微观应力下单晶镁{10-12}孪晶的可控形核机制研究

Previous studies have provided a number of insights about the frequent intersection between dislocation slip and twin boundary below the critical {10-12} twin lamellar structure which can effectively improve the mechanical properties of Mg. However, dense arrays of coherent twin boundaries are considered difficult to introduce in Mg alloys under the conventional process due to the high twin boundary energy. So the twin-nucleation mechanism controlled, which can improve the nucleation rate, will be crucial to create high-density twin boundaries. In this research, focused-ion-beam (FIB) technique with the controllable microstress is used to induce twin nucleation in magnesium single crystal with the aim of increasing the number of twin nucleus. Here, the pure Mg single crystals with a wide range of crystallographic orientations were subjected to the site specific sputtering of FIB with different values of the incident energy. And the condition and characterization of {10-12} twin nucleation, effect of stress field caused by FIB sputtering on twin nucleation and the mechanisms of twin-twin and twin-dislocation interactions are systematically studied by analyzing characteristics of twin nucleation. This project research and experiment will reveal the selection mechanism of twin variants based on the controllable twin nucleation. We expect to establish a quantitative relationship between microstructure evolution and the controllable microstress and effectively control twin nucleation, and eventually produce Mg alloy with high-density twin boundaries. Works to be carried out in this program can not only contribute much to the fundamental theory of twin-nucleation control of Mg alloy, but also provide the important suggestions for exploring the processing performances of the high strength and toughness Mg alloys and other new-type metals.

大量研究证明高密度{10-12}孪晶片层结构下孪晶与位错频繁交互作用能有效改善镁合金强韧性。但是高的孪晶界面能使镁很难在传统加工工艺下形成高密度孪晶片层结构，因此，实现孪晶形核可控性以提高形核率对于形成高密度孪晶片层结构至关重要。本项目利用具有微观可控应力特点的聚焦离子束系统来诱发单晶纯镁的孪晶形核。首先对不同取向的单晶纯镁进行多种能量的离子束入射，系统研究{10-12}孪晶形核条件及特征，然后考察聚焦离子束形成应力场对孪晶形核影响，详细分析孪晶间以及孪晶与位错间的交互作用机理。项目预期揭示孪晶可控形核的变体选择机制，建立组织结构演变与微观可控应力之间的定量关系，实现对孪晶形核以及微观组织结构演变的有效控制，尝试制备具有高密度孪晶片层组织结构的镁单晶材料。该课题的顺利开展，不仅为镁合金孪晶形核控制理论提供重要的基础研究积累，也为高强韧镁合金以及其它新型金属材料的设计提供理论依据。

高密度{10-12}孪晶片层结构下孪晶与位错频繁交互作用有效改善镁合金的力学性能。但是高的孪生界面能使镁合金很难在传统加工工艺下形成高密度孪晶片层结构，因此，实现孪晶形核可控性以提高形核率对于形成高密度孪晶片层结构至关重要。本项目利用具有微观可控应力特点的聚焦离子束系统以及具有宏观应力场的动态塑性变形系统来诱发单晶纯镁及AZ31镁合金的孪晶形核，从“聚焦离子束诱发{10-12}孪生的形核条件及特征”、“孪晶-孪晶交互作用下的界面结构分析”、“动态塑性变形下的孪晶形核机制”三个方面进行研究，研究结果表明：微观应力能够通过改变聚焦离子束的入射能量以及晶体位向关系来有效控制单晶纯镁的孪晶形核；多晶镁合金中的{10-12}孪晶交互作用形成的7.4°<-12-10>界面无法有效阻碍位错滑移运动，而且7.4°<-12-10>界面上具有错层连接的特点。这种低能界面的结构稳定性较差，在循环应力作用下易通过孪生或退孪生作用分解，或在退火作用下消失，所以导致其在镁合金力学性能改善方面作用有限；而宏观应力下的孪生不可控性，导致形成的孪晶结构不规律和不稳定，从而使镁合金力学性能改善不佳，这是由于宏观应力下形成的单一变体的孪晶结构不具有高密度界面及弱化织构的微观组织特征。通过该课题的研究，为镁合金孪晶形核控制理论提供了进一步的基础研究积累，具有重要的科学意义；同时也为高强韧镁合金以及其它新型金属材料的设计提供了理论依据，具有重要的工程意义。