Effect of aging treatments on microstructure evolution and mechanical properties of a new Fe-Ni-based superalloy

The effect of aging treatments on the microstructure evolution and mechanical properties of a newly developed Fe-Ni-based superalloy was investigated. It was observed that aging at 680℃ led to minimal γ′ nucleation, providing limited resistance to dislocation movement during tensile testing, which allowed significant plastic deformation. Consequently, the alloy exhibited low strength but high plasticity. In contrast, aging at 850°C promoted dense γ′ precipitation within the matrix, along with the formation of acicular precipitates and secondary TiC along grain boundaries. After aging at 680℃ followed by 850℃, the densely precipitated γ′ effectively hindered dislocation movement by forming Orowan loops, dislocation bowing and strongly coupled dislocation pairs. Simultaneously, acicular precipitate formation was significantly suppressed, while secondary TiC enhanced grain boundary strength, resulting in improved tensile strength and elongation. Conversely, after aging at 850°C followed by 680°C, the precipitation of M₂₃C₆ and coarsened acicular precipitates with axial dimensions exceeding 0.5 μm at the grain boundaries significantly deteriorated grain boundary integrity. In particular, the acicular precipitates hindered grain boundary sliding, resulting in dislocation pile-up and stress concentration. Consequently, intergranular cracking increased, causing a 41.4% reduction in elongation. The acicular precipitates formed when coarsened γ′ at the grain boundaries absorbed excess Ti, undergoing orientation rotation and directional growth. During aging at 680°C followed by 850°C, the γ′ precipitated at 680°C reduces the matrix supersaturation, decreasing the nucleation driving force and promoting more stable γ′ formation during the subsequent 850°C aging. As a result, fewer acicular precipitates are formed.