镁合金中18R-LPSO相的宽尺寸范围调控策略及其主导的强韧化机理转变

Mg-Y-Zn magnesium alloys with long period stacking ordered (LPSO) phase have been considered to be one of the most promising high-performance magnesium alloys due to their excellent mechanical properties. However, it is difficult to refine the 18R phase owing to its good ductility and tendency to be kinked during conventional plastic processing (larger than 20μm in length), which unfavorably suppresses its strengthening effects. To date, increasing the contents of alloying elements is the predominant approach to prepare high-strength alloys, but compromise the ductility and low-density for the alloys. To address this limitation, the proposed project intends to develop a new refining strategy for 18R phase, i.e., pre-kinking and large-strain hot drawing. This strategy could effectively refine the size of 18R phase into submicron level, endowing the low-18R-content alloys with both ultra-high strength and good ductility. A systematical study will be carried out to investigate the influence of various processing parameters on the morphology and dimensions of 18R phase, as well as the mechanical properties of the alloys. By establishing the intrinsic relations of microstructure, technology and property, the evolution mechanisms of 18R pre-kinking bands under the coupling effect of strain and heat will be elucidated, and the strengthening-toughening mechanisms as well as the transition induced by the morphology and dimensions of 18R phase will be illuminated. Finally, a controllable method of preparing Mg-Y-Zn alloys with 18R phase in the size range of submicron to 20μm will be developed. This research will deepen and broaden the strengthening-toughening theory of 18R phase, and thereby provide a theoretical basis for the high performance and novel microstructure design of 18R-containing magnesium alloys.

含有18R长周期堆垛有序结构的Mg-Y-Zn合金力学性能优良，是最具应用潜力的高性能镁合金之一。然而，18R相易扭折、塑韧性好，传统塑性加工时难细化（长度＞20μm），导致其强韧化潜力未充分发挥。现有强度提高途径多依靠合金元素含量的增加，牺牲了合金塑性和低密度优势。因此，本项目拟发展一种18R相细化新策略——预扭折+大应变热拉拔，将18R相尺寸调控下限扩展至亚微米，使含低18R相体积分数的合金获得超高强度和良好韧性。申请人以Mg97Y2Zn1合金为研究对象，系统研究工艺参数对合金微观组织和力学性能的影响规律，建立18R微结构-工艺-性能的内在关联，揭示应变-热耦合作用下18R预扭折带的演化规律及机理，阐明18R相形态和尺寸主导的强韧化机理及其转变，实现18R相尺寸在亚微米～20μm范围内可控制备。项目研究可深化18R相的强韧化理论，为含18R相镁合金的高性能化和微观组织设计提供理论依据。

实现18R长周期堆垛有序结构的细化/超细化是突破其强韧化效果，获得高性能Mg-Y-Zn基合金的关键。本项目主要以含有18R-LPSO+α-Mg典型两相组织的 Mg97Y2Zn1 (at%)镁合金为研究对象，发展了充分细化18R相的预扭折+大应变热挤压的组合加工策略，揭示了18R相的细化过程及机制；结合组织-工艺-性能的内在关联及一体化分析，实现了18R相的尺寸控制；开发了高强高韧镁合金，并阐明了18R相形态和尺寸主导的强韧化机理。本研究深化了18R相的强韧化理论，为含18R相镁合金的高性能化和微观组织设计提供了理论依据，为该类高强镁合金型材的开发和工业化应用提供了技术支持。..重要研究发现：（1）利用等通道转角挤压（ECAP）可以实现18R相的预扭折，预扭折加工后合金的成形性能提升，但预扭折+热轧制无法实现18R相的进一步细化；（2）预扭折结合大应变热挤压有效将18R相细化至亚微米级别，经16道次ECAP预扭折组合热加压将18R相的平均颗粒尺寸细化至1.4微米，合金的拉伸屈服强度、抗拉强度和断裂应变分别达到475 MPa、526 MPa和14.5%；（3）强化贡献率分析结果表明，18R相的显著细化、以及细化的18R相引起的尺寸更加细小的动态再结晶组织是合金强化的主要来源；（4）预扭折后沿试样不同方向进行热挤压具有不同的细化18R相的效果，经32道次ECAP加工后再沿ED方向热挤压可将18R相的平均尺寸细化至1微米，合金的抗拉强度进一步提升至550 MPa，断裂应变13.5%，媲美于高强铝合金。..本项目为期三年，执行期间已取得丰富的创新性成果，现已授权国家发明专利7项，在国内外行业核心期刊发表论文11篇。