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

摘要

The evolution of grain boundary phases during high temperature creep has been systematically investigated in a directionaliy solidified Ni-base superalloy. At the primary creep stage, precipitation of nanosize M5B3 boride from γ matrix and phase transformation from M23(C,B)6 borocarbide to M5B3 boride occur at grain boundaries. At the steady-state creep stage, precipitation of blocky M23(C,B)6 borocarbide and coarsening of M23(C,B)6 with the dissolution of M5B3 boride occur at grain boundaries. At the tertiary creep stage, precipitation of M5B3 boride from γ matrix or phase transformation from M23(C,B)6 borocarbide to MSB3 boride occur again at grain boundaries. The precipitation of M5B3 is due to diffusion of B atoms promoted by applied tensile stress during high temperature creep, and the phase transformation between M23(C,B)6 and M5B3 is thought to correlate strongly with the strain rate of different creep stages.