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Abstract
Polycrystalline nickel based superalloys are prone to grain boundary attack by atmospheric oxygen either in the form of time dependent intergranular cracking during dwell time within a low cycle fatigue loading spectrum, known as hold time cracking, or in the form of intercrystalline oxidation at higher temperatures. In the case of hold time cracking of IN718 it has been shown that the crack propagation velocity is determined by local microstructure and environmental conditions, reaching values up to 10 μm s−1 under four-point bending conditions at 650°C in air. The governing mechanism for this kind of time dependent quasi-brittle intergranular failure has been recognised to be 'dynamic embrittlement', i.e. diffusion of the embrittling element into the elastic stress field ahead of the crack tip, followed by stepwise decohesion. In a very similar way to intercrystalline oxidation, this damage mechanism seems to depend on the local microstructure. Assuming that oxygen grain boundary diffusivity is particularly slow for special coincident site lattice (CSL) grain boundaries, bending and oxidation experiments were carried out using specimens that underwent successive steps of deformation and annealling, i.e. grain boundary engineering. It has been shown that an increase in the fraction of special CSL grain boundaries yields a higher resistance to both intercrystalline oxidation and hold time cracking by dynamic embrittlement.