Microstructure evolution in the electron beam treated surface of a Ni-base single crystal

Electron beam melting and welding is becoming popular in nuclear, chemical and aerospace industries due to its localized melting, high penetration depth and fast cooling rate. Surface of DD8 has been melted by electron beam (EB). A scanning electron microscope (SEM) having the attachment of an energy dispersive system (EDS) has been employed to study the resulted microstructure. The microstructure of the molten zone (MZ) is mostly cellular dendritic whereas columnar dendritic structure is observed only at the edges of the MZ. The hardness of the MZ is found to be 50% less than that of as-received surface of the DD8 single crystal. The dendrites contain higher content of Cr and W and low content of Al, Ta and Ti while the dendrite boundaries are depleted in Cr, Co and W and rich in Al, Ta and Ti. The X-ray diffraction patterns of the MZ show shifting of peaks towards lower angle compared to the diffraction pattern of the as-received alloy. Broadening of the peaks is also observed in the diffraction pattern of the MZ.