GPa级高压作用下镁合金的异质晶核及凝固组织细化机制

The grain refiement is a effevtively method for strenthening Mg alloys. By applying the high pressure in the order of GPa during solidification of the Mg-Zn-Y-(Zr) alloys, the nucleation rate would be enhanced and the microstructure refinements by means of the influence of the high pressure on the size of the critical radius and the work of the critical nucleation and the influences of the formed phases with high melting temperature and the high pressure on the wetting between the the high melting temperature phase and the Mg matrix. The chemical composition, the structure, the distribution, the amount, the sizw and the formation process of the hetergenoeus nucleus in the alloys solidification under the high pressure and the effect of the high pressure on the wetting between the Mg matrix and the necleus substrate would be investigated to reveal the feature and the formation mechanism of the hetergenoeus nucleus in the Mg alloys. The effects of the pressure and the chemical composition on the size of the grain, the distribution of the alloying elements and the refined strucutre of the solidification would be reseached to characterize the high pressure solidification microstructure. The effects of the pressure and the chemical composition on solidification characteristic temperature would be analyzed to build the relation curve of pressure-chemical composition-supercooling-grain size and to explore the nature of the effect of the high pressure on the parameters of solidification thermodynamics and kinetics and the refinement mechanisum of the high pressure solidification Mg alloys. The bulk material of the superfinement grains of Mg alloys would be prepared under the high pressure directively. The factora and the mechanisum of the infleuncing and controlling the the high pressure solidification microstructural refinement would be understood. It is of the very significance to enrich the theory of the high pressure solidification and to develop new Mg alloys with high strengths.

晶粒细化是强化铸造镁合金行之有效方法。本课题拟通过在Mg-Zn-Y-(Zr)合金凝固过程中施加GPa级压力，利用高压对临界晶核半径和临界形核功的影响，高压下高熔点相生成及压力对高熔点相与镁基体润湿性等的影响，提高形核率，细化凝固组织。将研究高压下异质晶核的成分与结构、形貌与分布、数量与大小及生成过程，研究压力对镁基体与晶核衬底润湿性的影响，揭示高压下镁合金异质晶核特性及形成机制。研究压力和合金成分对合金晶粒尺寸、溶质元素分布规律及凝固组织精细结构的影响，表征高压凝固组织。分析压力和合金成分对凝固特征温度的影响，建立压力-化学成分-过冷度-晶粒度关系曲线，探讨高压对镁合金凝固热力学和动力学参量影响的本质，揭示高压凝固镁合金组织细化机制。利用高压凝固直接制备超细晶镁合金块体材料，认知影响、控制高压凝固组织细化因素及机制，对于丰富高压凝固理论、开发新型高强压铸镁合金有着重要意义。

凝固组织粗大的铸造镁合金增强、增塑是镁合金研究中的重要问题，也是镁合金研究领域的一个瓶颈。由于Hall-Petch公式中Mg的常数k值（280~320 MPa µm1/2）很大，细晶强化效果显著，因此晶粒细化成为镁合金增强增塑最有效的方法之一。.凝固过程直接决定合金凝固组织粗细。为了控制凝固过程，传统方法是通过调节化学成分和温度来改变凝固组织，而没有考虑影响凝固过程的另一个热力学参数—压力。压力与温度和化学成分一样，对合金凝固过程也有着重要的影响。因此，本项目主要研究了凝固压力（2 GPa ~6GPa）对Mg-Zn-Y合金凝固组织及力学性能的影响规律，研究了高熔点相作为有效异质晶核及其凝固组织细化机制；研究了高压凝固Mg-Zn-Y合金热变形行为及其高压扭转行为。研究了高压下Ca元素对镁合金凝固组织及力学性能的影响规律及影响机制，制备出“枝晶团”平均尺寸仅有24μm、抗压强度达520MPa且还维持一定塑性的铸造镁合金。为进一步深入系统地研究GPa级高压下，溶质浓度对镁合金凝固组织晶核数目、晶粒大小的影响规律，制备了十几种不同Al量的Mg-Al二元合金，研究了2GPa~6GPa高压下Mg-Al二元合金相图特征点、相组成及组织结构特征，建立了凝固压力-合金成分-晶粒度关系曲线；从热力学及凝固理论出发，研究了高压下Mg-Al合金凝固过程中溶质的再分配、有效分配系数以及与组织结构相关性，探讨了凝固压力、溶质浓度、晶体生长限制参数、成分过冷以及异质生核等对Mg-Al二元合金凝固组织细化的影响机制。在此基础上，制备出基于Al溶质晶界偏聚、具有固溶体型组织结构、强度突破500MPa并具有良好塑性（δ为25%）的细晶铸造Mg-Al二元合金，这对于开发新型高强韧压铸镁合金有着重要科学意义与实际工程意义。