基于GP区调控第二相强化Mg-RE-Zn系镁合金的设计、组织控制与力学性能

In this work, the solubility of RE and Zn in the alpha-Mg solid solution is to be accurately determined by diffusion couple technique in the Mg-RE(Y, Gd, Nd)-Zn ternary systems. Then the reliable thermodynamic parameters of each phase are to be optimized by the CALPHAD technique, in particular for the alpha-Mg solid solution. The metastable miscibility gaps are to be thermodynamically calculated, the existing temperature or composition ranges of the GP zone are to be achieved, and therefore, the compositions of the corresponding Mg-RE-Zn alloys with GP zone will be designed. The precipitation features of GP zone, and its effects on the density, size, distribution and interface nature of the subsequent metastable or stable strengthening phases are to be revealed, and then the scientific foundation of microstructure control is to be established in the Mg-based alloys strengthened by the second phase depending on the GP zone. And the outstanding mechanical properties of the corresponding Mg-based alloys are to be achieved, so that the role of GP zone is to be known well in the practical Mg-RE-Zn-Zr systems. At the same time, it also provides the valuable information of alloy design for the improvement of strength of other Mg-based alloys containing GP zone.

本项目通过扩散偶法精确测定Mg-RE(Y, Gd, Nd)-Zn系alpha-Mg基固溶体中RE和Zn溶解度；利用CALPHAD方法获得相关的每个相，尤其是alpha-Mg固溶体的准确可靠热力学参数；通过亚稳溶解度间隙的热力学计算，明确GP区存在的温度和成分范围，设计出具有GP区析出的Mg-RE-Zn合金；揭示GP区在单级、双级/多级以及应力/应变条件下的时效析出特征，及其对后续各类亚稳或稳态强化相密度、尺寸、分布和界面性质的影响规律，建立GP区调控第二相强化Mg合金组织学的科学基础；并在此基础上使GP区调控第二相强化Mg合金取得优异的力学性能，进而掌握GP区在实用镁合金Mg-RE-Zn-Zr体系的作用，也为改善其它含有GP区镁合金的强度提供有价值的合金设计依据。

本项目在精确测定Mg-Zn-RE系相平衡的基础上，获得了准确可靠的热力学数据。利用HAADF-STEM技术成功地揭示出Mg-RE镁合金中时效初期存在不同形态的原子团簇和“Z”字单链GP区。明确了Mg-Zn合金中棒状析出强化相具有二维5次准晶结构、Mg-Gd-Zn合金中析出强化相为五层原子组成的拓扑密堆单胞二十面体团簇结构。除了Mg-Y(Gd)体系外，在Mg-Nd（Sm）合金时效过程中也发现了Mg5RE结构的亚稳析出相的存在。而且，在Mg-RE(Nd, Sm, Gd)二元合金时效过程中，提出了次要强化相β1(fcc, Mg3RE)既可以通过溶质原子扩散直接形核，也可以通过切变的方式形核，即由六角链构成的hcp，Mg3RE (βH’和β’’)不再通过扩散的方式形成βS’相，而是直接通过切变的方式（约5°的切变角）及结构驰豫，转变为fcc, Mg3RE(β1)相。此外，应力时效可以显著地细化强化相，提高其析出密度。这为理解不同体系第二相强化机理提供了理论基础，对开发新型的高强高韧镁合金具有重要的实际意义。同时，开发出了含有GP析出的高强Mg-Ca基镁合金和高强耐蚀的单相Mg-Zn-Ag合金，其在骨植入器件（骨钉和骨板等）具有潜在的应用价值，从而对于扩大镁合金的应用范围具有重要的实际意义。