Micro-mechanical measurement of fracture behaviour of individual grain boundaries in Ni alloy 600 exposed to a pressurized water reactor environment

A micro-mechanical technique is used to determine values of the critical stress intensity factor for fracture for grain boundaries of various orientations in Ni alloy 600 exposed to Pressurized Water Reactor (PWR) primary water at 325 °C with a hydrogen partial pressure of 30 kPa. Pentagonal cross-section cantilevers 5 μm wide by 25 μm long were milled using a focused ion beam (FIB) at individual grain boundaries in unoxidised Alloy 600 samples and in samples that had been exposed to simulated PWR environment for 4500 h and for 1500 h. The cantilevers were notched at the grain boundaries using FIB and tested using a nanoindenter to deflect them in bending. The critical stress intensity factor for the fractured cantilevers in samples that had been exposed for 4500 h was measured to be between 0.73 and 1.82 MPa(m)1/2. No intergranular fracture occurred in the samples that had been exposed for 1500 h and in the unoxidised samples. No direct correlation was observed between the grain boundary misorientation angle and the critical stress intensity factor.