Growth competition of columnar dendritic grains: A phase-field study
Growth competition of columnar dendritic grains: A phase-field study
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DOI：
10.1016/j.actamat.2014.08.049
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发表时间：
2015-01-01
期刊：
影响因子：
9.4
通讯作者：
Karma, A.
中科院分区：
文献类型：
--
作者：
Tourret, D.;Karma, A.
我们报告的结果在两个维度的柱状枝晶晶粒的生长竞争的一个广泛的相场研究。我们调查的影响的温度梯度和晶粒bicallography上的晶粒和微观结构的选择，专注于一个几何形状与两个晶粒的主晶轴取向平行，并在一个有限的取向差角相对于轴的温度梯度。我们的第一个主要发现是，对于在低温梯度下形成的发育良好的枝晶结构，消除取向不良晶粒的速率是两种晶粒的枝晶尖端之间的过冷度差的非单调函数。因此，这个速率甚至不能用通常的假设来定性地预测，即消除速率随过冷度差而增加。这一假设的崩溃是特别引人注目的高度取向不良的树枝状和退化的结构，持续了很长时间，尽管在一个大得多的过冷度比良好取向的相邻晶粒的增长。我们的第二个主要发现是，微观热波动的起源侧支化可以引起显着变化的宏观轨迹的晶界（GB），从而使晶粒选择的随机过程，同时产生有限的变化，在选定的主要间距。相反，在没有波动的情况下，GB运动变得基本上是确定性的，并且晶粒消除被抑制。此外，我们的模拟再现定量的标度律推导出的实验的一次枝晶间距和枝晶生长方向的取向不良的晶粒。他们进一步揭示，在GB三级分支事件所选择的“intergrain”的主要间距系统大于“intragrain”的主要间距所选择的一个单一的晶粒内的初级分支之间的瞬态生长竞争，同时遵守相同的标度律。最后，事实上，在我们的2-D模拟中的颗粒消除的速度比在实验中慢，这表明，3-D颗粒bicallography在颗粒选择中起着关键作用。这一作用被解释在光的2-D模拟，阻碍侧支化的取向不良的粮食。(C)2014 Acta Materialia Inc.由爱思唯尔有限公司出版。保留所有权利。
We report the results of an extensive phase-field study of the growth competition of columnar dendritic grains in two dimensions. We investigate the influence of the temperature gradient and grain bicrystallography on the selection of both grain and microstructure, focusing on a geometry with two grains with principal crystal axes oriented parallel and at a finite misorientation angle with respect to the axis of the temperature gradient. Our first main finding is that, for well-developed dendritic structures forming at a low-temperature gradient, the rate of elimination of the misoriented grain is a non-monotonic function of the difference in undercooling between the dendrite tips of the two grains. Hence this rate cannot be predicted even qualitatively by the common assumption that the elimination rate increases with this undercooling difference. The breakdown of this assumption is particularly striking for highly misoriented dendritic and degenerate structures that persist for very long times despite growing at a substantially larger undercooling than the well-oriented neighboring grains. Our second main finding is that microscopic thermal fluctuations at the origin of sidebranching can induce significant variations in the macroscopic trajectories of grain boundaries (GBs), thereby making grain selection a stochastic process, while yielding limited variations in the selected primary spacings. In contrast, in the absence of fluctuations, GB motion becomes essentially deterministic and grain elimination is suppressed. In addition, our simulations reproduce quantitatively scaling laws deduced from experiments for both the primary dendritic spacing and the dendrite growth direction of misoriented grains. They further reveal that the "intergrain" primary spacing selected by tertiary branching events at GBs is systematically larger than the "intragrain" primary spacing selected by the transient growth competition between primary branches within a single grain, while obeying the same scaling laws. Finally, the fact that the rate of grain elimination is slower in our 2-D simulations than in experiments suggests that the 3-D grain bicrystallography plays a key role in grain selection. This role is interpreted in the light of 2-D simulations that hinder sidebranching on the misoriented grain. (C) 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.