Macroscopic behaviour of materials is often controlled by microscopic events; this has driven interest in testing and analysing increasingly smaller features. The ability to perform mechanical tests on the micron-scale, with modelling and high resolution chemical and structural analysis on the same scale, now makes it possible to investigate in detail the mechanisms controlling one of the most complex modes of fracture: stress corrosion cracking (SCC). In this paper, through such a multifaceted approach, we show that individual grain boundaries, preferentially oxidized after exposure to simulated pressurized nuclear reactor cooling water, can be mechanically tested and their resistance to fracture quantified. These results have direct consequences in understanding the mechanisms controlling SCC propagation and initiation.

Rolls-Royce has successfully established a capability for the hot isostatic pressing (HIP) of austenitic stainless steels in nuclear plant. The focus has now been extended to HIP of nickel based alloys, whose good material properties make their use in plant increasingly attractive. A development programme has been undertaken, comparing the mechanical property behaviour of HIP Alloy 600, 690 and 625 with their wrought counterparts. The results are promising, showing acceptable tensile properties and Charpy impact results for the HIP test pieces that are comparable to, or better than, the wrought data. It is concluded that HIP offers an attractive alternative manufacturing route that is ideal for the production of a small number of high integrity components. Hot isostatic pressing microstructures have the advantage of being isotropic and equiaxed, with uniformly fine grain sizes; properties not normally found in thick-section forgings.

The fracture behaviour of individual grain boundaries has been studied in order to understand the mechanisms controlling stress corrosion cracking in nuclear reactors. In particular, the role of oxidation in facilitating crack initiation and propagation has been reviewed. Nickel alloys from pressurized water reactors (PWRs) have been tested in simulated primary water conditions to induce grain boundary oxidation. Microcantilevers containing an oxidized grain boundary plane have been prepared and tested for fracture. The brittle nature of the oxide was demonstrated and the required stress to fracture measured.