Stress Accelerated Grain Boundary Oxygen Embrittlement on Creep Crack Growth of Nickel-Base Superalloys

Creep crack growth behavior in some nickel-base superalloys have been known to be sensitive to environments. Oxidizing environment can increase the creep crack growth rates by two or three orders of magnitude accompanied by intergranular fracture mode. This serious brittle fracture problem is suggested to be caused by “Stress Accelerated Grain Boundary Oxygen (SAGBO) Embrittlement” where, at high stress regions, oxygen may diffuse along grain boundaries and reacts with alloy elements (such as Nb, Cr, Al, Mo or Ti) to form brittle oxides that resulted in intergranular fracture. In this research, SAGBO embrittlement on creep crack growth behavior of Inconel 718 was investigated. High temperature moire interferometry (HTMI) was applied to obtain, in-situ, full-field crack tip displacement fields of the test specimens under constant loads (from 0.27 KIC to 0.39 KIC) at 650°C and elastic crack growth was observed for specimens tested in air. Based on the near-tip moire fringes, stress intensity factors and crack-tip plastic yield zones were evaluated and compared favorably with the theoretical predictions based on linear elastic fracture mechanics (LEFM). Post-mortem microstructural analyses showed the existence of Nb-rich oxide films at grain boundaries near the crack tip region. This indicated that SAGBO embrittlement was caused by oxygen diffusion to grain boundaries in high stress regions and reacting with Nb to form thin brittle Nb2O5 oxide layers on grain boundary surfaces. Chemical composition of the Nb2O5 layers was verified using Auger electron spectroscopy (AES). It is suggested that the formation of the brittle Nb2O5 oxide films resulted in SAGBO embrittlement on Inconel 718 superalloy.