Microstructural Characterization and Phase Separation Sequences During Solidification of Ni₃Al-Based Superalloy

Abstract

As-cast microstructure of a designed polycrystalline Ni₃Al-based superalloy is characterized using optical microscope, scanning electron microscope, transmission electron microscope equipped with selected area diffraction system, and the intermetallic phase transformations involved during solidification process are determined based on thermal analysis measurements. The as-cast microstructure is mainly composed of 80.63 vol% dendritic and 19.37 vol% interdendritic phases, and the dendrite is identified as quasi-cuboidal $\gamma_{\text{I}}^{\prime }$ phase connected by γ-channels where ultrafine $\gamma_{\text{II}}^{\prime }$ particles are distributed, and the interdendritic phases are determined as γ′–γ eutectic structure consisting of γ _E phase with dotted quasi-spherical $\gamma_{\text{E}}^{\prime }$ particles. During solidification, the dendrite firstly nucleates from liquid melt near 1348 °C; subsequently, the residual liquidoid is transformed into interdendritic phases around 1326 °C. Afterward, γ′ phase will precipitate from dendritic γ-matrix with two-stage characteristics, resulting in the distinct precipitation of $\gamma_{\text{I}}^{\prime }$ and $\gamma_{\text{II}}^{\prime }$ phases when approaching to 1190 and 1043 °C, respectively. The corresponding transformations involved during the solidification process can be translated as: