关于合金定向凝固中浮力诱导的振荡生长不稳定性的发生
On the occurrence of buoyancy-induced oscillatory growth instability in directional solidification of alloys
AI总结：
该研究通过DNN模型和Navier-Stokes求解器，在Al-Cu合金中重现了定向凝固中的浮力诱导振荡生长，并绘制了振荡状态图谱，揭示了振荡类型与流体速度比的关系。
摘要：
最近的凝固实验在Ni基高温合金CMSX4的定向凝固过程中，在给定的冷却速率范围内识别出一种振荡生长不稳定性。从建模的角度来看，由于涉及多个长度尺度，在对流条件下对枝晶生长进行定量模拟仍然具有挑战性。利用枝晶针状网络（DNN）模型，结合高效的Navier-Stokes求解器，我们重现了CMSX4定向凝固中观察到的浮力诱导的生长振荡。这些先前的结果表明，对于给定的合金和温度梯度，振荡发生在较窄的冷却速率（或拉速，$V_p$）范围内，并且所选的一次枝晶臂间距（$Λ$）在导致振荡的流动激活中起着关键作用。在这里，我们通过在Al-4at.%Cu合金中重现振荡行为，表明该振荡行为可以在适当的$(V_p,Λ)$范围内推广到其他二元合金。我们绘制了振荡状态随$V_p$和$Λ$变化的图谱，并识别了不同行为（例如持续振荡或衰减振荡）的发生区域及其对振荡特性的影响。我们的结果表明，振荡的发生存在一个最小的$V_p$，并确认了振荡类型（即衰减、持续或噪声）与平均流体速度$\bar V$与$V_p$之比之间的相关性。我们描述了观察到的不同生长状态，并强调了与先前CMSX4合金结果的相似性和对比。
英文摘要：
Recent solidification experiments identified an oscillatory growth instability during directional solidification of Ni-based superalloy CMSX4 under a given range of cooling rates. From a modeling perspective, the quantitative simulation of dendritic growth under convective conditions remains challenging, due to the multiple length scales involved. Using the dendritic needle network (DNN) model, coupled with an efficient Navier-Stokes solver, we reproduced the buoyancy-induced growth oscillations observed in CMSX4 directional solidification. These previous results have shown that, for a given alloy and temperature gradient, oscillations occur in a narrow range of cooling rates (or pulling velocity, $V_p$) and that the selected primary dendrite arm spacing ($Λ$) plays a crucial role in the activation of the flow leading to oscillations. Here, we show that the oscillatory behavior may be generalized to other binary alloys within an appropriate range of $(V_p,Λ)$ by reproducing it for an Al-4at.%Cu alloy. We perform a mapping of oscillatory states as a function of $V_p$ and $Λ$, and identify the regions of occurrence of different behaviors (e.g., sustained or damped oscillations) and their effect on the oscillation characteristics. Our results suggest a minimum of $V_p$ for the occurrence of oscillations and confirm the correlation between the oscillation type (namely: damped, sustained, or noisy) with the ratio of average fluid velocity $\overline V$ over $V_p$. We describe the different observed growth regimes and highlight similarities and contrasts with our previous results for a CMSX4 alloy.
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