With regard to interrupted cooling, we find that it affects the phase transformation rate of ε-ZrD1.8 phase and as a result leads to cracking. As mentioned above, based on the thermodynamic stability of phase structure, δ-ZrD1.66 is more stable at 900 °C, while ε-ZrD1.8 is more stable at 800 °C and 700 °C. So we conclude that at 900 °C and 2 atm δ-ZrD1.66 phase precipitates first, and then partially transforms to ε-ZrD1.8 phase with the next cooling. In the slow cooling, the rest of the δ-ZrD1.66 phase slowly transforms to ε-ZrD1.8, while in the furnace cooling, the rest of the δ-ZrD1.66 phase quickly transforms to ε-ZrD1.8 phase. Thus the martensitic transformation from δ-ZrD1.66 phase to ε-ZrD1.8 phase will take less time during furnace cooling. This will promote shear stress concentration and hence lead to cracking. Fig. 11 gives a schematic diagram of phase percentage in the samples at different interrupted temperatures, in which the needles represent ε-ZrD1.8 phase and the matrix is δ-ZrD1.66 phase. It can be seen from Fig. 11(a) that at the interrupted temperature of 900 °C there exist few needle-like ε-ZrD1.8 phases, while a large number of ε-ZrD1.8 phases precipitate later during the furnace cooling. From Fig. 11(b)–(d) it can be seen that with the decrease of interrupted temperatures, more ε-ZrD1.8 phase precipitate during the slow cooling, which causes less stress concentration and as a consequence reduces the tendency to crack, as shown by Fig. 8. Detailed mechanisms of deuterium absorption cracking in Zircaloy-4 alloys are summarized in Table 3 to provide a better understanding of its cracking behavior.