Effect of withdrawal rate on the microstructure and mechanical properties of a novel monocrystalline CoNi-based superalloy

Withdrawal rate is a critical parameter that affects the microstructure during single-crystal growth. In the present work, the effect of the withdrawal rate (3, 4.5, 6, and 9 mm/min) on the microstructure and mechanical properties of a new CoNi-based single-crystal alloy in the as-cast and heat-treated states was investigated in detail. As the solidification distance decreased with increasing withdrawal rate, the dendritic structures and γ′ phase in as-cast microstructures were significantly refined, the element (e.g. W, Al, Ta, Ti, etc.) segregation aggravated, and the volume fraction of the intercrystalline phase increased from 11.9% to 25.2%, which led to a decrease in the creep rupture life (from 90.4 h to 57.8 h) of the as-cast alloy at 1000 °C/150 MPa. By contrast, the microstructure and mechanical properties of the heat-treated alloys did not appear to be a strong function of the withdrawal rate, which was probably due to the fully heat treatment eliminated the effects of the withdrawal rate and homogenized microstructure. This investigation provides theoretical support and a practical basis for the growth of large-scale single crystals from CoNi-based superalloys.