Effects of pre-precipitation treatment temperature on the microstructure and mechanical properties of nickel-based powder superalloy

Abstract

The slow-cooling treatment has been observed to induce the formation of serrated grain boundaries, which effectively enhance the creep strength of nickel-based powder superalloys. However, this heat treatment also leads to the coarsening of γ′ precipitates in nickel-based alloys, which impairs their dispersion-strengthening effects. To address this issue, the high-temperature pre-precipitation treatment (HTPT) was employed to simultaneously form fine γ′ precipitates and serrated grain boundaries in the alloys. A systematic investigation was conducted to analyze the impact of various HTPT intermediate temperatures (ranging from 1150 to 1110 °C) on the microstructure and mechanical properties of the superalloy. In this study, a significant increase in the volume fraction of secondary γ′ with a butterfly-shaped morphology, from 1.6% to 27.7%, was observed as the intermediate temperature was decreased from 1150 to 1110 °C. Simultaneously, the average size of spherical tertiary γ′ precipitates decreased from 73.3 nm to 52.7 nm. Serrated grain boundaries were observed in the samples when the HTPT temperature was below 1140 °C, with increasing amplitudes ranging from 2.1 to 2.8 μm. To investigate the formation mechanism of these serrated grain boundaries, we focused on the interactions between γ′ precipitates and grain boundaries. At temperatures below 1135 °C, a fan-shaped structure consisting of finger-shaped γ′ dendrites and γ phases was observed, which potentially contributes to the formation of serrated grain boundaries. Among the samples tested, the sample subjected to an HTPT temperature of 1130 °C showed the best mechanical properties at room temperature (1542 MPa, 12.9% elongation), while the 1140 °C sample exhibited superior performance at 750 °C (1301 MPa, 6.5% elongation). These findings highlight the potential of heat treatment as a novel approach for enhancing the mechanical characteristics of nickel-based superalloys.