Ni-20%Cr alloy is used in the form of small-sized elements, as heating products, in which the mechanisms of creep at high temperature remains relatively poorly understood. Especially, the role played by the dynamic recrystallization, the impact of the initial microstructures, or the size effects on the mechanical behaviour at elevated temperature need to be more in depth studied. The main objective of this work and for the industry is therefore to predict the creep life of the material, and to understand the plasticity and the damage mechanisms at high temperature.

Firstly, mechanical tensile and creep tests (fig. 1(a)) were carried out to characterise the mechanical properties of this material in the form of thick rolled products and fine wire (0.65mm of diameter) for a temperatures range from 800°C to 1100°C, and for several stress levels in the elasticity domain.
Norton creep laws with two distinct coefficients are evidenced depending on the temperature. They are representative of the microstructural mechanisms of creep: the dislocation creep at low temperature and the diffusion creep at high temperature [Poi-85]. These two creep mechanisms are also identified according to an Ashby map [Ash-79]. All our results allow us to draw up a conventional Ashby map for the studied Ni20Cr in the selected stress and temperature ranges.
The second part of this study consists of reproducing the creep tests on smaller dimension specimens fine wire of 0.65mm diameter using an original device to access the fracture time in function of along the temperature and the applied stress. The obtained results are in good agreement with the behaviour of the thicker samples, despite a strong dispersion of the measurements. The size reduction does not seem to have any impact on the creep behaviour of the alloy.
Finally, a microstructure study was undertaken to understand the damage mechanisms (fig.1(b)) in the two identified creep domains. This characterisation will be completed by SEM and TEM analyses, which will allow us to understand the deformation mechanisms at the local scale and to better understand the role played by the plasticity and the diffusion mechanisms.
References
[1] M.F, Ashby C.Gandhi, D.M.R Taplin; Acta metallurgica, 1979, 27, 699-729.
[2] J.P, Poirier Cambridge university press, 1985, 1, 81-102.