微纳双尺寸内生相增强Mg-Sn基合金多相交互作用及高温蠕变行为研究

Mg-Sn based alloys have been a potential material applied in the automobile industry based on the high stiffness and good thermal stability. However, the coarse net-like Mg2Sn in as-cast Mg-Sn based alloys greatly dissevers the matrix and leads to the decrease of the elongation. This detrimental effect is more apparent with the increasing Sn content. While, the nano-sized qusai-crystal phase can refine the grain size and improve the strength and ductility, simultaneously. In this project，the nano-sized qusai-crystal phase and micrometer Mg2Sn phases co-reinforced Mg-Sn based alloys can be fabricated by large plastic process combined with the controlling of the composition, microstructure and the phase transformation. The relations of microstructure and mechanical properties of coreinforced Mg-Sn based alloys before and after forming process will be analyzed and multiphase interaction mechanism during forming process will be revealed. In addition, the high temperature creep mechanism and creep fracture mechanism of co-reinforced fine grained Mg-Sn based alloys will be illustrated through the way of investigating its creep property, microstructure evolution during creep and calculating stress exponent and creep activation energy after creep test. The essential of the improvement in strength, ductility and thermal stability of the nano-sized qusai-crystal pahse and micrometer Mg2Sn phases co-reinforced Mg-Sn based alloys in the microscale will be reavealed. The investigation of this project will have important theoretical value on the development, microstructures and mechanical properties control of high strength, high ductility and high thermal stable Mg-Sn based alloys.

基于轻质高强的优势，Mg-Sn基合金是汽车动力系统最具发展潜力的结构材料。但铸态Mg-Sn基合金中的Mg2Sn相粗大且呈网状分布, Sn含量愈高，其韧性愈低，限制了其应用。纳米准晶可以细化晶粒，同时提高材料的强度和韧性。本项目提出在Mg-Sn基合金中加入适当原子比的Zn和Al，通过大塑性成形技术获得内生纳米准晶相 (I-phase) 及微米Mg2Sn相增强的高强韧超细晶Mg-Sn基合金。研究纳米和微米相对成形前后合金微观组织结构与力学性能的影响；揭示双尺寸内生相在成形过程中与基体的交互作用机理；研究超细晶Mg-Sn基合金的高温蠕变行为，揭示双尺寸内生相提高镁合金高温抗蠕变性的微观本质。为高强韧耐热镁合金的开发、组织和性能控制提供理论依据。

本项目通过在Mg-Sn基合金中添加不同质量比的Zn和Al，通过预处理工艺参数优化+大塑性变形工艺，制备出微纳双相增强的Mg-Sn-Zn-Al（TZA）合金。研究结果表明：当合金中Zn/Al质量比大于3，合金中会生成Mg43.7Zn43.7Al12.3和Mg53.5Zn28.5Al18.1准晶相。对不同Zn/Al比的TZA合金（TZA852/862/872）进行等径角挤压（ECAP），结果表明：TZA862合金表现出最优的力学性能。对TZA862合金进一步进行预时效处理（APE）+ECAP，合金的性能进一步优化。因为挤压前时效不仅可以有效细化晶粒、提高细小微纳第二相粒子的体积分数来提高强度，而且可以减少粗大未变形粒子的数量来提高合金的伸长率。进一步增加该合金的挤压道次至6~8道次，合金的屈服强度可以增加到235MPa，最大伸长率达16.1%。需要指出的是，合金组织中发现了准晶相和Mg2Sn相之间的共存关系，且挤压后的微纳增强相均与基体存在一定的位向关系。此外，研究了挤压态TZA822合金中微纳相的强化机制，结果表明合金中析出相的非基面球状纳米相对合金的强化贡献最大。对挤压态合金在不同的温度下和应变速率下进行高温压缩，结果表明：合金的平均激活能为189.5 kJ/mol，合金的应力指数为6.45。合金的动态再结晶机制对应变速率的依赖较大，当应变速率小于0.1 s-1时，连续动态再结晶（CDRX）占主导作用，当应变速率增加到10s-1时，非连续动态再结晶（DDRX）占主导作用。最后，研究了TZA862合金在不同温度和应变下的蠕变行为，结果表明：在200℃/50~70MPa时n为6.08，在应力为50 MPa时，合金的蠕变激活能Q为30.1kJ·mol-1。合金的高温变形机制为晶界滑移控制的位错的攀移机制。本项目的研究成果为微纳颗粒增强镁基材料的制备和工业应用推广提供了一种可行的实验方法，为高性能镁基材料的开发、组织和性能控制提供了实验依据和理论指导。