Microstructure formation in γ–γ′ Co–Al–W–Ti alloys during directional solidification

Abstract

The microstructure formation process in Co–Al–W–Ti alloys was investigated through quenching performed during directional solidification. The phase transition route of the Co–Al–W–Ti alloys during directional solidification is determined as follows: L→L+γ→L+γ + (β+γ′)→L+γ+γ′+(β+γ′)→γ+γ′+β+(β+γ′). The eutectic (β+γ′), primary γ′ and β are orderly deposited in terminal stages of solidification. The eutectic γ′ and β are well distributed and nucleate in random directions. The γ′ particles are precipitated initially from the interdendritic Al-rich regions and subsequently from the dendritic region. The sizes of the γ′ particles close to the primary γ′ and β phases are greater than that of the particles far from them. Since the growth of eutectic colonies restricts the interdendritic liquid convection at earlier stage of the terminal solidification, Co–Al–W–Ti alloys are resistant to the grain defect formation and suitable for fabricating single crystal components.