Determination of terminal solid solubility of hydrogen in Zr-alloy pressure tube material using dilatometry technique

Hydrogen in excess of solid solubility precipitates out as hydride phase in Zirconium alloys. The maximum amount of hydrogen which can be retained in solution without forming hydride precipitates is called terminal solid solubility (TSS). Due to hysteresis associated with hydride precipitation and dissolution, TSS depends on the direction of approach of test temperature. TSS obtained during heating corresponds to the dissolution of hydrides and is called TSSD whereas TSS obtained during cooling corresponds to the precipitation of hydrides and is called TSSP. In this work, both TSSD and TSSP were determined 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 wppm. Change in length of cylindrical specimens of length- 10 mm and diameter -3 mm machined from the coupons were measured as a function of temperature using a dilatometer. The samples were heated at 2 degC/min to 430 degC, held for 30 min at 430 degC and cooled back to the ambient temperature at 2 degC/min. The transition temperatures corresponding to the end of dissolution of hydrides during heating and beginning of precipitation of hydrides during cooling, corresponding to each hydrogen concentration, in this alloy were determined from thermal strain (e) vs temperature (7), average slope (of e vs T plot) vs T and differential thermal strain vs 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 17.5- 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. (author)