偏晶合金Ni—31.44％Pb过冷组织粒化机制

利用溶融玻璃净化、循环过热相结合的方法对Ni-31.44%Pb(质量分数）偏晶合金宽过冷区间组织演化规律进行研究。结果表明：随过冷度的增大，凝固组织发生三次转变。其中当△T＞242K时，合金组织发生第三次转变，由细密枝晶骤燃粒化为过冷粒状晶。通过组织观察和过冷熔体枝晶生长过程的理论计算发现，快速凝固过程中液相变速率骤然增加，引起枝晶全面碎断，然后在枝晶块表面能和就变能的驱动下，晶界移动，发生碎晶合并－再结晶是形成过冷粒状晶的原因。

Structural Evolution inUndercooled Ni-31.44%Pb Monotectic Alloys

The evolution of solidification structures in undercooled Ni 31.44%Pb monotectic alloys was systematically investigated by using glass fluxing technique combined with cycle superheating.Within the achieved undercooling range,three consecutive structural transitions with the increase of undercooling were revealed.When the undercooling exceeded the critical value,242K,the third transition of structures from dendrites to undercooled granular grains happened.Based on the structural observation as well as the calculations of the maximum recalescence temperature,solid/liquid phase transition velocity,and undercooling using BCT model of the dendrite growth,it is concluded that the abrupt increasing of solid/liquid phase transition velocity is the main reason for the granulation transition.The granulation mechanism of undercooled granular grains is owing to dendrites breaking up and recrystallizing.