Mechanical and Microstructural Characterization of AF955 (UNS N09955) Nickel-Based Superalloy After Different Heat Treatments

The mechanical and strain-hardening behaviors of the new AF955 nickel-based superalloy were investigated through two different heat treatments. The first consisted of a solubilization with a subsequent precipitation heat treatment at 746 °C for 4 hours, while the second included an additional precipitation treatment at 621 °C for 8 hours, which further increased the AF955 yield stress by about 15 pct and the ultimate stress by about 9 pct. However, through analyzing the true stress and true strain flow curves, the Considere’s stresses of AF955 after the heat treatments were similar and the strain-hardening behaviors at high stresses were surprisingly comparable. The AF955 microstructures were observed after the two different heat treatments through transmission electron microscopy. The dimensions and volume fractions of the strengthening γ″ and γ′ particles were quantified through the imaging analysis technique, finding that there were only γ″ particles in AF955 with heat treatment at 746 °C, while with the additional heat treatment at 621 °C, there was a higher total volume fraction of the γ″ + γ′ phases. The microstructure quantification allowed modeling of the different yield behaviors of the alloy after the heat treatments through the Orowan model for nondeformable particles and the weak coupled dislocation (WCD) and strong coupled dislocation models for deformable particles. The WCD model for deformable particles described the yield behaviors of AF955 very well after both heat treatments. Moreover, the deformability of the γ″ and γ′ particles also explained the comparable strain-hardening behaviors at high stresses of AF955 after the two different heat treatments. Although mechanical properties are correctly assumed to be key parameters for classifying materials, the analysis of true stress and true strain flow curves always should be performed to properly rationalize the mechanical behaviors of metallic alloys.