定向凝固DD6单晶高温合金枝晶组织均匀性研究

EVALUATION OF THE UNIFORM DISTRIBUTION OF DENDRITIC MICROSTRUCTURE IN DIRECTIONALLY SOLIDIFIED SINGLE-CRYSTAL DD6 SUPERALLOY

采用一次枝晶间距、最小生成树（minimum spanningtree，MST）、Voronoi多边形、FFT以及RO—XRD和EBSD等方法．研究了DD6单晶高温合金枝晶分布的均匀性．结果表明：DD6单晶高温合金一次枝晶平均间距为325．7μm，变化率7．38％．Voronoi多边形法给出的最近邻枝晶个数对应在5．87～5．93，所占比例变化率超过30％：另外，MST的枝干长度变化也比较明显，达到2695％，不同位置处的FFT频谱也不同，说明实验中选晶法获得的DD6单晶高温合金组织均匀性有待于进一步提高．上述结果也表明，定向凝固单晶高温合金枝晶生长是一个动态调整的过程，通过测量一次枝晶间距结合Voronoi多边形和MST法可量证枝晶生长过程的变化程度，而通常采用一次枝晶平均间距来衡量凝固过程稳定性是不充分的．对选晶法获得的DD6单晶高温合金择优取向与轴向（Z轴）之间的偏离角进行了测量，发现其值在10。以内，EBSD测试的结果比RO-XRD相应结果大，这是由于RO-XRD在计算偏离角时选用了强度最大值所对应的衍射峰所致．

Homogeneous distribution of primary dendritic arm spacing （PDAS） is required to achieve uniform mechanical properties in final product of single-crystal superalloys. In this work, the dendrite characterization and orientation of Ni-based single-crystal DD6 superalloy have been deeply investigated using different methods, which include minimum spanning tree （MST）, Voronoi polygon-based approach, fast Fourier transform （FFT）, as well as EBSD and RO-XRD. The investigation results indicate that the mean PDAS of DD6 superalloy is about 325.7 pm and its variation ratio is 7.38%. The measured Voronoi polygon parameters suggest that the num- ber of nearest-neighbor dendrite ranges from 5.87 to 5.93, approximating six nearest neighbors in the spatial distri- bution of dendrite microstructures. However, the change in ratio of six nearest number proportion has exceeded 30% for the twenty specimens. The MST method shows that the change in branch length measured from the twenty specimens achieves 26.95%. Also, the analysis results of FFT imply that the dendrite microstructures of DD6 superalloy evolve apparently. These results give the proof that the dendrite microstructures of DD6 superalloy vary with the solidified distance. Additionally, the deviation angles between preferential orientations of DD6 with the axial di- rection of specimen were measured by EBSD and RO-XRD, respectively. The deviation angle values of DD6 super- alloy in this experiment are both within 10% The reason for the deviation angle measured by RO-XRD being small- er is well explained due to the fact of selecting the diffraction intensity maximum angles. Furthermore, the EBSD results indicate that the orientations of DD6 superalloy prepared by grain selector can be well controlled along the Z-axial direction, but do not work in other two X and Y directions.