HYDRIDE MORPHOLOGY AND STRIATION FORMATION DURING DELAYED HYDRIDE CRACKING IN ZR-2.5-PERCENT NB

These experiments were designed to study hydride formation at the crac k tip, acoustic emission (AE), potential drop (PD) and striation forma tion during DHC (delayed hydride cracking) in Zr-2.5% Nb. The test mat erial was taken from an especially extruded pressure tube, which showe d similar strength properties to normal pressure tube material but som ewhat coarser microstructure. In testing at K-I below 12 MPa root m at both 200 and 250 degrees C very large striations (> 40 mu m at 200 an d > 50 mu m at 250 degrees C) were produced. In simultaneous monitorin g with acoustic emission and potential drop, both AE and PD jumps were shown to be monolithic. The number of striations on the fracture surf ace corresponded to the number of monolithic AE/PD jumps. Tapered shap ed hydrides with the thick end adjacent to the crack tip were observed . These hydrides grew in size during the incubation period until they reached the striation length and then fractured monolithically. Howeve r, when K-I was increased beyond about 12 MPa root m for these same sp ecimens, the striation spacing decreased below 30 mu m, the monolithic jumping dissolved into more continuous changes in signals, although t he smaller striations were still visible on the fracture surface.