A Differential Scanning Calorimetry (DSC) Study of Phase Changes in an As-received Zr-2.5Nb Pressure Tube Material during Continuous Heating and Cooling

Differential scanning calorimetry (DSC) has been used to study the phase changes in samples of as-received Zr-2.5Nb pressure tube material by continuous heating and cooling. Two different heating rates (5 and 20 degrees C/min) were used to heat the sample up to 1050 degrees C. After a short time hold at 1050 degrees C, all the samples were continuously cooled to 300 degrees C at a rate of 20 degrees C/min. On continuous heating, the DSC signals obtained showed two endothermic transitions. The low-temperature transition, occurring between about 500 and 650 degrees C, is attributed to a thermal decomposition of metastable niobium-stabilized beta-phase. The higher-temperature transition, occurring between 600 and 950 degrees C, is due to phase transformations of hcp alpha-Zr to bcc beta-Zr, as previously confirmed in a companion study on the same pressure-tube material that was examined in-situ by neutron diffraction. The neutron diffraction results provided a positive identification of the two phases and also a quantification of the beta-phase present in the sample at different heating temperatures, and thus provided a guide to extract the volume fraction of beta-phase from the DSC signals obtained in this study. The DSC signals revealed only one exothermic transition which is correlated to the reverse transformation of beta-Zr to alpha-Zr, as previously identified in the companion neutron diffraction study of the same pressure tube material.