Evolution of Grain Boundary Precipitates in a Directionally Solidified Ni‐Base Superalloy during High Temperature Creep

Abstract

The evolution of grain boundary phases during high temperature creep has been systematically investigated in a directionally solidified Ni-base superalloy. At the primary creep stage, precipitation of nanosize M 5 BB 3 boride from γ matrix and phase transformation from M 23 (C,B) 6 borocarbide to M 5 B 3 B boride occur at grain boundaries. At the steady-state creep stage, precipitation of blocky M 23 (C,B) 6 borocarbide and coarsening of M 23 (C,B) 6 with the dissolution of M 5 BB 3 boride occur at grain boundaries. At the tertiary creep stage, precipitation of M 5 B 3 B boride from γ matrix or phase transformation from M 23 (C,B) 6 borocarbide to M 5 BB 3 boride occur again at grain boundaries. The precipitation of M 5 B 3 B is due to diffusion of B atoms promoted by applied tensile stress during high temperature creep, and the phase transformation between M 23 (C,B) 6 and M 5 BB 3 is thought to correlate strongly with the strain rate of different creep stages.