Are Denuded Zones the Root Cause of Stress Relaxation Cracking? SRC Mechanisms Part I

Stress Relaxation Cracking (SRC) is a cracking mechanism in the lower creep temperature range that leads to low ductility, intergranular fracture in many alloy systems. In high-temperature alloys, intergranular precipitates develop as well as fine intragranular precipitates that nucleate on pre-existing dislocation structures. So-called denuded zones (DZ) or precipitate-free zones (PFZ) are often observed around grain boundaries. It has been reasoned that they are responsible for localizing creep deformation into a “soft” grain boundary zone because the grain interior has hardened too much to allow creep deformation. In this paper, the historic and contemporary literature is critically examined to find evidence supporting this “soft denuded zone model”. It will eventually be concluded that this mechanism does not hold up to scientific scrutiny. The stress state in locations susceptible to SRC does not allow the model’s micromechanics to operate and the predictions of the model do not match real world failure cases. In a companion paper [1], an SRC mechanism model will be presented that matches failure case observations and is supported by empirical data and scientific literature.