Terminal solid solubility of hydrogen in Zr-alloy pressure tube materials

In this work, terminal solid solubility (TSS) of hydrogen for Zr-alloy pressure tube materials was determined corresponding to end of hydride dissolution (TSSD) and start of hydride precipitation (TSSP) using the dilatometry technique. For this, Zircaloy-2 and Zr-2.5Nb pressure tube alloy coupons were gaseously charged with controlled amount of hydrogen in the range 10-100 μg/g. Change in length of cylindrical specimens of length ∼10 mm and diameter ∼3.5 mm machined from the coupons were measured as a function of temperature using a dilatometer. The samples were heated at 2 °C/min to 430 °C, held for 30 min at 430 °C and cooled back to the ambient temperature at 2 °C/min. The transition temperatures corresponding to the end of dissolution of hydrides during heating and beginning of precipitation of hydrides during cooling in these alloys were determined from thermal strain (e) versus temperature (T), average slope (of e versus T plot) versus T and differential thermal strain versus T plots. The enthalpies of hydride dissolution and precipitation for Zircaloy-2 pressure tube material were found to be 30-34.5 and 25.9-26.3 kJ/mol, respectively, whereas the corresponding enthalpies for Zr-2.5Nb pressure tube material were found to be 35.44 and 17.2-22.8 kJ/mol, respectively. This difference in the enthalpies between TSSD and TSSP is explained in terms of the different roles played by the components of strain energy associated with the elastic and plastic deformation in the matrix and precipitate, as a result of hydride accommodation in this alloy during heating and cooling process.