纯镁及镁合金在室温及更低温度下的包套等径角挤压及组织演变规律

This project proposes a canned ECAP (equal channel angular pressing) technique for severe-plastic-deformation processing of pure magnesium at or below room temperature and further explores the severe plastic deformation behaviors of pure magnesium and magnesium alloys at room temperature and lower temperatures to overcome the technological difficulties in severe plastic deformation of large bulk pure magnesium and magnesium alloys due to their poor plasticity at room temperature. The project will focus on the followings: revealing the plastic deformation mechanism and the structure evolution of pure magnesium and magnesium alloys during severe plastic deformation at room temperature or lower temperatures, including the types of the activated dislocation slip systems and twinning systems and the proportion of their contribution to the total deformation, the mechanism and degree of dynamic recrystallization (DRX) and recrystallized grain size, etc.; revealing the behaviors of static recovery and grain nucleation and growth during static recrystallization of the severe deformed microstructures at different temperatures; determining the deformation conditions for the realization of severe plastic deformation of pure magnesium and magnesium alloys at room temperature and at lower temperatures; and finally establishing a theoretical model reflecting the relationship between the microstructures of ultrafine/nano grained pure magnesium and magnesium alloys (including grain size, shape, preferred orientation, the type, configuration and distribution of dislocations in grain interior and grain boundaries, the type and shape of twin crystals as well as the configurations of the grain boundaries and the twin interfaces) and the conditions for severe plastic deformation and the subsequent treatment, to provide theoretical basis for the fabrication of large bulk ultrafine/nano grained pure magnesium and magnesium alloys.

为突破纯镁和镁合金室温塑性差，难以实现大尺寸块体室温多道次大塑性变形的技术瓶颈，本项目在提出包套等径角挤压技术基础上，开展纯镁和镁合金室温及更低温度下的大塑性变形研究。揭示不同应力应变场条件下，纯镁和镁合金在室温和更低温度下大塑性变形的微观机制及组织演变规律，主要包括：所开动的位错滑移系统和孪生系统的类型及其在总体塑性变形中所占的比例、动态再结晶的机制及其进行的程度以及动态再结晶晶粒尺寸等；揭示大塑性变形组织在不同温度下的静态恢复、静态再结晶形核与长大规律；确立纯镁和镁合金室温及更低温度下的大塑性变形条件；建立超细晶/纳米晶镁和镁合金的组织状态（包括：晶粒尺寸、形态、择优取向，晶内及界面位错类型、组态、分布，孪晶类型、大小以及晶界和孪晶界的状态等）与大塑性变形条件及后续处理条件关系的理论模型，为通过大塑性变形制备大尺寸块体超细晶/纳米晶纯镁或镁合金提供理论依据。

镁及其合金室温下的塑性和延展性有限，塑性成形性能差，同时强度也有待进一步提高，这些使其应用受到很大限制。通过大塑性变形细化晶粒是提高金属材料强度和塑性的重要手段。为突破纯镁和镁合金室温塑性差，难以实现大尺寸块体室温大塑性变形的技术瓶颈。本项目基于所提出的包套等径角挤压技术，在结合有限元模拟和实验研究成功实现室温和更低温度下纯镁多道次等径角挤压（ECAP）变形的基础上，研究了纯镁室温及低温大塑性变形的微观机制、大塑性变形纯镁在后续热处理过程中的组织演变规律以及大塑性变形所制超细晶纯镁的力学行为。研究结果表明：纯镁在室温和更低温度下大塑性变形时，其微观机制仍然以基面滑移和拉伸孪生为主，但非基面滑移，包括<c+a>位错滑移也参与了变形，保证了多晶体晶粒间的变形协调，从而可使纯镁在室温和更低温度下经历大的塑性变形而不发生开裂。使纯镁产生大塑性变形的变形条件是借助强度高、塑性好的包套在大塑性变形过程中形成强的压应力状态，强制非基面滑移系统开动，保证多晶体塑性变形协调进行。其次，在室温和更低温度下，纯镁大塑性变形过程中会伴随很强的组织动态回复和动态再结晶，随着挤压道次增加，即塑性变形量的增加，晶粒尺寸不断减小，动态回复增强，而动态再结晶减弱，晶粒细化趋于饱和。降低ECAP挤压变形温度，可获得更细小的晶粒尺寸。在0℃大塑性变形后，纯镁的晶粒可细化至近1微米。变形组织在200℃下保温30分钟，可基本完成静态再结晶，获得均匀细小组织，强度和塑性得到同步提高；进一步延长保温时间，晶粒开始长大。此外，项目研究表明当挤压超过两道次后，变形组织中均包含Y型和P型织构组分，他们分别与<a>型位错的柱面滑移和<a+c>型位错的锥面滑移相对应。表明随着变形量的增加，晶粒尺寸的减小，非基面滑移逐渐变得活跃，而孪生倾向不断减弱。当晶粒尺寸减小到一定程度时，大塑性变形过程中位错运动一方面变得活跃，一方面易于运动至晶界而湮灭，使动态回复变得容易。