Phase transformations in metastable $\beta$ Zr15Nb alloy revealed by in-situ methods

TL;DR

This study investigates the complex phase transformations in metastable Zr15Nb alloy during heating using advanced in-situ techniques, revealing detailed structural changes and phase stability over a broad temperature range.

Contribution

It provides a comprehensive multi-technique analysis of phase transitions in Zr15Nb alloy, identifying distinct bcc phases and the temperature of omega phase dissolution.

Findings

Identification of two bcc beta phases with similar lattice parameters.

Determination of omega solvus temperature at approximately 555°C.

Observation of phase evolution from beta and omega to alpha during heating.

Abstract

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 ${\deg}$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 ${\deg}$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.