Hydrogen concentration limit and critical temperatures for delayed hydride cracking in zirconium alloys

An experimental study was carried out to determine the hydrogen concentration limit as a function of temperature at which delayed hydride cracking (DHC) commences in Zr-2.5 Nb pressure tube material. For a given hydrogen content of the specimen, two critical temperatures were observed in this work - a DHC initiation temperature, Tcat which DHC would initiate when approaching the test temperature from above the solvus (or terminal solid solubility) for hydride dissolution (TSSD) and a DHC arrest temperature, Th, obtained by heating the same specimen from Tcafter DHC had started. Both of Tcand Thare close to, but below, temperatures defined by TSSD for the specific hydrogen content of the specimen. A theoretical analysis was carried out to quantitatively derive the hydrogen concentration limit and these critical temperatures. The theoretical por Tcdepends sensitivity on the particular solvus or terminal solid solubility curve for hydride precipitation (TSSP) used, since there is a wide range of values for TSSP depending on the thermal-mechanical history of the material. It is also suggested that This governed by the TSSP for hydride growth, in contrast to Tc, which is governed by the TSSP for hydride nucleation. A model for a previously observed critical temperature (TA) is also proposed. TAis a DHC arrest temperature, obtained by approaching the test temperature from a lower temperature. The model suggests that TAis controlled by the energy difference between TSSD, TSSP and the hydrostatic stress at the crack tip.