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

研究了Al、Mg和Fe合金在室温和高温多向锻造和单道次压缩条件下大变形时细晶组织的形成过程和机制。分析了变形特征与组织演变过程的关系，得到以下主要结论：1.与单道次压缩相比，双道次压缩时，随着变形量的增加，新细晶组织的体积分数增加更快。2.单道次压缩时，新晶粒仅沿沿着原始晶界发育，并在晶界处形成细小的层状组织; 3.在临界应变以上，晶粒内部产生微剪切带和扭折带等多种变形带。随着应变的增加，晶界的密度和取向差增加，导致原始晶粒内部破碎，最终形成新的细晶粒。这种应变诱导晶粒的形成过程是由原位或连续动态再结晶控制的。4.形变带在三维空间中向不同方向发展，导致在大应变下形成细晶组织。5.这种应变诱导晶粒组织的退火不能通过常规的不连续再结晶（包括新晶粒的形核和长大）来控制，而可以通过原位或连续的静态再结晶（晶粒长大并伴随回复）来控制。

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.