An analysis of the phase decomposition in Nb–Zr alloy based on the system free energy theory

The behaviour of the phase decomposition in Nb–Zr binary alloy with isothermal ageing was investigated experimentally and theoretically. First, the microstructural changes in Nb-30, 40, 50 and 60 at.% Zr alloys aged at 723–973 K were observed by means of a transmission electron microscope (TEM), and the time-temperature-transformation (TTT) diagrams of each composition were obtained experimentally. As a result, it is found that the sequences of the phase transformation in this alloy are divided into four types. Furthermore, by estimating the total free energy of the microstructure and deciding the energetic minimum path, the phase transformation process was predicted theoretically. The diagrams obtained by calculation agreed well with experimental ones, and which suggest that an energetic analysis is useful to predict the sequences of phase transformation of alloys with ageing.