Effect of gradient microstructure pinned by δ phase on elevated temperature performances of GH4169

A microstructure of gradient grains pinned by gradient δ phase was obtained within GH4169 by ultrasonic rolling followed by a post heat-treatment at either 900 °C or 954 °C for 5, 15 or 30 min, respectively. In order to evaluate the high temperature stability of such gradient microstructure, the variations of grain size and gradient microhardness of GH4169 were measured after being kept at 650 °C for 10–100 h. Hot tensile tests at 650 °C were conducted to reveal the effects of refined grains and δ phase on mechanical properties. The results showed that the amount of δ phase decreased with the depth increasing, while recrystallized grain size presented the opposite trend. And the cross-sectional microhardness within strengthened layer decreases as depth increases after post heat-treatment. The ultrasonic rolled sample treated at 900 °C for 15 min (900–15) brings a relatively finer δ phase which distributes both in grains and grain boundaries. Such microstructure results in a better pinning effect compared with the one treated at 954 °C for 15 min (954–15) with less δ phase only distributed in grain boundaries. The gradient refined grains and δ phase contributes to the improvement on tensile strength at 650 °C in Sample 900–15 with YS (Yield strength) = 903 MPa, UTS (Ultimate tensile strength) = 988 MPa (21% and 6% improvement compared with Sample 0-900-30, respectively). The gradient microstructure with refined grains pinned by δ phase showed good thermal stability and mechanical properties at 650 °C.