应力诱导调控镁锡基合金中Mg2Sn相析出行为及合金强化机理研究

Mg-Sn based magnesium alloys are currently a typical low-cost high-strength wrought magnesium alloy material, which have dual effects of solid solution strengthening and aging strengthening. However, the main strengthening precipitate, Mg2Sn, is generally basal plate/lath-like, coarse in size and inhomogeneous distributed, and cannot effectively hinder dislocation slip. Meanwhile, the peak aging time of Mg-Sn based alloys is very long. Thus, the further application and development of such alloys have been greatly limited. Stress-induced aging can significantly regulate the precipitation behavior, but most of studies and applications focused on aluminum, nickel, and titanium based alloys. In this project, the regulation of the precipitation behavior of Mg-Sn based alloys by the stress-induced aging process will be studied. A unique method to decrease the size, increase the number density and homogeneity, change the habit plane and accelerate the precipitation kinetics of the Mg2Sn precipitate is proposed. This project uses advanced analytical techniques to focus on the mechanisms of the evolution of crystal defects, the characteristics and the precipitation kinetics of Mg2Sn under stress-induced aging. The influences of dislocation evolution and precipitation on the quasi-static evolution of grain structure of Mg-Sn based alloys are investigated. The intrinsic mechanism of stress-induced regulation of age hardening response of Mg-Sn based alloys is studied, which aims to provide important theoretical guidance and technical support for the development and application of high strength Mg alloys.

镁锡基合金具有固溶强化和时效强化双重效应，是典型的低成本高强镁合金材料。然而，其主要强化相Mg2Sn普遍为基面盘状/板条状，尺寸粗大且分布不均，不能有效阻碍位错滑移，对合金强度提升有限，且峰值时效缓慢，限制了该系列合金的应用范围和发展空间。应力诱导可显著调控时效析出相的析出行为，但大部分研究和应用集中在铝镍钛合金。本项目以镁锡基合金为研究对象，通过施加应力诱导调控合金的时效析出行为，以减小Mg2Sn析出相尺寸，提高密度和分布均匀性，改变析出相惯习面，加快Mg2Sn析出动力学，从而增强合金时效硬化响应的新思路和新途径。采用先进的分析技术重点研究应力诱导作用下镁锡基合金中晶体缺陷的演化、Mg2Sn析出相特征和析出动力学的变化规律和微观机制，探明位错和析出相对合金晶粒组织准静态演变的影响规律，研究应力诱导调控镁锡基合金时效硬化响应的内在机制，为高强镁合金的的开发应用提供重要的理论指导与技术支撑。

调控析出相特征，促进时效析出动力学是提高镁合金时效硬化响应的重要途径。为此，本项目提出了采用应力诱导调控时效析出行为的思想，通过研究应力加载状态对Mg-Zn合金析出行为及时效硬化响应、晶粒结构准静态演变及合金强韧性能的影响等，实现析出相特征的有效调控及合金强韧性能的协同提升。研究结果表明，拉应力时效合金中β1’相的峰值密度明显高于常规时效样品。弹性应力使合金发生了微弱的塑性变形，诱发了可观的位错，为第二相的析出提供形核点，并加快溶质扩散，最终促进析出动力学和提高时效硬化响应。在拉应力诱导时效合金中，形成大量细小的β1’相，而压应力诱导时效合金中以富Zn溶质团簇为主。由于拉应力诱导时效合金中存在高密度的细小β1’相，其屈服强度高于常规时效合金；而在压应力诱导时效合金中，大量富Zn团簇与位错的相互作用，使得该合金具有较高屈服强度的同时，仍然保留了较优异的拉伸塑性。对不同应力加载条件下合金的晶粒组织和晶体取向的研究发现，在无应力时效早期，<11-20>//ED晶粒的优先生长导致了<11-20>//ED纤维组分的增强。高能晶界上的大量沉淀析出钉扎了晶界，降低了晶界迁移率，从而导致无应力时效合金具有较弱的基面织构。当合金处于100MPa-160℃的拉应力诱导时效状态下，柱面<a>位错的激活导致晶体转动，诱发了连续的静态再结晶(CSRX)，致使弱基面织构和非基面织构共存。时效时间的延长，使预先存在的<10-10>//ED晶粒和CSRX形成的再结晶晶粒的优先生长促使<10-10>//ED纤维组分增强。当合金处于100MPa-160℃的压应力诱导时效状态下，由柱面<a>位错激活引起的<11-20>//ED晶粒取向偏转和晶粒的微弱长大，导致在时效早期<11-20>//ED的织构组分增强。经过长时间的压应力时效，<10-10>//ED晶粒的优先生长产生了<10-10>-<11-20>双纤维组分。此外，应力取向也会对合金的时效析出行为产生显著的影响。这项工作为外应力场调控强基面织构镁合金的析出行为提供了进一步的见解，有望促使镁合金实现高强度和高韧性的协同提升。