钕钆在Al2O3纤维强化的镁合金晶界/晶内竞争生长及材料高温蠕变行为

Magnesium alloys have wide potential applications in the aerospace, electronic packaging and automobile industries due to their light weight, good thermal and electrical conductivity, excellent damping capacity and electromagnetic shielding property. However, the creep resistance of the as-cast Mg-Al alloys decreases sharply once the temperature is above 120℃, thus hindering their applications. This project aims to improve the high-temperature creep property of Mg-Al alloys by dual strengthening by adding rare earth elements and ceramic fibers. The magnesium matrix composites with dense microstructure, good interfacial bonding, intermetallic compounds containing Nd or Gd and well-regulated precipitation phase are fabricated by optimizing the mass ratio of Nd to Gd, and vacuum infiltration process and heat treatment. The steady-state creep rate, creep stress exponent and creep activation energy are achieved by high-temperature creep tests under different load conditions. The microstructure before and after creep is characterized by high resolution transmission electron microscopy (HRTEM). The relationship between the composition, microstructure and high-temperature creep properties is established. The thermodynamics and dynamics of competitive growth for the intermetallic compounds containing Nd and Gd at boundaries and inside grains are clarified, and the mechanism that hinders the grain boundary slide and dislocation movement is disclosed. The results can provide theoretical support for the improvement onthe high-temperature creep properties of magnesium alloys.

镁合金具有质轻、导热导电性好、阻尼减振、电磁屏蔽性能优异等优点，在航空航天、电子封装和汽车领域有广阔的应用前景。然而，铸造Mg-Al合金的蠕变抗力在120℃以上时急剧下降，严重限制了其应用范围。本项目基于设计钕钆含量使金属间化合物在合金晶界/晶内竞争生长和添加纤维陶瓷相双重强化原理提高镁合金的高温蠕变性能。拟通过采用不同Nd/Gd比、真空热压熔渗工艺及热处理工艺，制备出组织致密、界面结合良好和在晶界晶内皆有Nd、Gd金属间化合物析出且沉淀相数量可控的镁基复合材料。通过高温蠕变性能测试，获得不同载荷温度下材料的稳定蠕变速率、蠕变应力指数和激活能。利用高分辨透射电镜表征材料蠕变前后的微观组织结构，揭示成分、微观组织结构与高温蠕变性能之间的内在联系。本项目通过对Nd、Gd金属间化合物竞争生长的热力学和动力学研究，确立其阻碍晶界滑动和位错运动的机制，为进一步提高镁合金的抗高温蠕变性能提供理论支撑。

铸造Mg-Al合金具有轻质、低成本、高的比强度和比刚度、良好的铸造性能等优点，已广泛应用于航空航天、汽车和电子领域。然而，铸造Mg-Al合金的蠕变抗力在120℃以上时急剧下降，严重限制了其应用范围。本项目基于设计钕钆含量使金属间化合物在合金晶界与晶内有效分布和添加纤维陶瓷相制备出组织致密、界面结合良好的镁基复合材料。通过高温蠕变性能测试和材料蠕变前后的微观组织变化，来揭示成分、微观组织结构与高温蠕变性能之间的内在联系，解释Nd、Gd高熔点金属间化合物在晶界和晶内沉淀析出数量对抑制晶内位错运动和晶界滑动的本质。得到以下主要结论：.（1）Mg-6Al-1Nd-1.5Gd合金在蠕变温度从150℃升高到200℃时，稳态蠕变速率从1.946×10-8s-1升高至2.128×10-7s-1，提高了近一个数量级；合金经高温蠕变后，组织中Al2Nd和Al2Gd相在晶界上偏聚；随着蠕变温度的升高，合金显微组织中的位错线逐渐增多，晶粒中出现明显的滑移“台阶”及滑移线，加剧了合金的滑移变形，使合金的抗蠕变性变差；当Nd/Gd比为2:1时Mg-6Al合金具有最优的综合力学性能。.（2）T4、T6两种热处理方式都能提高合金的抗高温蠕变性能，T4处理后合金的抗蠕变性能提升最高，T4态Mg-6Al-1Nd-1Gd合金的蠕变应变量和稳态蠕变速率在70MPa，200℃时分别降至1.8 %和2.333×10-8 s-1。.（3）在Mg-6Al合金中复合添加稀土元素Nd、Gd和Al2O3纤维后，基体中有Al2O3f、Mg2Si、Al2Nd和Al2Gd相生成，纤维和基体的界面出现了MgO反应物过渡层，界面结合良好，22vol.%Al2O3f/Mg-6Al-1Nd-1.5Gd成分下的复合材料具有最优的蠕变性能，抗蠕变性能最佳；复合材料的蠕变断裂受基体合金和纤维之间的界面结合强度影响，断裂机制为界面脱粘和纤维断裂共同控制。.综上，在镁合金中复合添加Nd、Gd稀土元素和Al2O3纤维，达到了进一步提高镁合金的高温强度和高温蠕变抗力的目的，使得镁合金在汽车领域得到更广泛的应用，本研究成果对节能减排、保护环境有着非常重大的现实意义。

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