The ballooning behavior of Zircaloy-4 fuel sheaths at high temperatures has been reported previously by Hardy. Similar tests have been done on Zr-2.5Nb tubes. In both materials, the importance of the phase transformation from hexagonal close-packed to body-centered-cubic structure is evident. Towards the upper boundary of the two-phase (α + β) region where β-phase material would dominate, there is a marked discontinuity in the strain versus temperature results. They indicate that the solid solution strengthening in the β-phase is more effective than the second phase particle strengthening in the α-phase.

At normal operating temperatures, Zr-2.5Nb is stronger than Zircaloy-4. However, above 600°C it enters the two-phase temperature region and becomes weaker than Zircaloy-4 which is still within the α-phase. In the β-phase, where Zr-2.5Nb contains about twice as many atoms in solid solution as Zircaloy-4, the former again becomes stronger. At 1000°C, the strain rate of Zr-2.5Nb is about two orders of magnitude less than that of Zircaloy-4 at the same stress.