薄板试样定向凝固枝晶竞争生长

Competitive Growth of Dendrite Arrays in the Directional Solidification of a Thin Sheet Sample

深入理解高温合金定向凝固杂晶演化具有重要的科学和工程意义,其中枝晶竞争生长是揭示晶界演化与淘汰的关键环节。定向凝固双晶竞争生长的实验研究证实了传统尖端过冷度理论的局限性,定量相场模拟的研究获得了大量统计规律,促进了人们对定向凝固双晶竞争生长的理解。本研究在薄板状试样的定向凝固过程中,同时获得了大量的不同取向竞争生长的枝晶,通过拼接金相显微组织同时获得多个晶界的取向演化规律。结果表明,枝晶列竞争生长主要通过晶界两侧不同取向一次枝晶臂及二次枝晶臂的交互作用实现。在汇聚生长过程中,择优枝晶一次臂阻挡非择优枝晶,并且择优枝晶自身无法形成三次臂,导致晶界方向沿择优枝晶生长方向。发散生长过程中,处于温度梯度方向一侧时,两列枝晶的竞争生长方式以非择优枝晶新生三次臂为主,晶界以沿择优枝晶方向生长为主;当两列枝晶处于温度梯度不同侧时,两列枝晶均有三次臂新生的概率,其晶界在两列枝晶取向之间。实验统计结果给出了晶界随两列竞争生长枝晶的取向的变化规律,与定量相场模拟结果吻合。商业软件ProCAST的CAFE计算晶粒定向竞争生长晶界演化统计结果与实验结果存在较大偏差,其主要原因是CAFE仅给出晶粒尺度的轮廓信息,未考虑溶质扩散及二次臂的竞争。本研究获得的薄板状试样枝晶竞争生长的统计规律,对认识高温合金定向凝固过程中晶界取向的演化具有重要的意义,对柱状晶叶片的制备具有指导意义。

It is of great scientific and engineering significance to understand the misoriented grain evolution in the directional solidification of superalloys, where the competitive growth of dendrite arrays with different orientation is the key phenomenon to understand the grain boundary evolution and elimination of dendrite. The experimental bicrystal growth in directional solidification identified the drawback of the traditional tip undercooling criterion. Moreover, quantitative phase field simulations showed more details of the bicrystal competitive growth and enhanced our understanding. In this research, we proposed a sheet sample with multiple bicrystals configurations. The microstructures were observed in a stitching picture. The results indicate that the competitive growth of two dendritic arrays is directly related to the interaction of primary arms and secondary arms in the grain boundary region. In the converging growth, the primary arm of the favorably oriented dendrite block the primary arm growth of unfavorably oriented dendrite and the new primary arm of the favorably oriented dendrite can not overgrowth in the grain boundary region. Then the grain boundary forms mainly along the growth direction of favorably oriented dendrite. In divergent growth, when the two dendritic arrays are on the same side of thermal gradient direction, the grain boundary is still mainly along the growth direction of favorably oriented dendrite where the new primary arm of unfavorably oriented dendrite is more likely to form than that of favorably oriented dendrite. When the two dendritic arrays are on the different sides of thermal gradient direction, the new primary arms of the both dendritic arrays can generate and the grain boundary is between the growth direction of the two arrays. The competitive growth and the grain boundary selection of dozens of biocrystals have been summarized within several samples. The statistical results agree with the phase field simulation. However, the CAFE simulation results from the ProCAST can not predict the grain boundary evolution due to there is only a profile of grains related to tip undercooling without solute diffusion and competition of side branches in CAFE. The method proposed here can be used to obtain the statistic information of the competitive growth with a high throughput way, and the results are helpful to understand the misoriented grains in fabrication of single crystal blade.