Effect of long term aging on microstructure and stress rupture properties of a nickel based single crystal superalloy

A nickel based single crystal superalloy with [001] orientation was prepared by the screw selecting method in the directionally solidified furnace. The long term aging (LTA) of the alloy after full heat treatment was performed at 1040 °C for 200–800 h. The microstructure and stress rupture properties at 980 °C/250 MPa and 1070 °C/140 MPa of the alloy long term aged (LTAed) for different time were investigated. The coarsening γ′ phase and the broadening γ matrix channel are observed in the samples LTAed at 1040 °C, but the γ′ morphology is still in cubic shape after LTA for 800 h. No TCP phase precipitates after LTA for 400 h, while needle shaped and granular TCP phase forms in dendritic core of the alloy after LTA for 600 h. With increasing aging time, the volume of the TCP phase increases and it grows from the dendritic core to the interdendritic region along a fixed direction. The composition of the TCP phase is mainly composed of Re and W. With increasing aging time the rupture life of the alloy at 980 °C/250 MPa and 1070 °C/140 MPa all turns shorter. Finally, the relationship between the microstructure and the stress rupture properties is discussed.