基于多道次孔型轧制镁铋基合金的组织调控与强韧化机理

Increasing interest in development of low-cost and high-strength wrought magnesium alloys meets the strong demands for weight reduction in the automotive and aerospace industries nowadays. Instead of Mg-Sn based system, the Mg-Bi alloy system seems a potential candidate to satisfy this requirement because it shows typical precipitation-type phase equilibrium and contains Mg3Bi2 phase, which has a high melting temperature of 821oC comparable to those thermal stable intermetallic phases in RE-bearing Mg alloy. In addition, the fine Mg3Bi2 plates on the prismatic plane were reported to be more effective for precipitation-hardening than that more commonly reported basal plates..In this study, a novel Mg-6Bi-based alloy with ultra-fine grain (UFG) and superior mechanical properties is fabricated by optimized cold pre-forging (CPF), aging prior rolling (APR) as well as multi-pass rolling using experiment and finite element method (FEM) simulation. The effect of twinning and Mg3Bi2 phases with various size, volume fraction and distribution on the microstructure and mechanical properties at room and elevated temperature of Mg-6Bi-based alloys are systematically investigated. In addition, the dynamic recrystallization (DRX) mechanism of Mg-6Bi-based alloy influenced by twinning and bimodal size Mg3Bi2 particles will be discussed in terms of microstructure variation and true strain-stress curves under different deformation circumstances. In particular, the interaction among twinning, second phases in nano/micro-size and matrix containing dislocation, twining, sub-grain, grain boundary, etc. during the rolling processing will be analyzed, which in turn, the grain refinement, texture evolution as well as strengthening and toughening mechanism of the Mg-6Bi-based alloy will be revealed. .In summary, the understanding of pre-treatment before caliber rolling will not only offer a significant theoretical and experimental foundation for developing the novel UFG Mg alloys, but also can provide crucial guidance for microstructure design and mechanical properties control of futuristic high strength RE-free wrought Mg alloys.

基于轻量化对高性能变形镁合金的需求，本项目选取新型镁铋基合金为研究对象，拟运用轧前预冷锻/预时效与多道次孔型轧制技术，充分发掘合金内在强韧化潜力和双向压应力条件下的大塑性变形能力，在优化第二相尺寸及改善轧制应变分布不均的基础上制备出低成本高性能超细晶材料。通过实验研究和数值模拟掌握预处理制度及轧制工艺参数对镁铋基合金显微组织和力学性能的影响规律；通过分析材料在不同高温变形条件下的组织演化和应力应变曲线，揭示预置孪晶及微纳双尺寸第二相对动态再结晶和高温变形行为的影响机理与微观本质；结合微纳观尺度观察，重点阐明大塑性变形过程中预置孪晶及微纳双尺寸内生相与基体中位错、孪晶、亚结构、晶界等的交互作用机制，揭示组织细化、织构演变规律及其强韧化机理。研究成果不但对大尺寸非稀土型超细晶镁合金的开发、组织与性能调控具有重要理论和现实意义，还可为发展高性能变形镁合金的制备加工技术开辟新的途径和提供技术支撑。

基于开发出低成本高强韧镁合金材料的需求，本项目在前期探索的基础上选取镁铋基合金作为研究对象，通过轧前预冷锻/预时效与多道次孔型轧制工艺相结合调控合金微观组织，在提高再结晶分数、优化第二相尺寸分布及改善轧制应变分布不均的基础上成功制备出晶粒细小，第二相分布合理的低成本高性能镁合金。通过系统和深入研究，得到以下主要结论：采用实验研究和有限元模拟明确了最佳的预处理制度和轧制工艺条件，探明了预处理制度及轧制工艺对Mg-Bi基合金显微组织和力学性能的影响规律；通过对不同热变形条件下合金的高温变形行为和组织演变的研究，计算了材料的热激活能及应力指数，构建了相关的本构方程和热加工图，揭示了预置孪晶对合金动态再结晶行为及晶界迁移的影响；通过分析大塑性变形中预置孪晶、微纳双尺寸第二相与基体间的交互关系，揭示了材料晶粒细化、织构演变规律及强韧化的内在本质。本研究可为制备轻量化高性能变形镁合金提供重要的理论指导和实验依据。