Fretting fatigue damage mechanism of Nickel-based single crystal superalloys at high temperature

A novel high temperature fretting fatigue test apparatus was developed to investigate the fretting fatigue mechanism of Nickel-based single crystal (NBSX) superalloys at elevated temperature. The fretting fatigue tests were carried out under 5 loading conditions at 600 degrees C. Results showed that the fretting fatigue life decreased with the increase of axial load and normal load. Metallographic observations revealed that surface peeling, delamination occur at the contact area. Energy dispersive spectrum (EDS) analysis showed that the surface material was oxidized, and the oxides will be compacted onto the contact surface and form a glazed layer. Besides, many micro cracks are observed at the contact surface that are nearly perpendicular to the fretting direction. These micro cracks would either be eliminated due to the large relative displacement or grow into the main crack, which are supposed to be the dominant source of fretting fatigue failure. Multiple fretting fatigue cracks initiate at the contact leading edge area and grow along the (100) plane that is nearly perpendicular to the fretting direction. Then the crack propagation direction will deflect from (100) plane to a series of {111} planes and the specimen eventually failed. The combined effect of fretting wear and crystallographic slip is the cause of fretting fatigue failure.