Influence of Hydrogen Content on Axial Fracture Toughness Parameters of Zr-2.5Nb Pressure Tube Alloy in the Temperature Range of 306-573 K

Tubes fabricated from dilute Zr-alloys serve as miniature pressure vessels in Pressurized Heavy Water Reactors and are subjected to stress, aqueous corrosion and intense irradiation during service. Hydrogen evolved during the corrosion reaction may enter into the material and precipitate as hydride phase, which acquire platelet shaped morphology in Zr-alloys and are known to embrittle the host matrix. Since hydride embrittlement is a major life limiting factor for the components made from these alloys, several theoretical and experimental studies have been carried out to understand the influence of hydrogen/hydride on the mechanical properties in general and micromechanisms assisting crack nucleation and its propagation in the presence of hydride, in particular. For ductile materials like Zr-alloys, crack initiation follows void nucleation and its growth in the plastic zone. Nucleation of voids is associated with fracture of second phase particle or separation of matrix-precipitate interface. Hydrides are suspected to be fracture initiating sites in Zr-alloys and the presence of hydride platelets normal to tensile load significantly influences crack propagation. However, the role of hydrides in crack nucleation and its propagation and influence of temperature on the same has not been delineated clearly. In this work, influence of hydrogen and temperature on the axial fracture toughness parameters of Zr-2.5Nb pressure tube alloys is reported. The fracture toughness tests were carried out using 17 mm width curved compact tension specimens machined from gaseously hydrogen charged tube-sections and tested in the temperature range of 306 to 573 K. Metallography of the samples revealed that hydrides were predominantly oriented along axial-circumferential plane of the tube. The fracture toughness parameters like JQ, J0.15, JMax, J1.5, dJ/da, KJC and KMax were determined as per the ASTM standard E-813, with the crack length measured using direct current potential drop technique. The plane strain K values were computed from the corresponding J values. The critical crack length for catastrophic failure was determined using a numerical method, which is widely used in literature. It is observed that for a given test temperature both the fracture toughness parameters representing crack initiation, such as JQ, J0.15 and KJC and crack propagation, such as JMax, J1.5, and KMax, decrease mildly with increase in hydrogen content whereas mean dJ/da is practically unaffected by hydrogen content. Also, for a given hydrogen content crack initiation fracture toughness parameters showed large scatter with a tendency to decrease with increase in test temperature whereas the crack propagation fracture toughness parameters increased with temperature to a saturation value