Phase transformations in metastable $\beta$ 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 $\mathrm{°}$C). Two distinct body-centered cubic (bcc) $\beta$ phases - ${\beta }_{Zr}$ and ${\beta }_{Nb}$ - with closely related lattice parameters are identified. At room temperature, the microstructure is characterized by a mixture of the metastable ${\beta }_{Zr}$ + ${\omega }_{ath}$ phase. Upon heating, ${\beta }_{Zr}$ progressively decomposes, giving rise to the formation of ${\beta }_{Nb}$. TEM observation revealed the cuboidal shape of the ${\omega }_{iso}$ particles resulting from the high lattice misfit between $\beta$ and $\omega$ phase. The $\omega$ solvus temperature is determined to be approximately 555 $\mathrm{°}$C, as evidenced by in situ HEXRD and abrupt changes in the alloy’s thermal and electrical properties. The growth of the $\alpha$ phase occurs after the dissolution of the $\omega$ phase, resulting in a pronounced increase in thermal expansion.