Protection of Ni-base superalloys against stress accelerated grain boundary oxidation (SAGBO) by grain boundary chemistry modification

Abstract

It is well established that the creep and hold-time fatigue crack propagation rate of many Ni-base superalloys is accelerated by up to two orders of magnitude in air relative to vacuum environment. This effect is associated with the embrittlement of grain boundaries by oxygen in the presence of tensile stresses. In the literature, it is often referred to as stress accelerated grain boundary oxidation (SAGBO). Measures that improve crack growth resistance in air are (1) enlarging the grain size, (2) overaging the microstructure, and (3) modifying the base metal chemistry, for example by addition of grain boundary strengtheners such as boron and zirconium. However, these measures adversely affect other important properties such as yield strength, ultrasonic inspectability or--in the last case--forgeability and weldability. An alternative approach is presented here. Utilizing the beneficial effect of grain boundary strengthening elements such as boron while avoiding their detrimental effect on manufacturability if added to the master melt. The basic idea is to diffuse these elements into the component by a deposition and heat treatment process after manufacturing. As a result, environmentally exposed grain boundaries can be chemically modified for improved resistance against SAGBO. This new approach is described in detail. The beneficial effectmore » on elevated temperature performance is demonstrated using constant strain rate tests at 600 C.« less