Effect of Heat Treatment on the Evolution of Serrated Grain Boundaries and Tensile Properties of a Casting Ni-based Superalloy

Abstract:

This work investigated the mitigation of intermediate temperature embrittlement in casting Ni-based superalloys for gas turbine blades. By tailoring the post-solution cooling rate and water quenching termination temperature, we analyzed the impact of the synergistic evolution between γ′ phase precipitation and grain boundary (GB) geometry on 650 °C tensile performance. It was found that lower cooling rates and quenching termination temperatures promote intragranular γ′ phase coarsening and a transition from planar to serrated GB morphologies. A cooling rate of 5 °C/min led to SGB-1 type serrated GBs via strong γ′ phase pinning, while 1 °C/min resulted in SGB-2 type serrated GBs through the synergistic migration of coarse γ′ phases and GBs. Although yield strength at 650 °C decreased with reduced cooling rates, ductility was significantly enhanced, with the SGB-2 sample achieving an elongation of 8.5%. The brittleness observed in air-cooled samples was attributed to a mismatch between strong grain interiors and weak grain boundaries. In contrast, slow-cooled samples effectively alleviated this brittleness by sacrificing some intragranular strength in favor of GB strengthening through serration, creating a beneficial "strong GB–weak grain interior" balance.