Re-investigation of phase transformations in the Zr-Excel alloy

The thermodynamically equilibrium precipitates that form in the Zr-Excel alloy after long-term (2500 h) annealing at 550 °C were investigated with transmission electron microscopy and X-ray diffraction techniques. The equilibrium precipitates were found to be a form of cubic Zr(Mo,Nb,Fe)2 with about 26.5 at.% Mo substituted by Nb. Differential scanning calorimetry (DSC) heating curves show that the phase transformation sequence is αZr + Zr(Mo,Nb,Fe)2 → αZr + Zr(Mo,Nb,Fe)2 + βZr → αZr + βZr → βZr upon heating the long-term annealing alloy. With the heating rate of 10 °C/min, the αZr is found to transform into βZr at 530 °C, and Zr(Mo,Nb,Fe)2 completely dissolves at a temperature higher than 771 °C, while the αZr disappears at 969 °C.