## Ag-Ni偏晶合金凝固过程研究

1. 中国科学院金属研究所 沈阳 110016

2. 中国科学院大学 北京 100049

3. 辽宁石油化工大学机械工程学院 抚顺 113001

## Study on the Solidification of Ag-Ni Monotectic Alloy

DENG Congkun1,2, JIANG Hongxiang1, ZHAO Jiuzhou,1, HE Jie1, ZHAO Lei3

1. Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China

2. University of Chinese Academy of Sciences, Beijing 100049, China

3. School of Mechanical Engineering, Liaoning Shihua University, Fushun 113001, China

 基金资助: 国家自然科学基金项目.  51771210国家自然科学基金项目.  51574216国家自然科学基金项目.  51774264辽宁省教育厅基本科研项目.  L2017LQN022

Corresponding authors: ZHAO Jiuzhou, professor, Tel: (024)23971918, E-mail:jzzhao@imr.ac.cn

Received: 2019-06-13   Revised: 2019-09-06   Online: 2020-01-19

 Fund supported: National Natural Science Foundation of China.  51771210National Natural Science Foundation of China.  51574216National Natural Science Foundation of China.  51774264Basic Research Project of Education Department of Liaoning Province.  L2017LQN022

Abstract

The Ag-Ni alloy has high electrical conductivity, good thermal conductivity, high specific heat capacity, and excellent electrical wear resistance if the Ni-rich phase is dispersedly distributed in the Ag-based matrix. It has been widely used in the medium load contactors, magnetic starters, relays, etc. However, Ag-Ni alloy is a typical monotectic system. Generally, the liquid-liquid phase transformation leads to the formation of a solidification microstructure with serious phase segregation. So far, there have been few studies on the solidification process of Ag-Ni alloys and powder-metallurgical techniques are commonly used to prepare Ag-Ni alloys in industry. In this work, casting experiments and microhardness test were carried out with the Ag-Ni monotectic alloy. The samples with composite microstructure, in which the Ni-rich particles dispersed homogeneously in Ag matrix, were obtained. The microhardness of Ag-Ni alloy increases with the increase of nickel content and the cooling rate of the sample during solidification. When the cooling rate during the liquid-liquid phase transition of the Ag-4.0%Ni alloy reaches 1800 K/s, the microhardness of the Ag-4.0%Ni alloy is close to that of the Ag-10.0%Ni sheet electrical contacts produced by powder metallurgy. A model describing the microstructure evolution during cooling Ag-Ni monotectic alloy melt has been proposed. The process of microstructure formation has been simulated and discussed in details. The results indicate that the cooling rate during the nucleation of the Ni-rich droplets/particles has a dominant influence on the solidification microstructure. The average radius of the Ni-rich particles increases with the increase of nickel content, while it decreases with the increase of the cooling rate during solidification. The average radius of the Ni-rich particles shows an inverse square root dependence on the cooling rate during the nucleation of the Ni-rich droplets/particles. The Ostwald coarsening of the Ni-rich droplets/particles is very weak during cooling Ag-Ni monotectic alloy melt. Rapid/sub-rapid solidification has a good application prospect in the preparation of the high-performance Ag-Ni contact materials.

Keywords： Ag-Ni monotectic alloy ; liquid-liquid phase separation ; solidification microstructure ; microhardness ; simulation

Congkun DENG, Hongxiang JIANG, Jiuzhou ZHAO, Jie HE, Lei ZHAO. Study on the Solidification of Ag-Ni Monotectic Alloy. Acta Metallurgica Sinica[J], 2020, 56(2): 212-220 doi:10.11900/0412.1961.2019.00192

## 2 实验结果

### 图1

Fig.1   Experimental results (symbols) and calculated results (solid lines) of the cooling curves for the center region of Ag-4.0%Ni alloys with different diameters (d) (T—temperature, t—time, Tm and Te refer to the monotectic reaction temperature and the eutectic reaction temperature of Ag-Ni alloy, respectively)

### 图2

Fig.2   Schematic distribution of the Ni-rich particles in Ag-4.0%Ni alloy with a diameter of 8 mm (a), measured volume fraction ($φsβ$) distributions of the Ni-rich particles along the axial z direction (b) and radial r direction (c)

### 图3

Fig.3   SEM images of the Ag-xNi alloys for the samples of 5 mm in diameter with x=1.25% (a), x=2.24% (b), x=3.0% (c) and x=4.0% (d)

### Fig.4

Fig.4   2D size distributions of the Ni-rich particles in Ag-xNi alloys for the samples of 5 mm in diameter with x=1.25% (a), x=2.24% (b), x=3.0% (c) and x=4.0% (d)

### 图5

Fig.5   Average 2D radius (<R>2D) of the Ni-rich particles in Ag-Ni alloys for the samples of 5 mm in diameter vs alloy composition

