Modeling peak-aged precipitate strengthening in Al-Mg-Si alloys

Strengthening by needle-shaped β′′ precipitates is critical in Al–Mg–Si alloys. Here, the strengthening is studied computationally at the peak-aged condition where precipitate shearing and Orowan looping are usually considered to have equal strengths. Pseudo-random precipitate microstructures are constructed based on experimental precipitate dimensions and volume fractions at peak aging. A Discrete Dislocation Dynamics method is then adapted to compute the Critical Resolved Shear Stress (CRSS) for Orowan looping of dislocations moving through the non-shearable precipitate field. The CRSS for Orowan looping is determined by a typical in-situ precipitate spacing that is smaller than the average spacing and by the dislocation core energy within a radius of ≈5b, a factor rarely considered. The matrix misfit stresses, volume fraction, and precipitate shape have small effects on the CRSS. With microstructure and property details introduced as faithfully as possible, the CRSS for Orowan looping using atomistically-calibrated core energies at room temperature is nonetheless ≈33% higher than experiments. This suggests that precipitate shearing controls strength, and analyses of (i) forces acting on the precipitates, (ii) misfit stresses inside the precipitates, (iii) first-principles results for the relevant precipitate fault energies, and (iv) simulations that mimic precipitate shearing indicate a shearing CRSS closer to experiments. Thus, Orowan looping only sets an upper bound for the CRSS even at peak aging, and further quantitative progress requires detailed modeling of precipitate shearing.

针状β''析出相强化是铝-镁-硅（Al–Mg–Si）合金的关键强化机制。本研究针对峰值时效状态下的强化行为开展计算研究，该状态下析出相剪切与奥罗万绕环通常被认为具有同等的强化强度。基于峰值时效时实验测得的析出相尺寸与体积分数，构建伪随机析出相微观结构。随后采用离散位错动力学方法，计算位错穿过非可剪切析出相场时发生奥罗万绕环的临界分切应力（Critical Resolved Shear Stress, CRSS）。奥罗万绕环的临界分切应力由小于平均间距的典型原位析出相间距，以及半径约为5b的位错核心能量所决定，这一因素此前极少被纳入考量。基体错配应力、析出相体积分数与形状对临界分切应力的影响均较小。尽管尽可能忠实地引入了微观结构与性能细节，采用室温下经原子尺度校准的位错核心能量计算得到的奥罗万绕环临界分切应力，仍比实验结果高出约33%。这表明析出相剪切主导了合金的强度，而以下四项分析：(i) 作用于析出相的受力、(ii) 析出相内部的错配应力、(iii) 相关析出相层错能的第一性原理计算结果，以及(iv) 模拟析出相剪切过程的仿真结果，均指向剪切临界分切应力与实验结果更为接近。因此，即便在峰值时效状态下，奥罗万绕环也仅能给出临界分切应力的上限，后续的定量研究需要针对析出相剪切开展精细化建模。