature creep-deformation mechanisms of the Ni-based superalloy. In this work, uniaxial tensile MD simulations are performed to investigate the evolution of dislocation network and dislocation motion at γ/γ phase interface of Ni-based single crystal superalloy at the temperature from 300K to 1000 K and strain rate from s 1 to s 1. The objective of the present work is to determine the influences of the temperature and strain rate on the evolution of dislocation network and mechanical properties, to explore the relationship between the microstructure evolution and the mechanical properties of alloys. 3. Modeling and Simulation In the present study, molecular dynamics simulations were performed for the idealized Nibased superalloy composed of Ni3Al cuboidal precipitates (L12 ϒ phase), and the pure Ni matrix (fcc ϒ-phase). A LAMMPS data file was created using VESTA for Ni3Al, as shown in Fig. 1. In Fig. 1, the blue region presents ϒ-Ni phase and the red region ϒ -Ni3Al phase. The periodic boundary condition was applied in three directions. The simulation box was included four thousand Figure 2 Ni3Al Structure atoms. Uniaxial tensile testing was performed to study the mechanical properties of γ/γ phase in the z-direction using MD simulation, and the loading process was a dynamic loading. The uniaxial tension processes proceed at the NPT ensemble. In this work, Embedded Atom Method (EAM) potential from Mishin was used for the simulation. The MD simulations are per-formed using the open-source Large-scale Atomic/Molecular Parallel Simulator (LAMMPS) code. Various temperatures and strain rates are applied to study the effects of temperature and strain rate on the microstructure deformation. The resulting structures and deformation mechanism are visualized by using visualization software OVITO in the atomistic simulations.

3 4. Simulation Results 4.1. Morphological evolution of dislocation networks under uniaxial loading Figure 3 Structural Evaluation during test Figure 3 shows the structural evaluation of crystal during tensile test. 1 st one is the undeformed crystal structure. 2 nd one is the slightly deformed structure, but dislocation is not started yet. In the 3 rd figure dislocation pile up near the regions of γ/γ interfaces and cause stress concentration. When the stress value at the stress concentration exceeds γ strength, the dislocation network is damaged, so that the deformed dislocations in the matrix channel shear and enter γ phase through damaged dislocation network in γ/γ interfaces. At this position stress starts to go down. At the end of the slope, 4 th structure, dislocation is completed so stress level becomes constant. And finally 5 th structure is the permanent deformed structure. From above the results mentioned, it can be concluded that the dislocation network can inhibit dislocations in the γ matrix cutting into the γ phase by gathering at the {111} phase. Meanwhile it absorbs the matrix dislocations to stabilize and strengthen itself which facilitates the stability of structure and increases the critical value of maximum stress.

4 Stress (Gpa) Stress (Gpa) Stress (Gpa) 4.2. Effect of Different Strain Rate 8 Highest Strain Rate Medium Strain Rate 8 8 Lowest Strain Rate Strain Strain Strain Figure 4 Effect of Different Strain Rate Figure 4 shows the stress strain curves of γ/γ phase at various strain rates, including s 1, s 1 and s 1. These simulations performed at a room temperature of 300K and the loading mode was strain dynamic loading. It was observed that the yield strain and yield stress increased with the strain rate increasing from the stress strain curves. At high strain rate of s 1, as the stress exceed the maximum yield point, began to slowly decline, so the plastic deformation of the system presented instability. However, when the strain rates were s 1, and s 1, the stress was falling fast. It could be concluded that the high strain rate increased the yield stress so that the plasticity of system decreased, however, with the loss of strain rate, although the yield stress decreased, the system presented better plasticity Effect of Temperature The temperature effect on mechanical properties of γ/γ monocrystal was investigated using MD simulation in the temperature range from 300 to 1000 K. The yield stress of the monocrystal was 6.56 GPa at the strain level of 0.11 at low temperature of 300 K, whereas the yield stress was 2.33 GPa at the strain level of 0.06 at high temperature of 1000 K. It could be concluded that the yield strain and yield stress of γ/γ monocrystal decreased as the temperature rose. At low temperature of 300 K, the γ/γ monocrystal had the maximum yield strain and yield stress. The elastic modulus of γ/γ monocrystal was obtained by fitting the stress strain curves in Figure 5, and it also decreased from 300 to 1000 K.

5 Figure 5 Effect of Temperature 5. Conclusion At room temperature, the γ/γ phase of nanophases experienced elastic and plastic deformation processes in uniaxial tension testing, and showed stability of the plastic deformation, so the γ/γ phase of nanophases had excellent mechanical properties. The shearing process caused by the dislocations motion was the dominant deformation mechanism in γ/γ phase. The effects of strain rates on the mechanical properties of γ/γ phase were studied at room temperature. The results showed that the high strain rate led to the increase of yield stress and the phase sizes had no significant influence on the maximum yield stress for γ/γ phase. In brief, Under the influence of the temperature, the initial mosaic structure of dislocation network gradually becomes irregular, and the initial misfit stress and the elastic modulus slowly decline as temperature increasing. On the other hand, with the increase of the strain rate, it almost has no effect on the elastic modulus and the way of evolution of dislocation network but contributes to the increases of the yield stress and tensile strength. Moreover, tension compression asymmetry of Ni-based single crystal superalloys is also presented based on MD simulations.

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