### 图6

Fig.6   SEM images showing solidification micro-structures of Ag-4.0%Ni alloys with diameters of 4 mm (a), 6 mm (b) and 8 mm (c)

### 图7

Fig.7   Experimental results (black symbols) and calculated results (black line) of <R>2D of the Ni-rich particles in Ag-4.0%Ni alloys and the cooling rate of the melt during the nucleation ($T.nuc$, red line and symbols) of the Ni-rich droplets as a function of sample diameter

### 图8

Fig.8   Microhardnesses of Ag-Ni alloys as a function of alloy composition for the samples of 5 mm in diameter (a) and as a function of sample diameter for the Ag-4.0%Ni alloys (b)

## 3 分析讨论

### 3.1 凝固过程的模型构建

$∂fi∂t+1r∂(uMrfd)∂r+∂∂R(vifi)=∂Ii∂RR=Ri*$

$Ii=N0⋅4nci23⋅6Dδ2⋅(ΔGci3πkBTnci2)12.exp(-ΔGcikBT)$

$-kmix∂T∂rr=rs=h(Ts-Tmould)$

$4π3∂r∫0∞uMfdCβ-CmR3dRr∂r=∂Cmix∂t$

$η=0.4532×10-3exp(22.2×103kBNAT)$

D通过Sutherland-Einstein关系式计算[32]

$D=kBT4πηrNi$

$σL-L=σ0(1-TTC)1.26$

Table 1  The thermophysical property parameters of Ag-Ni system[31,32]

ParameterValueUnit
Thermal conductivity of liquid Ag $klAg$122.29093+0.04259TW·K-1·m-1
Thermal conductivity of liquid Ni $klNi$57W·K-1·m-1
Thermal conductivity of solid Ag $ksAg$429W·K-1·m-1
Thermal conductivity of solid Ni $ksNi$90.7W·K-1·m-1
Density of liquid Ag $ρlAg$9330-0.91(T-1233.7)kg·m-3
Density of liquid Ni $ρlNi$7905-1.19(T-1727)kg·m-3
Density of solid Ag $ρsAg$10500kg·m-3
Density of solid Ni $ρsNi$8900kg·m-3
Specific heat of liquid Ag $cpl,Ag$283J·kg-1·K-1
Specific heat of liquid Ni $cpl,Ni$620J·kg-1·K-1
Specific heat of solid Ag $cps,Ag$235J·kg-1·K-1
Specific heat of solid Ni $cps,Ni$444J·kg-1·K-1
Latent heat of solidification of pure Ag $LAg$102809J·kg-1
Latent heat of solidification of pure Ni $LNi$292334J·kg-1

### 图9

Fig.9   Supersaturation (S, red line) of the matrix melt, nucleation rate (I, green line), number density (N, black line) and <R>2D (blue line) of the Ni-rich droplets/particles in the center region of the sample as a function of time during cooling the Ag-2.24%Ni alloy (solid line) and Ag-4.0%Ni alloy (dashed line) for the samples of 5 mm in diameter (a), and the enlargement of the microstructure evolution during the period from 55 ms to 265 ms (b)

### 图10

Fig.10   Cooling rate ($T.$, black line) and supersaturation (S, red line) of the matrix melt and nucleation rate (I, green line) of the Ni-rich droplets in the center region of the sample as a function of time during cooling a Ag-4.0%Ni alloy sample with a diameter of 4 mm (solid line) and 6 mm (dashed line)

### 图11

Fig.11   Experimental results (solid circles) and calculated results (open circles) of the average radius (<R>) of the Ni-rich particles in Ag-xNi alloys with x=1.25% (blue color), x=2.24% (green color), x=3.0% (red color), x=4.0% (black color) vs$T.nuc$ of the Ni-rich droplets/particles

## 4 结论

(1) 利用快速/亚快速凝固方法可制备富Ni相粒子弥散分布于Ag基体的Ag-Ni合金。

(2) Ag-Ni合金显微硬度随着合金Ni含量增加和试样凝固过程冷却速率升高而增大。当Ag-4.0%Ni合金液-液相变开始阶段熔体冷却速率达1800 K/s时，其显微硬度为48.13 HV，接近于粉末冶金生产的Ag-10.0%Ni的硬度。快速/亚快速凝固在高性能Ag-Ni触头材料制备上具有很好的应用前景。

(3) Ag-Ni合金的Ni含量越高、凝固过程中冷却速率越低，则凝固组织中富Ni相粒子平均尺寸越大。富Ni相粒子的平均半径(<R>)与富Ni相液滴/粒子形核阶段熔体冷却速率($T.nuc$)之间满足：<R>=A($T.nuc$)-B

(4) 在快速/亚快速凝固条件下，Ag-Ni合金凝固过程中富Ni相液滴/粒子的Ostwald粗化作用很弱，富Ni相液滴/粒子的尺寸主要受形核和长大控制。

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