Phase transformations in metastable β-Zr15Nb alloy revealed by in situ methods

This study examines the phase transitions occurring during linear heating of the Zr15Nb alloy through a comprehensive, multi-technique methodology comprising in situ high-energy synchrotron X-ray diffraction (HEXRD), electrical resistance measurements, differential scanning calorimetry (DSC) and thermal expansion analysis, supplemented by ex situ transmission electron microscopy (TEM). The findings reveal a complex sequence of phase transformations and corresponding structural changes over a broad temperature range (from room temperature up to 800 °C). Two distinct body-centered cubic (bcc) β phases β Zr and β Nb with closely related lattice parameters are identified. At room temperature, the microstructure is characterized by a mixture of the metastable β Zr + ω ath phase. Upon heating, β Zr progressively decomposes, giving rise to the formation of β Nb . TEM observation revealed the cuboidal shape of the ω iso particles, resulting from the high lattice misfit between β and ω phase. The ω solvus temperature is determined to be approximately 555 °C, as evidenced by in situ HEXRD and abrupt changes in the alloy’s thermal and electrical properties. The growth of the α phase occurs after the dissolution of the ω phase, resulting in a pronounced increase in thermal expansion.