Yang Haodong, Zhang Yifan, Zhang An, Stein Frank, Xu Zhengbing, Tang Zhichao, Ren Dangjing, Zeng Jianmin
State Key Laboratory of Featured Metal Materials and Life-Cycle Safety for Composite Structures, Guangxi University, Nanning 530004, China.
Max-Planck-Institut für Eisenforschung GmbH, Max-Planck-Str. 1, 40237 Düsseldorf, Germany.
Materials (Basel). 2023 Mar 28;16(7):2691. doi: 10.3390/ma16072691.
The mechanism of solid-state dendrite formation in high-aluminum Fe-Al alloys is not clear. Applying an in-situ observation technique, the real-time formation and growth of FeAl solid-state dendrites during the eutectoid decomposition of the high-temperature phase FeAl is visualized. In-situ experiments by HT-CSLM reveal that proeutectoid FeAl usually does not preferentially nucleate at grain boundaries regardless of rapid or slow cooling conditions. The critical radii for generating morphological instability are 1.2 μm and 0.9 μm for slow and rapid cooling, respectively. The morphology after both slow and rapid cooling exhibits dendrites, while there are differences in the size and critical instability radius , which are attributed to the different supersaturation and the number of protrusions The combination of crystallographic and thermodynamic analysis indicates that solid-state dendrites only exist on the hypoeutectoid side in high-aluminum Fe-Al alloys. A large number of lattice defects in the parent phase provides an additional driving force for nucleation, leading to coherent nucleation from the interior of the parent phase grains based on the orientation relationship {3¯30}//{1¯10}, <111¯>//<111¯>. The maximum release of misfit strain energy leads to the preferential growth of the primary arm of the nucleus along <111¯> {1¯10}. During the rapid cooling process, a large supersaturation is induced in the matrix, driving the Al atoms to undergo unstable uphill diffusion and causing variations in the concentration gradient as well as generating constitutional undercooling, ultimately leading to morphological instability and the growth of secondary arms.
高铝铁铝合金中固态枝晶形成的机制尚不清楚。应用原位观察技术，可视化了高温相FeAl共析分解过程中FeAl固态枝晶的实时形成和生长。通过高温-同步辐射激光熔化显微镜进行的原位实验表明，无论冷却速度快慢，先共析FeAl通常不会在晶界处优先形核。慢速冷却和快速冷却时产生形态不稳定性的临界半径分别为1.2μm和0.9μm。慢速冷却和快速冷却后的形态均呈现枝晶，但其尺寸和临界不稳定性半径存在差异，这归因于不同的过饱和度和凸起数量。晶体学和热力学分析相结合表明，固态枝晶仅存在于高铝铁铝合金的亚共析侧。母相中大量的晶格缺陷为形核提供了额外的驱动力，导致基于取向关系{3¯30}//{1¯10}，<111¯>//<111¯>从母相晶粒内部进行共格形核。失配应变能的最大释放导致晶核的一次臂沿<111¯>{1¯10}优先生长。在快速冷却过程中，基体中会产生较大的过饱和度，促使铝原子进行不稳定的上坡扩散，导致浓度梯度发生变化并产生成分过冷，最终导致形态不稳定和二次臂的生长。
