Study on Microstructure of Zr-2.5Nb Pressure Tube Material

Zr-2.5Nb alloy with hydrogen content 60 mg/g was produced by electrolytical hydrogenation. The microstructure of the Zr-2.5Nb alloy was investigated using X-ray diffraction（XRD）, optical microscope（OM）, and transmission electron microscopy（TEM）. Fractography was performed on the delayed hydride cracking（DHC） fracture surfaces using scanning electron microscopy（SEM）. The results have shown that the matrix of the Zr-2.5Nb alloy is close-packed hexagonal structure, and the grain size is larger（more than 5 mm） away from crack-tip, but smaller（approximately 1um） near the tip of the crack. The hydride with stripy feature is δ-ZrH_1.66 of face-centered cubic structure; it distributes parallel to the rolling direction. The second phase Nb particles, which have the body centered cubic, varying from 50 nm to 500 nm in size, are roundly shaped, appear quite homogeneously distributed in the inner region of the grains, but they were cluster on grain boundaries. When the preset crack direction is parallel to the rolling direction and the test temperature is 250℃, DHC fracture growth process is an intermittent process. Each step of crack propagation results in the crack extension by a distance approximately the length of the hydride, and the length of the hydride is about 20 mm.