Microscopic Substructure in Face-centered Cubic Metals Fatigued at Elevated Temperatures

Abstract

The fatigue deformation of 18-8 austenitic steel and aluminum due to high temperature has been investigated mainly from the stand point of cross slip using the replica and transmission electron microscopy technique. The main results obtained are as follows.(1) Remarkable difference is found in the configuration of slip lines in both 18-8 austenitic steel and aluminum when fatigued either at room temperature or at elevated temperatures. In the former of low stacking fault energy, the morphology of surface slip lines formed at room temperature is fine and straight, while at elevated temperatures it is characterized by the thick and wavy appearance. In the latter of high stacking fault energy, on the other hand, the waviness increases with rise in temperature so strikingly that remarkable irregularity on specimen surface occurs. The morphology of deformation structure at high temperatures depends also on the stacking fault energy.The cell structure is more distinct at elevated temperatures than at room temperature.(2) The fatigue crack is initiated at grain boundary in fatigued 18-8 austenitic steel at 500°C, while at 300°C it is initiated at well-developed intrusion and at the region near the grain boundary, where the intensified slip lines including many cross slip lines intersect each other, and finely divided substructures are thus formed.(3) The well-developed substructures are formed near the crack in fatigued 18-8 austenitic steel at elevated temperatures. Therefore the fatigue crack might grow with the same mechanism as that in metals of high stacking fault energy at elevated temperatures. The clear cellular structures on the surface are observed at the crack tip in fatigued aluminum at high temperatures, and fatigue crack grows through the boundaries of these cellular structures of the surface.(4) Many pores were observed on the surface of the specimen of aluminum fatigued at elevated temperatures. It is considered that the pores are formed beneath the surface oxide film by the condensation of vacancies which were abundantly created during fatigue deformation at the elevated temperatures.(5) The characteristic feature in fatigue at elevated temperatures can be explained by considering cross slipping enhanced by high temperature.