Acta Metall Sin  2020, Vol. 56 Issue (2): 212-220    DOI: 10.11900/0412.1961.2019.00192
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Study on the Solidification of Ag-Ni Monotectic Alloy
DENG Congkun1,2,JIANG Hongxiang1,ZHAO Jiuzhou1(),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
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.

 ZTFLH: TG111.4
Fund: National Natural Science Foundation of China(51771210);National Natural Science Foundation of China(51574216);National Natural Science Foundation of China(51774264);Basic Research Project of Education Department of Liaoning Province(L2017LQN022)
Corresponding Authors:  Jiuzhou ZHAO     E-mail:  jzzhao@imr.ac.cn
 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) 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) 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.5  Average 2D radius (2D) of the Ni-rich particles in Ag-Ni alloys for the samples of 5 mm in diameter vs alloy composition 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) Fig.7  Experimental results (black symbols) and calculated results (black line) of 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 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) Table 1  The thermophysical property parameters of Ag-Ni system[31,32] Fig.9  Supersaturation (S, red line) of the matrix melt, nucleation rate (I, green line), number density (N, black line) and 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) 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) Fig.11  Experimental results (solid circles) and calculated results (open circles) of the average radius () 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