Liquid Phase Separation Mechanism of Cu-40 wt.% Pb Hypermonotectic Alloys

Solidification microstructures of Cu-40 wt.% Pb alloy were examined under different undercooling degrees. The liquid phase separation mechanism in the systems with stable miscibility gaps mainly involved Ostwald ripening, Brownian motion, Marangoni migration, and Stokes motion. Stokes had little influence on the liquid phase separation in the early phase and played a leading role in the later period. The liquid phase separation mechanism of Cu-40 wt.% Pb hypermonotectic alloy was illustrated in detail.

A Homogeneous Solidification Criterion for Undercooled Ni-20at%Pb Hypermonotectic Melts

The kinetic process of liquid-liquid phase separation in the undercooled Ni-20at%Pb hypermonotectic alloy melts was analyzed theoretically. The results showed that liquid-liquid separation could not be inhibited due to smaller nucleation barrier and bigger nucleation rate of Pb-rich droplets. In the course of liquid-liquid phase separation, the volume fraction of Pb-rich droplets was thought as a function of time or temperature. At a certain cooling rate, the volume fraction was mainly controlled by the undercooling of Ni-Pb hypermonotectic melts. Based on the above results, a homogeneous solidification criterion for the undercooled Ni-20at%Pb hypermonotectic alloy melts was developed. Such a criterion predicted that the homogeneous microstructure could be obtained at the undercooling 263K, and the experimental results accorded with the predicting ones on the whole.

Gravity-Induced Convection during Directional Solidification of Hypermonotectic Alloys

A two-phase volume-averaging model developed for the simulation of the dynamic decomposition and solidification of hypermonotectic alloys was used to study the occurrence of convection phenomena during directional solidification of a hypermonotectic alloy (Al-10 wt.%Bi) under terrestrial condition. The model accounts for nucleation and growth of secondary phase droplets, Marangoni and Stokes forces, solute partitioning, heat release due to decomposition and solidification. It is shown that the appearance and growth of secondary droplets is accompanied with a continuous downwards motion of droplets, which rapidly becomes unstable. After relaxation of this dynamic motion the advancing solidification front freezes in the resulting non-uniform droplet distribution.

Microstructural Evolution of Undercooled Ni-40wt%Pb Hypermonotectic Alloy

Microstructural evolution of bulk undercooled Ni-40wt%Pb hypermonotectic alloy was systematically investigated by using molten glass purification and superheating cycling. Within the achieved undercooling range, the microstructures were classified into three categories. When the undercooling was less than 50K, the microstructure consisted of coarse dendritic grain and interdendritic Pb lumps. Increasing undercooling to the range 100 to 198K, macrosegregation was serious. When the undercooling was up to 292K, refined granular grain with homogeneously distributed fine Pb particles on it formed. The granulation mechanism of granular grains was owing to dendritic disintegration and subsequent recrystallization.