Effects of thermo-mechanical process on phase transitions, hydrogen solubility and corrosion of Ta-modified Zr-1Nb alloys

The feasibility of using Ta as a minor alloying element in Zr-1Nb alloys is addressed in terms of phase stability and resistance to corrosion of the microstructure obtained with the following two thermomechanical processes after a water quenching from β_Zr: a) annealed at 570°C for 3840 h and b) a combination of intermediate annealing temperatures and cold rolled steps. Hydrogen solubility was also measured, but only in alloys with the first process. It was found, through calorimetric measurements, that an increase of Ta replacing Nb in the nominal alloy composition increases the α→(α+β) phase transformation temperature in agreement with an estimated increase in the enthalpy of formation of the β_Nb/Ta phase, ΔH_βNb/Ta^f. As expected, the mass fraction of β_Nb/Ta measured in the rolled alloys is lower. Further, this diminution is larger in the alloys containing Ta due to the lower mobility of Ta and higher ΔH_βNb/Ta^f. The growth of the oxide layer in each alloy, at 400°C steam up to 14 days, follows the predictor calculated as the ratio between the atomic concentrations of Nb plus Ta in the alloy and ΔH_βNb/Ta^f. A slight decrease in hydrogen solubility was observed at low Ta content in the alloy.