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金属学报  2018, Vol. 54 Issue (9): 1322-1332    DOI: 10.11900/0412.1961.2017.00553
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基于三维离散位错动力学的fcc结构单晶压缩应变率效应研究
郭祥如1,2, 孙朝阳1,2(), 王春晖1,2, 钱凌云1,2, 刘凤仙3
1 北京科技大学机械工程学院 北京 100083
2 北京科技大学金属轻量化成形制造北京市重点实验室 北京 100083
3 清华大学航天航空学院应用力学教育部重点实验室 北京 100084
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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摘要: 

基于位错理论建立了Ni单晶微柱压缩变形过程的三维离散位错动力学模型,该模型考虑了晶体塑性变形过程中位错所受的外载荷、位错间相互作用力、位错线张力及自由表面镜像力的影响。应用该模型研究了Ni单晶微柱压缩变形过程中流动应力和变形机制的应变率效应,同时,结合理论分析研究了应变率对流动应力中有效应力、位错源激活应力和位错间弹性相互作用力的影响。结果表明:当应变率较低时,Ni单晶微柱压缩变形中位错源激活应力主导流动应力,位错源激活数量较少,初始位错密度对流动应力影响很小,呈现单滑移变形;随着应变率增加,晶体变形过程中的流动应力随之增加,流动应力中位错源激活应力所占比例逐渐减小,有效应力逐渐主导流动应力,同时激活多个滑移系内的位错源来协调塑性变形;应变率越高,各激活滑移系内的塑性应变贡献相差越小,单晶微柱变形逐渐由单滑移向多滑移机制转变;在高应变率条件下,晶体初始位错密度越高塑性变形过程中流动应力越小。

关键词 离散位错动力学塑性变形应变率流动应力变形机制    
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 wordsdiscrete dislocation dynamics    plastic deformation    strain rate    flow stress    deformation mechanism
收稿日期: 2017-12-22     
ZTFLH:  TG142.1  
基金资助:国家自然科学基金项目No.51575039和国家自然科学基金委员会-中国工程物理研究院联合基金项目No.U1730121
作者简介: 作者简介 郭祥如,男,1989 年生,博士生

引用本文:

郭祥如, 孙朝阳, 王春晖, 钱凌云, 刘凤仙. 基于三维离散位错动力学的fcc结构单晶压缩应变率效应研究[J]. 金属学报, 2018, 54(9): 1322-1332.
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.

链接本文:

https://www.ams.org.cn/CN/10.11900/0412.1961.2017.00553      或      https://www.ams.org.cn/CN/Y2018/V54/I9/1322

图1  位错曲线离散化示意图
图2  单晶Ni的12个滑移系中对称选取4个滑移系、初始位错源应力松弛过程中的塑性应变率演化(插图为位错松弛后的位错构型)
图3  晶粒尺寸为5 μm的Ni单晶微柱在恒定应变率2×102 s-1时单向压缩下的应力和位错密度随着应变演化的3D-DDD模拟结果和文献[4]中实验统计结果的对比
图4  变形后Ni单晶微柱的SEM像[4]与3D-DDD模拟的位错构型
图5  应变率为2×102 s-1下Ni单晶微柱不同压缩应变时的位错构型图
图6  晶粒尺寸为5 μm的单晶Ni沿着[001]取向在不同应变率载荷压缩下的真应力和位错密度随应变演化结果及不同应变率下应变为0.8%时的位错构型。
图7  应变率分别为2×102、1×103、1×104和 5×104 s-1下流动应力中各组成应力的贡献
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
表1  Ni单晶微柱在不同应变率载荷下不同应变时对应的位错密度
图8  Ni单晶微柱在应变率分别为2×102 和5×104 s-1时不同初始位错密度下流动应力和位错密度随应变演化结果
图9  初始位错密度为1.2×1011 m-2的Ni单晶微柱在应变率分别为2×102、 1×103、1×104和 5×104 s-1 的压缩变形过程中各滑移系累积的塑性应变
图10  应变率为5×104 s-1下不同应变时的位错构形图
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