Review of DHC initiation and arrest temperatures for Zr-2.5Nb tubes

As Shi and Puls proposed a DHC model where the driving force for a DHC is a tensile stress gradient, leading the hydrogen concentration at the crack tip to increase to the terminal solid solubility of precipitation (TSSP) of hydrogen accompanied by a decrease in the hydrogen concentration dissolved in the bulk region. Based on their DHC model, Shi et al. conducted a series of tests and a theoretical analysis with an aim to define the hydrogen concentration limit as a function of the temperature where the delayed hydride cracking commences and stops in the Zr-2.5Nb pressure tube. They reported that the DHC initiation and arrest temperatures, Tc and Th, respectively are close to but below, the temperatures defined by the terminal solid solubility for dissolution (TSSD) for the hydrogen concentrations of the specimens which is in contrast with the Coleman's result. Furthermore, they suggested that the reason for the closeness of the Tc to the TSSD temperatures resides in the shift of the solvus due to the hysteresis between the hydride precipitation and dissolution and the opposite effect of the stress by increasing the hydrogen concentration at regions of a high tensile stress. The problem with their DHC model is that the crack tip cannot reach the TSSP with the test temperature approached by heating under any stress gradient and the assumption that the tensile stress acts as a hydrostatic stress is unrealistic. Instead, Kim propose a new DHC model where the driving force for the DHC is a difference in the hydrogen concentration by nucleating the hydrides, preferentially only at the crack tip under an applied tensile stress which is attributed to a hysteresis of the terminal solid solubility of hydrogen on a heat-up and a cool-down. Thus, the objective of this paper is to reanalyze the hydrogen concentration limit and the critical temperatures for the DHC in zirconium alloys reported by Shi et al. using the Kim's DHC model