The Characteristics and Mechanisms of Ultra-Fine Grained Structure Formation during Severe Large Deformation.

研究了Al、Mg、Fe合金在常温和高温下多向锻造和单道次压缩过程中严重大变形时细晶组织的形成过程和机理。分析了变形特征与组织演变过程的关系，得到的主要结果如下：1.与单道次压缩相比，MDF变形过程中新细晶组织体积分数的增加速度更快。在单道次压缩过程中，只沿原始晶界生长出新的晶界，在晶内形成了细小的层状组织。3.在临界应变以上，晶内产生了微剪切带和扭结带等多种形变带。随着应变的增加，晶界的密度和取向错位增加，导致原始晶内的碎裂，最终形成新的细晶。这种应变诱发的晶粒形成过程是由原位或连续动态再结晶控制的。4.在MDF过程中，变形带在三维空间中向不同方向发展，导致大应变下细晶组织的形成。这意味着MDF是细晶组织形成的有效方法之一。5.这种应变诱发的晶粒组织的退火化不能由传统的非连续再结晶(包括新晶粒的形核和长大)控制，而是由原位或连续的静态再结晶控制，在原位或连续静态再结晶中，晶粒长大伴随着回复发生。

The process and mechanisms of fine-grained structure formation during severe large deformation was studied in multi-Directional forging (MDF) and a single pass compression at ambient and high temperatures by using Al, Mg and Fe alloys. The relationships between deformation characteristics and evolution process of microstructures were analyzed, and the main results obtained were summarized as follows.1.The volume fraction of new fine-grained structure evolved by MDF increases more rapidly with deformation compared with that in a single pass compression. Increase in a strain per each MDF results in rapid evolution of finer grained structure in lower strain.2.During a single pass compression, in contrast, new grains are developed only along the original grain boundaries and fine layered microstructures are formed in the grain interiors.3.Several kinds of deformation bands such as microshear and kink bands are generated in grain interiors in above a critical strain. The density and the misorientations of their boundaries increase with straining, leading to fragmentation of original grain interiors and finally formation of new fine-grains. The process of such strain-induced grain formation is concluded to be controlled by in-situ or continuous dynamic recrystallization.4.Deformation bands are developed in various directions in three-dimensional space during MDF, leading to formation of fine-grained structures in large strain. This means that MDF is one of effective methods for fine-grained structure formation.5.Annealing of such strain-induced grain structure can not be controlled by conventional discontinuous recrystallization including nucleation and growth of new grains, but also by in-situ or continuous static recrystallization, in which grain growth takes place accompanied with recovery.