Effects of Zr addition on the microstructure and mechanical behavior of a fine-grained nickel-based superalloy at elevated temperatures

In this study, the effects of the addition of Zr on the mechanical properties and fracture mechanism of a fine-grained TM-321 superalloy were investigated. The results indicated that the tensile properties of the TM-321 superalloy were enhanced by the addition of Zr at room and moderate temperatures, and the creep life of the TZ3 alloy was twice as long as that of the original TM-321 superalloy (TZ1) at 760 °C and 724 MPa because of three major factors. First, the grain size of the TM-321 superalloy was refined by adding Zr. Second, the higher content of Zr in the TM-321 super alloy might have prevented the formation of coarse and long script-like grain-boundary (GB) carbides, but facilitated the formation of fine and particle-like carbides and inhibited GB sliding. Moreover, Zr was effective in increasing GB cohesion and decreasing GB diffusion, thus exhibiting a superior GB strengthening effect. Third, the Zr addition increased the strength of the γ/γ′ microstructure and resulted in the inhibition of dislocation motion. The fracture modes of creep and tensile tests demonstrated that crack propagation along the GBs was reduced as the Zr content increased, whereas secondary cracks within the grain interior increased.