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Acta Metall Sin  2018, Vol. 54 Issue (9): 1322-1332    DOI: 10.11900/0412.1961.2017.00553
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Investigation of Strain Rate Effect by Three-Dimensional Discrete Dislocation Dynamics for fcc Single Crystal During Compression Process
Xiangru GUO1,2, Chaoyang SUN1,2(), Chunhui WANG1,2, Lingyun QIAN1,2, Fengxian LIU3
1 School of Mechanical and Engineering, University of Science and Technology Beijing, Beijing 100083, China
2 Beijing Key Laboratory of Lightweight Metal Forming, University of Science and Technology Beijing, Beijing 100083, China
3 Applied Mechanics Lab., School of Aerospace Engineering, Tsinghua University, Beijing 100084, China
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Abstract  

Microelectromechanical systems (MEMS) have become increasingly prevalent in engineering applications. In these MEMS, a lot of micro-components, such as thin films, nanowires, micro-beams and micropillars, are utilized. The characteristic geometrical size of those components is at the same scale as that of grain, the mechanical behavior of crystal materials exhibits significant size effect and discontinuous deformation. In addition, those MEMS are often subjected to high strain rate at work, such as collision and impact loading. The coupling deformation characteristics of small scale crystals and high strain rate makes their mechanical behavior more complicated. Accordingly, investigation of the effect of the strain rate on crystal materials at micron scale is significant for both the academia and industry. In this work, a plastic deformation model of fcc crystal under axial compression was developed based on three-dimensional discrete dislocation dynamics (3D-DDD), which considered the influence of externally applied stress, interaction force between dislocation segments, dislocation line tension and image force from free surface on dislocation movement during the process of plastic deformation. It was applied to simulate the plastic deformation process of a Ni single crystal micropillar during compression under different loading strain rates. 3D-DDD and theoretical analysis are carried out to extensively investigate the effect of strain rate on flow stress and deformation mechanisms during plastic deformation process of crystal materials. The results show that the flow stress and the dislocation density increased with the loading strain rate. In the case of low strain rate, the flow stress was dominated by the activation stress of Freak-Read (FR) source in plastic deformation. With the increase of strain rate, the contribution of activation stress of FR source to the flow stress decreases and the effective stress gradually dominated the flow stress. Under high strain rate loading, with the increase of the initial FR source, the dislocation density also increased at the same strain correspondingly, which makes it easier to meet the requirement of the loading strain rate, so the flow stress is smaller. In addition, under the low strain rate loading, a few activated FR sources can meet the requirement of the plastic deformation, a single slip deformation come up as a result. While, as the loading strain rate increases, more and more activated FR sources would be needed to coordinate the plastic deformation, the deformation mechanisms of the single crystal micropillar transformed from single slip to multiple slip.

Key words:  discrete dislocation dynamics      plastic deformation      strain rate      flow stress      deformation mechanism     
Received:  22 December 2017     
ZTFLH:  TG142.1  
Fund: Supported by National Natural Science Foundation of China (No.51575039) and Joint Fund of National Natural Science Foundation of China and Chinese Academy of Engineering Physics (No.U1730121)

Cite this article: 

Xiangru GUO, Chaoyang SUN, Chunhui WANG, Lingyun QIAN, Fengxian LIU. Investigation of Strain Rate Effect by Three-Dimensional Discrete Dislocation Dynamics for fcc Single Crystal During Compression Process. Acta Metall Sin, 2018, 54(9): 1322-1332.

URL: 

https://www.ams.org.cn/EN/10.11900/0412.1961.2017.00553     OR     https://www.ams.org.cn/EN/Y2018/V54/I9/1322

Fig.1  Discretization of dislocation line (The black solid line is the original dislocation line, the blue segments represent the discrete dislocation segments, bi is the Burgers vector of dislocation segment i)
Fig.2  Four slip systems are chosen symmetrically in twelve slip systems of single crystal Ni (a) and plastic strain rate evolution during stress relaxation (b) (Inset in Fig.2b shows the corresponding dislocation snapshot after stress relaxation)
Fig.3  Comparisons of 3D-DDD simulated and experimental statistical[4] results of stress and dislocation density with strain under constant strain rate 2×102 s-1 compression loading of Ni single crystal micropillar with a diameter of 5 μm
Fig.4  SEM image of Ni single crystal micropillar (a)[4] and dislocation snapshot after compression (b)
Fig.5  Dislocation snapshots corresponding to strains of 0.03% (a), 0.20% (b), 0.27% (c), 0.43% (d), 0.55% (e) and 0.70% (f) in the compression process of Ni single crystal micropillar
Fig.6  3D-DDD simulation results of stress (a) and dislocation density (b) with strain under different strain rates compression of Ni single crystal micropillar with a diameter of 5 μm, and dislocation snapshots corresponding to true strain 0.8% in the compression (ε˙—strain rate ) (c)
Fig.7  Respective contributions of τ*, τα and τFR to the flow stress during plastic deformation under strains rate of 2×102 s-1 (a), 1×103 s-1 (b), 1×104 s-1 (c) and 5×104 s-1 (d) (τ* is the effective stress on the dislocation, τα is the elastic interaction stress related to dislocation density, τFR is the critical resolved shear stress to activate FR source)
ε˙ / s-1 ε / % ρ / 1011 m-2
2×102 0.1 1.3
0.3 1.3
0.5 1.3
0.7 1.3
1×103 0.2 1.7
0.4 1.7
0.6 1.8
0.8 1.7
1×104 0.2 5.3
0.4 5.6
0.6 6.8
0.8 6.7
5×104 0.2 7.2
0.4 7.7
0.6 8.9
0.8 9.3
Table 1  Dislocation densities corresponding to different strains of Ni single crystal micropillar under different strain rates
Fig.8  Simulation results of flow stress and dislocation density with different initial dislocation densities ρ0 under strain rates of 2×102 s-1 (a) and 5×104 s-1 (b)
Fig.9  Plastic strains contributed by four slip systems with initial dislocation density of 1.2×1011 m-2 under loading strain rates of 2×102 s-1 (a), 1×103 s-1 (b), 1×104 s-1 (c) and 5×104 s-1 (d)
Fig.10  Dislocation snapshots corresponding to strains of 0.11% (a), 0.20% (b), 0.32% (c), 0.45% (d), 0.58% (e) and 0.78% (f) in the compression
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