双尺寸SiCp/AZ91镁基复合材料组织稳定性及高温变形行为研究

The aim of this project is to investigate the microstructure stability and elevated temperature deformation behavior of bimodal size particle reinforced magnesium matrix composite. By optimizing deformation process, it is hoped to obtain the bimodal size SiCp/AZ91 magnesium matrix composites with fine grains, uniform particle distribution and higher mechanical properties. The effect of micron SiCp size as well as the volume ratio between submicron and micron SiCp on the microstructure and mechanical properties of magnesium matrix composite is analyzed. The grain size, microstructure and mechanical properties of bimodal size SiCp/AZ91 magnesium matrix composite influenced by different annealing process is researched, thus the microstructure stability interval can be constructed based on temperature and time, which is help to illustrate the influence of bimodal size particles on microstructure stability of fine-grained magnesium matrix. Based on above analysis, the microstructure and properties of fine-grained magnesium matrix composite after elevated temperature compression are investigated. The dynamic recrystallization mechanism of fine-grained magnesium matrix influenced by bimodal size particles will be illustrated through the way of investigating the microstructure evolution of fine-grained magnesium matrix composites during compression process. Besides, the hot deformation activation energy and stress exponent of fine-grained magnesium matrix composites are calculated, then the effect of bimodal size particles on the high temperature deformation behavior of fine-grained magnesium matrix composite can be revealed. The investigation of this project will have important theoretical value on the development, microstructures and mechanical properties control of high strength magnesium matrix composite.

本课题拟就(亚微米+微米)双尺寸颗粒增强镁基复合材料组织稳定性及高温变形行为进行进行研究，通过优化热变形工艺，制备出晶粒细小、颗粒分布均匀且力学性能较高的双尺寸SiCp/AZ91复合材料。分析亚微米和微米SiCp体积比、微米SiCp尺寸对复合材料显微组织和力学性能的影响规律。研究双尺寸SiCp/AZ91复合材料经不同退火工艺后的晶粒尺寸、微观结构和力学性能，建立基于温度和时间的细晶镁基复合材料组织稳定区间，揭示双尺寸颗粒对细晶镁基体组织稳定性的影响规律。在此基础上，研究细晶镁复合材料高温压缩变形后的微观结构和性能。通过分析复合材料在高温变形过程中的组织演化，揭示双尺寸颗粒对细晶镁基体动态再结晶行为的影响机制；计算细晶镁基复合材料的变形热激活能和应力指数，揭示双尺寸颗粒对细晶镁基复合材料高温变形行为的影响机理。本研究对高强镁基复合材料的开发、显微组织和力学性能控制，具有重要的理论意义。

优化铸造和热变形工艺，制备出晶粒细小、颗粒分布均匀且力学性能较高的（亚微米+微米）双尺寸SiCp/AZ91镁基材料。室温变形过程中位错在亚微米和微米SiCp附近由于运动受阻而塞积，导致颗粒附近位错密度升高。在外加载荷作用下，微米SiCp端部和颗粒密集区易产生应力集中萌生微裂纹，而亚微米SiCp同基体界面结合较好，无微裂纹产生。微米SiCp附近弥散分布的亚微米SiCp对裂纹的扩展有一定的阻碍作用。通过与AZ91合金和（亚微米/微米）单一尺寸SiCp/AZ91镁基材料对比，研究了细晶双尺寸颗粒增强镁基材料组织稳定性及高温变形行为。研究结果表明，热处理的过程中，微米SiCp周围能够形成亚微米颗粒密集区，抑制晶粒长大，提高了镁基基体的高温组织稳定性。采用三种本构方程描述双尺寸颗粒增强镁基材料高温变形行为，发现指数本构方程和双曲正弦本构方程分别在低应力水平和高应力水平范围出现显著误差，而幂本构方程在较宽应力范围内可精确描述双尺寸颗粒增强镁基材料在高温变形过程中流变应力与应变速率之间的关系。双尺寸颗粒增强镁基材料的变形激活能随变形温度和应变速率的增大而增大。在应变速率为0.001s-1时，复合材料的变形机制为晶界扩散控制的位错攀移机制；在应变速率为0.01-1s-1时，复合材料的变形机制为位错攀移机制。与细晶AZ91合金和亚微米/微米单尺寸SiCp增强镁基材料相比，单一尺寸SiCp降低了镁的变形激活能，而双尺寸SiCp使镁的变形激活能提高，变形机制变形机制由位错滑移转变为位错攀移。亚微米与微米SiCp混合有利促进DRX形核，降低临界应力与临界应变，缩短了DRX时间。热变形的过程中，微米SiCp周围能够形成PDZ，促进DRX形核；而亚微米SiCp，一方面由于同基体变形不匹配而在其周围产生位错，另一方面通过阻碍位错运动使位错在颗粒周围塞积，两方面的共同作用导致亚微米SiCp附近位错密度增大，从而有利于促进DRX形核。