The Effects of Dwell on the LCF Behavior of IN617

Abstract

Much has been studied on the individual effects of creep and fatigue on alloy life. However, not much is known of the combined effects of these two mechanisms. Therefore, a study was put into place to determine the effects of dwell on fatigue life and deformation mechanisms, for IN617, a solid-solution strengthened Ni-base alloy used widely in the power generation and aerospace industries. Low cycle fatigue (LCF) tests were conducted from 649–982°C with either tensile or compressive dwell. Fracture surfaces of the test specimens as well as longitudinal and transverse sections were examined via scanning electron microscopy to determine the damage and failure mechanisms. Test results confirmed tensile dwell lives that were significantly lower than those seen in compressive dwell. The mechanics for the reduction in cyclic life for tensile dwell was attributed to creep damage accumulation at grain boundaries that led to widespread intergranular cracking and failure. Tensile dwell life reductions were largest in tests at moderate (649–760°C) temperatures. The failed specimens for this temperature range showed the most evidence of grain boundary cavitation and intergranular cracking. At higher test temperatures, the tensile dwell sensitivity for IN617 was significantly reduced or almost entirely eliminated at high temperatures (871–982°C). This was attributed to the lower stresses that developed at these temperatures for a given strain range. The LCF testing and subsequent analysis indicated that a substantial tensile stress during dwell time coupled with moderate temperatures, to allow for diffusion creep, lead to grain boundary damage that can reduce cyclic life.