Theoretical and Experimental study of Creep Damage in Alloy 800 at High Temperature

International audienceDepending on stress, temperature, and material, transitions in creep fracture mechanisms are extensively observed in metals and alloys. To investigate this transitions, Alloy 800 are subjected to numerous creep tests at temperatures ranging from 500 up to 760∘C.At first, experimental observations show two failure mechanisms: necking fracture for short term testing and intergranular fracture for long term testing.Then, the onset of necking is firstly analyzed using the Hart criterion of viscoplastic instability. The necking evolution with strain is then predicted which allows us to predict lifetime.The Riedel model considers that cavities nucleate continuously and grow by grain boundary diffusion mechanism. The intergranular cavity nucleation rate is assumed to be proportional to the creep strain rate, as stated by the well-known Dyson law. The Riedel model is used to predict long term lifetime.The results show that creep lifetimes are accurately predicted by the combination of the necking model and the Riedel model whatever the chemical composition, stress and temperature.Furthermore, the comparison between the experimental data obtained from different Alloy 800 batches shows that the higher the Ti + Al content the stronger the creep resistance. This phenomenon may due to differences inprecipitation and its influence on the creep strain rate. Such effect is well predicted through the combination of the necking and intergranular fracture models, which both are strain rate dependent.Finally, the measured and the predicted cavity size distributions are compared and the applicability of such combined modeling to various metals and alloys is discussed