| Zricaloy-4 specimens were heated in the region of β-phase and then cooled in iced brine, water, oil, air and furnace. From the microstructural observation of the dislocation and twin substructures, the characteristics of martensite, Widmanstatten and massive structure were investigated. At the cooling rate of 2,100℃/s (iced brine quenching), martensite having (1011) twin was formed. And at the cooling rate of 1,200℃/s (water quenching), Widmanstatten structure a having dense dislocations within lath, precipitates at lath boundaries and massive a along prior β grain boundaries were observed. This suggests that both diffusion and diffusionless transformations took place in this case. At a cooling rate lower than 1200℃/s, precipitates appeared along the lath boundaries of Widmanstatten αAs the cooling rate decreased, the crystal structure of the precipitates changed in the following manner : cubic→cubic+hexagonal→hexagonal. From the present study of crystal structures, it is thought that the stable structure of precipitates at room temperature was of HCP ZrCr₂ type. In addition, it was observed that the size of the precipitate and the volume fraction of the precipitate were increased as the cooling rate decreased. |
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