In-situ tensile deformation of Zircaloy-4 sheets with different hydrogen contents at RT and 300 °C

Highlights: • A ductile-brittle transition occurs at hydrogen content of 850 ppm at RT. • Plastic deformation is reduced with the increase of hydrogen content at RT. • Intersect between hydrides is the reason for the ductile-brittle transition. • The ductile-brittle transition can be inhibited below 1300 ppm hydrogen at 300 °C. In this paper, the in-situ tensile deformation behaviors of Zircaloy-4 sheets with different hydrogen contents were investigated at room temperature (RT) and 300 °C. The results showed that at RT, the ultimate tensile strength (UTS) increased from 407 MPa to 508 MPa and the reduction of area (RA) decreased from 51% to 29% with the hydrogen content increased from 0 ppm to 850 ppm. When the hydrogen content reached 1300 ppm, both the UTS and RA dropped abruptly, showing an obvious ductile-brittle transition. The in-situ observations showed that for the un-hydrided samples, with the increase of tensile deformation, there first appeared slip deformation, then grain distortion and elongation, and finally crack initiation at the grain boundaries. For the hydrided samples, due to the precipitation of hydrides, the slip deformation and grain distortion were reduced, and the cracks trended to initiate around the hydrides. At 300 °C, the UTS increased from 246 MPa to 318 MPa and the RA decreased from 66% to 55.7% with the hydrogen content increased from 0 ppm to 850 ppm. When the hydrogen content reached 1300 ppm, no ductile-brittle transition was observed. The in-situ observations showed that for the un-hydrided and hydrided samples, with the increase of tensile deformation, they both exhibited good plastic deformation except more cracks initiated around the hydrides in the hydrided samples. The transmission electron microscope observations showed that at RT, with the increase of hydrogen content, the number of hydrides increased, and when the hydrogen content reached to a high level, these hydrides would intersect or collide with each other. However, at 300 °C, these hydrides were observed to be twisted or kinked after tensile deformation, which showed a good deformation ability. Based on these observations, the tensile deformation behavior of the hydrided samples was discussed.