Effects of intermediate temperature on the grain boundary and γ' precipitates of nickel-based powder superalloy under interrupted cooling

The serrated grain boundaries obtained by solution treatment followed by slow-cooling can improve the creep properties of nickel-based superalloy, but such heat treatment simultaneously gives rise to coarse γ' precipitates and deteriorates the high-temperature strength. Herein, the interrupted cooling during solution treatment was used to obtain the serrated grain boundaries and the fine γ' precipitates in a nickel-based powder superalloy by regulating the intermediate temperature. The effects of intermediate temperature on the grain morphology and secondary γ' precipitate size distribution in the alloy were systematically investigated. Flat grain boundaries were formed in the sample at the intermediate temperature of 1140 °C. The serrated grain boundaries appeared in the samples with an intermediate temperature below 1120 °C. The amplitude of the serrated grain boundaries increased from 0.62 to 1.03 µm as the intermediate temperature reduced from 1120° to 1000°C. With the decrease of the intermediate temperature, the secondary γ' precipitates in the alloys changed from spherical to butterfly shapes, and then split. The size of secondary γ' increased from 312 to 456 nm for the alloys with the intermediate temperature between 1120 and 1000 °C. Meanwhile, the tertiary γ' precipitates were produced under the intermediate temperature of 1120–1050 °C, forming a multimodal size distribution of γ' precipitates in the superalloys. The microhardness values of the samples were improved with the increasing intermediate temperature during the interrupted cooling, which was closely related to the size of γ'. As the intermediate temperature rose from 1050 °C to 1120 °C, the tensile strength of the samples increased from 1297 MPa to 1425 MPa at room temperature and from 985 MPa to 1100 MPa at 650 °C.