Abstract. During mechanical tests at high temperature in an oxidizing atmosphere, the effects of strain rate on crack initiation are noticeable. This effect is due to a coupling between local mechanical loading and oxidation. Samples were machined in a turbine disk made of direct aged alloy 718. Tests were performed to understand the effect of these couplings on crack initiation and to ensure lifetime is optimized. This study compared the cracking resistance of two different specimen geometries at a given plastic strain and performed quantitative measurement of the mechanical loading conditions inducing crack initiation between 600• C and 650• C. Sample geometries consisted in tensile flat specimens and V-shaped samples. This specific geometry was used to localize strain and damage in the apex of the V and to reach strain rates lower than those possible on standard specimens. Digital image correlation technique was used to provide strain measurements. For each temperature and strain rate, finite element calculations using the identified constitutive law were performed to get a refined level of strain in different areas at the V apex. Tests were stopped after an imposed displacement corresponding to a given plastic strain distribution. SEM observations of the surface of the flat tensile samples revealed no crack initiation. On the contrary, SEM observations at the apex of V specimens for which the level of cumulative strain was close to the level of cumulative strain of flat samples reveal the presence of intergranular damage when the strain rate used was below a given level.

OATAO is an open access repository that collects the work of Toulouse researchers and makes it freely available over the web where possible. Nickel-based superalloys are sensitive to an oxidation-assisted intergranular crack (OAIC) growth mechanism. Crack initiation during slow strain rate tensile tests is investigated at 600 and 650 °C, at different strain rates, with or without oxidation on a direct-aged material. A V-shaped sample geometry is used to promote damage initiation for a specific stress triaxiality. The critical mechanical loading paths inducing intergranular crack initiation as well as the effect of oxidation are discussed.