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.

在铝-镁-硅合金中，针状β′′析出物的强化作用至关重要。本研究在峰值时效条件下对强化进行了计算分析，在此条件下，析出物的剪切变形与Orowan环状变形通常被认为具有相等的强度。基于实验确定的峰值时效析出物尺寸和体积分数，构建了伪随机的析出物微观结构。随后，采用离散位错动力学方法计算了通过不可剪切析出场移动的位错的Orowan环状变形的临界分辨剪切应力（CRSS）。Orowan环状变形的CRSS由典型的原位析出物间距决定，该间距小于平均间距，并由半径约为5b范围内的位错核心能量决定，这一因素很少被考虑。基体的不匹配应力、体积分数和析出物形状对CRSS的影响较小。尽管尽可能忠实地引入了微观结构和性能细节，但在室温下使用原子级校准的核心能量计算出的Orowan环状变形的CRSS仍比实验值高约33%。这表明析出物的剪切变形控制着强度，对（i）作用于析出物的力，（ii）析出物内部的不匹配应力，（iii）与相关析出物断裂能相关的一阶原理结果，以及（iv）模拟析出物剪切变形的模拟分析表明，剪切CRSS更接近实验值。因此，即使在峰值时效条件下，Orowan环状变形也仅设定了CRSS的上限，而进一步的定量进展需要详细建模析出物的剪切变形。