A Ti-stabilized cold-worked 15Cr-15Ni steel, called AIM1 (Austenitic Improved Material #1), has been selected as a candidate for the fuel cladding tubes of sodium-cooled fast reactors. This steel exhibits an unusual loss of ductility between 20 and 200 °C for both solution-annealed and cold-worked conditions, which is similar to that observed for Twinning Induced Plasticty steels and for the 200 and 300 series stainless steels. Therefore, a multi-scale study has been carried out to determine the deformation mechanisms that are active at different temperatures. Tensile tests have been performed to characterize the macroscopic material behavior, and Electron Backscattered Diffraction and Transmission Electron Microscopy characterization techniques have been used to investigate the meso and micro-scale phenomena, such as the deformation microstructures and the evolution of the lattice defects. The parameters governing the deformation mechanisms have been examined, with particular attention paid to the conditions for mechanical twinning activation. This work required an original study of the variation of Stacking Fault Energy with temperature, based on the measurement of the dissociation extension of dislocation nodes. An increase in the SFE was observed between 20 and 200 °C. After reviewing the existing models for predicting twinning, the present study proposes an approach based on the minimization of the total energy of the material to explain why twinning is not favorable at high temperatures. At 20 °C, both dislocation slip and twinning are active and efficient mechanisms to release the strain energy. However, at 200 °C, only dislocation slip is favorable and is often associated with dislocation cross-slip.