Behaviour and modelization of a 17-12 SPH stainless steel under cyclic, unidirectional and bidirectional anisothermal loadings

It is shown in this article that a knowledge of the mechanical behavior of the 17-12 SPH stainless steel for different isotherms (20 degrees C less than or equal to T less than or equal to 650 degrees C) is insufficient to describe its behaviour correctly under anisothermal thermomechanical loadings. Indeed, this alloy possesses a temperature history memorization effect at intermediate temperatures. This phenomenon is studied in detail with experiments performed under uniaxial and biaxial mechanical loadings (in-phase or out-of-phase tensile-torsion tests) for different isotherms belonging to judiciously chosen temperature history sequences. The results of cyclic torsion tests under anisothermal thermomechanical conditions are presented where strain and temperature loading evolve simultaneously in phase, in phase opposition, in quarter- and three-quarters-phase. The analysis of the totality of results shows that the material memorizes the maximal value of the cyclic stress peak reached in the zone where the derivative of the maximal cyclic stress with respect to the temperature is positive. This observation can also be applied to both uniaxial and biaxial cyclic tests. From a physical point of view, this behavior is closely related to the interaction phenomena between dislocations and point defects in insertion solid solutions. In order to describe this history effect, a simple mathematical formulation is proposed in the second part of this article which, after being integrated into a unified viscoplastic model developed elswhere and taking into account a few modifications, leads to an acceptable phenomenological representation of the different experiments presented earlier.