Precipitation and phase transformation mechanism of additive manufactured Ni-Co base superalloy

Microstructures of a newly developed Ni-Co base superalloy with different contents of Co manufactured by additive manufacturing (AM) have been investigated using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM). The results show that, the Ni-Co base superalloy made by AM presents a columnar microstructure. This columnar microstructure is achieved through epitaxial growth of fine parallel dendrites from a substrate. eta phase and gamma' phase precipitate during the deposition process. After heat treatments, there are primary gamma', secondary gamma' and tertiary gamma' precipitated. The heat treatments also promote the precipitation of eta phase. The inductively coupled plasma atomic emission spectroscopy (ICP-AES) investigation shows that the loss and segregation of elements can account for the formation of eta phase. The results also indicate that the addition of Co promotes the precipitation of gamma' and impedes the precipitation of eta phase. Electron probe microanalyser (EPMA) results show that increasing of Co content decreases the segregation of Ti in the heat-treated state and is beneficial for decreasing the precipitation of eta phase. TEM analysis confirms that the phase transformation mechanism is mainly the reordering of atoms and the formation of stacking faults.