液液结构转变对熔体分形特征和凝固行为的影响

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1、合肥工业大学博士学位论文液-液结构转变对熔体分形特征和凝固行为的影响姓名：丁国华申请学位级别：博士专业：材料加工工程指导教师：祖方遒20090201象，由粗大树枝晶变为等轴晶，而溶质分配系数较小的Pb．90wt％Sn合金液一液结构转变后枝晶尺度虽然也减小，但其形貌仍为树枝晶。对于组织以等轴晶形式存在的Bi．10wt％Sb合金，液态结构转变使凝固组织形貌由不均匀不规则变为均匀三角星状，且使晶粒尺寸明显减小。第四，自制了定向凝固装置，成功实现了Pb-80wt％Sn、Pb．90wt％Sn和Pb．99wt％Sn的定向凝固实验。发现熔体结构转变减小了溶质非平衡分配系数，增加了固液界面前沿的溶质富集程度，

2、减小了胞晶间距和二次枝晶间距。液．液结构转变对晶体生长取向的影响主要是随凝固长度增加，液．液结构转变使合金实现高指数晶面取向，加剧了低指数晶面分支现象。液．液结构转变增大了Pb．80wt％Sn、Pb．99wt％Sn合金的定向凝固固液界面不稳定性。在水淬冷速条件下时，熔体结构变化后的Pb．80wt％Sn合金枝晶尖端、Pb-99wt％Sn合金胞晶尖端能够进入其固液界面前沿的成分过冷区，继续向液相生长；而液．液结构转变前合金的枝晶和胞晶尖端被阻止伸入固液界面前沿的成分过冷区，从而能够保留较完好的固液界面。关键词：液一液结构转变；自由凝固；定向凝固；分形理论；R／S分析ABSTRACTPolymorp

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