Formation and Evolution of Stray Grains on Remelted Interface in the Seed Crystal During the Directional Solidification of Single-Crystal Superalloys Assisted by Vertical Static Magnetic Field

Nickel-based superalloys, especially single-crystal (SC) ones, have long been recognized as important materials for turbine blades used in aerospace and gas engines. Static magnetic fields are effective at controlling the material forming. The use of static magnetic fields during solidification has evolved as a sophisticated approach for efficiently controlling the microstructures and mechanical performance of metallic materials. In recent years, studies have shown that static magnetic fields have a complex effect on dendrites in SC superalloys. However, the mechanism of static magnetic fields regulating stray grains on remelted interface needs to be investigated further. This work studied the generation of stray grains near the seed remelted zone and the evolution mechanism during the directional solidification of the SC superalloy assisted by a magnetic field by tracing the solidification microstructure. The stray grains of large orientation that deviated from the <001> direction appeared on the remelted zone interface of the solidification microstructure when the magnetic field was applied, accompanied by the formation of a large-angle grain boundary (LAGB). Most of the stray grains were distributed at the sample edge. The increase in magnetic field intensity and pulling speed increased the number of stray grains and the length of the LAGB. As the solidification progressed, the large-orientation stray grains and the LAGBs were eliminated at a fast speed and evolved into small-orientation dendrites. During the following solidification, the orientation of the dendrites became even smaller and the evolution speed decreased sharply. The increase in withdrawal speed intensified the evolution process. The stray grains formed in the remelted zone can be attributed to the twisting dendrite by the thermoelectric magnetic force. The distribution of more stray grains around the sample was caused by the circulation from thermoelectric magnetic convection at the macroscopic scale.