## 含水条件下单晶Cu的应力松弛及弹性恢复

1. 西安稀有金属材料研究院有限公司 西安 710016

2. 西安交通大学金属材料强度国家重点实验室 西安 710049

3. 浙江大学宁波理工学院 宁波 315000

## Stress Relaxation and Elastic Recovery of Monocrystalline Cu Under Water Environment

SHI Junqin1, SUN Kun2, FANG Liang2, XU Shaofeng,3

1. Xi’an Rare Metal Materials Institute Co. , Ltd. , Xi’an 710016, China

2. State Key Laboratory for Mechanical Behavior of Materials, Xi’an Jiaotong University, Xi’an 710049, China

3. Ningbo Institute of Technology, Zhejiang University, Ningbo 315000, China

 基金资助: 国家自然科学基金青年科学基金项目.  (No.51605432)浙江省自然科学基金青年科学基金项目.  (No.LQ16E050007)宁波市自然科学基金项目.  (No.2015A610097)

Corresponding authors: XU Shaofeng, Tel: 15824200943, E-mail:10925066@zju.edu.cn

Received: 2019-02-20   Revised: 2019-05-14   Online: 2019-07-24

 Fund supported: Young Scientists Fund of National Natural Science Foundation of China .  (No.51605432)Young Scientists Fund of the Natural Science Foundation of Zhejiang Province, China .  (No.LQ16E050007)Natural Science Foundation of Ningbo, China.  (No.2015A610097)

Abstract

The stress relaxation and elastic recovery have an important effect on the mechanical and electrical properties of metallic crystal materials, which restricts the range of application and working life of materials. However, during plastic deformation of materials, the relaxation and elastic recovery behaviors are still not very clear at the nanoscale. In this work, the stress relaxation and elastic recovery of monocrystalline Cu under water environment is studied by molecular dynamics simulation. The results indicate the stress acting on Cu surface decreases at constant strain, meaning the occurrence of stress relaxation phenomenon. The stress relaxation increases with water film thickening compared with no-water environment. The separation between Cu atoms dramatically decreases with the increasing indentation depth at indenting stage, and there is no clear change in the nearest interatomic separation at stress relaxation stage, but the separation increases rapidly due to the release of elastic energy and dislocation energy at the unloading stage. The nucleated dislocations within Cu coated by water film are obviously more than that without water, which suggests the water film increases the unrecovered deformation in the total nanoindentation process. During unloading, partial dislocations disappear because of the deformation energy release, while the water film impedes the elastic recovery and plastic release.

Keywords： stress relaxation ; elastic recovery ; monocrystalline Cu ; molecular dynamics

Junqin SHI, Kun SUN, Liang FANG, Shaofeng XU. Stress Relaxation and Elastic Recovery of Monocrystalline Cu Under Water Environment. Acta Metallurgica Sinica[J], 2019, 55(8): 1034-1040 doi:10.11900/0412.1961.2019.00041

## 1 模拟方法

### 图1

Fig.1   The initial model of nanoindentation of monocrystalline copper under water environment

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$E=D0exp-2αr-r0-2exp-αr-r0$

Table 1  Parameters for TIP4P and Lennard-Jones potential[30,31,32]

Atom pair

ε

kJ·mol-1

δ

nm

Cutoff distance
nm
O—O0.6505[32]0.31655[30]0.6
Cu—O1.1335[30,31]0.28877[30,31]0.5
C—O0.4186[32]0.32750[32]0.5

Note: ε—depth of potential well, δ—zero-crossing distance

### 图2

Fig.2   Schematic of displacement-controlled nano-indentation

## 2 结果与讨论

### 图3

Fig.3   Load-indentation depth curves of without (a), and with H=1.0 nm (b), 2.0 nm (c) and 3.0 nm (d) water films (H—water film thickness)

### 图4

Fig.4   The nearest interatomic separation among Cu atoms in indenting region under different water environments

### 图5

Fig.5   The slice configurations of monocrystalline copper before (orange) and after (blue) unloading of without (a), and with H=1.0 nm (b), 2.0 nm (c) and 3.0 nm (d) water films

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### 图6

Fig.6   Default configurations within monocrystalline copper after loading (left), after stress relaxation (middle), and after unloading (right) of without (a), and with H=1.0 nm (b), 2.0 nm (c) and 3.0 nm (d) water films (blue—undeformed atom, red—surface and partial dislocation atom, green—stacking fault atom)

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## 3 结论

(1) 压头悬停阶段作用于压头上的载荷明显减小，即发生应力松弛现象。无水膜时单晶Cu的应力松弛量明显小于含水情况，水膜厚度为1.0和2.0 nm时，应力松弛增长率达到44.0%和8.2%。

(2) 纳米压入过程中，Cu原子间距随压入深度增加而快速减小；应力松弛阶段，Cu原子间的最邻近距离先增大后保持不变；卸载过程中，初期Cu原子间距因变形区域弹性能及位错能的释放而迅速增大，后期基本恒定。

(3) 含水条件下加载结束时，单晶Cu内部的位错、层错等缺陷数量明显多于无水情况，说明压痕内的不可恢复性变形因为水膜的存在而加剧；卸载结束时单晶Cu的变形能量得到释放，促进了部分位错、层错等缺陷的消失或向其它缺陷的转变，但是水膜对弹性恢复和塑性变形能量的释放行为具有一定的阻碍作用。

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