Mechanisms of strain-induced grain structure formation-Reconsideration of recrystallization

研究了铜、铝、镁、铁合金在多向锻造和高温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。系统地分析了变形特征与组织演变过程之间的关系，取得的主要成果如下：1.研究结果。在超过某一临界应变时，在晶界内部产生了微剪切带和扭结带等多种变形带。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。在MDF过程中，变形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。这说明MDF是细晶组织形成的有效方法之一。宽温区间新细晶的形成过程可分为三个阶段：(1)第一阶段(ε&lt；2)伴随变形带(DBS)胚胎的常规位错亚结构的形成；(2)大规模DBS的发展，随后在第二阶段(2&lt；ε&lt；4)沿中等应变DBS形成新的晶粒(2&lt；ε&lt；4)；(3)在阶段3(ε&GT；4)在高应变下发展新的晶态结构。这种应变诱发晶粒组织的退火不能由传统的非连续再结晶控制，包括新晶体的形核和长大，而是由原位或连续静态再结晶控制，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 low and high temperatures by using Cu and Al, Mg and Fe alloys. The relationships between deformation characteristics and evolution process of microstructures were systematically analyzed, and the main results obtained were summarized as follows.1. 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.2. 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.3. The process of new fine grain formation in a wide temperature interval can be classified into the three stages : (1) formation of conventional dislocation substructures accompanied with embryos of deformation bands (DBs) in stage 1 (ε<2) ; (2) development of large scale DBs followed by grain fragmentation, leading to new grain formation along DBs at moderate strains in stage 2 (2<ε<4) ; (3) development of new grained structures at high strains in stage 3 (ε>4).4. 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.