Sequence of phase transformations in metastable ß Zr–12Nb alloy studied in situ by HEXRD and complementary techniques

Abstract

Phase transformations in a metastable beta Zr–12Nb alloy were investigated by high-energy X-ray diffraction (HEXRD) measured simultaneously with thermal expansion in situ during linear heating from room temperature to 800 °C. Complementary in-situ methods of electrical resistance and differential scanning calorimetry, which were performed using the same heating conditions as in the HEXRD experiment, provided additional information on the transformation sequence occurring in the Zr–12Nb alloy. Two bcc phases with a different lattice parameter, ßZr and ßNb, were observed in the investigated temperature range and identified using the phase diagram of the Zr–Nb system. In the initial solution-treated condition, metastable ßZr phase and athermal ¿ particles are present in the material. At about 300 °C, Nb-rich ßNb phase starts to form in the material and the original ßZr phase gradually disappears. Ex-situ observations of the microstructure using transmission electron microscopy revealed a cuboidal shape of the ¿ particles, which is related to a relatively large misfit between the ¿ and ß phases. At 560 °C, ¿ solvus was observed, identified by an abrupt dissolution of ¿ particles which was followed by growth of the a phase.