Study On Non-equilibrium Solidification And Abnormal Grain Growth In Ni-Pb Alloy

As for material preparation, liqulid/solid transformation and solid-state transition(e.g. grain growth) are two basic and correlated processes. How to propose a relation between liqulid/solid transformation and the subsequent solid-state grain growth and how to guide material preparation are still open questions. In this paper, a systematic investigation in purification process and micro-purification mechanism of Ni-Pb alloys has been performed, by employing glass slag purification incorporated with cyclic superheating techniques. With the aid of infrared radiation thermometer, TEM, SEM, EDS analytical methods, the formation of crystallite, the growth mechanism and thermal stabilization in the undercooled Ni-Pb alloy were studied. The main conclusions were as follows:1. The microstructure evolution of undercooled Ni-Pb alloy in the obtained undercooling 20K<ΔT<300K were investigated by employing the glass fluxing technique in combination with cyclical superheating. When ΔT<80K, Ni-Pb alloy was made of developed dendrites; When 80K<ΔT<100K, the microstructure of the alloy was changed from developed dendrites to first kind of granular crystal which was tiny and interface for irregular surface. When 100K<ΔT<150K, the solidification microstructure was changed from first kind of granular crystal to first kind of undercooled dendrites. When ΔT >150K, the solidification microstructure in undercooled dendrites was change into second type of granular crystal. With the increase of undercooling, the grain size decreases.2. The normal/ abnormal grain growth has been studied by employing annealing at different temperatures and times. According to the rate of grain growth, the whole process of grain size evolution could be rationally categorized as three stages: 1) The slow growth stage; 2) The rapid growth stage; 3) The stable growth stage.3. The thermal-dynamic model of nano-scale grain growth under the condition of dynamic grain growth segregation has been extented and been applied in the whole process of Ni-Pb grain growth. It can be confirmed from the model calculation and calibration of the TEM pictures, slow grain growth was induced by the reduction of grain boundary energy(i.e. the reduction of grain growth driving force) which caused by the segregation of Pb. Rapid grain growth occurred accompanying the increase of grain boundary energy which caused by the precipitation of saturated Pb atoms and the formation of second phase particles. The stable grain growth was controlled by the Zener drag.