SAGBO Mechanism on High Temperature Cracking Behavior of Ni-base Superalloys

Abstract

Many Ni-base superalloys such as Inconel 718 show a time dependent behavior of crack growth at elevated temperatures above 500°C. The environmental interaction at the crack tip plays an important role on the crack growth rate, and the stress assisted grain boundary oxidation (SAGBO) is the acting mechanism for these alloys cracking in air. In this research, the SAGBO effect on the creep crack growth behavior of fatigue pre-cracked single edge-notched (SEN) Inconel 718 specimens tested in air and oxygen-free environments is investigated. High temperature moire interferometry (HTMI) was applied to obtain in-situ crack tip displacement fields of the test specimens at 650°C under constant loading conditions. The diffusion of oxygen along grain boundaries in front of the crack tip governs the size and degree of environmental damage. The formation of a SAGBO-induced damage zone at the crack tip causes the cracking in a brittle mode as verified by our experimental results. The damage zone induces intergranular fracture and accelerates the crack growth rate. The coarsening of the damage zone is a linear function with time under given temperatures, oxygen partial pressure, and stress intensity factor. Superalloys 718,625, 706 and Various Derivatives Edited by E.A. Loria The Minerals, Metals & Materials Society, 1997 679