Zirconium alloys are used extensively in nuclear reactor cores. During their service a part of hydrogen produced through the corrosion reaction of Zr with hot coolant is absorbed by materials. Hydride induced embrittlement significantly influences the in-service performance of the Zr-alloy components. Delayed hydride cracking (DHC) is a localized form of hydride embrittlement, consequently, hydrogen atoms in the solid solution will diffuse into this region ahead of the crack tip subjected to a triaxial state of stress, which may lower the chemical potential of the region. Once the hydrogen concentration in this region reaches the terminal solid solubility (TSS), hydrides will start to form and grow. When the hydrides at the crack tip reach a critical size, the main crack will propagate through this hydrided region. The crack front finally is arrested at the end of the hydrided region by the ductile zirconium matrix, and the whole process repeats itself. Most of the investigations on DHC in zirconium alloys are focused on Zr-Nb alloys. Few literatures were found on the subject of DHC in Zr-Sn alloys. The purpose of the present study was to investigate critical temperature for initiating and arresting delayed hydride cracking in Zr-Sn-Nb alloy. A critical temperature for DHC study was carried out to determine the critical temperature for initiating and arresting in N18 zirconium alloy (Zr-Sn-Nb alloy). For a given hydrogen concentration of a specimen, the two critical temperatures were observed-a DHC initiation temperature, T-c at which DHC would initiate when approaching the test temperature from above the terminal solid solubility (C-d) temperature in hydride dissolution and a DHC arrest temperature, T-h, obtained by heating the same specimen from T-c after DHC had started. T-c slightly below T-h. Both T-c and T-h fall below the dissolution solvus temperature and above the precipitation solvus temperature. A theoretical analysis was carried out to quantitatively determine the hydrogen concentration limit and these critical temperatures using the method of Dutton and Plus, a key assumption in the method is that, while the local crack tip stress concentration causes a local enhancement of the hydrogen concentration in solution, the hydride precipitation solvus is unaffected by stress. Good agreements are obtained between measured and predicted values of critical temperatures, which support the Dutton-Plus theory.
