The growth mechanism of grain boundary carbide in Alloy 690

•A transition phase was observed at the coherent interfaces of M23C6 and matrix.•The transition phase has hexagonal structure, and is coherent with matrix and M23C6.•The M23C6 transforms from the matrix directly at the incoherent phase interface.

The growth mechanism of grain boundary M23C6 carbides in nickel base Alloy 690 after aging at 715 °C was investigated by high resolution transmission electron microscopy. The grain boundary carbides have coherent orientation relationship with only one side of the matrix. The incoherent phase interface between M23C6 and matrix was curved, and did not lie on any specific crystal plane. The M23C6 carbide transforms from the matrix phase directly at the incoherent interface. The flat coherent phase interface generally lies on low index crystal planes, such as {011} and {111} planes. The M23C6 carbide transforms from a transition phase found at curved coherent phase interface. The transition phase has a complex hexagonal crystal structure, and has coherent orientation relationship with matrix and M23C6: {111}matrix//{0001}transition//{111}carbide, <112¯>matrix//<21¯10>transition//<112¯>carbide. The crystal lattice constants of transition phase are ctransition=3×amatrix and atransition=6/2×amatrix. Based on the experimental results, the growth mechanism of M23C6 and the formation mechanism of transition phase are discussed.