Fatigue & Fracture behaviour of Zr-alloy processed through SPD

Abstract

In Nuclear fuel complex, Hyderabad the bars of Zircaloy-4 (Zr-4) are used for the production of endcaps which is a critical component in terms of quality for Pressurised heavy water reactor (PHWR) fuel assembly. These endcaps are produced using cold swaging process. The cold swaging process comprises several steps for reduction in diameter of bars including several intermediate vacuum annealing process. These endcaps are used to seal the nuclear fuel rod from both the ends. Presence of even a minute crack will lead to leakage of radioactive substance into coolant during operation. In the present work a deep analysis has been carried out on microstructural and mechanical properties of Zircaloy 4 at each of these intermediate steps so that the gradual changes in the properties can be determined. The Zircaloy-4 bar was subjected to cold swaging to reduce its cross section gradually from 23 mm diameter to 16 mm diameter via several passes of swaging followed by intermediate vacuum annealing at 732° for 3hr. Tensile test has shown increase in Tensile strength of the swaged sample with decrease in ductility. The dislocation density and fine grains formed during cold swaging impart improved yield strength to the deformed alloy. EBSD analysis helps to in understanding the grain orientation, size as well as twinning which helps in deformation. While TEM analysis was conducted to visualize the presence of dislocations density inside the material. Amount of dislocation density was determine using XRD and KAM studies. Corrosion test was conducted to determine the corrosion rate and it is found that the corrosion rate is decreasing with the intermediate steps and found to be least for the final product. Fracture analysis has been carried out on few samples to determine the fracture toughness at crack initiation and simulated fatigue results also indicates that the fatigue life is improving for the final product. All these results indicates that the process which is carried out to reduce the diameter of the rod via several swaging and annealing treatment helps in improving the desired properties of the material.