Minimum energy path of a solute atom diffusing to an edge dislocation core in Al-Mg alloys based on empirical atomic potential
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摘要: 鋁鎂合金在制造業中應用廣泛, 但其在特定應變率下的塑性失穩不利于加工應用. 溶質原子與位錯的交互作用是塑性失穩的微觀機理. 本文采用勢能曲面過渡態搜索技術計算了鋁鎂合金中替代型溶質鎂原子向位錯芯遷移的過渡態, 確認了溶質原子與位錯芯的交互作用范圍, 并采用過渡態理論估算了遷移擴散所需的時間, 且區分了無空位及有空位參與遷移兩種情況. 結果表明, 位錯壓應力區內的溶質原子遷移無明顯規律, 而在位錯拉應力區內, 隨著溶質原子與位錯間距的縮短, 遷移勢能壘和系統總能量均逐漸降低. 說明目前廣泛采用的經驗原子勢可以很好地反映溶質原子易朝位錯拉應力區偏聚這一現象. 溶質原子遷移的過渡態證實遷移過程中的微觀結構變化因溶質原子所處位置不同而各異, 而交互作用范圍不超過約2 nm. 空位參與對遷移的輔助作用被量化為遷移熱激活時間的縮短, 并得出其可在微秒量級. 當溶質原子完成遷移穩定至位錯芯附近, 并不傾向于沿位錯線密集分布.Abstract: 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.
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Key words:
- aluminum magnesium alloy /
- solute atom /
- dislocation /
- diffusing /
- migration energy
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圖 1 溶質鎂原子遷移之前的模型局部圖
Figure 1. Relative position of the Mg solute and the edge dislocation before migration
圖 2 溶質鎂原子在滑移面上向位錯擴散時發生置換反應的最低能量路徑
Figure 2. MEP of a Mg solute migration to dislocation on the glide plane without a vacancy mechanism
圖 3 溶質鎂原子在滑移面內的置換遷移過渡態(R13為亞穩過渡態，R21為鞍點過渡態)
Figure 3. Transition states of a Mg solute migration to dislocation on the glide plane without a vacancy mechanism (R13 is a metastable state, and R21 is the saddle point state)
圖 4 溶質鎂原子位于位錯拉壓應力區的置換遷移最低能量路徑及過渡態.(a)拉應力區(θ=-90°)；(b)壓應力區(θ=+90°)
Figure 4. MEP and transition states of a Mg solute migration to dislocation above and below the dislocation line without a vacancy mechanism: (a) tension stress field (θ=-90°); (b) compression stress field (θ=+90°)
圖 5 位錯周圍應力場對溶質鎂原子擴散行為的影響
Figure 5. Different migration energy and potential energy changes after migration in the stress fields of an edge dislocation core
圖 6 溶質鎂原子位于位錯拉壓應力區時借助空位向位錯芯擴散的遷移最低能量路徑及過渡態. (a)拉應力區(θ=-90°)；(b)壓應力區(θ=+90°)
Figure 6. MEP and transition states of a Mg solute migration to dislocation below and above dislocation line with a vacancy mechanism: (a) tension stress field (θ=-90°); (b) compression stress field (θ=+90°)
圖 7 本征空位存在時位錯拉應力場對溶質鎂原子擴散行為的影響
Figure 7. Different migration energy and potential energy change after the migration in the stress fields of an edge dislocation core with a vacancy mechanism
圖 8 位錯正下方沿位錯線方向分布的兩個鎂原子之間距離對體系能量變化的影響
Figure 8. Potential energy change induced by the distance variation between two Mg solutes along the dislocation line in the tensile stress field
表 1 空位機制對遷移勢能壘(EBF)的削減及相應的熱激活時間
Table 1. Reduction in migration energy by a vacancy mechanism and the evaluated thermal activation time required for migration
角度/(°) 間距原子個數 置換機制能壘/eV 空位機制 能壘/eV 時間，t/s -45 1 1.77 0.39 3.59×10-6 2 2.81 0.59 8.25×10-3 3 3.5 0.42 1.15×10-5 4 3.64 0.45 3.66×10-5 5 3.92 -90 1 2.13 0.38 2.44×10-6 2 2.45 0.45 3.66×10-5 3 3.58 0.46 5.39×10-5 4 3.65 0.47 7.93×10-5 5 3.72 0.48 1.17×10-4 6 4.04 0.49 1.72×10-4 7 4.07 
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