Microstructural and Mechanical Characterization of Directionally Solidified Conventional and Nb-Modified Mar-M247 Superalloy

Abstract

This study analyzes the effects of replacing Ta by Nb (at.% basis) in the phase transformation temperatures, Scheil solidification behavior, and microstructural mechanical features of a Nb-modified as well as conventional Mar-M247 superalloys. Both alloys were directionally solidified at a withdrawal rate of 18 cm/h under a thermal gradient of about 80 °C/cm. DSC results were compared to thermodynamic simulations of phase transformation temperatures and showed a good correlation. The replacement of Ta by Nb has not altered the solidification path of Mar-M247, keeping the sequence L → L + γ → L + γ + MC → L + γ + MC + γ/γ′ eutectic. Due to the segregation of Hf and Zr to the very last liquid to solidify, a low melting point γ/Ni5(Hf,Zr) constituent was observed. The as-solidified microstructures of both Mar-M247(Nb) and Mar-M247 were constituted of columnar dendrites of gamma matrix (γ) phase with cuboidal gamma-prime (γ′) precipitates. In addition, γ/γ′-eutectic was observed in the interdendritic region along with MC carbides in blocky, script (blocky dendritic), needle-like and fine nodular morphologies. Both alloys contained about 1.5 vol.% of MC carbides. The MC carbides in Mar-M247(Nb) and Mar-M247 are, respectively, Nb-rich and Ta-rich, with only a limited amount of Hf being detected in the MC phase. The yield stress of Nb-modified Mar-M247 showed higher values up to 750 °C. For higher temperatures, above 800 °C, a conventional Mar-M247 presented a higher value of yield stress.