Effect of Strain Range on the Low Cycle Fatigue in  Alloy 617 at High Temperature

Abstract

The aim of this study is to investigate the fully‐reversed low cycle fatigue properties of Alloy 617 in the air at 950 °C; these tests were conducted at total strain ranges from 0.9% to 1.5% with a constant strain rate of 10−3/s. The result of the fatigue tests showed a decrease in fatigue resistance with an increasing total strain range. The reduction of fatigue resistance was due to the effect of the total strain range and microstructure evolution during high temperature, such as brittle oxides cracking. At all testing conditions, the cyclic softening mechanism was observed as a function of the total strain range in the current high temperature condition. An analysis of low cycle fatigue resistance was performed using the Coffin–Manson relationship and the total strain energy density; it was found that Alloy 617 followed these relationships well. In addition, this study compared well with previous work reported in the literature for a similar testing condition. Post‐fracture analysis on the fracture surfaces of failed specimens revealed a more severe damage cracking at the periphery of specimens due to the increase in the total strain range. The surface connected grain boundary cracks induced by oxidation were obvious at low strain range. Thus, the primary crack propagation occurred in transgranular mode from persistent slip bands.

Keywords

• Alloy 617
•  nickel‐based superalloy
•  very high temperature gas‐cooled reactor (VHTR)
•   low cycle fatigue
•  high temperature
•  fatigue damage
•  life prediction
•  solute drag creep
•  oxidation