INFLUENCE OF THE MICROSTRUCTURE EVOLUTION ON FERRITIC/MARTENSITIC STEELS DURING FATIGUE TESTS

M.F. GIORDANA; P.-F. GIROUX; I. ALVAREZ-ARMAS; M. SAUZAY; A.F. ARMAS

Montpellier

Congreso; European Congress and Exhibition on Advanced Materials and Processes, EUROMAT 2011; 2011

The fast decay characteristics of induced radioactivity imposed on candidate steels of fusion reactors have led to the development of the reduced-activation ferritic/martensitic (RAFM) steels. The Fe-(7-9) Cr ferritic/martensitic steels, such as the European heat EUROFER97 and the commercial Nb-free 12% Cr steel DIN 1.4923, have been studied. Although pronounced ageing did not produce marked changes in the tempered martensite lath structure of the steels, it is evident from previous results that continuous cycling produces changes in the microstructure and a marked cyclic softening. This effect could become a significant engineering problem affecting creep, swelling and segregation phenomena during irradiation. Although the origin of the effect and the kinetics of the softening behaviour are not well understood, it was mainly attributed to the gradual elimination of obstacles, such as dislocation or subgrain/lath boundaries, to the motion of dislocations. The cyclic behavior of these steels as a function of the imposed plastic strain amplitude and temperature has been analyzed in details between 20ºC and 5500C. Under low-cycle fatigue tests these steels show, after the first few cycles, a pronounced cyclic softening accompanied by microstructural evolution. The rate of softening increases with temperature, being very pronounced at temperatures above 500ºC. Although both steels show similar behaviors under these test conditions, the cyclic softening presented in DIN 1.4923 is clearly weaker than the softening exhibit by EUROFER97. The evolution of the flow stress during cycling was studied by analyzing the stress components, the “back” and “friction” stresses, obtained from the hysteresis loops. From this analysis and TEM observations, it can be concluded that the strong cyclic softening observed is produced by the softening exhibited by both stresses. Based on the identified mechanisms of softening a mean field polycrystalline model is proposed to predict both the microstructure evolution and the macroscopic softening at room temperature. Particularly, two different softening mechanisms are modeled: the decrease of the dislocation density inside the subgrains and subgrain size growth. The subgrain growth is mainly due to the disappearance of low-angle boundaries. Using physical parameters determined by microstructural observations, the model predicts the evolution of the dislocation microstructures.