The microstructure, size, elemental composition and hardness of the surface eutectic layer formed during directional solidi?cation of a Ni-based single crystal superalloy were studied. The formation mechanisms of the surface eutectic on the outer surface of the casting were also discussed. The metal/mould interactions did not play any role in the formation of the surface eutectic. The formation cause of surface eutectic layer was attributed to the interdendritic residual liquid for excretion caused by solidi?cation shrinkage.
A nickel-based superalloy was deposited onto a single crystal substrate based on epitaxial laser metal forming(E-LMF). The microstructure development in two depositions has been researched. For the first time, the crystal orientation of dendrites varying beyond 20° was found when the dendrites deflected in deposition. In addition, a new grain boundary was found between different orientation dendrites in a grain, and the detected grain boundary angle was 23°. The result shows that flowing field in laser pool is responsible for this phenomenon.
Guowei WangJingjing LiangYizhou ZhouLibin ZhaoTao JinXiaofeng Sun
A geometric analysis technique for crystal growth and microstructure development in single-crystal welds had been previously developed.And the effect of welding conditions on the tendency of stray grains formation during solidification was researched.In the present work,these analytical methods were further extended.Combined with an original vectorization method,a 3D Rosenthal solution was used to determine thermal conditions of the welds.Afterward,the dendrite growth orientation,the dendrite growth velocity and the thermal gradient along dendrite direction were calculated and lively plotted.Finally,the tendency of stray grains formation in the solidification front was forecasted and its distribution was presented with a 3D plot.The results indicate that substrate orientation has some impacts on the crystal growth pattern,dendrite growth velocity,distribution of thermal gradient and stray grain.Based on the research methods proposed in this work,any substrate crystallographic orientation can be studied,and predicted stray grains distribution can be visualized.
Guo-Wei WangJing-Jing LiangYi-Zhou ZhouTao JinXiao-Feng SunZhuang-Qi Hu
Precipitation of topologically close-packed phases in a Re-containing Ni-base single crystal superalloy was studied under elevated temperature by in situ transmission electron microscopy.Above 1150?C,a new intergrowth structure called C phase was found within μ phase with a defined crystallographic orientation relationship.Elements mapping analysis reveals that C phase has a similar element composition as μ phase,but contains a lower level of Cr,W and Re.With increasing temperature,the proportion of C phase increases gradually.At 1250?C,C phase becomes the dominant precipitate.It is demonstrated that C phase is more thermodynamically stable than μ phase above 1150?C that leads to an intergrowth pathway from μ phase to C phase.
The pattern of dendritic growth and distortion of dendritic network in the platform have been investigated by one mold casting with different platform length during directional solidification. As the platform length elongates, the symmetry of dendritic growth along left and right edges gradually worsens in platform base. While the platform length reaches 14 mm, the distortion of dendritic network is first observed in outward platform. It is found that the distortion of dendritic network along platform inside is more serious than that along platform edges. Both[001] deviation and accumulated misorientation along platform inside, up to 9 and 16.3, respectively, are far greater than those along lefte outwarde right edges. The deformation of dendritic network in a platform may be caused by the asymmetry of the solidification front at the mush zone.
Xiaoli ZhangYizhou ZhouYanyun HanTao JinXiaofeng Sun
The influence of active elements C and Hf on the interface reactions and wettability between a Ni_3Albased superalloy and the ceramic mould material was studied by using a sessile drop experiment.The microstructure of the alloy interface was investigated by scanning electron microscopy analysis and the phase identification was performed by X-ray diffraction analysis.The results show that interface reactions occur as C and Hf contents reach a critical value.The critical values for C and Hf to cause interface reactions are 0.12 wt%and 1.17 wt%,respectively.The reaction products contain HfO_2 and 9Al_2O_3-Cr_2O_3.Adsorptions of Hf and interface reactions improve the wettability obviously.
