Mg-Zn-Zr合金铸造凝固组织中枝晶生长约束因子、枝晶相干点固相分数与晶粒尺寸之间的关系

The refinement of grain size of magnesium alloys can be realized by adding modifier which can effectively increase the number of nuclei, and also by methods that are favored for restricting grain growth, such as increasing the concentration of solute atoms. So far, for binary magnesium alloys at low solute content, Q value theory that characterizes the impeding effect of solute atoms on the growth of grains is usually adopted to explain the change in grain size. In Q value theory, diffusion of solutes during grain growth is taken into condition.However, the impact of dendritic morphological changes on the rate of dendrites growth were ignored in Q value theory, and it should be noted that the dendritic morphological changes can be significantly induced by different solute concentrations and casting process. So when part of the alloys shows a high solute content, i.e., Q value is large, the grain coarsening phenomenon contradicted this theory. By calculating the dendritic consistency Point, changes in solute concentratio and dendrite morphology, this project aiming to characterize the influence of elements for heterogeneous nucleation, solute concentration and casting process on α-Mg dendrite morphology. Combining wit thermodynamic calculations, this project also establishes the relationship between Mg-Zn-Zr alloy compostion, α-Mg dendritic morphology, dendritic consistency Point, and Q value, providing a novel thinking for controlling the dendritic morphology, predicting the grain size, and developing new method of grain refinement of magnesium alloys.

在镁合金中往往通过添加变质剂增加形核中心、增加溶质浓度等约束晶粒长大等方法细化镁合金组织中晶粒尺寸。目前,在二元镁合金中，当溶质含量较低时，铸态下合金的晶粒尺寸随着溶质含量的增加而减小，一般采用表征溶质元素对晶粒长大抑制作用的Q值理论解释晶粒尺寸的变化，但Q值理论只考虑了晶粒长大过程中溶质的扩散，而忽略了溶质浓度和铸造工艺不同引起的枝晶形貌变化对枝晶长大速率的影响，因此部分合金中溶质含量较高时，即Q值较大时，出现晶粒粗化的现象与该理论矛盾。本项目通过计算合金凝固过程中枝晶相干点固相分数的变化和枝晶形貌特征参数表征异质形核元素、溶质浓度和铸造工艺对α-Mg枝晶形貌及晶粒尺寸的影响产生的影响。结合热力学计算，建立Mg-Zn-Zr合金α-Mg枝晶形貌、晶粒尺寸、枝晶相干点固相分数与Q值之间的关系，为控制枝晶形貌，预测晶粒尺寸，发展新型镁合金晶粒细化方法提供思路。

首先采用冷却曲线分析法结合热力学计算，探究了 Mg-Zn-Zr 合金的凝固特征点及凝固组织的变化规律。结果表明，随着 Zn 含量的增加，合金的液固相线温度逐渐降低，凝固区间先增加后趋于稳定，枝晶干涉固相分数整体呈降低趋势，但在 Zn 含量为 4 wt.%时存在峰值。针对 Mg-Zn-Zr 合金中出现的晶粒尺寸随 Zn 含量增加先减小后增大的现象，从生长约束和枝晶干涉的角度进行了详细的分析。究其原因，这是形核和生长竞争的结果。当 Zn 含量为 3 wt.%时，由溶质富集造成的适当的成分过冷一方面为快速形核创造了条件，另一方面使晶粒仍以等轴晶形式生长并较慢地达到枝晶干涉点，从而使得 ZK31 获得最细的晶粒和最佳的综合力学性能。