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

研究了Al、Mg和Fe合金在常温和高温下多向锻造和单道次压缩大变形过程中细晶组织的形成过程和机理。分析了变形特征与微观组织演化过程之间的关系，总结了主要研究结果：1。与单次压缩相比，MDF形成的新细晶组织体积分数随变形的增加更快。增加每个MDF的应变导致较低应变的更细粒度结构的快速演变。相比之下，在单道次压缩过程中，新晶粒仅沿着原晶界发育，晶粒内部形成精细的层状组织。在临界应变以上，晶粒内部产生微剪切和扭结等多种变形带。随着应变的增加，晶粒的密度和边界的取向偏差增加，导致原有晶粒内部破碎，最终形成新的细晶粒。这种应变诱导晶粒形成的过程是由原位或连续动态再结晶控制的。在MDF过程中，变形带在三维空间中向各个方向发展，导致在大应变下形成细晶结构。这意味着MDF是形成细粒结构的有效方法之一。这种应变诱导晶粒结构的退火不能通过常规的不连续再结晶（包括形核和新晶粒的长大）来控制，而可以通过原位或连续静态再结晶（晶粒长大伴随着恢复）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.