Effect of Microstructure on the High Temperature Fatigue Properties of Two Ni-based Superalloys

Abstract

There is significant need for Ni-based superalloys in the next generation automotive engine components such as exhaust valves. High temperature, high cycle fatigue life is one of the important properties required for such applications. The focus of this work is to evaluate the effect of microstructure on the high cycle fatigue properties of two Ni-based alloys, alloy 751, an alloy used in these applications at lower temperatures, and Waspaloy. High cycle fatigue lives of the alloys were evaluated using in-situ high temperature fully reversed fatigue tests at 870 o C and a nominal frequency of 30 Hz. Scanning electron microscopy and transmission electron microscopy were used to characterize the microstructure of the alloys. Computational modeling was used to calculate the equilibrium microstructure and microstructural coarsening at 870 o C. Correlation of fatigue properties with microstructure of the alloys shows that for the experimental conditions used in the study, the fatigue life of Waspaloy, which has greater high temperature strength and larger γ' volume fraction, is better than that of alloy 751. IntroductionThere has been a significant interest in recent years in understanding the fatigue performance of materials in the Very High Cycle Fatigue (VHCF) range or Ultrahigh-cycle fatigue range (UHCF) with typical lifetimes greater than 10 7 cycles. In this range of lifetimes, it has been emphasized that fatigue crack initiation becomes much more important than crack propagation [1]. Fatigue failures have been observed at stresses below the conventional high cycle fatigue limit, resulting in significant motivation to understand the microstructural origins of such failures [1,2]. The role of microstructural characteristics on crack initiation in the VHCF regime has been studied in a wide range of materials, including polycrystal and single crystal Ni-based superalloys [1][2][3].Very high cycle fatigue life at high temperatures is an important property for automotive components such as exhaust valves. With the demand for increased thermal efficiencies in engines, there is a need for materials to withstand higher operating temperatures and pressures 559 7th International Symposium on Superalloy 718 and Derivatives Edited by: