合金元素调控镁中变形孪晶形核与生长的基础研究

How to obtain high-density twin structure has become a main issue in Mg for enhancing strength and ductility through twin boundary. Segregation of solute atoms into twin boundaries could effectively pin twins from further growing. Furthermore, pseudoelastic behavior of Mg alloys could be utilized to adjust twin thickness. As a result, nucleation of new twins may probably be activated under suitable stress field exerted by the pinned twins. Based on this, a new idea has been proposed to increase the density and decrease the size of twins in Mg through solute atoms. Mg alloy systems will be selected on the basis of twin-boundary segregation energies and solute-diffusion activation enthalpies of alloying elements. The twin nucleation, microstructure evolution and deformation behavior of the alloy systems during deformation will be investigated. The effects of twin variants, twin thickness, solute atoms and grain size, together with the deformation conditions, on the nucleation density, location and distribution will be detailed. Moreover, MD simulation will be carried out to simulate the interaction and stress field of twins with different structures, including different twin variant, twin boundary structures with different twinning dislocations, and twin thickness, in order to obtain basic theory on controlling the nucleation and growth of twins through solutes. Research results will not only be helpful to develop fundamental knowledge on plastic deformation mechanism and twin boundary strengthening mechanism, but also provide a new path and basis for new alloy design.

如何获得高密度孪晶微结构，已成为制约在镁中实现孪晶界面强韧化的一个核心和难点问题。本项目创新性地提出了利用溶质原子在孪晶界面的钉扎阻碍孪晶生长、利用镁合金的伪弹性行为调控孪晶片层厚度，进而在合适的孪晶界面前沿应力场作用下激发产生新的孪晶形核，从而提高孪晶形核密度、减小孪晶尺寸的新思路。在孪晶偏聚能和扩散激活能计算的基础上，选择典型合金体系，研究变形过程中孪晶形核、微结构演变和变形力学行为，考察起始孪晶组态、片层厚度、溶质原子、晶粒尺寸、变形条件等相关诸因素，影响孪晶形核数量、位置和分布等的关系规律，同时采用分子动力学，模拟不同结构孪晶（不同孪晶组态、孪晶位错界面结构、片层厚度等）之间的交互作用和应力场，获得利用合金元素调控镁中变形孪晶形核与生长的基础共性科学规律。研究结果不仅有助于发展和完善塑性变形和孪晶界面强韧化机制的一些共性基础问题和规律，并且将为新型合金设计提供新的途径和理论依据。

如何获得高密度孪晶微结构，已成为制约在镁中实现孪晶界面强韧化的一个核心和难点问题。项目采用第一性原理计算得到了描述溶质原子在孪晶界偏聚倾向的孪晶偏聚能和描述原子从晶内扩散至孪晶界难易程度的扩散激活能，获得了溶质原子强化孪晶位错的定量数据，构建了孪晶偏聚强化设计图，为优化选择合金元素、实现有效偏聚和界面钉扎提供了重要依据，其中Nd、Sm、Gd等元素显示了最佳的强化效果。在此基础上，选择典型合金系，系统考察了应变量、应变路径与孪晶形核、退孪生之间的相互关联，建立了孪晶片层厚度与变形过程参量之间的关系规律，获得了调控孪晶片层厚度的依据。采用分子动力学模拟首次获得了孪晶核心的三维结构特征和长大特征，与目前的二维模拟结果相比，孪晶界面类型和迁移机制均有较大差别。孪晶核心长大涉及pure-shuffle和glide-shuffle两种孪生机制；{10-11}界面沿±[1-120]方向的迁移比{10-12}和BP / PB界面沿相关方向的迁移更容易。采用模拟和实验相结合的方法获得了变形过程中界面结构特征及其演化规律，获得了变形过程中新形核孪晶的形核机制、新形核孪晶与已有孪晶的交互作用，由此获得了通过合理利用孪晶之间的交互作用阻碍孪晶长大、促进形核的新途径和思路。实验确定了典型合金系实现溶质原子在孪晶界面扩散偏聚的合适的扩散退火热处理工艺，测试计算了溶质原子孪晶界面钉扎和强化效果，与模拟预测结果吻合得很好。建立了已有孪晶应力场和溶质原子对于后续孪晶形核数量、位置和分布的影响规律，以及后续变形过程中孪晶形核与长大的竞争规律，获得了调控孪晶的依据和手段。表明合适的变形路径可以提高孪晶晶内形核的能力、提高孪晶形核率；中间退火能够同时强化孪晶形核和长大；一定的合金元素和退火工艺的结合可以增加后续变形过程中形核对孪生的贡献。研究工作还探索了不同热处理方式对孪生演变的影响，表明感应加热淬火的方式同时抑制了孪晶长大和退孪生，进一步为调控孪晶尺寸和数量提供了新的途径和思路。