经验原子势下铝镁合金中溶质原子向位错芯迁移的最低能量路径

铝镁合金在制造业中应用广泛, 但其在特定应变率下的塑性失稳不利于加工应用. 溶质原子与位错的交互作用是塑性失稳的微观机理. 本文采用势能曲面过渡态搜索技术计算了铝镁合金中替代型溶质镁原子向位错芯迁移的过渡态, 确认了溶质原子与位错芯的交互作用范围, 并采用过渡态理论估算了迁移扩散所需的时间, 且区分了无空位及有空位参与迁移两种情况. 结果表明, 位错压应力区内的溶质原子迁移无明显规律, 而在位错拉应力区内, 随着溶质原子与位错间距的缩短, 迁移势能垒和系统总能量均逐渐降低. 说明目前广泛采用的经验原子势可以很好地反映溶质原子易朝位错拉应力区偏聚这一现象. 溶质原子迁移的过渡态证实迁移过程中的微观结构变化因溶质原子所处位置不同而各异, 而交互作用范围不超过约2 nm. 空位参与对迁移的辅助作用被量化为迁移热激活时间的缩短, 并得出其可在微秒量级. 当溶质原子完成迁移稳定至位错芯附近, 并不倾向于沿位错线密集分布. 

Minimum energy path of a solute atom diffusing to an edge dislocation core in Al-Mg alloys based on empirical atomic potential

Al-Mg alloys are widely used in manufacturing. But at specific temperatures and strain rates, their plastic instability is not conducive to processing applications. The microscopic mechanism of plastic instability is the interaction between solute atoms and dislocations which induce a pinning-unpinnning effect. This effect, reflected on the microscopic scale, is also called dynamic strain aging (DSA). The DSA phenomenon causes negative strain-rate sensitivity and leads to plastic instability, which is harmful to its production. In this paper, the climbing image nudged elastic band method was adopted to explore the transition states along the minimum potential energy path, revealing a detailed evolution of atomic structures. The interaction range relies on the relative position and energy barrier of the transition, when a substitutional solute diffuses to an edge dislocation core in its stress field. Both substitution and vacancy-assisted migration are considered. The thermal activation time required for diffusion was estimated using transition state theory. The results indicate that there is no obvious law of solute atom migration in the compressive stress field. However, with the distance of the solute atom and the dislocation shortening, the migration potential energy barrier and the total energy of the system were gradually reduced. The present widespread empirical atomic potential can well estimate the phenomenon that the solute atom is prone to gathering in the tensile stress field. The transition states of migration confirmed the microstructure changes, depending on the position of the solute atom. The interaction region was no more than 2 nm. The migration energy was significantly reduced by vacancy mechanism, and the corresponding thermal activation time was shortened to microseconds. When the solute atoms finally migrated and stabilized near the dislocation core, there existed a maximum linear density. That is to say a dense arrangement along the dislocation line was not energetically preferred.