微量元素对Mg-Sn系合金时效双峰行为的作用机制

Double-peaking aging behavior is an important method to improve the properties of magnesium alloys at high temperature. The effect of microelements on the double-peaking aging of magnesium alloys is seldom. Recently, we found that there is a character of double-peaking aging appeared on the Mg-Sn alloys of treatment by T6, however, the corresponding mechanism of formation is not clear. Therefore, the project begins from this and the mechanism of formation for double-peaking aging will be affirmed by studying the precipitations behaviors (nuclei, growth, coarsening) and precipitations characters (size, distribution, morphology and orientation relationships) of Mg-Sn alloys at different aging stages completely using the morden analysis methods such as HRTEM and SAED and so on. Then, alloyed Mg-Sn system double-peaking alloys are selected, the effects of microelements (Zn, Y, Ca)on the double-peaking aging behaviors will be studied; the mechanism of action of microelements on the precipitations behaviors,characters，double-peaking aging behaviors and microstructure of alloys will be explored; last, with the estmation of mechanical properties, the formation mechanism of double-peaking aging of Mg-Sn alloy will be affirmed and the action mechanism of microelements on the double-peaking aging behaviors of Mg-Sn system alloys will be explained in order to afford a new clue for research and development of magnesium alloys served at high temperature.

时效双峰行为是改善镁合金高温使用性能的重要手段。微量元素对镁合金时效双峰行为的影响报道较少。最近，我们发现经T6处理的Mg-Sn合金具有时效双峰特征，但其形成机理尚不清楚。本研究以此为切入点，通过HRTEM、SAED等现代材料研究方法全方位探索其沉淀相的析出动力学（形核、长大、析出序列）规律与沉淀相特征（形态、尺寸、数量、分布、位相关系、界面结构等）的演化规律；进而以合金化的Mg-Sn系双峰合金为研究对象，研究微量元素（Zn、Y、Ca等）对双峰合金时效析出行为、沉淀相特征变化、时效双峰行为和微观组织演化的影响规律；结合力学性能评价，阐明Mg-Sn合金时效双峰形成的本质原因，揭示微量元素对Mg-Sn系双峰合金的时效双峰行为的作用机制，为研发新型高温服役镁合金提供一个新的线索。

Mg-Sn合金是一种可沉淀强化的镁合金，项目通过采用添加合金化元素Zn及热处理工艺等手段调控Mg-Sn合金的时效析出行为，并利用现代研究方法获得了不同时效阶段的沉淀相β特征，结合拉伸性能测试，重点揭示了少量Zn对Mg-Sn合金析出行为、微观组织和力学性能的影响规律。研究发现，T6处理的Mg-Sn-(Zn)合金均表现出时效双峰特征，第一个时效峰值主要是非连续析出β相的硬化效果，时效谷主要是非连续析出β相粗化的结果，第二个时效峰值主要是晶内连续析出β相的结果。与Mg-Sn二元合金相比，1.0 wt % Zn的添加使其时效峰值硬度提高约14HV，同时其室温抗拉强度、屈服强度分别提高22.4%和52%，延伸率略有下降。通过添加少量的Ca和Y，T6处理Mg-Sn-Ca和Mg-Sn-Y合金时效硬化曲线并未表现出明显的时效双峰特征，其原因在于其合金凝固时在晶界处形成的高熔点的MgSnCa/Y相在固溶处理时很难固溶入基体，减弱了合金的时效硬化能力和沉淀强化效果。.基于以上，进一步研究了Al、Zn共掺对Mg-Sn合金时效行为的影响，获得了一种高强度铸造Mg-Al-Sn-Zn镁合金，其室温屈服强度达到166MPa，抗拉强度达到287MPa，延伸率为7.9%，重点揭示了Zn含量对合金组织、时效硬度与拉伸性能的影响规律与作用机制，该合金的强化效果得益于连续与非连续析出的β与γ相，其中非连续析出的γ相起主要强化作用，其与母相的位向关系为Burgers OR。