Mg-Al系合金熔体内生尖晶石型氧化夹杂调控及其孕育细化机制研究

Metal melts are easily oxidized at high temperature, leading to residual oxides in the final products as harmful inclusions. Under certain condition, some oxide inclusions can act as nuclei during solidification due to their high thermodynamic and structural stability, resulting in the refinement of solidified microstructure. However, for Mg based alloys, the behavior of oxide inclusions in solidification process has not yet been systematically studied. Our previous work showed that the spinel type oxides can act as potent nuclei for α-Mg grains. However, there are still some key issues in order to be addressed to use these oxide inclusions as effective grain refiner. Up to now, there is no controllable technique for forming the in-situ oxide inclusions. The inoculant mechanism and the dominant factors are also not clear. This project aims to develop a technique to prepare controllable spinel oxide inclusions in Mg-Al alloy melt. The exact composition, size and distribution characteristics of spinel oxide type inclusions acting as potent nuclei will be disclosed. The detailed structure of nucleating oxide inclusion and the interface between nuclei and Mg matrix will be deeply characterized. The thermodynamic and kinetic mechanism of the formation and evolution of oxide inclusion nuclei and the dominant factors will be systematically investigated. The physicochemical properties of spinel type oxide inclusion with high nucleation potency and the interface with Mg matrix will be studied using first-principles calculations. The nucleation potency and the influencing factors will be evaluated. The refining mechanism of inoculated by oxide inclusions for Mg-Al based alloys will be comprehensively investigated. This research is helpful to realize the beneficial utilization of oxide inclusions and develop the grain refining theory of Mg-Al alloy. The achievement can also be used to guide the development of Mg-Al alloy melt inoculation technology and grain refiner with high efficiency.

氧化夹杂是金属熔炼中产生的有害相，其热力学和结构稳定，可参与凝固形核从而细化组织，但针对Mg基合金氧化夹杂在凝固过程中的作用行为尚未进行系统研究。申请人前期研究证实尖晶石型氧化物可作为α-Mg相的有效晶核，但针对镁熔体内氧化夹杂的可控生成、孕育形核机制及其控制因素等的研究还不够深入。本课题拟开发Mg-Al系合金熔体内生尖晶石型氧化夹杂调控技术，分析尖晶石型有效氧化夹杂晶核的物相、尺度和分布特征，研究氧化夹杂晶核与基体间界面的精细结构，揭示氧化夹杂晶核的生成与结构演变过程的热力学和动力学机制及其控制因素。基于模拟计算厘清尖晶石型氧化夹杂晶核及其与基体间界面的物理化学性质，研究氧化夹杂孕育晶核的形核活性及其影响因素，全面揭示氧化夹杂孕育细化机制。本研究可实现氧化夹杂的有益化利用，拓展Mg-Al系合金晶粒细化理论，对Mg-Al系合金熔体氧化孕育技术和晶粒细化剂开发具有一定的指导意义。

Mg合金熔炼过程中易于产生有害氧化物夹杂，项目提出利用氧化夹杂实现Mg晶粒孕育细化。系统研究外加氧化夹杂和原位内生氧化夹杂孕育技术，优化氧化夹杂孕育细化工艺，系统研究影响因素及其孕育细化机制。. 商用MgO和MgAl2O4粉体，镁合金屑氧化燃烧粉末均可有效细化Mg-Al系合金。MgO不能有效细化纯Mg，MgAl2O4表现出更优的孕育形核能力，可以有效细化纯Mg。熔体中含有足够的Al(≥2%)时MgO才可充分发挥细化作用，MgO与Al反应生成MgAl2O4颗粒作为α-Mg异质核心。基于FIB-TEM分析揭示了MgO作为有效晶核的外延生长机制。理论分析表明，MgO有效形核需溶质过冷驱动作用下才可实现有效形核。超声作用可进一步提高氧化夹杂细化效果。晶粒细化后合金的力学性能显著改善，实现强度和塑性同步提升，提升幅度均超过50%。氧化物孕育改变熔体凝固特征，孕育细化效果越好，初始形核温度越高，再辉现象减弱甚至消失。. 将O2/Ar混合气体引入镁熔体内部原位内生氧化夹杂，并作为异质晶核实现α-Mg晶粒有效细化。孕育细化效率与熔体中存在的溶质元素类型密切相关，对含Al和Zn的二元合金有效，其中3%Al时效果最佳，但无法细化Mg-Ca合金。内生氧化孕育处理优先生成MgO夹杂颗粒，含Al熔体还会内生MgAl2O4，Al含量越高MgAl2O4粒子数量越多。第一性原理计算表明Al原子可促进Mg原子在氧化夹杂晶核表面吸附，而Zn/Mn/Fe等则表现出相反的趋势。因此Al提高氧化夹杂的异质形核效率，而Zn/Mn/Fe等弱化氧化夹杂的形核效率。热力学计算表明在含Zn/Mn/Fe的Mg-Al熔体中，不利于惨杂型尖晶石稳定生成。对于Fe/Si两种杂质，提出Al2Fe可作为α-Mg晶粒形核核心的新机制，并发现Mg2Si相可作为Mg17Al12的异质形核核心。. 项目研究证实氧化物夹杂可有效细化α-Mg晶粒，成功开发了一种原位内生氧化孕育新工艺，厘清了影响氧化夹杂有效孕育的工艺因素，揭示氧化夹杂孕育细化机制。研究结果为氧化物夹杂的有益化利用提供了新的思路，也为Mg-Al系合金晶粒细化剂及孕育细化工艺开发提供了一条新的途径。