Nano-twinning and uncommon ??-martensite formation as a result of very high cycle fatigue of metastable austenitic stainless steel at 573 K

Microstructural changes were investigated regarding the existence of the true endurance limit of a cyclic loaded metastable austenitic stainless steel AISI 347 in the very high cycle regime at 573 K. The fatigue tests were performed in stress control with a stress load ration R =-1 using a servo-hydraulic testing system with a load frequency of 980 Hz. Local cyclic plastic deformation and slip bands formation were observed even in the specimens that achieved the endurance limit at 5 x 108 cycles. Moreover, in these specimens further microstructural effects like nano-twins and alpha '-martensite formation were detected by transmission electron microscopy. While nano-twin formation takes place in the bulk material, the alpha '-martensite nucleates at the specimen's surface. Therefore, the alpha '-martensitic transformation is not the main mechanism responsible for the true endurance limit observed in the investigated AISI 347 at 573 K.

Automated summary

The microstructural changes of cyclic loaded AISI 347 stainless steel were investigated to determine the true endurance limit at 573 K. Fatigue tests were conducted using stress control and a load frequency of 980 Hz. Even in specimens that reached the endurance limit at 5 x 10^8 cycles, local cyclic plastic deformation and slip bands formation were observed. Transmission electron microscopy revealed additional microstructural effects such as nano-twin formation and alpha '-martensite formation. However, the alpha '-martensitic transformation is not the main mechanism responsible for the observed true endurance limit in AISI 347 at 573 K.