Loading Rate Effects on the Hydrogen Enhanced Cracking Behavior of Ni- and Co-based Superalloys for Marine Applications

Marine fastener alloys can be susceptible to hydrogen environmentally assisted cracking (HEAC) in cathodically polarized seawater environments [1]–[3]. Understanding the controlling mechanisms of HEAC is critical to developing accurate predictive models for structural management of these material systems. Prior work has demonstrated that there is a potentially strong influence of loading rate on the HEAC susceptibility in environments pertinent to engineering applications [4]. The current work uses linear elastic fracture mechanics (LEFM) based testing to characterize the stress intensity rate (dK/dt) dependence of HEAC behavior for Monel K-500 (Ni-based superalloy) and MP98t (Co-based superalloy) in cathodically polarized solutions. This work has been divided into three tasks to rigorously analyze the material behavior.
Task 1 addresses laboratory testing challenges and develops a functional protocol for characterizing the HEAC resistance of high toughness, relatively low strength materials in mildly aggressive environments using LEFM based techniques. In this paradigm, direct current potential difference (dcPD) uniquely enables high fidelity detection of crack advance but was found to be adversely influenced by specimen plasticity and non-HEAC based crack advance (e.g. crack tip blunting, ductile tearing, etc.). A mitigation protocol was developed to truncate data prior to the remaining ligament reaching the proportional limit and to decouple the effect of crack tip plasticity from real crack growth. Task 2 utilized the protocol above to quantify the effect of the stress intensity rate, dK/dt, on standard susceptibility metrics: the threshold stress intensity, KTH, and the stage II crack growth rate, da/dtII, for Monel K-500 and MP98t [4]. MP98t was found to be much less susceptible to HEAC than Monel K-500. For both materials, the KTH did not exhibit a strong dependence on dK/dt, whereas the da/dtII increased as a power law function of dK/dt.
Task 3 explored the mechanistic underpinnings that govern the effect of loading rate on the stress corrosion cracking (SCC) behavior of each alloy. Traditional loading rate dependent arguments such as H diffusion limitation and crack tip film rupture were not found to be consistent with the data and the materials/environment/loading conditions. The dK/dt dependence was analyzed in the context of prominent H-enhanced damage mechanisms: hydrogen enhanced local plasticity (HELP) and hydrogen enhanced decohesion (HEDE) [5]. The observed increase in da/dt with increasing dK/dt is considered to be due to one or both of the following two failure criteria, provided a dK/dt independent of H concentration at the crack tip. In the first, failure requires a critical crack tip strain to be exceeded for crack advance; and in the second, accelerated work-hardening increases the crack tip stresses which enable decohesion. In both cases, elevated dK/dt would cause elevated da/dt due to exceedance of the critical value in a shorter time increment. These arguments do not influence KTH because crack growth does not occur below this value.